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4.1 CT Image Display For radiologists, the most important output from a CT scanner is the image itself. As was discussed in Chapter 2, although the reconstructed images represent the linear attenuation coefficient map of the scanned object, the actual intensity scale used in CT is the Hounsfield unit (HU). For convenience, we replicate the mapping function below: CTnumber=μ−μ water μ water ×1000. The linear attenuation coefficient is magnified by a factor over 1000 (note the division by μwater). When specified in HU, air has the value of −1000 HU; water has the value of 0 HU; and bones, contrast, and metal objects have values from several hundred to several thousand HU. Because of the large dynamic range of the CT number, it is impossible to adequately visualize it without modification on a standard grayscale monitor or film. Typical display devices use eight-bit grayscales, representing 256 (28) different shades of gray. If a CT image is displayed without transformation, the original dynamic range of well over 2000 HU must be compressed by a factor of at least 8. Figure 4.1(a) shows a reconstructed image of a head phantom in which the minimum and maximum pixel intensities are −1000 HU and 1700 HU, respectively. When this dynamic range is linearly mapped to the dynamic range of the display device (0 to 255), the original grayscale is so severely compressed that little intensity variation can be visualized inside the human skull. This is clearly unacceptable.
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