HALF-FANBEAM COLLIMATORS COMBINED WITH SCANNING POINT SOURCES FOR SIMULTANEOUS EMISSION-TRANSMISSION IMAGING

One type of SPECT system often used for simultaneous emission-transmis sion tomography is equipped with parallel-hole collimators, moving lin e sources (MLS) and electronic windows that move in synchrony with the sources. Although downscatter from the emission distribution is reduc ed by the use of the electronic window, this still can represent a siz able fraction of the transmitted counts. These systems have relatively poor spatial resolution and use costly transmission sources. Methods: Using a two-head SPECT system, with heads at right angles, two Gd-153 line sources (5800 MBq each) were replaced by two Gd-153 point source s of only 750 MBq each and positioned to move along the focal lines of two half-fanbeam collimators. A suitable acquisition protocol for a m oving point source (MPS) system was selected by considering the result s of a simulation study. With this protocol, physical phantom experime nts were conducted. Results: Simulations showed that by using two half -fanbeam collimators, a gantry rotation of 90 degrees, such as used fo r 180 degrees acquisition with parallel-beam collimators for cardiac i maging, was insufficient. A gantry rotation of 180 degrees resulted in attenuation maps where only an area to the posterior of a 400-mm wide thorax phantom was affected by truncation. The MPS system had a 14.7 times higher sensitivity for transmission counts than the MLS system. Despite the smaller sources in the MPS system, the number of acquired transmission counts was a factor 1.91 times higher compared with the M LS system, resulting in reduced noise. The relative downscatter contri bution from Tc-99m (140 keV) in the Gd-153 moving electronic window (1 00 keV) was reduced by a factor of 1.81. Transmission images of a rod phantom with segments containing acrylic rods of different diameters s howed an improvement of resolution in favor of the MPS system from abo ut 11 mm to about 6 mm (five instead of two segments of rods were clea rly visible). In addition, the noise level in the MPS thorax transmiss ion images was significantly lower. Conclusion: The MPS system has imp ortant advantages when compared with the MLS system. The use of low-ac tivity point sources is economically beneficial when compared with lin e sources and reduces radiation exposure to staff and patients.