Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

246 8Tissue Thickness (cm)FIGURE 10-8. The relative image brightness(which is related to contrast) as a functionof patient thickness is dependent onthe subject contrast, tlie detector contrast,and the overall brightness of the light box.The combined influence of all the imageformation steps yields a sigmoidal relationshipbetween image brightness reachingthe radiologist's eyes and patient thickness.There is sufficient difference between digital imaging systems and screen-film radiographythat the output measure of contrast in the image is defined separately. Forscreen-film radiography and mammography, the analog film is the output and thecontrast that is seen on the film is called radiographic contrast:Radiographic Contrast = ODA - OD B [10-7]where A and B refer to adjacent regions on the radiograph. While a more completediscussion of optical density is found in Chapter 6, it is worth mentioning here thatradiographic films are placed over light boxes for display. In this process, the numberof light photons hitting the radiologist's eyes is proportional to B lO-OD, whereB is the brightness of the light box. In radiography, the contrast cannot be adjustedor enhanced on the analog film. So how does the final light signal that reaches theradiologist's eyes depend on patient thickness? The mathematical relationships relatingto subject contrast, detector contrast, and B lo- 0D were combined to demonstratethe relationship between brightness on the radiograph and tissue thickness,shown in Fig. lO-8.Digital Image Contrast (Contrast-to-Noise Ratio)Once a digital image is acquired, for many types of detector systems, a series of processingsteps is performed automatically as part of the acquisition software. Thedetails are dependent on the modality and detector technology, but one commontype of processing is the subtraction of a constant number (call this number k) fromthe image. Let the average density of a small square region on a digital image bedenoted as A, but after the processing this becomes A - k. The average value of anadjacent region can be denoted B - k, and assume that A > B. If we apply previousnotions of contrast to these values, such as for subject contrast (Equation lO-1), weimmediately run into problems when the processing is considered: Contrast = ([A- k] - [B - k])/[A - k] = (A - B)/(A - k). Notice that if k = A/2, contrast is doubled,and ifk =A, contrast would be infinite (division by zero). Ifk is negative, contrastis reduced. Thus, depending on the somewhat arbitrary choice of k, contrast