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135 terms

Computed/digital imaging

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To be suitable for use in intensifying screens, a phosphor should have which of the following characteristics?
1. High conversion efficiency
2. High x-ray absorption
3. High atomic number
Intensifying-screen phosphors
have a high atomic number are more likely to absorb a high percentage of the incident x-ray photons and convert x-ray photon energy to fluorescent light energy.
How efficiently the phosphors detect and interact with the x-ray photons is termed
quantum detection efficiency
How effectively the phosphors make this energy conversion is termed
conversion efficiency
With all other factors constant, as digital image matrix size increases,
1. pixel size decreases.
2. resolution increases.
A digital image is formed by
a matrix of pixels (picture elements) in rows and columns.
A matrix that has 512 pixels in each row and column is a
512 x 512 matrix
The term field of view is used to describe
how much of the patient (eg, 150-mm diameter) is included in the matrix.
The matrix and the field of view can be changed independently, without one affecting the other, but changes in either will change
pixel size
As in traditional radiography, spatial resolution is measured in
line pairs per mm (lp/mm)
As matrix size is increased, there are more and smaller pixels in the matrix, and therefore improved
resolution
Fewer and larger pixels result in
a poor resolution, "pixelly" image, that is, one in which you can actually see the individual pixel boxes.
Most laser film must be handled
in total darkness
Most laser film is sensitive to both the
Wratten 6B and the GBX (green, blue, x-ray) safelight filters. Laser film will fog if it is handled under these safelight conditions.
Most laser film is loaded into a film magazine in
total darkness.
Processing temperatures for laser film are the same as
those for regular x-ray film.
The violet light emited by the photostimulable phosphor (PSP) is transformed into the image seen on the CRT by the
ADC
The exposed CR cassette is placed into the CR scanner reader
where the PSP/imaging plate is automatically removed.
The latent image appears as the PSP is scanned by
a narrow high-intensity helium-neon laser to obtain the pixel data.
As the plate is scanned in the CR reader, it releases
a violet light—a process referred to as photo-stimulated luminescence.
The luminescent light is converted to
electrical energy representing the analog image
The electrical energy is sent to an analog-to-digital converter (ADC) where
it is digitized and becomes the digital image that is eventually displayed (after a short delay) on a high-resolution monitor and/or printed out by a laser printer.
The digitized images can also be
manipulated in postprocessing, electronically transmitted, and stored/archived
In digital imaging, pixel size is determined by
dividing the field of view (FOV) by the matrix; the FOV and matrix size are independent of one another, that is, either can be changed and the other will remain unaffected.
pixel size is affected by
changes in either the FOV or matrix size
if the matrix size is increased, pixel size
decreases
If FOV increases, pixel size
increases.
Pixel size is inversely related to
resolution
As pixel size increases, resolution
decreases
Bone densitometry is often performed to
1. measure degree of bone (de) mineralization.
2. evaluate results of osteoporosis treatment/therapy.
Dual x-ray absorptiometry (DXA) (bone densitometry) imaging is used to
evaluate bone mineral density (BMD).
Bone densitometry, DXA, can be used to evaluate
bone mineral content of the body, or part of it, to diagnose osteoporosis or to evaluate the effectiveness of treatments for osteoporosis
DXA uses two photon energies
one for soft tissue and one for bone.
DXA is the most widely used method of
bone densitometry—it is low dose, precise, and uncomplicated to use/perform
the following statements regarding dual x-ray absorptiometry are true
1. two x-ray photon energies are used.
2. photon attenuation by bone is calculated.
Since bone is more dense and attenuates x-ray photons more readily, their attenuation is calculated to represent
the degree of bone density.
Soft tissue attenuation information is not used to measure
bone density
the main function of a CR cassette is to
support and protect the IP that lies within the CR cassettes.
CR cassettes do not contain:
intensifying screens or film and therefore do not need to be light tight.
The photostimulable IP is not affected by
light.
One of the biggest advantages of CR/DR is
the latitude it offers.
The characteristic curve of typical film emulsion has a "range of correct exposure," limited by
the toe and shoulder of the curve.
