spiral ct and its quality control procedures (mp-01)-report

8/14/2019 Spiral CT and Its Quality Control Procedures (MP-01)-Report

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Spiral CT and its Quality Control Procedures

By

Ali Adeel (Medical Physics MP-01)

Report submitted to Dr. Tariq Majeed in partial fulfillment of

requirements for the course of Physics of Nuclear Medicine

Department of Physics and Applied Mathematics,

Pakistan Institute of Engineering & Applied Sciences,

Nilore, Islamabad, Pakistan

December, 2012


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Dedication

This report is dedicated to my parents who taught me well

instead of having financial problems. It is also dedicated to my

teachers especially Mr. Muhammad Asif and Dr. Bilal Masood

for their guidance. It is also dedicated to my friends Muhammad

Umer Asif, Farhan Ijaz Ahmad, Muhammad Jamil, Khawar

Sultan and all those who collectively enabled me to gain thestatus I have today.


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Acknowledgement

First of all thanks to ALLAH ALMIGHTY , WHO has blessed me with too many

abilities while I have not requested for any of these, so I am able to do this.

After that I acknowledge my Parents and all Teachers , who supported me both

morally and technically, especially supervisor Dr. Tariq Majeed , who helped me at

every step in this report.

I also acknowledge to Dr. Rehan Abdullah, he taught me about paragraph formatting.

I also acknowledge to www.shaunakelly.com from where I learned multi-level listing,

list numbering and modifying heading styles. I acknowledge to all authors, editors,

publishers etc. of reference material.

Ali Adeel
http://www.shaunakelly.com/http://www.shaunakelly.com/http://www.shaunakelly.com/http://www.shaunakelly.com/


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Table of Contents

List of Figures ............................................................................................................... iv List of Tables ................................................................................................................. v Abstract / Executive Summary ..................................................................................... vi 1) Introduction ............................................................................................................ 1

1.1) From X-rays to CT .......................................................................................... 3 1.2) Basic Principles of CT..................................................................................... 3 1.3) Generations of CT ........................................................................................... 5

2) Material and Methods ............................................................................................ 7 2.1) Spiral or Helical CT Scanning ........................................................................ 7 2.2) Multislice Spiral CT ...................................................................................... 11 2.3) Dose Calculation in Spiral CT ...................................................................... 13 2.4) Quality Control Procedure in Spiral CT ........................................................ 14

2.4.1) CT Numbers or Hounsfield Units .......................................................... 15 2.4.2) Other Quality Control Checks ............................................................... 16

3) Result Discussion ................................................................................................. 17 3.1) Advantages of Spiral CT ............................................................................... 17 3.2) Disadvantages of Spiral CT........................................................................... 17 3.3) Importance of Spiral CT Quality Assurance ................................................. 17

4) Summary & Conclusion ....................................................................................... 19 References .................................................................................................................... 20 Vita ............................................................................................................................... 21


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List of Figures

Figure 1.1: Posteroanterior and lateral chest radiographs give three-dimensionalinformationconcerning the location of an abnormality [2]. ........................................... 4 Figure 1.2: Scan motions in computed tomography [1]. ............................................... 6 Figure 2.1: Spiral CT scanning [1]. ............................................................................... 8 Figure 2.2: Working principle of slip rings. .................................................................. 8 Figure 2.3: Application of slip ring in spiral CT. .......................................................... 8 Figure 2.4: Variation in slice thickness with respect to pitch. ..................................... 10 Figure 2.5: A schematic of a fixed-array detector geometry for a multislice spiralscanner (Left). Four configurations connecting the data acquisition channels to singleor multiple elements of the arrayed detectors produce four different slice thicknesses(Right) [3]. ................................................................................................................... 12

Figure 2.6: CT image of a quality control phantom [1]. .............................................. 15
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List of Tables Table 1-1: Energy Sources and Tissue Properties Employed in Medical Imaging [1]. . 2 Table 2-1: Common Quality Control Measurements for Computed Tomography [1]....................................................................................................................................... 15


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Abstract / Executive Summary This report explores spiral CT and its quality control procedures . First of all question

of why we need medical imaging (as CT is one of these) is addressed and differentimaging modalities with their principle are described. After that principle of

conventional X-rays imaging are discussed to some extent, because CT evolved from

this modality.

