arrt & other digital terms defined supplement to hw assignment 2014 255 lect 4

ARRT & Other DIGITAL

Terms Defined

Supplementto HW assignment

2014 255

Lect 4

CRT vs LCD• CRT• Luminance higher in the

center• Lower measurable black

levels• Phosphor granularity

adds to spatial noise• Viewable area smaller

than stated size • Better color reproduction• More responsive on

redraw• More rugged• Aspect ratio 4:3

• LCD• Less veiling glare• Consumes less energy• Increased spatial

resolution• Larger viewing area by

described size• Display limited to

designed resolution• Can position screen• Smaller footprint and

lighter• Widescreen aspect ratio

16:9

Image Display= MONITORS

• viewing conditions– (i.e.,luminance,ambient lighting)

• spatial resolution

• contrast resolution/dynamic range

• DICOM gray scale function

• window level and width function

MONITORS: Spatial Resolution

• Spatial resolution refers to the amount of detail present in any image.

• Phosphor layer thickness and pixel size determines resolution in CR.

• The thinner the phosphor layer is, the higher resolution.

• Film/screen radiography resolution at its best is limited to 10 line pairs per millimeter (lp/mm).

• CR resolution is 2.55 lp/mm to 5 lp/mm, resulting in less detail.

ARRT DEFINITIONS

MONITOR RESOLUTION

MODULATION TRANSFER FUNCTION - MTF

• A measure of the ability of the imaging system to preserve signal contrast as a function of the spatial resolution.

• Every image can be described in terms of the amount of energy for each of its spatial frequency components.

• MTF often is regarded as the ideal expression of the image quality provided by a detector.

digital image characteristics

• spatial resolution• sampling frequency• DEL (detector element size)• receptor size and matrix size• image signal (exposure related)• quantum mottle• SNR (signal to noise ratio) or• CNR (contrast to noise ratio)

ARRT definitions

Spatial Resolution• A radiograph typically

does not show soft tissue structures

• Digital image shows not only the soft tissue but also the edge of the skin.

• Giving the appearance of more detail.

MONITORScontrast resolution

/dynamic range

• Appearance of more detail is due to the wider dynamic recording range / contrast resolution

• and does not mean that there is additional detail

• Because so many more densities are recorded in CR (wide dynamic range), images appear more detailed.

contrast resolution• The contrast resolution of a monitor is the

difference between the maximum and

minimum luminance of the display.

Exposure Latitudeor Dynamic Range

• CR and DR• Contain a detector that

can respond in a linear manner

• as compared to S shape of H&D Curve

• Exposure latitude is wide, allowing the single detector to be sensitive to a wide range of exposures.

Why do digital systems havesignificantly greater latitude?

• Linear response give the imaging plates greater latitude

• Area recieving little radiation can be enhanced by the computer

• Higher densities can be separated and brought down to the visibile density ranges

• (Brightness in DR replaces density)

Monitors - RESOLUTION• Pixel is a basic picture element on a display.

• A pixel is “any of the small discrete elements that together constitute an image.”

• Resolution -# of pixels contained on a display • Relationship:• More pixels in an image, the higher the

resolution & more information that can be displayed.

• Resolution also is defined as the process or capability of distinguishing between individual parts of an image that are adjacent.

10 PIXEL DENSITY

WHAT IS THE NYQUIST FREQUENCY= ?

Nyquist frequency ?

The Nyquist Theorem

• Theorem states that when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be nearly perfect.

• At least twice the number of pixels needed to form the image must be sampled.

• If too few pixels are sampled, the result is a lack of resolution.

Nyquist frequency• The highest spatial frequency that can be

recorded by a digital detector.

• is determined by the pixel pitch.

• The Nyquist frequency is half the number of pixels/mm.

• A digital system with a pixel density of 10 pixels/mm would have a Nyquist frequency of 5 line pair/mm.

Sampling Frequency ? Define ……

sampling frequency

• The frequency that a data sample is acquired from the exposed detector.

• It is expressed in pixel pitch and pixels per mm. • Sampling frequency may be determined by

receptor size depending on the vendor. • KODAK 8x10 better detail than 14x17• Use of the smallest imaging plate possible for

each exam results in the highest sampling rate.• When the smallest possible imaging plate is selected, a corresponding matrix is used

by the computer algorithm to process the image.

ARRT definitions

Pixel “picture element,” • the smallest area represented in a digital

image.

• A digital radiography image consists of a matrix of pixels which is typically several thousand pixels in each direction.

• Pixel density – A term that describes the number of pixels/mm in an image. Pixel density is determined by the pixel pitch.

• a pixel or picture element.

• The typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500.

• The more pixels there are, the greater the image resolution.

• The image is digitized by position (spatial location) and by intensity (gray level).

DEL (detector element size)receptor size and matrix size

DELs – detector elements

• DELs collect electrons that are extracted from the detector assembly and converted into a digital value by an ADC. That process creates the image that displays on our monitor.

• DEL size controls the recorded detail, or spatial resolution, for the flat-panel device. The technologist can’t change the size of the DEL, which is fixed for that piece of equipment.

• .

Detective Quantum Efficiency

• How efficiently a system converts the x-ray input signal into a useful output image is known as detective quantum efficiency, or DQE.

• DQE is a measurement of the percentage of x-rays that are absorbed when they hit the detector.


