Image Quality
The more exposure received by a specific portion of the
image receptor, the darker that portion of the image will be. The visibility of
the radiographic image depends on two factors: the overall blackness of the
image and the differences in blackness between the various portions of the
image. The clarity and sharpness of the image is a true representation of the
subject. These features make up the four elements of radiographic quality:
density, contrast, detail, and distortion.
Radiographic or Optical Density
The overall blackness of the image is referred to as the
radiographic density or optical density (OD). When the radiographic density is
optimum, the image is both dark enough and light enough for you to see the
anatomic details clearly on the viewbox. In conventional film / screen system,
density is controlled by the exposure factors, primarily the mAs. Because
exposure darkens the image, an increase in mAs will result in a darker
radiograph, while a decrease will cause it to be lighter. In filmless
radiographic systems the radiographic density of the image is controlled by the
computer; an increase or decrease in mAs will not darken or lighten the image.
An increase or decrease in exposure can only be detected by looking at the
exposure indicator number.
Take care not to confuse radiographic density with tissue
density, which refers to the mass density of anatomic parts. While increased
optical or radiographic density indicates that the image is darker, an increase
in tissue density will result in an image that is lighter. To avoid errors, try
not to use the word density without an appropriate descriptor.
Radiographic Contrast
The difference in the optical density of adjacent structures
within the image is referred to as the radiographic contrast. Even when a radiograph
has the proper optical density, it is possible that structures may be too
similar in density to be easily distinguished from one another. Image below
shows radiographs with high, low-contrast image has a black and white appearance.
Structures in the
grey areas are easily distinguished, but no details can be seen in the very
dark or the very light portions of the image. The low contrast image has an
overall grey appearance, and the structures tend to blend into one another. The
optimum contrast image shows details within all areas of the image, although
the contrast in some areas is less pronounced.
Kilovoltage
Kilovoltage is the primary contrast control factor, but
radiographic contrast is influenced by a number of other factors as well. These
include the nature of the subject, the characteristics of the film and or the
image receptor, and the amount of scatter radiation impacting the image
receptor. High kilovoltage produces an xray beam that penetrates more
completely, leaving no white areas in the image. The dark, easily penetrated
portions of the subject are not quite as dark when the kVp is high because less
mAs is needed to obtain the desired radiographic density. When more (higher)
contrast is desired, the kVp is decreased. Because this will result in less
penetration by the xray beam, a beam of greater intensity is needed, and the mAs
must be increased. Contrast is best evaluated when the overall radiographic
density is optimum.
Image Detail
The third element of image quality is image detail. This refers
to the sharpness of the image. When detail is high, the edges and lines that
make up the image are crisp and precise; with low detail, these lines and edges
are less distinct and appear somewhat blurred or “out of focus”. Among the
factors that affect image detail are the distance between the source of xray
and the image receptor, referred to as the source/image distance (SID); the
distance between the object and the image receptor, referred to as the object/
image distance (OID); the size of the screen crystals and the thickness of the phosphor
layer when intensifying screens are used, or the size of the pixels in digital
systems; the focal spot size ( the smaller the focal spot the greater the
detail); and whether the patient is able to hold still during the exposure.
Distortion
The fourth element of image quality is distortion. This refers
to a variation in size or shape of the image in comparison to the object in
represents. Size distortion is always in the form of magnification, and all
radiographic images are magnified to some degree. The factors that affect
magnification are the OID and the SID. The angulation of the diverging xray that
define the edges of a subject affects the degree of magnification.
When the image tube is farther from the image receptor, the
central, more parallel rays will define the subject, resulting in less
magnification. When the SID is shorter, the rays that define the subject are
those that diverge at a greater angle, increasing the magnification. The closer
the object to the receptor, the less magnification there will be.
Shape Distortion
Shape distortion is the result of unequal magnification of
various parts of the subject. The least shape distortion occurs when the plane
of the object is parallel to the plane of the image receptor and the central
ray is perpendicular to it. Angulation of the xray beam, the image receptor, or
the object in relation to the image receptor will all cause some degree of
distiortion.
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