Afterwards basic principles of conventional CT are elaborated; different generations

of CT are also briefly explained so that a sequence may be established between

different generations. Although there were technological developments from one

generation to other but spiral CT has to wait for slip ring technology.

Working principle of spiral or helical computed tomography is discussed in material

and method section. Although spiral CT is superior to conventional CT in many

aspects but there were difficulties to attain same image quality using spiral CT.

Related problems and corresponding solution are also summarized. Multislice with its

important parameters is also briefly touched.

Relation of dose with different parameters of spiral CT is established so that risk and benefit analysis can be done. Parameters used for quality control procedures, allowed

values for these parameters and frequency for their testing is also described.

In the end advantages and disadvantages associated with spiral CT are briefly

summarized, no doubt spiral CT is superior in comparison with conventional CT in

many aspects and will replace it with time.


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1) Introduction Natural science is the search for truth about the natural world. In this denition,

truth is dened by principles and laws that have evolved from observations andmeasurements about the natural world. The observations and measurements are

reproducible through procedures that follow universal rules of scientic

experimentation. They reveal properties of objects and processes in the natural world

that are assumed to exist independently of the measurement technique and of our

sensory perceptions of the natural world. The mission of science is to use observations

and measurements to characterize the static and dynamic properties of objects,

preferably in quantitative terms, and to integrate these properties into principles and,ultimately, laws and theories that provide a logical framework for understanding the

world and our place in it [1] .

As a part of natural science, human medicine is the quest for understanding one

particular object, the human body, and its structure and function under all conditions

of health, illness, and injury. This quest has yielded models of human health and

illness that are immensely useful in preventing disease and disability, detecting and

diagnosing illness and injury, and designing therapies to alleviate pain and suffering

and to restore the body to a state of wellness or, at least, structural and functional

capacity. The success of these efforts depends on (a) our depth of understanding of

the human body and (b) the delineation of ways to intervene successfully in the

progression of disease and the effects of injuries [1] .

Progress toward these objectives has been so remarkable that the average life span of

humans in developed countries is almost twice its expected value a century ago.

Greater understanding has occurred at all levels, from the atomic through molecular,cellular, and tissue to the whole b ody, and includes social and lifestyle inuences on

disease patterns. The human body is an incredibly complex system. Acquiring data

about its static and dynamic properties results in massive amounts of information.

One of the major challenges to researchers and clinicians is the question of how to

acquire, process, and display vast quantities of information about the body so that the

information can be assimilated, interpreted, and utilized to yield more useful

diagnostic methods and therapeutic procedures. In many cases, the presentation ofinformation as images is the most efcient approach to addressing this challenge. As


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humans we understand this efciency; from our earliest years we rely more heavily on

sight than on any other perceptual skill in relating to the world around us. Physicians

increasingly rely as well on images to understand the human body and intervene in the

processes of human illness and injury. The use of images to manage and interpret

information about biological and medical processes is certain to continue its

expansion, not only in clinical medicine but also in the biomedical research enterprise

that supports it [1] .

Images of a complex object such as the human body reveal characteristics of the

object such as its transmissivity, opacity, emissivity, reectivity, conductivity, and

magnetizability, and changes in these characteristics with time. Images that reveal one

or more of these characteristics can be analyzed to yield information about underlying

properties of the object, as depicted in Table 1-1. For example, images

(shadowgraphs) created by x rays transmitted through a region of the body reveal

intrinsic properties of the region such as effective atomic number Z, physical density

(grams/cm 3), and electron density (electrons/cm 3). Nuclear medicine images,

including emission computed tomography (ECT) with pharmaceuticals releasing

positrons [positron emission tomography (PET)] and single photons [single-photon

emission computed tomography (SPECT)], reveal the spatial and temporal

distribution of target- specic pharmaceuticals in the human body [1] .

