Transmission computed tomography imaging of the head with a SPECT system and a collimated line source

Transmission images of relatively high resolution as compared to SPECT are needed for brain SPECT quantification to provide skull thickness, attenuation coefficients and anatomical correlation. Consequently, a technique to acquire transmission CT images with a SPECT system by using a collimated line source positioned at the focal line of a fanbeam collimator (FBC) has been developed.

Computer simulations that model the transmission imaging system optimized the system resolution and tested the validity of a equation for the geometric efficiency of the line source collimator (LSC). Based on the computer simulations, a LSC was constructed with tantalum septa 100 mm long, 0.5 mm thick and spaced 1.0 mm apart. A 600-mm focal length FBC was used. Experiments were conducted to measure the system resolution and to determine the effect of the LSC on the amount of detected scatter.

The simulations showed that without a LSC the transmission images have a longitudinal resolution (LR) characterized by the resolutions of the FBC (depth-dependent, approximately 8 mm FWHM at 150 mm) and the detector (approximately 4 mm). However, with an optimally designed LSC, the contribution of the FBC to the system resolution can be made negligible, creating a system with a LR that is comparable to the detector resolution and independent of object depth. Resolution experiments conducted with a lucite rod phantom showed that the LR and TR are better than 4.8 mm and confirmed the results of the computer simulations.

Brain transmission images of relatively high isotropic resolution can be obtained using a SPECT system, a FBC and an optimized LSC.