tennessee x ray · 2020. 9. 21. · x-ray news volume 26, issue 2 page 5 ann watson, r.t. (r)...

Puzzle Challenge 3

X-Ray Quiz 3

Amazing Facts 3

Online Access Instructions 4

Answer Sheet Instructions 4

DR-101 Article 5

DR-101 Post-Test 24

Link to XRN Order Form 27

Link to RES Enrollment 27

Link to CE Enrollment 27

Change of Address Form 28

INSIDE THIS ISSUE

Volume 26, Issue 2

Apr-Sept 2020

Tennessee X-Ray

Direct Reading DR-101

Radiographic Image

Quality

The Tennessee Board of Radiologic Imaging and Radiation Therapy usually tries to meet 4 times each year. The minutes from those meetings are available on the x-ray board site at https://www.tn.gov/health/health-program-areas/health-professional-boards/xray-board.html. The board has completed updating the x-ray rules for x-ray operators. These proposed new rules are set to go through a lengthy rulemaking process in December to become law in 2021. By reading the minutes, x-ray operators can see what changes might be coming to their license. Here are some of the changes that will immediately affect your license:

1. The passing score for the limited examinations will change from 65% to 70%.

2. The due date for proof of attendance and completion of the required continuing education hours will be each license holder’s biennial renewal due date.

3. The 2 special credits on rules and ethics will be removed from the rules. Scope of practice for full licenses will be limited to only the modality in which they are certified. There will be a 2 year exemption period for anyone working on a modality to upgrade their ARRT license.

4. Required continuing education (CE) will increase to 24 credits.

Rulemaking Process to Begin

If you renew your license next year in 2021, you need to complete your 20 CE credits by December 31, 2020 to be compliant with current state rules.

Even though you must complete your CE credits within the 2 preceding calendar years, the due date for proof of that completion is not until your biennium renewal due date.

Renewing in 2021?

Limited Scope Exam

Limited Scope Testing (LST) provides application processing services for eligible students wishing to sit for the American Registry of Radiologic Technologists (ARRT) Limited Scope of Practice in Radiology examination and the Bone Densitometry Equipment Operator examination for the state of Tennessee. Information is provided at www.limitedscopetesting.com.

https://www.tn.gov/health/health-program-areas/health-professional-boards/xray-board.html



http://www.limitedscopetesting.com

X -RAY NEWS VO LUME 26, ISSUE 2

1. You must obtain 20 continuing education credits every biennial period. The easiest way to accomplish this is to subscribe to X-Ray News on a yearly basis. X-Ray News will provide 10 CE credits each year if completed timely.

2. These credits must be completed in the 2 calendar years prior to your renewal date i.e. by the end of December the year before you renew. If you renew your license next year in 2021, then

you will need to complete your 20 CE credits by December 31, 2020.

3. Your renewal period is every 2 years with the renewal date being the last day of your birth month. If you were born in an odd year then your renewal year will always fall in an odd year and vice versa.

4. Be sure to keep your address current with the state so you will receive your notice to renew. This is your responsibility. The state does not forward mail. You can update

your address online at www.x-raynews.com under TN X-Ray License on the top menu. Under the heading TN State X-Ray Links click on the link License Renewal & Address Update.

A Continuing Education Publication for Limited Radiographers

Important Facts to Remember About Your TN Medical X-Ray License

X-Ray News Office and Staff

X-Ray News Staff

Donna Smith, editor Ann Watson, technical editor James Becker, production assistant

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is published 3 to 4 times per year with each issue containing 2 or 4 credits.

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• X-ray technology typically focuses on detecting small details inside the human body, but in 1979, the technology was turned towards larger targets when NASA used x-ray technology to take amazing images of outer space. NASA helped pioneer the development of x-ray astronomy and continues to operate today.

• Until it was known that x-rays were dangerous for those exposed to them, they were used as novelty at carnivals and circuses like modern photo booths. Shoe stores even used x-rays to ensure a better fit of shoe using the shape and size of the bones?

• The x in x-ray stands for unknown. Roentgen named them x-rays because he detected the rays but didn’t know quite what they were.

Puzzle Challenge

Amazing Facts

X-Ray Quiz

There are far better things ahead than

any we leave behind.

C.S. Lewis

The words used in this puzzle are taken from the article, DR-101: Radiographic Image Quality. Words may read normally, from right to left, bottom to top, top to bottom, or on any diagonal. Solution is below.

Puzzle Words

absorption anatomy anode attenuation brightness density

distortion exposure fog grid milliamperage oid

photons quantum roentgen scatter sid

2. Higher kVp settings produce images with: A. long-scale contrast B. short-scale contrast C. high contrast D. little or no recorded

detail

1. If the radiographic image is overexposed, which of the following changes in exposure factors should be used to correct the problem? A. decrease kVp B. increase kVp C. increase mAs D. decrease mAs

3. A change from the small focal spot to the large focal spot results in: A. decreased resolution B. magnification C. distortion D. increased contrast

1. D 2. A 3. A


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Ann Watson, R.T. (R) (ARRT) and Donna H. Smith, B.S.

Radiographic Image Quality Direct Reading

DR-101 Approved for

4.0 CE Credits

X-rays were accidentally discovered in Germany on November 8, 1895, during cathode tube experiments conducted by Wilhelm Conrad Roentgen. Through experimentation, Dr. Roentgen realized this new ray was able to penetrate material, including the human body as well as the walls of his laboratory. Roentgen delivered a paper entitled: On a New Kind of Rays, detailing his findings to the Wurzburg Physico-Medical Society. In the paper, he acknowledged that he did not know the precise nature or purpose of these new rays. He chose to name this new discovery x-ray, since x is the mathematical symbol for unknown. X-rays are also known as Roentgen rays in honor of Dr. Roentgen. For his discovery, Roentgen was awarded the first Nobel Prize in Physics in 1901.

At the time, most people did not understand the magnitude of Roentgen's achievement. However, it soon became clear that he had stumbled upon something vastly significant for the advancement of not only medicine, but also for science. The news of Roentgen’s discovery of this powerful investigative tool spread quickly throughout the world. The medical community was eager to know and understood the impact of his finding. The possible diagnostic uses of x-rays were astounding. Six months following Roentgen's breakthrough, physicians were using x-rays to locate bullets and other foreign objects in patients. What was not known at the time was that the unseen x-rays were causing permanent damage

and often led to drastic consequences for early experimenters. Within the first few years after the discovery of x-rays, the lethal effects became apparent. Unknowingly and unfortunately, some of the early pioneers became radiation martyrs, suffering overexposure that required the amputation of limbs, caused malignancies, and even caused death. These tragedies led to greater awareness of radiation hazards for health care workers. Today protective measures for x-ray operators from overexposure to x-rays include: 1. minimizing the time of personal exposure, 2. maximizing the distance from the x-ray tube, and 3. maximizing the use of shielding.

Scientists everywhere could duplicate Roentgen’s experiment because the cathode tube was very well known. In January of 1896, Dr. Henry Louis Smith, a physics professor at Davidson College in North Carolina, read about the discovery of x-rays. He realized that Davidson College possessed the right equipment to duplicate Roentgen’s x-ray experiments. Dr. Smith was soon able to use x-rays to assist doctors at a local hospital. Dr. Smith used his x-ray machine to locate a broken needle in a man’s knee. This procedure is one of the first documented medical uses of x-rays in the United States.

Radiography is the art and the science of producing radiographic images. X-ray operators simultaneously use their knowledge of anatomy, patient positioning, physics, exposure factors, communication, radiation protection, and patient care skills. The use of radiographic imaging is an integral part of diagnostic medicine. A high-quality radiographic image is an invaluable asset to the clinician as a diagnosis is determined. The quality of a radiographic image is not easy to define and cannot be precisely measured. The term radiographic quality

Fig. 1 Wilhelm Conrad Roentgen

Fig. 2 Early experimenters taking an x-ray Fig. 3 X-ray operator


refers to the accuracy with which the anatomic structure being examined is imaged on the radiograph. Therefore, a radiograph that authentically reproduces anatomic structures is generally identified as a high-quality radiograph. Understanding how the radiographic image is produced is essential for the radiographer. A primary responsibility of the radiographer is to evaluate radiographic images to determine if sufficient information exists for a diagnosis. Evaluating the image includes understanding the 4 primary factors that directly affect how a radiographic image looks: density or brightness, contrast, spatial resolution, and distortion. This article introduces the nature of the radiographic image and the factors that affect its diagnostic quality. Since the discovery of x-rays, this permanent image has resided on a variety of media, including glass plates, radiographic film, and the most recent, digital images stored on computer systems. The radiograph must meet certain requirements to be of any diagnostic value. The overall technical value of a radiograph depends on the compatibility that exists among these 4 primary factors.