Table 1-1: Energy Sources and Tissue Properties Employed in Medical Imaging [1] .


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1.1) From X-rays to CT

In conventional radiography, subtle differences of less than about 5 percent in subject

contrast (i.e., x-ray attenuation in the body) are not visible in the image. This

limitation exists for the following reasons [1] :

1. The projection of three-dimensional anatomic information onto a two-

dimensional image receptor obscures subtle differences in x-ray transmission

through structures aligned parallel to the x-ray beam. Although conventional

tomography resolves this problem to some degree, structures above and below

the tomographic section may remain visible as ghosts in the image if they

differ signicantly in their x -ray attenuating properties from structures in the

section.

2. Conventional image receptors (i.e., lm, intensifying and uoroscop ic screens)

are not able to resolve small differences (e.g., 2%) in the intensity of incident

radiation.

3. Large-area x-ray beams used in conventional radiography produce

considerable scattered radiation that interferes with the display of subtle

differences in subject contrast.

To a signicant degree, each of these difculties is eliminated in computed

tomography (CT). Hence, differences of a few tenths of a percent in subject contrast

are revealed in the CT image. Although the spatial resolution of a millimeter or so

provided by CT is notably poorer than that provided by conventional radiography, the

superior visualization of subject contrast, together with the display of anatomy across

planes (e.g., cross-sectional) that are not accessible by conventional imaging

techniques, make CT exceptionally useful for visualizing anatomy in many regions of

the body [1] .

1.2) Basic Principles of CT

The mathematical principles of CT were first developed by Radon in 1917. Radon's

treatise proved that an image of an unknown object could be produced if one had an

infinite number of projections through the object. Mathematical details are not

discussed in this report, we can understand the basic idea behind tomographic

imaging with an example taken from radiography [2] .


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With plain film imaging, the three-dimensional (3D) anatomy of the patient is reduced

to a two-dimensional (2D) projection image. The density at a given point on an image

represents the x-ray attenuation properties within the patient along a line between the

x-ray focal spot and the point on the detector corresponding to the point on the image.

Consequently, with a conventional radiograph of the patient's anatomy, information

with respect to the dimension parallel to the x-ray beam is lost. This limitation can be

overcome, at least for obvious structures, by acquiring both a posteroanterior (PA)

projection and a lateral projection of the patient as shown in Figure 1.1. For example

the PA chest image in Figure 1.1 yields information concerning height and width,

integrated along the depth of the patient, and the lateral projection provides

information about the height and depth of the patient, integrated over the width

dimension. Imagine that instead of just two projections, a series of 360 radiographs

were acquired at 1-degree angular intervals around the patient's thoracic cavity. Such

a set of images provides essentially the same data as a thoracic CT scan. However, the

360 radiographic images display the anatomic information in a way that would be

impossible for a human to visualize: cross-sectional images. If these 360 images were

stored into a computer, the computer could in principle reformat the data and generate

a complete thoracic CT examination [2] .

The tomographic image is a picture of a slab of the patient's anatomy. The 2D CT

image corresponds to a 3D section of the patient, so that even with CT, three

Figure 1.1: Posteroanterior and lateral chest radiographs give three-dimensional informationconcerning the location of an abnormality [2].


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dimensions are compressed into two. However, unlike the case with plain film

imaging, the CT slice-thickness is very thin (l to 10 mm) and is approximately

uniform. The 2D array of pixels (short for picture elements) in the CT image

corresponds to an equal number of 3D voxels (volume elements) in the patient.

Voxels have the same in-plane dimensions as pixels, but they also include the slice

thickness dimension. Each pixel on the CT image displays the average x-ray

attenuation properties of the tissue in the corresponding voxel [2] .

1.3) Generations of CT

Early (rst -generation) CT scanners used a pencil-like beam of x-rays and a

combination of translational and rotational motion to accumulate the many

transmission measurements required for image reconstruction (Figure 1.2- a).

