Improvements in SPECT technology for cerebral imaging

Advancement in three major areas of SPECT (single photon emission computed tomography) technology have resulted in improved image quality for cerebral studies. In the first area, single-crystal camera electronics, extensive use of microprocessors, custom digital circuitry, an data bus architecture have allowed precise external control of all gantry motions and improved signal processing. The new digital circuitry permits energy, uniformity, and linearity corrections to be an integral part of the processing electronics. Calibration of these correlations is controlled by algorithms stored in the camera's memory. In addition, digital signals can be routed directly to interface circuitry of auxiliary computer systems without analog-to-digital conversion. Look-up tables, downloaded to the interface from the central processing unit (CPU), permit computer-controlled real-time processing of coordinate signals, including truncation, magnification, and spatial calibration. The second area of improved SPECT technology is camera collimation and related imaging techniques. In this area, system resolution has been improved without loss of sensitivity by decreasing the air gap between patient and collimator surface. Rotating the detector in close apposition to the head has required various stratagems to avoid detector-shoulder contact: the selective reduction of camera shielding, the use of long bore collimators, and the 30 degrees angulation of the camera head for slant hole collimation. Since cerebral studies characteristically image high-contrast regions less than 1 cm in size, image quality has been improved by increasing collimator resolution even at the expense of sensitivity. Increased resolution also improved image contrast for studies using 123I-labeled pharmaceuticals with 3% to 4% 124I contamination. Such studied acquired with low energy or medium energy collimation and a window centered on the 159 keV 123I photopeak contain appreciable septal breakthrough signals originating from Compton scatter of high energy photons primarily from 124I. The third area of advancements in technology, multidetector instrumentation, offers the promise of increased sensitivity and resolution. For the dynamic computer-assisted tomograph (DCAT) system, which was especially designed for regional blood flow studies with 133Xe or 127Xe, a count rate of 170,000 counts per microCi/cc for three slices has been achieved. This system consists of four detector banks each with 16 rectangular NaI crystals. An alternative system at Harvard uses an array of 12 moving detectors with focused collimators to acquire a single slice.(ABSTRACT TRUNCATED AT 400 WORDS)