X-Ray Production

X-rays are produced inside a glass envelope called an x-ray tube. The tube contains 2 electrodes: a negatively charged cathode and a positively charged anode. X-rays are created when free electrons are accelerated from the cathode to the anode. When these high velocity electrons collide with the metal anode, x-rays are created. In addition to creating x-rays, this process also creates large quantities of heat.

To produce a radiographic image, x-ray photons must pass through the patient’s anatomy and interact with an image receptor (IR). The IR can be described as a device that receives the radiation exiting the patient. A radiographic image can be acquired from 2 different types of IR: digital and film-screen. The process of creating an image is the same for both digital and film; however, the acquisition, processing, and display is different.

A radiographic image is created by exposing a

patient to a primary x-ray beam. The x-ray beam may be referred to as beam, photons, or radiation. The primary x-ray beam leaves the x-ray tube and passes through the collimator toward the patient. As the primary beam passes through the patient’s anatomy, it will lose some of its energy. This decrease in the energy of the primary x-ray beam is called attenuation. Beam attenuation occurs because the x-ray photons interact with the patient’s anatomy. Three distinct processes occur during beam attenuation: absorption, scattering, and photon transmission.

1. Absorption

As the primary x-ray beam passes into the patient, some of the x-ray photons are completely absorbed by the patient. In this scenario, all the photon’s energy is absorbed by the patient. The absorption characteristics of the anatomy are determined by its composition, such as its thickness and atomic number. The probability that a photon will be completely absorbed by the patient increases as the thickness or the atomic number of the anatomy increases. Therefore, in dense structures such as bone, absorption of photons is highly likely. Absorption of photons in less dense structures such as air-filled cavities is less likely to occur. When photons are absorbed by the anatomy, they contribute to the radiation exposure of the patient.

2. Scattering

Not all the primary x-ray photons are absorbed by the patient. Instead, some of the photons interact with the patient’s anatomy. These photons lose energy while traveling within the patient and their direction of travel is changed. Photons with a new direction of travel have less energy than primary beam photons. This process is called scattering. Photons with a changed direction of travel are called scattered photons or scattered x-rays or scatter radiation. Scattered photons may be absorbed within the anatomic tissue of the patient and increase the patient’s exposure. Or they can either leave the patient to interact with the IR resulting in fog on the image, or they can exit the patient and contribute to the radiation exposure of anyone near the patient,

Fig. 4 X-ray tube

Fig. 5 Radiography


including the radiographer. Scatter radiation is detrimental to the radiographic image and it is detrimental to human tissue.

3. Transmission

If an incoming x-ray photon passes through the anatomic part without any interaction with the atomic structures, it is called transmission. These transmitted photons that emerge from the patient are referred to as exit or remnant radiation. Scatter radiation can also be a part of the remnant rays. Remnant photons exiting the patient will do so with varying energies. The information carried by this remnant beam (varying amounts of scattered and transmitted photons) must be transferred to an IR to become the invisible, latent image. The latent image is converted to a permanent, visible image by processing.

Areas within the anatomy that absorb primary photons (such as bone) create the light areas on the radiographic image. Areas in the patient that absorb many photons are called radiopaque. Primary photons that are transmitted through the patient (such as lungs) create the black areas on the radiographic image. Areas in the patient that allow x-rays to easily be transmitted are called radiolucent. Anatomic tissues that vary in absorption and transmission (such as soft tissue) create a range of dark and light areas - varying shades of gray on the image.

This variation of beam attenuation as radiation passes through different structures in the body is known as differential absorption. With differential absorption, some of the x-ray beam is absorbed by the anatomy and some of the beam passes through the anatomy. The term differential is used because varying anatomic parts absorb the x-ray photons differently. Differential absorption of the primary x-ray beam will create a radiographic image that structurally represents the area of interest (AOI).

Several factors combine to determine the quality of the radiographic image. By manipulating these factors, the radiographic image can be changed. There is no one perfect combination of factors that must be used for each exposure. Rather, several possible combinations can produce very acceptable diagnostic radiographs. The radiographer is

responsible for finding an appropriate combination of factors unique for each exam that will produce a quality radiograph. This combination should keep the patient’s radiation exposure to a minimum, while producing a high-quality radiograph for interpretation by the physician.

The radiograph can be described as a two-dimensional image composed of a variety of black, gray, and white shadows. The amount of the x-ray beam absorbed (attenuated) by the anatomy determines the radiographic density or brightness of the shadows on the radiograph. The black areas on the image represent less dense anatomic structures that have allowed many of the primary x-rays to pass through the anatomy to the IR. The gray areas on the image represent moderately dense anatomic structures that have absorbed the x-ray beam to varying degrees. The white areas on the image represent dense anatomic structures that have absorbed many of the primary x-rays.

The extent to which the IR is affected during an x-ray exposure depends on the number of x-rays reaching it. A diagnostic radiograph should have adequate density or brightness (overall darkness), appropriate contrast (shades of gray), recorded detail or spatial resolution (definition and resolution) and minimal distortion or magnification (size and shape) of the anatomy being examined.

Density or Brightness

Density and brightness refer to the same image quality factor but are defined differently. Density is the amount of overall blackness on the processed film image, while brightness is the amount of luminance of a display monitor. The image must have sufficient density or brightness to visualize the anatomic structures of interest. An image that is too light has insufficient density or increased brightness. Conversely, an image that is too dark has extreme or excessive density or decreased brightness. In either case, the anatomy is not well demonstrated. Digital

Fig. 6 Spine radiograph

Fig. 7 Resulting gray-scale image


imaging can adjust for exposure errors, the image can display appropriate levels of brightness, yet be over or under exposed. Extreme exposure errors can affect image quality and be visible through quantum noise when the exposure is too low or saturation when the exposure is overexposed and cannot be properly processed. This is accomplished using a windowing function called the window level. The display monitor has a window level function that allows the image brightness to be increased or decreased through a range of brightness levels in the displayed image.

The radiographer should evaluate the overall density or brightness on the image to determine whether it is adequate to appropriately visualize the anatomy. The radiographer must also establish whether the radiographic image is diagnostic or is unacceptable. The ability to determine when a radiographic image is unacceptable because of either insufficient or excessive density or brightness requires knowledge of radiographic factors that manipulate density or brightness, as well as clinical experience. If an image is deemed unacceptable due to a density or brightness problem, it is necessary to determine which factor(s) contributed to the error. Knowledge of factors that affect radiographic density or brightness is critical to developing effective problem-solving skills. Factors that directly affect density or brightness are considered controlling factors, whereas factors that indirectly affect density or brightness are considered influencing factors.

1. Milliamperage and Exposure Time (mAs)

The primary controlling factor for radiographic density or brightness is milliamperage-seconds (mAs). The product of milliamperage (mA) and exposure time (s) has a direct relationship with the quantity of x-rays produced. Milliamperage and exposure time together control the number of photons in the primary x-ray beam. They are multiplied and referred to as mAs. The radiographer can control the quantity of exposure with the mAs selected for each radiographic exposure. The amount of exposure increases as the quantity of photons in the primary beam is increased. The quantity of photons in the primary beam can be increased by increasing the mAs. The quantity of

exposure decreases as the quantity of photons in the primary beam is decreased. The quantity of photons in the primary beam can be decreased by decreasing the mAs. Therefore, the radiographer can manipulate radiographic density or brightness by adjusting the mAs. Because mAs is the product of milliamperage and exposure time, changing milliamperage or time has the same effect on the quantity of exposure.

To demonstrate radiographic density, a coconut was radiographed (Fig. 9). This image displays appropriate radiographic density. It is neither too light nor too dark. The coconut was radiographed in the anteroposterior (AP) projection on top of the x-ray table using 2 mAs, 52 kVp, and a 40-inch source to image receptor distance (SID). The primary x-ray beam was directed perpendicularly to the center of the coconut, and the beam was closely collimated. The anatomy of the coconut is well visualized because of the appropriate radiographic density.

A change in mAs results in a corresponding direct change in radiographic density. For example, when the mAs is increased, radiographic density is increased; when the mAs is decreased, radiographic density is decreased. When an image is too dark, it has excessive density, and a decrease in mAs is necessary to correct the density error (Fig. 10). To produce this image, 4 mAs and 52 kVp were used.

For a radiographic image that must be repeated due to excessive density, the mAs used on the repeat image is adjusted by a factor of 2. Decreasing either milliamperage or exposure time by ½ will decrease the mAs by ½. Decreasing the mAs by ½ will decrease the radiographic density by ½. For the repeat, 2 mAs should be used and will produce the same density found on the first image.

When an image is too light, it has insufficient density, and an increase in mAs is necessary to correct the density error (Fig. 11). In general, for a radiographic image that must be repeated due to insufficient density, the mAs used on the repeat

Fig. 9 This radiograph displays appropriate density or brightness.

Fig. 10 This radiograph demonstrates excessive radiographic density or brightness.