Although this approach yields satisfactory images of stationary objects, considerable

time (4 to 5 minutes) is required for data accumulation, and the images are subject to

motion blurring. Soon after the introduction of pencil-like beam scanners, fan-shaped

x-ray beams were introduced so that multiple measures of x-ray transmission could be

made simultaneously (Figure 1.2- b). Fan beam geometries with increments of a few

degrees for the different angular orientations (e.g., a 30-degree fan beam and 10-

degree angular increments) reduced the scan time to 20 to 60 seconds. Fan beam

geometries also improved image quality by reducing the effects of motion. CT

scanners with x-ray fan beam geometries and multiple radiation detectors constitute

the second generation of CT scanners [1] .

The third and fourth generations of CT scanners eliminate the translational motion of

previous scanners and rely exclusively upon rotational motion of the x-ray tube and

detector array (third generation Figure 1.2- c) or upon rotational motion of the x-ray

tube within a stationary circular array of 700 or more detectors (fourth-generation

scanner, Figure 1.2- d). With these scanners, data accumulation times as short as 1

second are achievable [1] .


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Figure 1.2: Scan motions in computed tomography [1].


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2) Material and Methods Several approaches to even faster CT scans have been pursued. Until recently,

multiple scan sequences to produce contiguous image slices required that the x -ray

tube stop its rotation and reverse its direction because the maximum extension of the

high-voltage cables had been reached. Thus, a successive slice-by-slice accumulation

technique was used to produce multi-slice images. In this technique, the total image

acquisition time is signicantly longer than the beam -on time because the table

increments (moves) to the next slice location and the patient breathes between slices.

2.1) Spiral or Helical CT Scanning

In the conventional CT systems described above, only a single slice can be acquired at

one time. If multiple slices are required to cover a larger volume of the body, the

entire thorax, for example, then the patient table is moved in discrete steps through the

plane of the X-ray source and detector. A single slice is acquired at each discrete table

position, with an inevitable time delay between obtaining each image. This process is

both time-inefficient and can result in spatial misregistrations between slices if the

patient moves [3] .

In the early 1990s, the design of third- and fourth-generation scanners evolved to

incorporate slip ring technology. A slip ring is a circular contact with sliding brushes

that allows the gantry to rotate continually, untethered by wires as shown in

Figure 2.1 1. The use of slip-ring technology eliminated the inertial limitations at the

end of each slice acquisition, and the rotating gantry was free to rotate continuously

throughout the entire patient examination. This design made it possible to achieve

greater rotational velocities than with systems not using a slip ring, allowing shorter

scan times. A typical spiral CT scheme is shown in Figure 2.2 2 and Figure 2.3 [2] .

1 From Fundamentals of Physics by Halliday, Resnick and Walker

2 http://www.mdtmag.com/articles/2007/09/compressing-storage-demands-ct-imaging


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Figure 2.1: Working principle of slip rings.

Figure 2.2: Application of slip ring in spiral CT.

Figure 2.3: Spiral CT scanning [1].


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The trajectory of the X-ray beam through the patient traces out a spiral, or helix:

hence the name. This technique represented a very significant advance in CT because

it allowed scan times for a complete chest and abdominal study to be reduced from

~ 10 min to ~1 min . In addition, a full three-dimensional vascular imaging dataset

could be acquired very shortly after injection of an iodinated contrast agent, resulting

in a significant increase in the SNR of the angiograms. Incorporation of this new

technology has resulted in three-dimensional CT angiography becoming the method

of choice for diagnosing disease in the renal and the pulmonary arteries as well as the

aorta [3] .

The instrumentation for spiral CT is very similar to conventional third-generation CT

scanners (some companies employ a fourth-generation design). However, because both the detectors and the X-ray source rotate continuously in spiral CT, it is not

possible to use fixed cables to connect either the power supply to the X-ray source or

the output of the photomultiplier tubes directly to the digitizer and computer. Instead,

multiple slip-rings are used for power and signal transmission. Typical spiral CT

scanners have dual-focal-spot X-ray tubes with three kVp settings possible [3] .