• Known as the fill factor, the larger the area of the TFT photodiodes, the more radiation can be detected and the greater amount of signal generated.

• Consequently, the greater the area of the TFT array, the higher the DQE.

• Over 1 million pixels are read & converted

FILL FACTOR• A field-effect transistor (FET) or silicon TFT

• Isolates each pixel element • Reacts like a switch to send the electrical charges to

the image processor


• In other words, CR records all of the phosphor output. Systems with higher quantum efficiency can produce higher-quality images at a lower dose.

• Indirect and direct DR capture technology has increased DQE over CR.

• However, DR direct capture technology, because it does not have the light conversion step and consequently no light spread, increases DQE the most.

SNR (signal to noise ratio) orCNR (contrast to noise ratio)

• SNR (signal to noise ratio): there is always a very small electric current flowing in any circuit - is called background electronic noise.

• It is similar to the fog on a radiograph in that it conveys no information and serves only to obscure the electronic signal.

• CNR (contrast to noise ratio): measure for assessing the ability of imaging an procedure to generate clinically useful image contrast.

• gives an objective measure of useful contrast.

ARRT definitions

Image Display

• spatial resolutioncontrast resolution/dynamic range

• What is a 3-D array of Pixels ?

• A voxel (a volumetric pixel) is a volume element, representing a 3-D value space. A pixel which represents 2D image data.

Pixel Pitch

• The space from the center of a pixel to the center of the adjacent pixel. It is measured in microns (μm).

• Pixel pitch is determined by sampling frequency for cassette-based PSP systems and by DEL spacing for TFT flat panel.

Monitors: Display Workstations

• Pixels are arranged in a matrix. • Common screen resolutions found on today’s

monitors are the following:• 1280 × 1024 (1K)• 1600 × 1200 (2K)• 2048 × 1536 (3K)• 2048 × 2560 (5K)

Monitors – DOT PITCH

• Dot pitch is the measurement of how close the dots are located to one another within a pixel.• The smaller the dot pitch of a display, the finer the

resolution. • Dot pitch may be expressed as aperture grille pitch or

slot pitch.

Monitors – REFRESH RATE

• Refresh rate or vertical scanning rate• Refresh rate is a measure of how fast the monitor

rewrites the screen or the number of times that the image is redrawn on the display each second.

• Refresh rate helps to control the flicker seen by the user.

• The higher the refresh rate, the less flicker will be seen.

• Most refresh rates on today’s computer are set between 60 and 75 Hz; the image is redrawn 60 to 75 timers per second.

Image Display= MONITORS

• viewing conditions– (i.e.,luminance,ambient lighting)

• spatial resolution

• contrast resolution/dynamic range

• DICOM gray scale function

• window level and width function

MONITOR RESOLUTIONDICOM gray scale function – calibration of monitors to the same standard & communication of images•Window level affects brightness (density) - B•Window width function affects contrast/gray scale - A

“windowing and level”

MONITOR Image Manipulation and Enhancement Functions

• Window/level

• This is a default function of the left mouse button.• By depressing and holding down the mouse button

and moving the mouse up and down and left and right, the window and level can be adjusted.

• Window (width) represents the range of gray values.• Level represents the center value of the range. • A change in the window and level appears to change

the brightness and contrast of the image.

Image Manipulation and Enhancement Functions

• Other Tools: Annotations• Annotations are NOT to be used • to label left or right to indicate the patient’s side.• Annotations are used to indicate prone or

supine, 30 minutes, upright or flat.• Any other image information is appropriate.• Radiologist will place arrows or circles around

pathologic or questionable areas.

MONITOR Image Manipulation and Enhancement Functions

• Pan, zoom, and magnify • Tools are used primarily by the

radiologist to increase the size of an area on the image.

• Magnify usually magnifies a square area of the image

• Pan and zoom functions are usually used together.

• Image is first zoomed up to the desired magnification level then Pan icon is activated.

• Zoomed image can be moved around to view the different areas of the image.


• Measurements• Size of a pixel is a known

so the software

has the ability to measure

structures on the image

based on this.• Angle measurement.

– Can give an angle measurement between two structures

– Commonly used when reading spine studies


• Measurements– Region of interest– Measurement tool determines the pixel intensity of a

certain area.– Each type of tissue or fluid has a different intensity of

reading.– Radiologist can make a determination whether

something is solid or fluid.

– Each pixel can have a gray level between 0 (20) and 4096 (212). The gray level will be a factor in determining the quality of the image

Monitor :Navigation Functions• Hanging protocolsCan be viewed:1:1 4:1 etc

• Protocol can also be specified to show the previous exam on one monitor and the current exam on the other

• Once set, the most efficient study navigation is determined.

Image Management Functions • Patient demographics• Patient demographics• must be correct.

– If demographics are not correct at the archive level, the images could be lost.

• Changes should only be made when the information is absolutely known to be wrong.

• Many hospitals allow only certain persons the access to change demographics just to keep the errors to a minimum.

Image Management Functions

• Query/retrieve icon• Used to retrieve on demand

any studies from the archive• Allows user to query a study

on multiple fields– Patient’s name or identification– Date of service– Modality– Diagnosis code or comment

field

WINDOW LEVEL / WIDTHWhich one controls

Denisty (brightness) ?Contrast

What else control these in DIGITAL IMAGING?

arrt & other digital terms defined supplement to hw assignment 2014 255 lect 4

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