Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 22-10. Image of a cylinder filledwith a uniform radionuclide solution, showinga ring artifact due to a nonuniformity.Artifact is the dark ring toward the center.As mentioned in the previous chapter, the primary intrinsic causes of nonuniformityare (a) spatial nonlinearities, which "stretch" the image in some areas, reducing thelocal count density, and "compress" other areas of the image, increasing the count density;and (b) local variations in the efficiency of light collection, which produce systematicposition-dependent variations in the amplitudes of the energy signals and thus localvariations in the fraction of counts rejected by the energy discrimination circuits. Allmodern SPECT cameras incorporate digital circuits that apply correction factors,obtained from internal "lookup" tables, to the X, Y, and energy signals from each interaction.However, these correction circuits cannot correct nonuniformity due to localvariations in detection efficiency, such as dents or manufacturing defects in the collimators.If not too severe, nonuniformities of this type can be largely corrected. A very highcount uniformity image must be acquired. The ratio of the average pixel count to thecount in a specific pixel in this image serves as a correction factor for that pixel. Followingthe acquisition of a projection image during a SPECT study, each pixel of the projectionimage is multiplied by the appropriate correction factor before COR correctionand filtered backprojection. For the high-count uniformity image, at least 30 millioncounts should be collected for 64 2 images and 120 million counts for a 128 2 format.These high-count uniformity images should be collected every 1 or 2 weeks. Separatecorrection images must be acquired for each camera head. For cameras from some manufacturers,separate correction images must be acquired for each radionuclide. The effectivenessof a camera's correction circuitry and of the use of high-count flood correctionimages can be tested by acquiring a SPECT study of a large plastic jug filled with a wellmixedsolution ofT c 99m and examining the transverse images for ring artifacts.The camera head or heads must be exactly parallel to the AOR. If they are not, aloss of spatial resolution and contrast will result from out-of-slice activity beingbackprojected into each transverse image slice (Fig. 22-11). The loss of resolutionand contrast in each transverse image slice will be less toward the center of the sliceand greatest toward the edges of the image. If the axis of rotation of the camera isaligned to be level when the camera is installed and there is a flat surface on the