Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

These phantoms are very useful for the semiquantitative assessment of spatial resolution,image contrast, and uniformity. They are used for acceptance testing of newsystems and periodic testing, typically quarterly, thereafter. Table 22-1 provides asuggested schedule for a SPECT quality-control program.Positron emission tomography (PET) generates images depicting the distributionsof positron-emitting nuclides in patients. Figure 22-13 shows a PET scanner. In thetypical scanner, several rings of detectors surround the patient. PET scanners useannihilation coincidence detection (ACD) instead of collimation to obtain projectionsof the activity distribution in the subject. The PET system's computer thenreconstructs the transverse images from the projection data, as does the computerof an x-ray CT or SPECT system. Modern PET scanners are multislice devices, permittingthe simultaneous acquisition of as many as 45 slices over 16 cm of axial distance.Although there are many positron-emitting radiopharmaceuticals, the clinicalimportance of PET today is largely due to its ability to image theradiopharmaceutical fluorine 18 fluorodeoxyglucose (FDG), a glucose analog usedfor differentiating malignant neoplasms from benign lesions, staging malignantneoplasms, differentiating severely hypoperfused but viable myocardium from scar,and other applications.FIGURE 22-13. A commercial PET scanner. (Courtesy of Siemens Medical Systems, Nuclear MedicineGroup.)