Fig.8 Digital display monitor


image is adjusted by a factor of 2. To produce this image, 1 mAs was used which produced insufficient radiographic density. Doubling either milliamperage or exposure time will double the mAs and therefore double the radiographic density. For the repeat, 2 mAs should be used and will produce the same density as found on the first image.

The image brightness of digital IRs is not directly affected by mAs, because they can detect a wider range of radiation intensities. During computer processing, if the mAs is too low, image brightness is adjusted to achieve the desired level, but increased quantum noise (graininess) might be visible within the image. If the mAs selected is too high, the brightness can be adjusted but the patient has been exposed to significantly more radiation than necessary. Regardless of IR type, to best visualize the anatomic AOI, the mAs selected must produce enough radiation, as an insufficient or excessive amount would adversely affect image quality and patient radiation exposure.

2. Collimation

The remnant radiation received by the IR is from 2 sources: primary x-rays and scattered x-rays. Scatter radiation that reaches the IR does not provide any diagnostic information about the anatomic area. Scatter radiation reaches the IR from all directions and produces a uniform density over the entire image. This veil or haziness is in addition to the density produced by the primary x-rays striking the IR and is known as fog. When fog is present, the effect is as if the image is being viewed through a cloud. Therefore, the image is degraded. Any change in the size of the primary x-ray field will alter the amount of primary radiation reaching the patient. Collimation is a process where the primary x-ray beam is reduced in size to the size of the AOI. In most modern x-ray machines, a collimator is attached to the x-ray tube housing and can be easily used to manipulate (collimate) the size of the x-ray beam. A collimator consists of 2 pairs of lead

shutters at right angles to each other. These lead shutters can be adjusted to resize the beam to correspond to the size of the anatomy being imaged. When properly adjusted, the collimated light field represents the radiation field.

Increased collimation results in a smaller primary x-ray beam size. This tighter collimation of the primary x-ray beam has the effect of reducing the number of primary photons exiting the collimator and thus reduces the patient exposure. Because less primary radiation reaches the patient, this in turn reduces the amount of scatter radiation produced within the patient. This decrease in primary and scatter photons reduces the number of photons reaching the IR. Fewer photons reaching the IR decreases the amount of density or brightness on the radiograph.

A larger field size, or decreased (less) collimation, increases the amount of tissue irradiated and increases the amount of scatter radiation (produced primarily by the patient) reaching the IR. This in turn increases the amount of density on the radiograph. This larger primary beam allowed more primary radiation to reach the IR. Additionally, the increased number or primary photons caused more scatter to be created and reach the IR. This increase in the amount of remnant radiation reaching the IR resulted in a greater radiographic density or brightness. Therefore, one method of reducing the amount of scattered radiation produced, maintaining appropriate radiographic density, and reducing the patient’s dose is to reduce the field size (increase collimation) with collimators, cones, or other beam-limiting devices. The effect of collimation on radiographic density is more visible when imaging large anatomic areas, performing examinations without a grid, or when using a high kilovoltage.

3. Kilovoltage Peak (kVp)

The energy of primary x-rays created during an exposure is controlled by the kilovoltage peak (kVp) selected. The higher the kVp, the greater the energy of the primary x-rays created. Since kilo means 1,000, kilovoltage means 1000 volts. The p stands for peak. Therefore, kVp means kilovoltage peak, or the peak energy of the primary x-rays created. The

Fig. 11 This radiograph exhibits insufficient density or brightness.

Fig. 12 Scatter radiation

Fig. 13 Collimation


term kVp is often shortened to kV.

Kilovoltage peak influences radiographic density because it alters the penetrating ability of the x-ray beam. Increasing kVp increases the number of primary photons penetrating the anatomy and exiting as remnant photons. This increased quantity of radiation reaching the IR increases radiographic density. Adjusting radiographic density can be achieved with kilovoltage by using the 15% rule. The 15% rule states that increasing kVp by 15% doubles the radiographic density on an image and decreasing kVp by 15% decreases radiographic density by one-half.

Kilovoltage peak affects not only the radiographic density but also other aspects of the image, such as contrast; therefore, kilovoltage is not the primary factor to manipulate for changes in radiographic density. It is, however, sometimes necessary to adjust radiographic density by changing the kVp. Whenever possible, a high kVp, low mAs technique should be used for quality diagnostic images and to reduce patient exposure.

Although adequate penetration of the AOI is important when using digital IRs, changing the kVp does not affect the digital image the same as a film-screen image. Because kVp affects the amount of radiation reaching the IR, its effect on the digital image is like the effect of mAs which is primarily controlled during computer processing. When the kVp selected is too low, the brightness and contrast are adjusted, this could cause quantum noise on the image. When the kVp is too high, the brightness and contrast are adjusted, but increased patient exposure could result as well as increased scatter radiation to the IR which may adversely affect image quality.

4. Source to Image Receptor Distance

The distance from the source of the radiation in the x-ray tube to the IR is known as the source to image receptor distance (SID). The routine SID for procedures performed on the x-ray table is 40 inches. The SID selected has a significant influence

on density. Because of the divergence of the x-ray beam, the intensity of the radiation will vary at different distances. The x-ray intensity decreases as the SID increases. This relationship between distance and x-ray beam intensity is best described by the Inverse Square Law (ISL). The ISL states that the intensity of the x-ray beam is inversely proportional to the square of the distance from the x-ray source. Because beam intensity varies as a function of the square of the distance, SID affects the quantity of radiation reaching the IR. As SID is increased, the x-ray intensity is spread over a larger area. This decreases the overall intensity of the x-ray beam reaching the IR. As SID increases, density decreases; as SID decreases, density increases. Because of the diverging properties of x-rays, changes in SID also affect other qualities of the image and should not be used routinely to manipulate density.

The ISL can be written as the formula:

In the ISL formula, l1 equals the intensity of the beam at the first distance; l2 equals the intensity of the beam at the second distance; (D1)

2 equals the square of the first distance; and (D2)

1 equals the square of the second distance. Essentially, this states that the amount of radiation decreases as the distance from the tube increases. Using this formula, when the distance is doubled, the intensity at the new (doubled) distance is one-fourth the initial value.

A change in SID requires that a new mAs must be used to maintain the same original exposure on the radiograph. Maintaining consistent densities when the SID is altered requires that the mAs be adjusted to compensate. The Exposure Maintenance Formula, also known as the mAs-Distance Compensation Formula, provides a mathematical formula for adjusting the mAs when changing the SID. The following formula represents how mAs should be changed to compensate for a change in

Fig. 14 This radiograph illustrates the change in density in Fig. 9 when kVp is increased by 15% .

Fig. 15 This radiograph illustrates the decrease in density caused by a 15% decrease in kVp in Fig. 9.

Fig. 16 Inverse square law


distance to maintain the original exposure:

In this formula, mAs1 equals the mAs used at the first distance; mAs2 equals the mAs used at the second distance; (SID1)

2equals the square of the first distance; and (SID2)

2 equals the square of the second distance.

Because increasing the SID decreases x-ray beam intensity and therefore the density, the mAs must be increased accordingly to maintain density. The SID was increased to 80 inches from the original 40 inches used in Fig. 9 (Fig. 17). Notice the significant decrease in radiographic density as a result of the 80-inch SID. This image would need to be repeated to obtain the correct density. For the repeat, when using an 80-inch SID the mAs should be increased.

To use this formula, mAs1 would be 2. The correct mAs for the 80-inch distance is represented by mAs2 which would be currently unknown as indicated by the X. The original distance of 40 inches is represented by (SID1)

2. The new distance of 80 inches is represented by (SID2)

2. Substituting the correct numbers into the formula would look like the following:

The first step in solving this formula is to square the distances or multiply the distances by themselves. This would result in:

or

The next step is to cross multiply:

To solve for X, 12,800 is divided by 1600 or:

or

Therefore, if the same technical factors used for Fig. 9 are used when the SID is changed to 80 inches (doubled), the radiographic density will be

insufficient (4 x too little) as demonstrated in Fig. 17. Using the Exposure Maintenance Formula, 8 mAs should be used when changing the SID from 40 inches to 80 inches. Fig. 18 is the same as Fig. 17, except the mAs is changed to 8 as calculated using the Exposure Maintenance Formula. This change in mAs increases the radiographic density allowing this image to look like Fig. 9. When it is necessary to change SID, mAs should also be changed to maintain density.

When the SID is decreased, the density increases; therefore, the mAs must be decreased accordingly to maintain density. Fig. 19 demonstrates how density changes when the SID is decreased. All technical factors used for Fig. 9 were kept the same in Fig. 19, except the SID was decreased to 20 inches (cut in half). This change in the SID causes excessive radiographic density. Because Fig. 9 has appropriate density, when changing the SID, mAs will also need to be changed. To repeat Fig. 19 using a 20-inch SID but maintaining appropriate density, the mAs should be decreased.

