Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 22-9. Center-of-rotation (COR) misalignment. Small misalignments cause blurring(center), whereas large misalignments cause point sources to appear as "tinydoughnut" artifacts (right).the image; this projection of the AOR into the image is called the center ofrotation(COR). Ideally, the COR is aligned with the center, in the x-direction, of each projectionimage. However, there may be a misalignment of the COR with the centerof the projection images. This misalignment may be mechanical; for example, thecamera head may not be exactly centered in the gantry. It can also be electronic. Themisalignment may be the same amount in all projection images from a single camerahead, or it may vary with the angle of the projection image.If a COR misalignment is not corrected, it causes a loss of spatial resolution inthe resultant transverse images. If the misalignment is large, it can cause a pointsource to appear as a tiny "doughnut" (Fig. 22-9). (These doughnut artifacts are notseen in clinical images; they are visible only reconstructed images of point or linesources. The doughnut artifacts caused by COR misalignment are not centered inthe image and so can be distinguished from the ring artifacts caused by nonuniformities.)The COR alignment is assessed by placing a point source or line source inthe camera field of view, acquiring a set of projection images, and analyzing theseimages using the SPECT system's computer. If a line source is used, it is placed parallelto the AOR. The COR misalignment may be corrected by shifting each imagein the x-direction by the proper number of pixels prior to filtered backprojection.When a line source is used, the COR correction can be performed separately foreach slice. If the COR misalignment varies with camera head angle, instead of beingconstant for all projection images, it can only be corrected if the computer permitsangle-by-angle corrections. Separate assessments of the COR correction must bemade for different collimators and, on some systems, for different camera zoom factorsand image formats (e.g., 64 2 versus 128 2 ). The COR correction determinedusing one radionuclide will only be valid for other radionuclides if the multi energyspatial registration is correct.The uniformity of the camera head or heads is important; non uniformities that arenot apparent in low-count daily uniformity studies can cause significant artifacts inSPECT. The artifact caused by a nonuniformity appears in transverse images as aring centered about the AOR (Fig. 22-10).Multihead SPECT systems can produce partial ring artifacts when projectionimages are not acquired by all heads over a 360-degree arc. Clinically, ring artifactsare most apparent in high count-density studies, such as liver images. However, ringartifacts may be most harmful in studies such as TI-201 myocardial perfusionimages in which, due to poor counting statistics and large variations in count density,they may not be recognized and thus may lead to misinterpretation.