CR and DR Image Characteristics

CR and DR Image Characteristics

A. Wide exposure latitude; range of exposure techniques

that will result in an acceptable image

B. Exhibit good visualization of soft tissue and bone

C. Digital image is composed of numerical values indicating

the variety of tissue thicknesses, densities, and atomic

numbers through which the x-ray photons are attenuated

D. Digital image is composed of rows and columns called a

matrix

1. Matrix size equals the total number of pixels in the

image

2. Smallest component of the matrix is the pixel (picture

element)

3. Each pixel corresponds to a shade of gray representing

an area in the patient called a voxel (volume element)

4. Pixel density—number of pixels/mm in the image;

determined by pixel pitch

5. Pixel pitch—space from the center of one pixel to the

center of an adjacent pixel

E. A histogram (graphic display) is constructed to show the

radiographer the distribution of pixel values (indicating

low, proper, or high exposure)

F. The correct processing algorithm (mathematical formula)

must be chosen so that the computer can

reconstruct the image specific to the exam that was

performed

G. As a digital image, the information can be manipulated

through various postprocessing steps (subtraction, edge

enhancement, contrast enhancement, and black/white

reversal)

1. Subtraction: Removal of superimposed or unwanted

structures from the image

2. Contrast enhancement: Altering of image to display

varying brightnesses

3. Edge enhancement: Improves visibility of small, highcontrast

areas

4. Black/white reversal: Reversal of the gray scale in the

image

H. Postprocessing can compensate for overexposures or

underexposures of considerable degree (−100% to +500%)

1. Because radiographers comply with the as low as

reasonably achievable (ALARA) concept, patients

should never be overexposed with the intention of

correcting the resultant images in the postprocessing

mode (dose creep)

I. Image may be printed onto film with a laser camera

J. Resolution is finer with DR than with CR because DR

involves less conversion of the information

K. In both CR and DR, visibility of the resolution depends

on the monitor being used to display the image

L. Changing the window level (midpoint of densities)

adjusts the image brightness (lighter or darker) throughout

the range of densities in postprocessing mode; this is

a direct relationship

1. As window level increases, image brightness increases

2. As window level decreases, image brightness decreases

M. Changing the window width adjusts the radiographic

contrast in postprocessing mode

1. As window width narrows, contrast increases

2. As window width widens, contrast decreases

N. Spatial frequency resolution: Level of detail or sharpness

on the CR image

O. Look-up table (LUT): Histogram of pixel values from

image acquisition that can be used to correct or enhance

luminance values

P. Beam-part-receptor alignment latitude—crucial so that

the software can detect collimated edges

Q. Quantum mottle (“grainy” or “noisy” image)

1. Is a source of noise in the image

2. Caused by too few x-ray photons hitting the image

receptor (normally from too low kVp)

R. Artifacts may be present in digital radiography

1. Artifacts related to the image receptor

a. Any object on the surface of the image receptor

(e.g., hair, dust, adhesive) may result in a light area

on the image the shape of the object; may also be

caused by dust on the CR plate reader rollers

b. Mishandling of the image receptor may result in

bending or cracks in the plate that may cause areas

of dead pixels or a row of dead pixels; dead pixels

are unable to display the information deposited

in their region of the image; dead pixel correction

software may provide some correction

c. Inadequate erasure of an image receptor, or incorrect

erasure settings, may result in “ghosting,” the

appearance of anatomy imaged on the previous

exposure (also called image lag)

1. This can occur more often with DR plates as

opposed to CR cassettes since there is usually a

shorter time period between exposures using DR

d. Excessive backscatter radiation may result in the

electronics of the DR plate being imaged and visualized,

even with additional lead foil backing on

the plate

e. Malfunctioning rollers in the CR plate reader may

result in vertical or horizontal lines across the image

f. A CR imaging plate that “stutters” or pauses while

in the reader may result in wavy lines across the

image

g. Incorrectly oriented grids may cause a moiré pattern

on the image; seen more often with low ratio grids

h. Digital imaging plates are extremely sensitive to

scatter radiation; control of scatter is critical

2. Artifacts related to software

a. Overprocessing the digital image may lead to loss

of information

b. Overcompression of the image may result in a loss

of information in the image

3. Artifacts related to technical errors

a. Improper collimation may lead to exposure field

recognition error and histogram analysis errors; the

image may appear too light, too dark, or too noisy

b. Misalignment of the exposure field may lead to

exposure field recognition error and histogram

analysis errors; the image may appear too light, too

dark, or too noisy