To use this formula, mAs1 would be the original mAs used for Fig. 9, which would be 2. The correct mAs for the 20-inch distance is represented by mAs2 which would be currently unknown as indicated by the X. The original distance used to produce Fig. 9 is represented by (SID1)

2 and would be 40 inches. The new distance is represented by (SID2)

2 and would be 20 inches. Substituting the correct numbers into the formula would look like the following:

or

The next step is cross multiplying:

Solving for X requires 800 be divided by 1600 or:

Fig. 17 This radiograph shows a significant decrease in density as a result of increased SID.

Fig. 18 By increasing the mAs to 8, the density is increased.

Fig. 19 This radiograph shows how density changes when SID is decreased.


Therefore, if the same technical factors used for Fig. 9 are used when the SID is changed to 20 inches (halved), the radiographic density will be excessive (4 times too much) as demonstrated in Fig. 19. However, if the mAs is also changed when the SID is changed, the radiographic density is appropriate as demonstrated in Fig. 20. For Fig. 20, 0.5 mAs was used (instead of the original 2 mAs) due to the 20-inch SID, and the density is appropriate.

Changes in the SID create changes in x-ray intensity and therefore changes in radiographic density. The Exposure Maintenance Formula can be used to compensate for the effect that changes in distance have on radiographic density. As with kVp, SID affects not only the radiographic density but also other aspects of the image, such as detail and distortion; therefore, SID should not be used as the primary factor to make changes in radiographic density.

5. Object to Image Receptor Distance

The object to image receptor distance (OID) is the distance from the object being radiographed to the IR. The OID has a slight effect on radiographic density. An exceptionally large OID may decrease density. A large OID is sometimes called the air-gap technique because of the large gap of air between the object and the IR. This air gap can act as a grid by helping to prevent scatter from reaching the IR, thereby decreasing radiographic density. In some situations, it is difficult to minimize OID because of factors or conditions beyond the radiographer's control. For example, when radiographing the cervical spine in the lateral position, there is a considerable distance between the cervical spine and the IR (large OID) due to the shoulders. The most significant impact an increased OID has on an image is magnification. For this reason, it is imperative that the radiographer position each patient so that the AOI is as close to the IR as possible.

For Fig. 21, the technical factors were kept the same as Fig. 9 except the coconut was raised 4 inches above the IR. Fig. 22 shows the coconut placed on a radiolucent sponge. The sponge created an OID of 4 inches. As demonstrated, this increase in OID had minimal impact on radiographic density. However, the increased OID caused magnification of the anatomy. In most situations, changes in the OID are not adequate to cause substantial density changes. Although the amount of OID necessary to visibly affect image density has not been standardized, the radiographer should strive to minimize the OID whenever possible.

6. Grids

A radiographic grid is a device that is placed between the patient and the IR to absorb scatter radiation exiting the patient. Limiting the amount of scatter radiation reaching the IR improves the quality of the radiograph. The first grid, a grid diaphragm, was invented by Dr. Gustav Bucky in 1913. This grid was stationary and due to its construction, it was not very practical. Dr. Hollis Potter improved Dr. Bucky’s invention by creating a system that would move the grid during the radiographic exposure. Most x-ray tables are equipped with a moving grid permanently mounted underneath the tabletop and above the Bucky tray. When the IR is placed in the Bucky tray (Fig. 23), the grid is used. This grid is mounted in a frame and moves under the x-ray table during the radiographic exposure. The grid and the movement mechanism are known as a reciprocating grid. It is also called a Potter-Bucky diaphragm after the inventors but is usually referred to simply as a Bucky.

In addition to absorbing scatter radiation, grids unfortunately also absorb some of the primary radiation. When a grid is used, it is placed under the x-ray table, between the patient and the IR. A grid is made of a series of radiopaque strips which alternate with radiolucent interspace material. The radiopaque strips are made of a dense material, usually lead.

Fig. 20 This radiograph shows that when SID changes, the mAs must also change accordingly to maintain appropriate density or brightness.

Fig. 21 This radiograph used the same technical factors as Fig. 1, except the coconut was raised 4 inches above the IR.

Fig. 22 The coconut is placed on a radiolucent sponge to create an OID of 4 inches.

Fig. 23 IR shown in the Bucky tray.


The radiolucent interspace material is often plastic or aluminum. These materials are pressed together and produced as flat sheets. Grids are designed to allow the divergent primary beam to pass between the lead strips and the scatter to be absorbed by the lead. This grid construction allows the primary beam to reach and interact with the IR, while absorbing the scatter radiation. The efficiency of each grid is determined by the grid ratio, which is defined as the ratio of the height of the lead strips and the distance between them

Typical grid ratios are 6:1, 8:1, 12:1, and 16:1. Higher ratio grids (16:1) absorb more scatter, preventing it from reaching the IR. Therefore, higher ratio grids are more efficient in removing scatter from the image. Higher ratio grids also require somewhat more accuracy in patient positioning. Each grid ratio is assigned a grid conversion factor to indicate its efficiency. Grid conversion factors are used to help determine the proper mAs to use with a specific grid. Lower grid ratios (5:1) have lower grid conversion factors, and thus require less mAs than higher ratio grids. The following table lists commonly used grids and their grid conversion factors:

Because of their construction, in addition to absorbing scatter, grids also absorb some of the primary beam. Because some primary photons are absorbed and do not reach the IR, density decreases when using a grid (Fig. 24). This exposure was made exactly like Fig. 9, except a 12:1 grid was used. Notice the decrease in density due to the absorption of some primary radiation and scatter by the grid. Therefore, when using a grid, changes to the technical factors are required to maintain density. It is

preferable to always change mAs for grid changes, except in chest radiography.

To determine how much to increase the mAs, the following grid conversion factor formula is used:

In this formula, mAs1 represents the mAs used without a grid, mAs2 represents the mAs used with the grid (it is the unknown), GCF1 represents the grid conversion factor for no grid, and GCF2 represents the grid conversion factor for the grid being used. In Fig. 9, 2 mAs was used without a grid and the density is appropriate. If the mAs is not changed when the use of a grid is changed, the density will be insufficient. To produce an image with appropriate density while using a grid, the grid conversion factor formula can be used to determine the necessary mAs.

To use this formula, the GCF1 is 1 (see the grid conversion table above) since no grid was used for the first image; the GCF2 is 5 because a 12:1 grid is used; mAs1 is the mAs used for Fig. 9 which is 2; and mAs2 is unknown and will be indicated as X.

The next step is to cross multiply which yields:

By using the grid conversion factor formula, the appropriate mAs to image this coconut when using a 12:1 grid can be calculated and is found to be 10 mAs (Fig. 25). This increase in mAs produces the necessary density, but it also causes an increase in the patient’s dose. Because of this increased dose to the patient, every effort should be made to expose the patient correctly the first time to avoid repeats. Additional ways to reduce the patient dose would be to increase collimation and use a lead shield for the patient whenever possible. The problem of scatter increases when imaging large anatomic areas such as the abdomen, pelvis, hips, and chest. Therefore, it is important to use a grid when imaging thick parts of the body. Using a grid will absorb much of the scatter produced and generate a more acceptable image.

Fig. 24 This radiograph shows decreased density when a grid is used.

Fig. 25 This radiograph shows the density produced by using a 12:1 grid and 10 mAs.

Table 1

Grid Ratio Grid Conversion Factor

(GCF)

No Grid 1

5:1 2

6:1 3

8:1 4

12:1 5

16:1 6


Here is another way to use the grid conversion factor. In instances as above, your original mAs is 2 with no grid and you make a second exposure using a 12:1 grid. Since the grid conversion factor for a 12:1 grid is 5, simply multiply 5 times the original mAs. The new mAs is 5x2, or 10 mAs. This is a simple method unless you are changing from one grid ratio to another grid ratio. In this case, the formula works great.

7. Quantum Noise

Quantum noise is a concern in digital imaging and is visible as brightness or density fluctuations on the image. It only serves to detract from the quality of an image and is photon dependent. The fewer the photons reaching the IR, the greater the amount of quantum noise on the digital image. When an exposure is too low to the IR, computer processing can alter the digital image to make the brightness acceptable, but this will increase quantum noise. Although the computer can adjust for exposures that are too low causing quantum noise or too high causing excessive radiation exposure, the radiographer is still responsible for choosing exposure techniques that produce acceptable image quality while making sure the patient’s exposure remains as low as reasonably achievable (ALARA).

Contrast

Contrast is defined as the difference in radiographic density or brightness to differentiate among anatomic tissues. The primary function of contrast is to make recorded detail visible. The degree of difference in density or brightness between 2 adjacent structures determines whether an image has high contrast or low contrast. High contrast images have few gray shades and are mostly black and white. Images with few shades of gray are considered to have short scale contrast. Low contrast images contain many shades of gray and demonstrate long scale contrast.