The main instrumental challenge in spiral CT scanning is that the X-rays must be

produced continuously, without the cooling period that exists between acquisition of

successive slices in conventional CT. This requirement leads to very high

temperatures being formed at the focus of the electron beam at the surface of the

anode. Anode heating is particularly problematic in abdominal scanning, which

requires higher values of tube currents and exposures than for imaging other regions

of the body. Therefore, the X-ray source must be designed to have a high heat

capacity and very efficient cooling. If anode heating is too high, then the tube current

must be reduced, resulting in a lower number of X-rays and a degraded image SNR

[3] .

X-ray detector design is also critical in spiral CT because highly efficient detectors

reduce the tube currents needed and help to alleviate issues of anode heating. The

detectors used in spiral CT are either solid-state, ceramic scintillation crystals or

pressurized xenon-filled ionization chambers. Scintillation crystals, usually made

from bismuth germanate (BGO), have a high efficiency (75-85%) in converting X-

rays to light and subsequently to electrical signals via coupled photomultiplier tubes.


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Gas-filled ionization chambers have a lower efficiency (40-60%), but are much easier

and cheaper to construct. The total number of detectors is typically between 1000

(third-generation scanners) and 5000 (fourth-generation systems) [3] .

A number of data acquisition parameters are under operator control, the most

important of which is the spiral pitch p. The spiral pitch is defined as the ratio of the

table feed d per rotation of the X-ray source to the collimated slice thickness S :

d p

S (1)

The value of p lies between 0 and 2 for single-slice spiral CT systems. For p values

less than 1, the X-ray beams of adjacent spirals overlap, resulting in a high tissueradiation dose. For p values greater than 2, gaps appear in the data sampled along the

long axis of the patient. For large values of p, image blurring due to the continuous

motion of the patient table during data acquisition is greater as shown in Figure 2.4 3.

A large value of p also increases the effective slice thickness to a value above the

width of the collimated X-ray beam: for example, at a spiral pitch value of 2, the

increase is of the order of 25%. The value of p typically used in clinical scans lies

between 1 and 2, which results in a reduction in tissue radiation dose compared to a

single-slice scan by a factor equal to the value of p [3] .

Due to the spiral trajectory of the X-rays through the patient, modification of the back

projection reconstruction algorithm is necessary in order to form images that

3 From lecture of Prof. Dr. Lothar Schad, Faculty of Medicine Mannheim, University of Heidelberg

12/9/2008

Figure 2.4: Variation in slice thickness with respect to pitch.


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correspond to those acquired using a single-slice CT scanner. Reconstruction

algorithms use linear interpolation of data points 1800 apart on the spiral trajectory to

estimate the data that would have been obtained at a particular position of a stationary

patient table. Images with thicknesses greater than the collimation width can be

produced by adding together adjacent reconstructed slices. Images are usually

processed in a way which results in considerable overlap between adjacent slices.

This has been shown to increase the accuracy of lesion detection, for example,

because with overlapping slices there is less chance that a significant portion of the

lesion lies between slices [2] .

2.2) Multislice Spiral CT

The efficiency of spiral CT can be increased further by incorporating an array of

detectors in the z direction, that is, the direction of table motion. Such an array is

shown in Figure 2.5. The increase in efficiency arises from the higher values of the

table feed per rotation that can be used. Multislice spiral CT can be used to image

larger volumes in a given time, or to image a given volume in a shorter scan time,

compared to single-slice spiral CT. The collimated X-ray beam can also be made

thinner, giving higher quality three-dimensional scans. The spiral pitch P ms for a

multislice CT is defined slightly differently from that for a single-slice CT system:

ms single

d P

S (2)

where S single is the single-slice collimated beam width. For a four-slice spiral CT

scanner, the upper limit of the effective spiral pitch is increased to a value of eight. In

multislice spiral CT scanning the effective slice thickness is dictated by the

dimensions of the individual detectors, rather than the collimated X-ray beam width

[3] .


