Silicon detectors use state-of-the-art electronics to take advantage of the semiconductor properties of silicon to produce very high resolution radiation detectors. These detectors have been a fundamental part of high energy, nuclear, and astroparticle physics experiments for decades, and they hold great potential for significant gains in both PET and SPECT applications. Two separate prototype nuclear medicine imaging systems have been developed to explore this potential. Both devices take advantage of the unique properties of high resolution pixelated silicon detectors, designed and developed as part of the CIMA collaboration and built at The Ohio State University.
The first prototype is a Compton SPECT imaging system. Compton SPECT, also referred to as electronic collimation, is a fundamentally different approach to single photon imaging from standard gamma cameras. It removes the inherent coupling of spatial resolution and sensitivity in mechanically collimated systems and provides improved performance at higher energies. As a result, Compton SPECT creates opportunities for the development of new radiopharmaceuticals based on higher energy isotopes as well as opportunities to expand the use of current isotopes such as 131I due to the increased resolution and sensitivity.
The Compton SPECT prototype consists of a single high resolution silicon detector, configured in a 2D geometry, in coincidence with a standard NaI scintillator detector. Images of point sources have been taken for 99mTc (140 keV), 131I (364 keV), and 22Na (511 keV), demonstrating the performance of high resolution silicon detectors in a Compton SPECT system. Filtered back projection image resolutions of 10 mm, 7.5 mm, and 6.7 mm were achieved for the three different sources respectively. The results compare well with typical SPECT resolutions of 5-15 mm and validate the claims of improved performance in Compton SPECT imaging devices at higher source energies. They also support the potential of silicon detectors to serve as the electronic collimator in these systems.
The second prototype is a high resolution PET system. By inserting a silicon PET ring inside a conventional scintillator PET ring, it has been proposed that both image resolution and system sensitivity can be increased. To investigate these claims, a partial BGO ring with clinical PET dimensions (50 cm inner diameter) has been constructed that can be used to evaluate a variety of system configurations. Initial investigations use two back-to-back high resolution silicon detectors in a 2D geometry with a small (10 cm) field of view. This configuration is used to demonstrate the potential performance of a specialized small animal imaging device for medical research applications.
Initial filtered backprojection images of a 22Na point source have shown the spatial resolution of the system to be 950 μm for the pure silicon events, 1.8 mm for the hybrid silicon-BGO events, and 7 mm for the pure BGO events. The performance is consistent with expectations and, as the first real images from this type of device, the results provide motivation to continue the investigation of the high resolution PET concept.