Adequate contrast is a key factor in the visibility of detail. In film images, contrast is a difference in densities, at least 2 different densities must be present for contrast to exist. It is possible to have density without contrast; however, it is not possible to have contrast without density. Images with low contrast have little density difference. These images have a flat, gray appearance, making it difficult to differentiate one density from another. Images with too much contrast have a more black and white appearance. These images demonstrate some areas that are very dark and others that are exceptionally

light. It is extremely difficult to see detail on images with extremely low contrast or extremely high contrast. Optimum contrast provides sufficient difference in density to be able to easily identify details.

In digital imaging, the different shades of gray that can be stored and displayed on a computer system is called grayscale. This grayscale or contrast can be altered because the digital image is processed and reconstructed in the computer as digital data. Digital images can display a range of gray levels from high to low contrast. Digital image receptors have an improved ability to distinguish between objects with similar subject contrast compared to film image receptors. This ability is termed contrast resolution. Once the digital image is processed, the window width control is used to adjust the contrast. When a wide window width is used, the image has lower contrast, or more shades of gray. When a narrow window width is used, the image has higher contrast, or fewer shades of gray.

The overall contrast seen on a radiographic image is referred to as radiographic or image contrast. Radiographic contrast is the combination of multiple factors associated with anatomy, radiation quality, IR capabilities, and, in digital imaging, computer processing and display. Radiographic contrast depends on the following factors: the subject contrast, and the image receptor contrast. The tissue density in the patient and the kilovoltage (kVp) used during the exposure create the subject contrast. The ability of a receptor to show adequately the information that the photons transmit through the subject create the image receptor contrast. Radiographic contrast can be significantly influenced by changes in either subject contrast or receptor contrast. In the clinical setting, typically the receptor contrast is standardized, and the subject contrast is altered by the kVp based on the needs of each examination.

A penetrometer is a radiographic testing device, usually made of aluminum, consisting of a series of steps of increasing thickness and can be used to visually illustrate contrast. Because of its shape, a penetrometer is also referred to as a step wedge. When radiographed, the thinnest steps of the penetrometer will appear dark on the

Fig. 26 Aluminum penetrometer with 16 steps.


resulting image. This is because most of the x-rays are not absorbed by the thin aluminum and eventually hit the IR. The thicker aluminum steps will absorb many of the x-rays, preventing them from reaching the IR. This absorption will cause the thicker steps to appear lighter on the image. Steps located between thin and thick will appear as different shades of gray, depending on their thickness. The difference in the adjacent shades of gray on the penetrometer image illustrates contrast. Long scale contrast is demonstrated in a radiograph of the 16-step penetrometer. The lighter steps at the top of the image represent the thickest part of the penetrometer. The darker steps at the bottom of the image represent the thinnest part of the penetrometer. The many shades of gray between the top and the bottom demonstrate contrast, the difference in radiographic density between adjacent portions of the image.

1. Subject Contrast

An important characteristic of x-ray photons is their ability to penetrate human anatomy. The x-rays (photons) found in a diagnostic x-ray beam have a variety of energy. Therefore, the penetrating ability of each individual x-ray varies, depending on its energy. X-rays with higher energy can easily penetrate anatomy and reach the IR. The x-rays with lower energy are unable to penetrate anatomy and are absorbed by the patient. If all x-rays entering a patient were able to penetrate the entire anatomy and reach the IR, the radiographic image would contain only one very dark density and would not provide a useful image. Contrast would not exist if all x-rays in the beam penetrated anatomic parts equally.

2. Kilovoltage Peak (kVp)

Kilovoltage (kV) and kilovoltage peak (kVp) refer to the same thing. The kVp determines the penetrating power of the x-ray beam. If the kVp is low, x-rays with lower penetrating ability are produced. If the kVp is high, the x-rays produced will have increased penetrating ability. By manipulating the energy of the beam with kVp, contrast can be changed. The kVp selection is under the control of the x-ray operator. Since kVp controls the penetrating ability of the x-ray beam, the controlling factor for contrast is kVp. When kVp is increased,

the penetrating ability of the x-rays increases, and fewer x-rays are absorbed by the patient. Because of this more uniform penetration of the anatomy, more photons reach the IR. When more x-ray photons reach the IR, radiographic contrast decreases and many shades of gray are visible on the image (long scale contrast) as seen in Fig. 27 where 70 kVp was used. On the image, many shades of gray are visualized and the difference between adjacent steps is minimal.

When kVp is decreased, x-rays are more easily absorbed by the thicker parts of the patient and will only penetrate the thinner parts. The result of this decreased penetration is an increased contrast, a more black and white image displaying high (short scale) contrast as seen in Fig. 28 where 40 kVp was used. On the image, the thicker steps at the top absorbed all the x-rays, resulting in several of the thickest steps appearing identical. Few different shades of gray are visualized. Fig. 29 places the image of the short scale/high contrast on the left, and the image of the long scale/low contrast on the right for comparison.

Appropriate contrast is not the same for all body parts. The major consideration when evaluating contrast is verification that a proper range of densities is visible on the image to demonstrate the anatomy of interest. Proper contrast must demonstrate enough distinctly different densities to sufficiently visualize the anatomy. The ability to recognize appropriate contrast is mostly the result of reliable clinical experience.

Both chest images demonstrate the effect that changes in kVp have on contrast (Fig. 30, Fig 31). Fig. 30 was exposed using 110 kVp and demonstrates low (long scale) contrast. This is due to the high kVp setting. This high energy x-ray beam was able to penetrate even the thickest anatomy to reach the IR resulting in a grayer image.

Fig. 29 Examples of short scale contrast on left and long scale contrast on right for comparison.

Fig.27 Radiograph of a penetrometer demonstrating long scale contrast.

Fig. 28 Radiograph of a penetrometer demonstrating short scale.


Because of the many shades of gray, many anatomical structures are visualized such as gas bubbles in the abdomen, individual thoracic vertebrae, ribs, scapulae, and the heart. Long scale contrast increases visibility of detail and is usually preferred for chest radiography.

Fig. 31 was exposed using 50 kVp and demonstrates high contrast or short scale contrast. The lower kVp does not provide adequate energy for the x-rays to properly penetrate the chest. More of the lower energy x-rays are absorbed by the anatomy rather than reaching the IR. Most of the x-rays that reach the IR penetrated the air-filled lungs. Therefore, this image is very black and white, representing high contrast or short scale contrast. Because of the limited shades of gray, many anatomical structures are not visualized. For instance, the gas bubbles in the abdomen are barely visualized and the thoracic vertebrae appear the same shade of gray as the heart and are not clearly visualized. Short scale contrast works well for extremity radiography.

The kVp also affects the radiation exposure (dose) received by the patient. When the kVp is increased, the dose to the patient can and should be decreased. Selection of the appropriate kVp becomes a balance between optimal image contrast and the lowest possible radiation dose (mAs) to the patient. In most situations, the highest kVp and lowest mAs that will yield sufficient diagnostic information should be used on each radiographic examination. The combination of high kVp and low mAs that results in the least exposure to the patient is referred to as high kV, low mAs technique. It is also one of the most important factors toward affecting the ALARA principle.

3. Scatter

X-rays creating the image are from 2 sources: primary x-rays and scatter x-rays. Scatter x-rays are

created when primary x-rays hit the patient’s anatomy and their direction of travel is changed. Scatter x-rays do not provide any useful diagnostic information on the image. Scatter x-rays reach the IR from all directions and produce a uniform density over the entire image. This increased density over the entire image causes a decrease in contrast. The presence of scatter on a radiographic image decreases contrast by adding undesirable grays.

As already stated, collimation is a process where the primary x-ray beam is reduced in size to the size of the AOI. The purpose of using collimation is to reduce unnecessary exposure to the patient and to improve radiographic contrast by reducing scatter interactions in the patient. An overabundance of scatter causes increased fog on the image which in turn produces poor radiographic contrast. Proper collimation is the responsibility of the x-ray operator.

4. Grids

Although collimation is effective in improving radiographic contrast by reducing scatter, even if proper collimation is used, the anatomy being radiographed will produce enough scatter to cause fog on the image. Preventing scatter from reaching the IR will improve the quality of the image by increasing contrast. A grid is a device that is placed between the patient and the IR to absorb scatter.

The use of a grid complements the effects of collimation. Preventing scatter from reaching the IR will increase contrast. Together, collimation and grids provide the most effective means of reducing the amount of scatter reaching the image and thus increasing contrast. The problem of scatter increases when imaging large anatomic areas such as the abdomen, pelvis, hips, chest, and skull. Therefore, it is particularly important to use a grid when imaging these parts of the body.