In CR/DR, there is a linear relationship between the exposure, given the PSP and
its resulting luminescence, as it is scanned by the laser
CD/DR affords much greater
exposure latitude; technical inaccuracies can be effectively eliminated
Overexposure of up to
500% and underexposure of up to 80% are reported as recoverable, thus eliminating most retakes.
The same exposure factors as screen-film systems, or less, are generally recommended for
CR/DR
Better resolution is obtained with
high SNR
Spatial resolution increases as
SNR (signal-to-noise ratio) increases.
A high SNR (e.g., 1000:1) indicates that there is far more signal than
noise
A lower SNR (e.g., 200:1) indicates a
"noisy" image.
Windowing allows for
contrast and density post-processing manipulation
Image matrix has a great deal to do with
resolution
A larger image matrix (1800 x 1800) offers better resolution than
a smaller image matrix (700 x 700).
Smaller image matrices look
"pixelly."
Characteristics of digital radiographic imaging include
1. solid state detector receptor plates.
2. direct-capture imaging system.
3. immediate image display.
digital radiographic imaging requires the use of
somewhat different equipment.
CR uses
traditional x-ray devices to enclose and protect the PSP image plate
DR does not use
cassettes or a traditional x-ray table; it is a direct-capture system of x-ray imaging
DR uses
solid state detector plates as the x-ray image receptor (instead of a cassette in the Bucky tray) to intercept the collimated x-ray beam and form the latent image
The solid state detector plates are made of
barium fluorohalide compounds similar to that used in CR's PSP image plates
DR affords the advantage of
immediate display of the image,compared to CR's delayed image display
In general, as the intensification factor increases
radiographic density increases
Factors that contribute to an increase in the intensification factor generally function to
reduce resolution.
Slow-speed (detail or "extremity") screens resolve more
line pairs per millimeter (lp/mm) than much faster screens
The use of fast screens results in some loss of
recorded detail
As intensification factor increases, radiographic density generally
increases.
Advantages of computed radiography (CR) over digital radiography (DR) include
1. CR is useful for mobile imaging.
2. CR is compatible with existing equipment.
CR is less expensive primarily because
it is compatible with existing equipment
DR requires existing equipment to be
modified or new equipment purchased
Because there is an image plate (IP), CR can be used for
mobile studies
DR cannot be used with mobile studies because
no cassettes are used
DR offers the advantage of
immediate visualization of the x-ray image; in CR, there is a short delay
The process of "leveling and windowing" of digital images determines the image
contrast
The digital image's scale of contrast, or contrast resolution, can be changed electronically through
leveling and windowing of the image
The level control determines the
central or mid density of the scale of contrast, while the window control determines the total number of densities/grays (to the right and left of the central/mid density)
Matrix and pixel sizes are related to
(spatial) resolution of digital images
One advantage of digital imaging in fluoroscopy is the ability to perform "road-mapping." Road-mapping
1. keeps the most recent fluoroscopic image on the screen.
2. aids in the placement of guidewires and catheters.
3. reduces the need for continuous x-ray exposure to the patient.
There are several advantages of electronic/digital fluoroscopy
1. Electronic/digital fluoroscopic images are produced with less patient exposure and can be postprocessed (windowed to improve/enhance the image).
2. The fluoroscopic still-frame images can be stored and/or transmitted to a TV monitor.
Another advantage is
the ability to perform "road-mapping."
In this procedure
the most recent fluoroscopic image is retained on the screen/monitor
Road-mapping is particularly useful in procedures that require
guidewire/catheter placement
The frame-hold function eliminates the need for
continuous fluoroscopy, thereby reducing patient exposure
Which possesses the widest dynamic range?
CR
One of the biggest advantages of CR is
the dynamic range, or latitude, it offers.
In CR, there is a linear relationship between
the exposure, given the PSP (photostimulable phosphor, or image plate) and its resulting luminescence, as it is scanned by the laser
This affords much greater
exposure latitude and technical inaccuracies can be effectively eliminated
Intensifying screens used in screen-film x-ray imaging tend to produce high
contrast
The faster the screens, the higher the
contrast; higher contrast is often associated with decreased latitude
AEC refers to
automatic exposure control and is unrelated to dynamic range or latitude
Resolution in computed radiography increases as
1. laser beam size decreases
2. PSP crystal size decreases
Spatial resolution in CR is impacted by
the size of the PSP, the size of the scanning laser beam, and monitor matrix size.
