The most popular SPECT systems are based on single or multiple scintillation cameras mounted on a rotating gantry. The successful design was developed almost simultaneously by three separate groups [Budinger and Gullberg, 1977; Jaszczak et al., 1977; Keyes et al., 1977]. In 1981, General Electric Medical Systems offered the first commercial SPECT system based on a single rotating camera and brought SPECT to clinical use. Today, there are over ten manufacturers (e.g., ADAC, Elscint, General Electric, Hitachi, Picker, Siemens, Sopha, Toshiba, Trionix) offering an array of commercial SPECT systems in the marketplace.
An advantage of camera-based SPECT systems is their use of off-the-shelf scintillation cameras that have been widely used in conventional nuclear medicine. These systems usually can be used in both conventional planar and SPECT imaging. Also, camera-based SPECT systems allow truly three-dimensional (3D) imaging by providing a large set of contiguous transaxial images that cover the entire organ of interest. They are easily adaptable for SPECT imaging of the brain or body by simply changing the radius of rotation of the camera.
A disadvantage of a camera-based SPECT system is its relatively low counting rate capability. The dead time of a typical state-of-the-art scintillation camera gives rise to a loss of 20% of its true counts at about 80K counts per second. A few special high-count-rate systems give the same count rate loss at about 150K counts per second. For SPECT systems using a single scintillation camera, the sensitivity per image slice is relative low compared with a typical multidetector SPECT system
FIGURE 13.8 Examples of camera-based SPECT systems. (a) Single-camera system. (b) Dual-camera system with the two cameras placed at opposing sides of patient during rotation. (c) Dual-camera system with the two cameras placed at right angles. (d) Triple-camera system. (e) Quadruple-camera system.
Recently, SPECT systems based on multiple cameras became increasingly more popular. Systems with two [Jaszczak et al., 1979a], three [Lim et al., 1980, 1985], and four cameras provide increased sensitivity per image slice that is proportional to the number of cameras. Figure 13.8 shows the system configurations of these camera-based SPECT systems. The dual-camera systems with two opposing cameras (Figure 13.8b) can be used for both whole-body scanning and SPECT, and those with two right-angled cameras (Figure 13.8c) are especially useful for 180-degree acquisition in cardiac SPECT. The use of multicameras has virtually eliminated the disadvantages of camera-based SPECT systems as compared with multidetector SPECT systems. The detection efficiency of camera-based SPECT systems can be further increased by using converging-hole collimators such as fan, cone, and astigmatic collimators at the cost of a smaller field of view. The use of converging-hole collimators in SPECT will be described in a later section.