Spatial Resolution or Recorded Detail

In addition to demonstrating proper contrast and brightness, a quality radiograph should also demonstrate the anatomy with maximum sharpness (definition). This sharpness is also referred to as spatial resolution or recorded detail. Spatial resolution describes the clarity of small anatomic structures on the radiographic image. With sufficient detail, the smallest parts of anatomy are clearly visible, and abnormalities are more easily diagnosed.

In digital imaging, a digital image is displayed as a

Fig. 30 Chest image using 110 kVp demonstrating long scale contrast..

Fig. 31 Chest image using 50 kVp demonstrating short scale contrast.


matrix, which is a combination of rows and columns of small square elements called pixels with various brightness levels. Spatial resolution is improved with a greater number of pixels per unit area or pixel density. A major factor of spatial resolution is the pixel size and spacing, which is determined by different types of digital image receptors that use different methods of transforming the continuous exit radiation intensities into an array of pixels for image display. There are 2 types of spatial resolution: visibility and sharpness.

1. Visibility

Visibility of detail is the ability of the human eye to see fine detail on the radiographic image. Visibility of detail exists because of appropriate brightness and contrast, permitting structural details of the anatomy to be recognizable. Visibility of detail can be inhibited by any factor that causes the obscuring or deterioration of detail. Fog in the form of light or scatter radiation will obscure anatomy which decreases visibility of detail. Essentially, any factor that affects density or contrast will affect visibility of detail. The use of grids and primary beam restriction are 2 manipulations the x-ray operator can make to improve visibility of detail.

Any change in the size of the x-ray beam will alter the amount of radiation reaching the patient. The x-ray beam size can be changed by the operator with the help of a beam restricting device (BRD), also called beam limiting device (BLD). The leading BLD used today is the collimator. Increased collimation results in a smaller beam size. This tighter collimation of the primary x-ray beam reduces the number of primary x-rays reaching the patient and thus reduces the exposure to the patient. Because less primary radiation interacts with the patient’s tissue, this in turn reduces the amount of scatter radiation produced within the patient. A smaller beam size produces less scatter radiation, reduces the dose to

the patient, and increases visibility of detail.

Although collimation is effective in improving visibility of detail by reducing scatter, even when proper collimation is used, the anatomy being radiographed can produce enough scatter to cause fog on the image. Preventing scatter from reaching the IR will improve the quality of the image by increasing visibility of detail. The use of a grid complements the effects of collimation. Preventing scatter from reaching the IR will increase visibility of detail. Together, collimation and grids provide the most effective means of reducing the amount of scatter reaching the IR.

2. Sharpness

Spatial resolution or recorded detail is the amount of geometric sharpness or the accuracy of anatomy recorded on the radiographic image. It describes the sharpness of small structures on the radiograph. With adequate detail, even the smallest parts of anatomy are visible. Spatial resolution or recorded detail may also be referred to as sharpness, definition, or simply detail. Maximum detail describes an image that illustrates anatomic structures extremely well. Maximum detail will result when the image displays minimal distortion. There are inherent limitations in radiography that prevent the production of a perfect image. It is imperative for the x-ray operator to correctly manipulate the many number of factors required to produce a radiographic image to maximize recorded detail. Factors that influence detail are geometric factors such as focal spot size, patient motion, SID, and OID. Excellent recorded detail may exist even when it cannot be seen, due to poor visibility of detail.

X-rays are created within the x-ray tube. Electrons from the filament portion of the cathode are directed at high speeds toward the anode where x-rays are created. This electron beam is responsible for creating the x-ray beam; however, most of these electrons end up creating heat. Less than 1% of the electron beam ends up as x-rays. The focal spot is the area of the anode hit by the fast-moving electrons. The focal spot size is primarily controlled by the size of the filament. Larger filaments result in larger focal spots. Most x-ray tubes have 2 filaments, resulting in 2 focal spots. By using the small filament and thus the small focal spot, detail is increased. The use of a small focal spot is important when small anatomic parts are being examined. Because of the large quantities of heat created

Fig. 33 Collimator attached to the x-ray table housing.

Fig. 32 Pixels


during the process of x-ray production, the small focal spot cannot always be used. To prevent overheating the anode when large anatomic parts are being exposed, the large filament and the large focal spot should be used, and indeed, will be used due to safety devices in the x-ray machine. Using the large focal spot decreases the recorded detail.

Motion unsharpness is the term used to describe image unsharpness caused by patient motion. Motion negatively affects recorded detail. With motion, the image appears blurred and permits little detail to be observed. Patient motion can be caused by voluntary or involuntary motion of the patient. Voluntary motion is under the direct control of the patient, such as being able to hold his breath. Appropriate communication between the x-ray operator and the patient is the best way to reduce voluntary motion. Involuntary motion is not under the control of the patient. Examples of involuntary motion are cardiac movement and intestinal peristalsis. Because involuntary motion is not controlled by the patient, it is best reduced by using short exposure times.

The image below identifies the SID as well as OID. As the x-ray tube (source) gets farther away from the IR, the SID increases. As the patient (object) gets closer to the IR, the OID decreases. Both distances influence detail. For maximum detail, a short OID and a long SID should be used.

Distortion

Distortion refers to differences between the size and/or shape of the actual anatomy and its radiographic image. Because real anatomy is three-dimensional (3D), and the radiographic image is flat or two-dimensional (2D), all radiographic images have some degree of distortion. Distortion can be a misrepresentation of either the size (magnification) or shape of the anatomic part. When the image is distorted, spatial resolution is reduced. The radiographic image can misrepresent the actual anatomy due to the positioning of the x-ray tube, the anatomy, or the IR.

Distortion is categorized as size distortion or shape distortion. Size distortion is always in the form of magnification (enlargement) of the anatomy. Shape distortion is the result of unequal magnification of the actual shape of the anatomy. The chart below illustrates the factors that control distortion. The objective of radiography is to provide accurate images of anatomic parts. Minimizing distortion is vitally important to an accurate diagnosis. Distortion can be minimized and controlled by following some basic principles. To minimize distortion, the x-ray operator must pay close attention to the distances used during radiographic examinations, as well as the alignment of the central ray (CR), the anatomy, and the IR.

1. Size Distortion (Magnification)

The term size distortion refers to a change in the image size compared with the original or actual size of the anatomy. Radiographic images can never be smaller than the actual size of the anatomy being imaged. Therefore, size distortion in radiography refers to magnification of the anatomy on the image. Some amount of magnification is a normal occurrence on radiographic images. The goal during radiographic procedures is to minimize

Fig. 35 SID and OID

Fig. 36 Factors that control distortion

Fig. 34 Focal Spot


magnification whenever possible.

Magnification is a result of the geometry of the imaging setup. Distances used during the radiographic examination play a critical role in minimizing the amount of size distortion, or magnification, produced on the image. The amount of magnification is determined by how the OID and the SID are manipulated. Anytime there is magnification, there will be a loss of detail.

A. OID

The first controlling factor for size distortion is the OID. The OID is the most critical distance for magnification and is the distance between the anatomy being radiographed and the IR. An increase in OID (the anatomy is farther from the IR) automatically increases magnification of the image. The OID must be minimized to decrease magnification. For most radiographic procedures, the anatomy being examined is positioned so that it is as close as possible to the IR. Examinations of body parts with an inherently large OID, such as the kidneys and the heart, use positioning techniques to reduce the OID. The kidneys are bean-shaped organs located in the posterior abdomen, slightly above the waist. To minimize the OID, the kidneys are radiographed with the patient in the supine (posterior) position to place the kidneys as close as possible to the IR. The heart is situated in the anterior portion of the thoracic cavity, just posterior to the sternum. Similarly, chest radiography is routinely performed in the anterior position, for a PA projection, in order to place the heart nearest the IR. Additionally, the lateral chest position of preference for most patients is the left lateral, since this position places the heart as close as possible to the IR. It is imperative for the x-ray operator to have a good understanding of anatomy, and to position the patient so that the OID is as small as possible for each radiographic examination.

The OID has a significant effect on magnification and is directly related to magnification. As the OID increases, magnification also increases. Anytime there is magnification, there will be a loss of detail. The OID must routinely be minimized to reduce size distortion (magnification) as much as possible.

B. SID

The SID is the distance between the x-ray source (tube) and the IR. Although the OID has the greatest effect on size distortion, the SID is still an important factor for the x-ray operator to control to lessen size distortion. The SID is inversely related to

magnification. As the SID increases, size distortion (magnification) decreases; as the SID decreases, size distortion increases.

The x-ray operator must establish the SID when performing a procedure. It has been a long-standing common practice to use 40 inches as the standard SID. Early x-ray operators used from 20 inches to 36 inches routinely. Chest radiography has customarily been performed at a 72-inch SID for many years. There will be less magnification of the heart when a 72-inch SID is used. Any radiographic examination that permits a horizontal x-ray beam to be used, can easily be performed at a SID greater than 40 inches. By increasing the SID, magnification is reduced. Examinations of body parts with a large inherent OID (chest, lateral cervical spine) should use a long SID to minimize magnification caused by the unusually large OID.