High-resolution monitors (2-4 MP, megapixels) are required for
high-quality, high-resolution image display
The larger the matrix size, the better the
image resolution
Typical image matrix size (rows and columns) used in chest radiography is
2048 x 2048
As in traditional radiography, spatial resolution is measured in
line pairs per mm
As matrix size is increased
there are more and smaller pixels in the matrix therefore improved spatial resolution
Other factors contributing to image resolution are
the size of the laser beam and the size of the PSP/IP phosphors
Smaller phosphor size improves resolution in ways similar to that of intensifying screens
anything that causes an increase in light diffusion will result in a decrease in resolution
Smaller phosphors in the PSP plate allow
less light diffusion
the scanning laser light must be the correct
intensity and size.
A narrow laser beam is required
for optimum resolution
In digital imaging, as the size of the image matrix increases:
1. pixel size decreases
2. spatial resolution increases
The FOV and matrix size are
independent of one another, that is, either can be changed and the other will remain unaffected.
pixel size is affected by changes in either the
FOV or matrix size
if the matrix size is increased, pixel size
decreases.
If FOV increases, pixel size
increases.
Pixel size is inversely related to
resolution
As pixel size decreases, resolution
increases
Types of shape distortion include
1. elongation.
2. foreshortening.
Size distortion (magnification)
is inversely proportional to SID and directly proportional to OID.
Increasing the SID and decreasing the OID decreases
size distortion.
Aligning the tube, part, and IR so that they are parallel reduces
shape distortion
There are two types of shape distortion
IR results in foreshortening of the object.
Tube angulation causes
elongation of the object.
The radiographer can manipulate (change, enhance) digital images displayed on the CRT through postprocessing. One way to alter image contrast and/or density is through
windowing
The term windowing refers to
some change made to window width and/or window level
Change in window width affects
change in the number of gray shades, that is, image contrast
Change in window level affects
change in the image brightness, that is, optical density
Windowing and other postprocessing mechanisms permit the radiographer to affect changes in the image and to produce "special effects" such as
edge enhancement, image stitching (useful in scoliosis examinations), image inversion, rotation, and reversal.
Which of the following terms refers to light being reflected from one intensifying screen, through the film, to the opposite emulsion and screen?
Crossover
If fluorescent light from one intensifying screen passes through the film to the opposite emulsion and intensifying screen, the associated diffusion creates a type of distortion called
crossover
ntensifying screens do need a degree of
reflectance to enhance their speed.
Scatter and filtration are unrelated to
intensifying screens.
Any images obtained using DXA bone densitometry
are used to evaluate accuracy of the ROI
DXA imaging is used to
evaluate BMD (bone mass density).
Any images obtained in DXA/bone densitometry are strictly to evaluate:
the accuracy of the ROI (region of interest); they are not used for further diagnostic purposes—additional diagnostic examinations are done for any required further evaluation
That portion of a CR cassette that records the radiologic image is the
photostimulable phosphor
Inside the IP (CR cassette) is the
photostimulable phosphor (PSP) image storage plate.
This PSP with its layer of europium-activated barium fluorohalide serves as the:
image receptor as it is exposed in the traditional manner and receives the latent image
The PSP can store the latent image for:
several hours; after about 8 hours, noticable image fading will occur
Once the IP is placed into the CR processor (reader/scanner), the PSP plate is automatically
removed
The latent image on the PSP is changed to a manifest image as :
it is scanned by a narrow high-intensity helium-neon (or solid state) laser to obtain the pixel data
As the plate is scanned in the "reader," it releases a violet light—a process referred to as
photo- (or light) stimulated luminescence.
What will result from using single-emulsion film in an image receptor having two intensifying screens?
Decreased density
If single-emulsion film is placed in a dual-screen cassette:
the emulsion will receive only one-half of the intended exposure, and the resulting image will exhibit decreased density