Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Factors Affecting the Availability Of PETThe main factors limiting the availability of PET are the relatively high cost of adedicated PET scanner and, in many areas of the countty, the lack of local sourcesof F 18 FDG. However, cyclotron-equipped commercial radiopharmacies are nowlocated in many major metropolitan areas. Multihead SPECT cameras with coincidencecircuitry and SPECT cameras with high-energy collimators provide lessexpensive, although less accurate, alternatives for imaging FDG. PET imaging usingFDG has become routine at many major medical centers.Combined PET/X-Ray CT and SPECT/Coincidence Imaging/X-Ray CT SystemsPET imaging, particularly in oncologic applications, often reveals suspiciouslesions, but provides little information regarding their exact locations in the organsof the patients. In such situations, co-registration of the PET images with imagesfrom another modality, such as x-ray CT or magnetic resonance imaging (MRI),that provides good depiction of anatomy, can be very useful. However, this usuallyrequires moving the patient from the PET system to the CT or MRI system and,once the patient has been moved, it is very difficult to accurately align the PET datawith the CT or MRI data. To solve this problem, systems have been developed thatincorporate a PET system and a conventional x-ray CT system in a single gantry.The patient lies on a bed that passes through the bores of both systems. A series ofCT images is acquired over the same section of the patient as the PET scan, eitherbefore or after the PET image acquisition. Because the bed moves in the axial direction,but the patient lies still on the bed, each CT image slice corresponds to a transversePET image, permitting accurate co-registration. Furthermore, the x-ray CTinformation can be used to provide attenuation correction of the PET information.In the co-registered images, the PET information is usually superimposed in coloron gray-scale CT images.One manufacturer provides a gantry containing two scintillation camera headsand a simple x-ray CT machine. The scintillation camera heads can be used forSPECT imaging of conventional radiopharmaceuticals and, when placed in the180-degree orientation, for coincidence imaging of positron-emitting radiopharmaceuticals.The x-ray CT device provides CT image data for image co-registrationand for attenuation correction.Bax JJ, Wijns W Editorial: Fluorodeoxyglucose imaging to assess myocardial viability: PET,SPECT, or gamma camera coincidence imaging.] Nucl Med 1999;40:1893-1895.Cherry SR, Phelps ME. Positron emission tomography: methods and instrumentation. In: SandIerMp, et al., eds. Diagnostic nuclear medicine, 3rd ed., vol. 1. Baltimore: Williams & Wilkins,1996:139-159.Gelfand MJ, Thomas SR. Effictive use a/computers in nuclear medicine. New York: McGraw-Hill,1988.Groch MW, Erwin WD. SPECT in the Year 2000: Basic principles.] Nuclear Med Tech2000;28:233-244.Groch MW, Erwin WD, BieszkJA. Single photon emission computed tomography. In: Treves ST,ed. Pediatric nuclear medicine, 2nd ed. New York: Springer-Verlag, 1995:33-87.