Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

heads of a SPECT system produced ideal projection images (i.e., no attenuation bythe patient and no degradation of spatial resolution with distance from the camera),projection images from opposite sides of the patient would be mirror images andprojection images over a 180-degree arc would be sufficient for transverse imagereconstruction. However, in SPECT, attenuation greatly reduces the number ofphotons from activity in the half of the patient opposite the camera head, and thisinformation is greatly blurred by the distance from the collimator. Therefore, formost noncardiac studies, such as brain SPECT, the projection images are acquiredover a complete revolution (360 degrees) about the patient. However, most nuclearmedicine laboratories acquire cardiac SPECT studies, such as myocardial perfusionstudies, over a 180-degree arc from the 45-degree right anterior oblique (RAO) viewto the 45-degree left posterior oblique (LPO) view (Fig. 22-2). The 180-degreeacquisition produces reconstructed images of superior contrast and resolution,because the projection images of the heart from the opposite 180 degrees have poorspatial resolution and contrast due to greater distance and attenuation. Althoughstudies have shown that the 180-degree acquisition can introduce artifacts, the 180-degree acquisition is more commonly used than the 360-degree for cardiac studies.SPECT projection images are usually acquired in either a 64 2 or a 128 2 pixelformat. Using too small a pixel format reduces the spatial resolution of the projectionimages and of the resultant reconstructed transverse images. When the 64 2format is used, typically 60 or 64 projection images are acquired and, when a 128 2format is chosen, 120 or 128 projection images are acquired. Using too few projectionscreates radial streak artifacts in the reconstructed transverse images.The camera heads on older SPECT systems followed circular orbits around thepatient while acquiring images. Circular orbits are satisfactory for SPECT imagingof the brain, but cause a loss of spatial resolution in body imaging because the circularorbit causes the camera head to be many centimeters away from the surface ofthe body during the anterior and posterior portions of its orbit (Fig. 22-3). NewerSPECT systems provide noncircular orbits (also called "body contouring") thatkeep the camera heads in close proximity to the surface of the body throughout theorbit. For some systems, the technologist specifies the noncircular orbit by placingthe camera head as close as possible to the patient at several angles, from which thecamera's computer determines the orbit. Other systems perform automatic bodyLPO ,/ ',',/ , ,/ , ,, ,"" ":'," , ":', , ,, , \" '",, ,,/, ,/