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In a multislice system the focal-spot-to-isocenter and the focal-spot-to-detector

distances are shortened compared to those in a single-slice scanner, and the number of

detectors in the longitudinal direction is increased from one long element to a number

of shorter elements. There are two basic types of detector arrangements, called fixed

and adaptive. The former consists of 16 elements, each of length 1.25 mm, giving a

total length of 2 cm. The signals from sets of four individual elements are typically

combined. With the setup shown in Figure 1.33, four slices can be acquired with

thicknesses of 1.25, 2.5, 3.75, or 5 mm. These types of systems are typically run in

either high-quality (HQ) mode with a spiral pitch of 3 or high-speed (HS) mode with

a spiral pitch of 6. The second type of detector system is the adaptive array, which

consists of eight detectors with lengths 5, 2.5, 1.5, 1, 1, 1.5, 2.5, and 5 mm, alsogiving a total length of 2 cm. As for the fixed detector system, four slices are usually

acquired with 1, 2.5, or 5 mm thickness. Unlike the fixed detector system, in which

only specific pitch values are possible, the pitch value in an adaptive array can be

chosen to have any value between 1 and 8 [3] .

Fan-beam reconstruction techniques, in combination with linear interpolation

methods, are used in multislice spiral CT. One important difference between single

slice and multislice spiral CT is that the slice thickness in multislice spiral CT can be

Figure 2.5: A schematic of a fixed-array detector geometry for a multislice spiralscanner (Left). Four configurations connecting the data acquisition channels to singleor multiple elements of the arrayed detectors produce four different slice thicknesses(Right) [3].


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chosen retrospectively after data acquisition, using an adaptive axial algorithm. The

detector collimation is set to a value of 1, 2.5, or 5 mm before the scan is run. After

the data have been acquired, the slices can be reconstructed with a thickness between

1and 10 mm. Thin slices can be reconstructed to form a high-quality three-

dimensional image, but the same dataset can also be used to produce a set of 5-mm-

thick images with a high SNR [3] .

2.3) Dose Calculation in Spiral CT

The radiation dose delivered during a CT scan is somewhat greater than that

administered for an equivalent radiographic image. A CT image of the head requires a

dose of about 1 to 2 rad, for example, whereas an abdominal CT image usually

requires a dose of 3 to 5 rad. These doses would have to be increased signicantly to

improve the contrast and spatial resolution of CT images. The relationship between

resolution and dose can be approximated as:

2

3

s D a

e b

(3)

where D is the patient dose, s is the signal/noise ratio, e is the spatial resolution, b is

the slice thickness, and a is a constant. From above equation the following are

apparent:

1. A twofold improvement in the signal-to-noise ratio (contrast resolution)

requires a fourfold increase in patient dose.

2. A twofold improvement in spatial resolution requires an eightfold increase in

patient dose.

3. A twofold reduction in slice thickness requires a twofold increase in patientdose.

In multislice computed tomography, patient dose is described as the CT dose index

(CTDI). When the distance that the patient moves between slices (the couch

increment CI) equals the slice thickness ST, the CTDI equals the dose averaged over

all slices (multislice average dose MSAD). When the couch increment is less than the

slice thickness, the MSAD is the CTDI multiplied by the ratio of the slice thickness

(ST) to the couch increment (CI); that is,


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ST MSAD CTDI

CI

(4)

Patient dose decreases signicantly outside of the slice. A conservative rule of thumb

(i.e., an overestimate) is that the dose is 1% of the in-slice dose at an axial distance of

10 cm from the slice [1] .

2.4) Quality Control Procedure in Spiral CT

Many electronic components and massive amounts of data processing are involved in

producing a CT image. A consequence of the separation between data acquisition and

image display is the difculty of observing and investigating imaging system

problems through observation of the image alone. In such a complex system, imagequality can be ensured only through prospective monitoring of system components

and tests of overall system performance with standard phantoms. These measurements

should be correlated with patient dose to ensure that the proper balance is maintained

among variables that affect contrast, spatial resolution, image noise, and patient

radiation dose [1] .

