Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

pixels are averaged to go from a 512 X 512 image to a 256 X 256 image, and thento a 128 X 128 image, and so on. Image-processing techniques are then applied toeach of the smaller-resolution images, and they are then recombined to get a single,processed 512 X 512 image.Medical images sometimes exhibit a very large range of contrast; for example, theaverage CT numbers in the lung fields are markedly different than in the liver, andthese are different from the CT numbers of bone. With such a broad range of grayscale, the radiologist often reviews CT images with two or three different window/levelsettings, because one windowllevel setting is insufficient to visualize allparts of the image. This problem can be rectified in some types of images by adaptivehistogram equalization (AHE). Essentially, AHE algorithms apply differentwindowllevel settings to different parts of the image, with a smooth spatial transition.AHE images have an appearance similar to the image shown in Fig. 11-160.11.10 CONTRAST VERSUS SPATIAL RESOLUTIONIN DIGITAL IMAGINGWhen an analog image is partitioned into a digital matrix of numbers, compromisesare made. As an example, screen-film mammography produces images with measurablespatial resolution beyond 20 cycles/mm. To obtain this resolution with a digitalimage, 25-llm pixels would be required, resulting in a 7,200 X 9,600 pixel image.Images of this size cannot be displayed (at full resolution) on any monitor, and storinghuge images is expensive as well. Consequently, digital radiographic images areinvariably lower in spatial resolution than their analog screen-film counterparts (Fig.11-17), both for mammography and for general diagnostic radiography.Images in digital format have advantages as well. The ability to transmit imageselectronically, to produce identical copies that can be in multiple locations at the100~~0--8060LLI-~ 40200 0 1234567Spatial Frequency(cycles/mm)FIGURE 11-17. The modulationtransfer functions (MTFs) are shownfor several modalities. SF, screenfilm;CR, computed radiography; SFMammography, screen-film mam-8 mography; TFT Digital, direct detection100-flm pixel flat panel detectorsystem.