Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

can be radically changed. Therefore, the whole notion of contrast on a digital imagehas to be rethought.A more meaningful and frequently used measure in assessing digital images,related to contrast, is the contrast-to-noise ratio (CNR):where the noise in the image is designated as 0". Noise will be discussed in moredetail below, but a couple of observations are warranted here. The CNR is notdependent on k, and that is good since the selection of k (an offset value) can bearbitrary. More importantly, because of the ability to postprocess digital images(unlike analog radiographic images), the CNR is a more relevant description of thecontrast potential in the image than is contrast itsel£Displayed Contrast (Digital Images)One of the most powerful attributes of digital images is that their displayedappearance can be easily changed. Figure 10-9 illustrates a digital chest radiographdisplayed using two different display curves. The graph in Fig. 10-9 illustratesa histogram of the image, and the two curves A and B illustrate the actualdisplay curves used to produce the two images. The graph in Fig. 10-9 shows thelook-up table used to convert ("map") the digital image data residing in the com-FIGURE 10-9. One of themajor benefits of digitalimages is that displayed contrastcan be manipulated,usually in real time, by changingthe way in which thegray-scale values are convertedinto brightness on thedisplay hardware. Here thesame chest radiograph is displayedwith two differentlook-up tables (lines A and Bon the lower graph). Thegraph below the two imagesillustrates a histogram of thegray-scale value in this image.The steeper display look-uptable (B) yields a higher contrastimage.