Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

msec is preprocessed, mathematically filtered, and backprojected to create a CTsubframe. This new CT subframe (an image) is added (pixel by pixel) to the fiveprevious subframes, creating a complete CT frame ready for display. Therefore,every 167 msec, a new subframe is created and an old subframe is discarded, so thatthe most recent six subframes are summed to produce the displayed CT frame.Although 6 frames per second was used in this example, higher frame rates are clinicallyavailable.Once the CT images from a patient's examination have been reconstructed, theimage data must be conveyed to the physician for review and diagnosis. There aresome basic postprocessing techniques that are applied to all CT images (i.e., windowingand leveling). For CT studies that have contiguous or interleaving imagesover a length of the patient, the volume data set lends itself to other forms of postproceSSIng.Windowingand LevelingCT images typically possess 12 bits of gray scale, for a total of 4,096 shades of gray(CT numbers range from -1,000 to +3,095). Electronic display devices such ascomputer monitors have the ability to display about 8 bits (256 shades of gray), andlaser imagers used for "filming" CT studies have about 8 bits of display fidelity aswell. The human eye has incredible dynamic range for adapting to absolute lightintensities, from moonless nights to desert sunshine. Much of this dynamic range isa result of the eye's ability to modulate the diameter of the light-collecting aperture(the pupil). However, for resolving relative differences in gray scale at fixed luminancelevels (fixed pupil diameter), as in viewing medical images, the human eyehas limited ability (30 to 90 shades of gray), and 6 to 8 bits is considered sufficientfor image display.The 12-bit CT images must be reduced to 8 bits to accommodate most imagedisplay hardware. The most common way to perform this postprocessing task(which nondestructively adjusts the image contrast and brightness), is to windowand level the CT image (Fig. 13-32). The window width (W) determines the contrastof the image, with narrower windows resulting in greater contrast. The level(L) is the CT number at the center of the window. Selection of the values of LandW determines the inflection points PI and P 2 , as shown in Fig. 13-32, where PI =L - 1/2W and P 2 = L + 1/2W All the CT numbers lower than PI will be saturated touniform black, and all the CT numbers above P 2 will be saturated to uniform white,and in these saturated areas all image information is lost. It is routine to display CTimages using several (two or three) different window and level settings for eachImage.A stack of axial CT images represents anatomic information in three dimensions;however, the axial display is the most common. For studying anatomic features thatrun along the cranial-caudal dimension of the body, such as the aorta or spinal cord,