Although magnification of body parts is not desirable in radiographic imaging, there are sometimes when magnification is intentionally created in order to better demonstrate a part. For example, a common body part that is often magnified is the odontoid process. Reducing the SID from the usual 40 inches to 30 inches or less to magnify the part, enables the surrounding bony area to be spread outward and the field of view of the odontoid to increase.

All images are magnified slightly, because the body part is always above or in front of the IR. Size distortion, therefore, is controlled by positioning the body part as close to the IR as possible and using the longest SID as practical.

Fig. 37 is a photograph of a hand positioned for a posterior-anterior (PA) projection radiograph. The hand is positioned in close contact with the IR and the SID is situated at 40 inches. The radiographic image of the hand is shown in Fig. 38. On the radiograph, the interphalangeal and metacarpophalangeal joints are open and are well

Fig. 37 Hand positioned for a PA projection.

Fig. 38 Radiograph image of the hand in Fig. 37.


demonstrated. Magnification of the anatomy is minimal. Images discussed later in this article should be compared to these images as the standards.

Fig. 39 is a photograph of a hand positioned for a PA radiograph. In this image, the hand is placed on a radiolucent sponge, increasing the OID to 5 inches. The radiographic image of the hand is shown in Fig. 40. Because of the increased OID, the radiographic image displays significant size distortion in the form of magnification. This magnification is caused by the increased OID. Anytime there is magnification, there will be a loss of detail. It is the responsibility of the x-ray operator to position each patient so that the OID is as small as possible to minimize magnification.

2. Shape Distortion

Shape distortion is the misrepresentation of the anatomy by unequal magnification of the actual shape. Shape distortion displaces the projected image of the anatomy from its actual position and can be described as either elongation or foreshortening. With elongation, the anatomy appears to be longer than it is and occurs when either the x-ray tube or the IR is improperly aligned. Foreshortening causes the anatomy to appear shorter than it is and occurs when the anatomy is improperly aligned. An image with shape distortion is usually the result of inadequate preparation by the x-ray operator. Evaluating shape distortion requires a detailed knowledge of normal anatomy by the x-ray operator, and the ability to recognize normal size and shape on radiographic images. Sometimes, shape distortion is used to an advantage in particular procedures. An example of intentional shape distortion is the Towne’s view of the skull. In this position, the CR is angled approximately 30° which greatly distorts the shape of the skull. However, the occipital bone is visualized with little

superimposition of other anatomy. This shape distortion aids in viewing the occipital bone.

Radiographic positioning is the process of placing the anatomy in the proper position to create the desired radiographic image. Creation of each diagnostic image also requires precise alignment of the CR to the anatomy, as well as properly positioning the IR under the anatomy. For most procedures, the CR is directed perpendicularly to the anatomy as well as to the IR. With this arrangement, the anatomy and the IR are parallel to each other. Any deviation from this configuration will cause shape distortion on the radiographic image. When the anatomy does not permit the CR to be perpendicular, creative positioning by the x-ray operator must be performed.

Usually, the CR will be directed perpendicularly to the anatomy and to the IR. Whenever the CR is not perpendicular, some degree of shape distortion will occur. Because the x-ray beam diverges or spreads out as it travels, only the photon in the center of the x-ray beam is perpendicular to the anatomy. Therefore, any anatomy that is not located directly under the CR will be distorted. The farther the anatomy is from the CR, the greater the distortion. Occasionally, the anatomy of interest is superimposed over other anatomic structures within the body. When this occurs, the CR can be angled to intentionally distort the image to provide an image of a structure that would otherwise be impossible to differentiate from overlying structures. When the CR is angled, distortion occurs in the form of elongation. When a CR angle is required, it is essential to maintain the correct relationship between the anatomy and the IR.

For most radiographic procedures, the anatomy being radiographed should be perpendicular to the CR and parallel to the IR. Any variation from this relationship will cause distortion. If the anatomy is improperly aligned, distortion in the form of foreshortening occurs. Some routine procedures are designed to vary from this standard to avoid superimposition of anatomy. When the anatomy cannot be properly aligned, it is essential for the x-ray operator to maintain the correct relationship between the CR and the IR.

Fig. 41 is a photograph of a hand positioned incorrectly above the IR. The hand is angled rather than lying flat. This angulation of the hand will cause shape distortion. Fig. 42 is the radiograph of the angled hand, and shape distortion is apparent.

Fig. 39 Hand positioned for a PA projection on a radiolucent sponge, increasing the OID to 5 inches.

Fig. 40 Radiograph of the hand in Fig. 39 displays significant size distortion in the form of magnification.


When the anatomy is improperly aligned, the shape distortion that occurs is foreshortening. Foreshortening causes the fingers in this image to appear shorter than normal.

The IR should be positioned parallel to the anatomy and perpendicular to the CR. If the IR is not positioned so that it is parallel to the anatomy, shape distortion results in the form of elongation.

X-ray tube angulation refers to the direction and degree the CR is moved from its normal perpendicular position to the IR. Several routine radiographic procedures require tube angulation. Angling the CR will prevent the superimposition of overlying anatomic structures. Angulation of the CR is designed to produce a controlled amount of shape distortion to avoid superimposition. When consistent angulation is routinely used, the result is a diagnostic image that is comparable to similar prior images. Angulation of the tube will also cause a change in the SID, which must be adjusted.

When the tube is directed toward the patient in any direction other than perpendicularly, it is a tube angle. The direction of the tube angle is described according to how it is directed toward the patient. CR angulations are usually either toward the patient’s head, or toward the patient’s feet. When the tube is angled toward the patient’s head, it is called a cephalic tube tilt or angulation. Another form of the word cephalic is cephalad. When the tube is angled toward the patient’s feet, it is called a caudal tube tilt or angulation. Another form of the word caudal is caudad.

Any angulation of the tube/CR from perpendicular will cause elongation of the image. Several routine radiographic procedures require a tube angle. The result is a radiographic image with an expected amount of shape distortion to avoid superimposition.

The degree of tube angulation is a way to describe

Fig. 42 Radiograph of the angled hand showing shape distortion caused by the angulation.

Fig. 41 Hand positioned incorrectly, angled rather than lying flat.

the amount of tube angulation, and therefore determines the degree of CR angulation. When the CR is directed perpendicularly to the anatomy, it is at a 90° angle. If a procedure requires the CR to be angled 15° toward the patient’s feet, the CR would be called a 15° caudal angle. The CR would be directed 15° from perpendicular and directed toward the feet. Likewise, a 20° cephalic angle would find the CR directed toward the patient’s head 20° from perpendicular. When a CR angle is part of the routine procedure, it is important to maintain the correct degree of angulation to produce the expected amount of shape distortion.

Fig. 43 is a photograph of a hand positioned for a radiograph with the CR angled 45°. The hand is positioned so that the beam moves through the hand from side to side as it enters the IR. This type of tube angulation causes shape distortion in the form of elongation of the anatomy on the image.

Fig. 44 is the radiograph produced by the equipment arrangement seen in Fig. 43. This tube alignment causes the joint spaces to be oblique and the bones of the hand to be superimposed. Comparing this image to our standard found in Fig. 38, the shape distortion is very evident.

Fig. 45 is a photograph of a hand positioned for a radiograph with the CR angled 45° as in the previous image, except the hand is positioned differently. Because

Fig. 45 Hand positioned differently . The CR is also angled 45°.

Fig. 44 Radiograph of Fig. 43. Notice shape distortion of the anatomy in the form of elongation.

Fig. 43 Hand positioned for a radiograph with the CR angled 45°.

Fig. 46 Radiograph of a hand positioned using the setup of Fig. 45.


of the angle of the CR, this arrangement will also produce elongation.

Fig. 46 is the radiograph produced by this setup. The shape distortion or elongation of the hand is obvious. The joint spaces are not open, and the fingers appear elongated when compared to Fig. 38.

Summary

In this article, we have discussed the 4 radiographic image properties. This knowledge should be incorporated with other principles of imaging to have a broad understanding of the production of quality radiographs. It is essential to understand the principles of imaging, but it is critical to use it in the x-ray room. The technical skill, knowledge, judgment, and integrity of the radiographer play significant roles in the radiographic images produced. Properly exposing a radiograph is often a trial and error process, because of the many variables that affect the final radiograph. The radiographer must develop a procedure that routinely produces an acceptable density or brightness, regardless of the patient size or anatomy imaged.

