Computed/digital imaging

135 terms by JenFrankland

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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

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