SPECT systems

PET imaging systems are somewhat more complex, and therefore more expensive than are SPECT systems, and the price factor is generally between two and three. The primary cost premium associated with these systems, however, is the need for a cyclotron and its attendant staff combined with the relative complexity of radiopharmaceutical preparation for short half-life isotopes. As of 1996, there are considerable hurdles blocking widespread regulatory approval and full reimbursement of PET studies. 

The resolution and sensitivity of an imaging device depend on a number of variables, for example, spatial resolution in SPECT is limited primarily by collimator resolution. Typical gamma camera SPECT systems have in-plane spatial resolution in the final reconstructed image of about 12 mm for high-resolution imaging. Resolution and sensitivity in PET is given in Table 58-3. 

Fig. 4.7 Schematic set-up of a SPECT system. One or more gamma cameras slowly rotate around the patient. The gamma cameras detect gamma ray photons that are emitted by a radiopharmaceutical injected into the patient.

Fig. 4.8 Combined animal CT and SPECT system, which allows high-resolution SPECT imaging of mice (spatial resolution <2 mm). The image on the left shows the anatomy of a mouse, while the image on the right shows a SPECT overlay on the anatomy. (Data courtesy of ImTek, Inc., Knoxville, TN, USA).

A recently developed pinhole high-resolution SPECT system and a 1.5 T clinical MRI scanner with a specially developed surface coil were used for rats (n = 9) after injection of the dopamine transporter ligand I-FP-CIT. The SPECT images showed clear striatal uptake. On the MR images, cerebral and extra-cerebral structures could be easily delineated. 

A SPECT system consists of one or more scintillation cameras mounted to a gantry that can revolve about a fixed horizontal axis (the axis of rotation) " (Fig. 27.13). SPECT studies are usually acquired over a full 360° arc, although myocardial perfusion studies typically use only data from the 180° arc that minimizes tissue attenuation. SPECT acquisitions are performed with the scintillation camera located at preselected angular locations (step-and-shoot mode), or in a continuous rotation mode. In the step-and-shoot mode, the detector rotates to each angular position and collects data in a preselected frame duration while the detector is motionless. In the continuous-rotation mode, the smdy duration is selected and the rotation speed is adjusted to complete the orbit during this time. Projections are collected as the detector rotates and are binned into 60 to 120 frames over 360°. 

FIGURE 27.14 SPECT system configurations. Although a single scintillation camera can be used to acquire SPECT data, multiple detectors improve the overall sensitivity. Two detectors arranged at either ISO or 90° are the most common configuration. 

Although most SPECT imaging samples more or less static distribution of radionuclides, some SPECT systems can perform rapid sequential studies to mcmitor tracer clearance. An example of this is determination of regional cerebral blood fiom the clearance of Multiple 1-minute... 

SPECT studies are acquired over a 10-minute interval. When one acquisition sample is completed, the next begins automatically. In order to minimize time, SPECT systems that perform these studies can alternately reverse the acquisition direction, although at least one SPECT system utilizes slipring technology so that the detectors can rotate continuously in the same direction. 

McEhoy DP, MacDonald LR, Beekman FJ, Wang Y, Patt BE, Iwanczyk JS, Tsui BMW, Hoffman EJ. Performance evaluation of A-SPECT a high resolution desktop pinhole SPECT system for imaging small animals. IEEE Trans Nucl Sci 2002 49 2139-2147. 

Meikle SR, Kench P, Wojcik R, Smith MF, Weisenberger AG, Majewski S, Lerch M, Rosenfeld AB. Performance evaluation of a multipinhole small animal SPECT system. IEEE Med Imaging Conf 2003 ... 

Dilmanian FA, Weber DA, Coderre JA, Joel DD, Shi K-C, Meinken GE, Som P, Tang Y-N, Volkow ND, Yee C, BriU AB, Watanabe M, Inuzuka E, Oba K, Gerson R, lida H, Hiruma A. A high-resolution SPECT system based on a microchannel-plate imager. IEEE Trans Nucl Sci 1990 37 687-695. 

KaUd K, Blankespoor SC, Brown JK, Hasegawa BH, Dae MW, Chin M, Stillson C. Myocardial perfusion imaging with a combined X-ray CT and SPECT system. J Nucl Med 1997 38 1535-1540. 

The above example points out the importance of complementary production technologies for large-size crystals. This kind of technology does not require development of dedicated sophisticated equipment, and it is not capital intensive, which is different from the hardware used in continuous single-crystal growth. However, when combined, they allow, for example, for rapid R D and fabrication of detectors for SPECT systems. 

PET and SPECT systems enable molecular and cell/tissue 3D visualization to be widely applied in clinical diagnosis and research studies. Studies supported by these techniques should allow the development of new therapies, such as inununo- and gene therapies, cell trafficking, infectious diseases, and cancer. PET and SPECT are very sensitive techniques that allow sections of a body to be imaged independently of physical properties and, therefore, there seans to be no limit to the range of applications. The reasons for this versatility are related to the availability of many different probes and of reporter molecules to target each kind of molecule. Both... 

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