Collimatorless coincidence imaging.

Abstract

This dissertation explores a novel design for a surgical probe, a collimatorless coincidence imaging system designed to aid in tumor detection in nuclear medicine. Surgical probes can be maneuvered close to a suspected tumor site, thereby achieving higher resolution and sensitivity than external gamma cameras. However, conventional probes cannot distinguish between distant background variations and small tumors near the probe. Collimatorless coincidence imaging is a new method for suppressing the effects of variations in the background radiation. This decidedly unconventional imaging system images without a collimator or aperture of any kind. The probe design consists of a 10 x 10 array of collimatorless gamma-ray detectors connected by coincidence circuitry. The probe is used with a radionuclide that emits multiple photons per decay, such as ¹¹¹In. The coincidence circuitry triggers data collection only when two photons strike the detectors within a short time interval. Because the photons are emitted independently, the probability of coincident hits on two detectors is proportional to the product of the solid angles subtended by the two detectors. Therefore distant sources have a very low probability of contributing to the data, making them all but invisible to the probe. Data collection from such a system was simulated using a Monte Carlo routine that included absorption, the slight correlation between the directions of the emitted photons, and the presence of accidental coincidences. The data were reconstructed into object representations using the pseudoinverse obtained by singular value decomposition (SVD). The images showed a significant suppression of distant sources when compared to a probe equipped with a conventional parallel-hole collimator. We confirmed in the laboratory, using a point source of In-111 and two CdTe detectors connected by an AND gate, that the falloff in sensitivity was inversely proportional to the fourth power of the distance to the source and that the proportion of true to accidental coincidences followed the predicted relationship to the source activity. We conclude that collimatorless coincidence imaging is promising approach for tumor detection using surgical probes.