Radiographic Image Quality: Optical Density, Image Detail and Distortion

Thursday, July 26, 2018


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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