Half-fanbeam collimators combined with scanning point sources for simultaneous emission-transmission imaging

One type of SPECT system often used for simultaneous emission-transmission tomography is equipped with parallel-hole collimators, moving line sources (MLS) and electronic windows that move in synchrony with the sources. Although downscatter from the emission distribution is reduced by the use of the electronic window, this still can represent a sizable fraction of the transmitted counts. These systems have relatively poor spatial resolution and use costly transmission sources.

Using a two-head SPECT system, with heads at right angles, two 153Gd line sources (5800 MBq each) were replaced by two 153Gd point sources of only 750 MBq each and positioned to move along the focal lines of two half-fanbeam collimators. A suitable acquisition protocol for a moving point source (MPS) system was selected by considering the results of a simulation study. With this protocol, physical phantom experiments were conducted.

Simulations showed that by using two half-fanbeam collimators, a gantry rotation of 90 degrees, such as used for 180 degrees acquisition with parallel-beam collimators for cardiac imaging, 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 MLS system, resulting in reduced noise. The relative downscatter contribution from 99mTc (140 keV) in the 153Gd moving electronic window (100 keV) was reduced by a factor of 1.81. Transmission images of a rod phantom with segments containing acrylic rods of different diameters showed an improvement of resolution in favor of the MPS system from about 11 mm to about 6 mm (five instead of two segments of rods were clearly visible). In addition, the noise level in the MPS thorax transmission images was significantly lower.

The MPS system has important advantages when compared with the MLS system. The use of low-activity point sources is economically beneficial when compared with line sources and reduces radiation exposure to staff and patients.