Typical measurements of CT performance are given in Table 2-1, and examples are

shown in Figure 2.6. The fundamental system performance indicators are CT number,

resolution, noise, and patient dose. Figure 2.6 shows CT image of a quality control

phantom. Image quality is evaluated by analysis of regions of interest and by visual

inspection. The mean and standard deviation of pixel values in region 1 indicate CT

number calibration, while comparison of region 2 with region 1 yields contrast

information. The serrated patterns at 3 and 9 oclock on the image indicate slice

thickne ss and alignment. The rows of small dark circles (low CT number) at 1 oclock

is an indication of high contrast resolution [1] .


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Table 2-1: Common Quality Control Measurements for Computed Tomography [1] .

Figure 2.6: CT image of a quality control phantom [1] .

2.4.1) CT Num bers or Houn sf ie ld Units

Mter CT reconstruction, each pixel in the image is represented by a high-precision

floating point number that is useful for computation but less useful for display. Most

computer display hardware makes use of integer images. Consequently, after CT

reconstruction, but before storing and displaying, CT images are normalized and

truncated to integer values. The number CT( x,y) in each pixel, ( x,y), of the image is

converted using the following expression:

( , )( , ) 1000 water

water

x yCT x y

(5)

Where ( , ) x y is the floating point number of the ( x,y) pixel before conversion, water

is the attenuation coefficient of water, and CT( x,y) is the CT number (or Hounsfield


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unit) that ends up in the final clinical CT image. The value of water is about 0.195 for

the x-ray beam energies typically used in CT scanning. This normalization results in

CT numbers ranging from about -1000 to +3000, where -1000 corresponds to air, soft

tissues range from -300 to -100, water is 0, and dense bone and areas filled withcontrast agent range up to +3000 [1] .

The accuracy of CT numbers is measured by scanning a water- lled phantom at least

monthly. The CT number for water should be zero over a 20-cm-diameter phantom,

with a variation of less than 1 CT number. Deviation from the expected CT number of

0 for water at any energy is adjusted by applying a correction factor for the pixel

value. Constancy of the value should be monitored with a daily scan [1] .

2.4.2) Other Quality Con trol Check s

An overall check of system performance is obtained from semiannual measurements

of CT image noise, dened as the standard deviation of CT numbers in a region of

interest. Constancy of performance is checked by evaluation of the standard deviation

in the daily water scan mentioned previously. Resolution is measured by scanning

phantoms on a monthly basis. Of particular importance is low contrast resolution,

which is a sensitive indicator of changes in component performance as they affectnoise. Patient dose is evaluated semiannually. Specially designed ionization chambers

provide measurements from which the dose may be calculated for the exposure

conditions (narrow beam, variable slice thickness) used in CT. The values should

agree wit h manufacturers specications to within 20% [1] .

A variety of physical and mechanical factors such as patient couch positioning and

indexing should be measured as part of a comprehensive quality control program. The

performance of the hard-copy device and system monitors should be checked for

distortion, brightness, contrast adjustment, and so on. The accuracy of image analysis

features such as distance measurements and measurements of bone density should

also be independently evaluated. Additional information on quality control in CT is

available in the publications of a number of advisory groups and individuals [1] .


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3) Result Discussion

3.1) Advantages of Spiral CT

Improved lesion detection due to elimination of respiratory misregistration. Reduced amount of contrast. It is because of short time required for acquiring

the data set. This has obvious benefits related both to cost as well as the

incidence of adverse reactions. Reduction in amount of contrast required is up

to half the volume used in conventional CT.

Ability to scan a particular phase of contrast delivery. Reduced patient time. Higher quality of multi-planar and three dimensional reformations. It is

possible due to reduction of motion artifacts [4].

3.2) Disadvantages of Spiral CT

Increased image noise. This is related to both the interpolation technique and

the decreased power of the X-ray tube, necessitated by continuous scanning.

Volume-averaging artifacts. As the pitch increases, partial volume averaging

increases.

Additional processing time. The large amount of raw data leads to an increase

in the processing time, which can also temporarily interrupt patient scanning

[4].

