A Compton camera for low energy gamma ray imaging in nuclear medicine applications

C-SPRINT is a prototype electronically-collimated imaging system that has been built using pixellated, low-noise, position-sensitive silicon as the first detector, and a sodium iodide scintillation detector ring as the second detector. The camera was intended to characterize potential performance gains of Compton cameras in nuclear medicine applications. The system consists of a single 4.5 x 1.5 x 0.03 cm3 silicon pad detector module with 2 keV energy resolution centered at the front face of a 50 cm diameter, 12 cm long NaI detector annulus. Calculations of the Uniform Cramer-Rao lower bound show that a "design Compton camera" based on our prototype can challenge existing mechanically-collimated systems at low to medium energies (∼140.5 - 400 keV) despite the deleterious effects of Doppler broadening. Measurements with our current system have yielded system sensitivity and spatial resolution estimates using 99mTc and 131I isotopes. Results showed an absolute efficiency of 1.8 x 10 -7 for 99mTc and 1.2 x 10-6 for 131I. The 99mTc value is an order of magnitude lower than predicted because of a combination of worse than expected silicon detector triggering performance, timing resolution issues, and system dead time effects. After correcting for these, efficiency predictions based on Monte Carlo analysis fall within 10% of the measured values. Spatial resolution estimates are also within 10% of analytical predictions. Measured resolution for the 99mTc point source was 15 min FWHM while in the 131I case, resolution improved to 8 mm FWHM. Extended source imaging was performed to characterize system performance under more challenging conditions. Images obtained were compared with measurements using a clinically-available mechanically collimated Anger camera. A resolution-variance study was also conducted for both isotopes. The results showed that the C-SPRINT camera performance on a per-detected photon basis was worse than the Anger camera for 99mTc but was similar for 131I, as predicted by theory. Potentially large gains in raw system sensitivity of a Compton camera similar in design to C-SPRINT could lead to substantial improvements in noise-equivalent performance of electronically-collimated cameras over mechanical systems, particularly in the energy range above 200 keV.