There are several special SPECT systems designs that do not fit into the preceding two general categories. The commercially available CERESPECT (formerly known as ASPECT) [Genna and Smith, 1988] is a dedicated brain SPECT system. As shown in Figure 13.9a, it consists of a single fixed-annular NaI(TI) crystal that completely surrounds the patient’s head. Similar to a scintillation camera, an array of PMTs and electronics circuitry are placed behind the crystal to provide positional and energy information about photons that interact with the crystal. Projection data are obtained by rotating a segmented annular collimator with parallel holes that fits inside the stationary detector. A similar system is also being developed by Larsson et al. [1991] in Sweden.

Several unique SPECT systems are currently being developed in research laboratories. They consist of modules of small scintillation cameras that surround the patient. The hybrid designs combine the advantage of multidetector and camera-based SPECT systems with added flexibility in system configuration. An example is the SPRINT II brain SPECT system developed at the University of Michigan [Rogers et al., 1988]. As shown in Figure 13.9b, the system consists of 11 detector modules arranged in a circular ring around the patient’s head. Each detector module consists of 44 one-dimensional (1D) bar NaI(TI) scintillation cameras. Projection data are required through a series of narrow slit openings on a rotating lead ring that fits inside the circular detector assemblies. A similar system was developed at the University of Iowa [Chang et al., 1990] with 22 detector modules, each consisting of four bar detectors. A set of rotating focused collimators is used to acquire projection data necessary for image reconstruction. At the University of Arizona, a novel SPECT system is being developed that consists of 20 small modular scintillation cameras [Milster et al., 1990] arranged in a hemispherical shell surrounding the patient’s head [Rowe et al., 1992]. Projection data are acquired through a stationary hemispherical array of pinholes that are fitted inside the camera array. Without moving parts, the system allows acquisition of dynamic 3D SPECT data.

FIGURE 13.9 Examples of novel SPECT system designs. (a) The CERESPECT brain SPECT system consists of a single fixed annular NaI(TI) crystal and a rotating segmented annular collimator. (b) The SPRINT II brain SPECT system consists of 11 detector modules and a rotating lead ring with slit opening.

FIGURE 13.10 Collimator designs used in camera-based SPECT systems. (a) The commonly used parallel-hole collimator. (b) The fan-beam collimator, where the collimator holes are converged to a line that is parallel to the axis of rotation. (c) The cone-beam collimator, where the collimator holes are converged to a point. (d) A varifocal collimator, where the collimator holes are converged to various focal points.