To determine the exact cause of contrast problems, the x-ray operator must use knowledge and be successful at diagnosing the problem so that proper adjustments can be made before any repeat examinations are made. Contrast is one of the 2 photographic properties that comprise visibility of detail. Visibility of detail refers to the fact that the radiographic image is visible only because sufficient contrast exists to permit anatomic details to be perceived. Remember that the desired scale of contrast is dependent on the anatomy being examined, and how many tissue densities that anatomy must demonstrate. The major consideration in evaluating contrast is verification that a proper range of densities is visible throughout the image. The anatomy can be visualized only when sufficient contrast exists.

To produce the sharpest detail on a radiographic image, the x-ray operator should use the smallest focal spot size possible, minimize patient motion, use the longest appropriate SID, and use the shortest possible OID. Additionally, to improve visibility of detail the operator should tightly collimate the x-ray beam to the AOI, using a grid when appropriate. While optimizing all imaging factors is the responsibility of the x-ray operator, the imaging process itself always inherently produces some loss of recorded detail.

Distortion is the difference between the actual anatomy and the radiographic image. Distortion creates a misrepresentation of the size and/or the shape of the anatomy. This misrepresentation can be either size or shape distortion. When the factors that affect size and shape distortion are changed, distortion will occur. Distortion misrepresents the size and/or the shape of the anatomy being imaged. All radiographic images will contain some degree of distortion. The degree of distortion can be great or small. Understanding the relationship between the CR, the anatomy, and the IR, and how their arrangement affects distortion, is essential for x-ray operators.

The process of producing a diagnostic radiograph has multiple stages and includes a variety of types of equipment. Any one of these may be a source of variability resulting in a suboptimal image. Images that are suboptimal require repeat examinations, resulting in increased radiation exposure to the patient and increased cost. It is imperative that the radiographer have substantial knowledge of essential x-ray equipment and imaging principles, and the ability to put that knowledge to use to properly manipulate the equipment and controlling factors to obtain optimal radiographic results. By the accurate utilization of a range of sophisticated equipment to produce high quality images, the radiographer provides radiographic images so that physicians and other health care professionals can better diagnose and treat injuries and disease. Radiographers must use their expertise to assess the patient, develop optimal radiographic techniques, and evaluate resulting radiographic images to determine if additional images are warranted. The goal of radiography is to provide maximum diagnostic information on the image with the least amount of exposure to the patient and to the x-ray operator.

References

1. Bushong SC. Radiologic Science for Technologists. 11th ed. St. Louis, MO: Elsevier; 2017.

2. Fauber TL. Radiographic Imaging and Exposure. 5th ed. St. Louis, MO: Elsevier; 2017.

3. Johnston JN, Fauber TL. Essentials of Radiographic Physics and Imaging. 3rd ed. St. Louis, MO: Elsevier; 2020.

4. Abdulla S. Image Quality. Radiology Café Web site. https://www.radiologycafe.com/radiology-trainees/frcr-physics-notes/image-quality. Updated May 25, 2020. Accessed June 2, 2020.

5. Image Production. Radiology Key Web site. https://radiologykey.com/image-production/. Accessed June 22, 2020.

https://www.radiologycafe.com/radiology-trainees/frcr-physics-notes/image-quality

https://www.radiologycafe.com/radiology-trainees/frcr-physics-notes/image-quality

https://radiologykey.com/image-production/

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6. Image Characteristics. Radiology Key Web site. https://radiologykey.com/image-characteristics/. Accessed June 22, 2020.

7. Radiographic Exposure Technique. Radiology Key Web site. https://radiologykey.com/radiographic-exposure-technique/. Accessed June 22, 2020.

8. Seals T. The Coconut-Demonstrating Radiographic Density. X-Ray News. Vol 15 No 1. Published Jan-Mar 2009.

9. Seals T. Controlling Radiographic Contrast through Understanding. X-Ray News. Vol 15 No 2. Published Apr-May 2009.

10. Seals T. Detail and Distortion. X-Ray News. Vol 15 No 3. Published Jul-Sep 2009.

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Radiographic Image Quality Direct Reading DR-101 Post-Test

Approved for 4.0 CE Credits

1. How many months following Roentgen’s discovery, did physicians use x-rays to locate bullets?

A. 2 B. 4 C. 6 D. 8

2. What is the name of the device that receives the radiation exiting the patient after an x-ray exposure?

A. anode B. densitometer C. image receptor D. collimator

3. What are x-ray photons that interact with matter and change their direction of travel called?

A. scatter radiation B. attenuation C. transmitted photons D. radiopaque

4. Differential absorption can be described as:

A. the redirection of a photon B. the device containing the latent image C. the termination of the exposure when the

appropriated amount of radiation is received D. the variation of beam attenuation as it passes

through different structures

5. Which function on a display monitor allows the image brightness to be increased or decreased?

A. window width B. window level C. shuttering D. dynamic range

6. What is the primary controlling factor for radiographic density or brightness?

A. SID B. kVp C. mAs D. OID

7. Reducing the size of the primary x-ray beam to the AOI is known as:

A. collimation B. attenuation C. absorption D. the Inverse Square Law

8. The energy of primary x-rays created during an exposure is controlled by:

A. mA B. exposure time C. SID D. kVp

9. Which of the following will provide a high-quality radiographic image with the least patient exposure?

A. high kVp, high mAs B. high kVp, low mAs C. low kVp, high mAs D. low kVp, low mAs

10. The Inverse Square Law states that:

A. the intensity of the x-ray beam is inversely proportional to the square of the distance

B. the distance from the beam varies because the intensity changes

C. a 15% change will change density or brightness

D. the mAs should be changed to compensate for a change in distance

11. If the SID for a radiographic examination is increased and nothing else is changed, what happens to the radiographic density?

A. decreases B. increases C. no change D. need more information


12. A radiograph is exposed using 2 mAs and a 40-inch SID. If the SID is changed to 80 inches, what mAs would be required to maintain the density found on the original image?

A. 0.5 B. 1 C. 4 D. 8

13. Increasing the ____ will cause magnification of the anatomy on the radiographic image.

A. mAs B. SID C. exposure time D. OID

14. The material found within a grid that absorbs radiation is made of:

A. plastic B. aluminum C. lead D. copper

15. The efficiency of a grid is determined by the:

A. weight of the grid B. width of the grid C. age of the grid D. grid ratio

16. During digital image display, the contrast can be lowered (decreased) by increasing _____________.

A. pixel density B. grayscale C. window level D. window width

17. Subject contrast refers to which of the following?

A. the patient only B. grayscale C. mAs D. the patient and the kVp used

18. How will a grid affect radiographic contrast?

A. eliminates contrast altogether B. as no effect on contrast C. increases contrast by absorbing scatter D. decreases contrast by absorbing scatter

19. When using low kVp, what type of contrast will result?

A. low contrast, long scale contrast B. high contrast, short scale contrast C. low contrast, short scale contrast D. high contrast, long scale contrast

20. When using high kVp, what type of contrast will result?

A. low contrast, long scale contrast B. high contrast, short scale contrast C. low contrast, short scale contrast D. high contrast, long scale contrast

21. Which of the following tools is used to visually illustrate contrast?

A. densitometer B. sensitometer C. penetrometer D. ionization chamber

22. Which of the following terms describes good visualization of small anatomic parts?

A. density B. contrast C. spatial resolution D. distortion

23. In digital imaging, a digital image is displayed as a combination of rows and columns of small square elements called:

A. voxels B. pixels C. detectors D. windows

24. Which of the following will decrease patient motion on a radiograph?

A. less mAs B. short exposure time C. decreased OID D. increased SID

25. The appearance of scatter radiation that obscures detail on a radiographic image is called:

A. after glow B. visibility of detail C. fog D. collimation


26. Recorded detail can be improved by which of the following?

A. use a larger focal spot size B. increasing the OID C. selecting a smaller focal spot size D. decreasing the SID

27. If you have anatomy that has an inherently large OID, what is the only technique that will help decrease the automatic magnification?

A. further increase the OID B. increase the SID C. use a small focal spot D. decrease the SID

28. To obtain the greatest spatial resolution, the OID should always be:

A. as long as possible B. in direct proportion to the SID C. as short as possible D. changed for each exam

29. How will increasing the SID affect spatial resolution?

A. increase distortion B. increase detail C. decrease detail D. decrease distortion

30. Changing the focal spot size from 1 mm to 3 mm will:

A. increase magnification B. decrease magnification C. increase detail D. decrease detail

31. Misrepresentation of the true size or shape of an object is the definition of:

A. unsharpness B. magnification C. minification D. distortion

32. What is causing the significant size distortion in the form of magnification displayed in Fig. 40?

A. increased SID B. increased OID C. a small focal spot D. decreased OID

33. Tube angulation will produce:

A. elongation B. foreshortening C. magnification D. minification

34. Which of the following terms describes size distortion?

A. magnification B. foreshortening & elongation C. foreshortening & magnification D. minification

35. Which of the following terms describes shape distortion?

A. magnification B. foreshortening & elongation C. foreshortening & magnification D. minification

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