Robotic SPECT for region-of-interest imaging onboard radiation therapy systems and in other applications

Objectives Most new external-beam radiation therapy systems include an onboard cone-beam x-ray transmission imaging system, for use in aligning the tumor target with the radiation therapy beam. Tumor however is often best differentiated from healthy tissue by molecular and functional characteristics, and accordingly, nuclear medicine modalities may have high potential for onboard therapy target localization. Conventional, gantry-based SPECT or PET systems are challenging for onboard use in part because of the space they consume. Therefore it is proposed that a robotic arm could extend a SPECT detector into the vicinity of the patient and the radiation therapy machine, implementing a SPECT imaging trajectory, and then retracting so as to allow radiation therapy. In order to provide high photon detection efficiency and fast scan times, a multi-pinhole system is proposed for region-of-interest SPECT imaging. The objective of the present study is to investigate the ability of this system to image different anatomical sites.

Methods Digital radiotracer and attenuation distributions were generated based on the XCAT phantom, with spherical hot spots, 7 to 12 mm in diameter, added. SPECT projection data were computer-simulated for 4-minute scans of a 49-pinhole system and a parallel-hole-collimated system. Several pinhole focal lengths were considered. Lung imaging, breast imaging of prone and supine patients, and prostate imaging were investigated. Images were reconstructed by OSEM and were assessed for hot spot visualization and, in the breast region, for localization.

Results For imaging with the 49-pinhole system, the 10 to 12 mm hot spots were well visualized and, in most of the breast region, were localized to within 2 mm.

Conclusions The results suggest that onboard SPECT is feasible for many anatomical sites and in clinically acceptable scan times. Multi-pinhole collimator sets for several different pinhole focal lengths may improve imaging. These SPECT methods may be useful also for treatment-planning and diagnostic imaging applications.

Research Support This work is supported by PHS/NIH/NCI grant R21-CA156390.