3.3) Importance of Spiral CT Quality Assurance

A Quality Assurance (QA) program, which includes quality control tests, helps to

ensure that high quality diagnostic images are consistently produced while

minimizing radiation exposure. The QA program covers the entire x-ray system from

machine, to processor, to view box. Quality assurance enables the facility to recognize

when parameters are out of limits, which will result in poor quality images and can

increase the radiation exposure to patients. Simply performing the quality control tests

is not sufficient. When quality control test results exceed established operating

parameters, appropriate corrective action must be taken immediately and documented

[5].


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Product manufacturers, vendors, and service companies all have information available

in the form of leaflets, videos and hands-on help. If the facility finds that they need

more instruction than there are guides which provide required one, please use these

companies and the medical physicist as resources [5].

The responsibility for the quality control tests should be assigned to a QA program

coordinator to ensure consistency in test methodology and interpretation of the data.

More than one person may perform the tests but one person should assume overall

responsibility for the day to day operation of the program. This leads to better

understanding of when to repeat tests, call for service, or consult with the practitioner

or medical physicist. The physician, medical physicist, and QC personnel, working

together as a team, are the key to providing optimum quality radiographic images [5].


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4) Summary & Conclusion Because of the technical advantages of Spiral CT, clear indication for using it in the

study of those areas in which breath-hold speed of acquisition and the need or

opportunity to perform volumetric renderings are prominent. Specifically in

examination of chest and abdomen, it leads to high quality, more meticulous

examination with helical CT than with conventional CT. It is well suited for detection

and evaluation of small lesions e.g. pulmonary or renal mass as small as 5 mm in size

can be detected.

It also helps to perform needle localization phase of CT guided interventional

procedures more rapidly. It is also superior in CT angiography to produceextraordinary images of the abdominal vasculature and organs as well as noninvasive

evaluation of the carotid arteries and intracranial vasculature. 'Virtual endoscopy' is

another most exciting prospect for spiral CT where 3D spiral CT data sets of a hollow

viscous (e.g. colon or tracheobronchial tree) are obtained. 'Endoscopic' images of the

viscous are then generated by a computer. One can visualize the smaller parts of

tracheobronchial tree with 'virtual bronchoscopy' where the fibro-optic bronchoscope

cannot reach because of smaller size of airway or pathological stenosis. Spiral CT is

inferior to conventional CT in imaging of motionless structures like brain and

musculoskeletal structures.

Scanning in spiral mode can be considered a mature technology. Further

improvements in the technical scanning parameters, increase in X-ray power and

refinements in data processing algorithms aimed at higher Z-axis resolution will lead

to better Images.


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References

[1] E. Russell Ritenour William R. Hendee, Medical Imaging Physics , 4th ed. New

York, USA: Wiley-Liss, 2002.

[2] Jerrold T. Bushberg, The Essential Physics of Medical Imaging , 2nd ed. USA:

Lippincott Williams & Wilkins, 2002.

[3] Andrew Webb, Introduction to Biomedical Imaging , Metin Akay, Ed. New Jersey,

United States of America: John Wiley & Sons, 2003.

[4] Mandeep Singh Sudan, "Spiral Computed Tomography," JK Science (New

Horizons) , vol. 1, pp. 138-139, January 1999.

[5] New Jersey Department of Environmental Protection Bureau of Radiological

Health. (2011, January) Compliance Guidance for Computed Tomography Quality

Control. Document.


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Vita

Author of this report was born in Lahore (Punjab). He did his matriculation from

Punjab School System and intermediate from Shalimar College Lahore, with 1 st

division. He completed his B.Sc [honors] in Computational Physics from Centre for

High Energy Physics, University of the Punjab, Lahore and got merit scholarship

regularly for 4 years. In November, 2011, he was selected for PAEC fellowship and

now doing MS in Medical Physics from PIEAS, Islamabad.

Ali Adeel

C-113 PIEAS Hostels,

Nilore, Islamabad

spiral ct and its quality control procedures (mp-01)-report

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