Surgical probe design for a coincidence imaging system without a collimator

Surgical probes used in nuclear medicine can be maneuvered close to a suspected tumor site, thereby achieving higher resolution and sensitivity than external gamma cameras. Tomography with handheld probes is not possible, however, so it is difficult to determine whether an increase in the photon count rate is caused by features near the probe or by variations in the distant background radiation. Our group has experimented with several probe designs that address this problem. This paper describes a probe that images nearby objects without a collimator, and which is insensitive to inhomogeneities in the distant background level. The probe, which is used with a radionuclide that emits multiple photons per decay, such as ¹¹¹ In, consists of a collimatorless array of gamma-ray detectors connected by coincidence circuitry. We used a Monte Carlo routine to simulate data collection from such a system and reconstructed the images using the pseudoinverse obtained by singular value decomposition. The images show a significant suppression of distant sources when compared to a probe equipped with a conventional parallel-hole collimator.