Design, development, and analysis of semiconductor-based instrumentation for nuclear medicine

Abstract

Nuclear medicine imaging using a gamma camera is a sensitive tool for mapping various physiological and biological processes in vivo. In some respects, the instrumentation for gamma-ray imaging is highly developed. Nevertheless, current technology in nuclear medicine has some significant limitations in the area of spatial resolution. Scintillator-based imaging systems most likely have reached their limits of spatial resolution. Achieving higher spatial resolution will require the use of semiconductor detectors. The first part and major focus of this dissertation is the development of a prototype imaging system based on modular CdZnTe semiconductor arrays. Each modular array is approximately 1.5 mm thick, and is patterned on one surface into a 64 x 64 array of pixels with 380-micron pitch. We present details of the design, the electronics, and system performance. The second part of this dissertation presents results on a coincidence-type surgical probe. The sensitivity of a surgical probe for tumor detection is often limited by spatial variations in radiotracer uptake in normal tissue. We are developing a probe for use with 111In that uses coincidences between the 171 keV and 245 keV gamma rays for background suppression. The performance of a coincidence probe was compared to that of single-gamma probe for the task of detecting radiolabeled tumor models in a water phantom containing an inhomogeneous background. A single-element NaI(Tl) probe was placed in random locations throughout the tank; the tumor was attached to the probe in half of the trials. Count data were recorded in three channels: 171 keV, 245 keV, and 416 keV. A linear discriminant was calculated from the data. The detectability index, d', was derived from the data and used to compare the optimal linear discriminant against the single-gamma energy peaks for counting times up to 30s. For a realistic 15s exposure time, d' for the linear discriminant attains a near-perfect value of 3. In contrast, the single-photon channel d' is always near zero, so this channel is worthless for background discrimination. Coincidence detection using linear discriminants shows promise for in vivo tumor localization with 111In-labelled radiopharmaceuticals.