Modular gamma cameras: Improvements in scatter rejection, and characterization and initial clinical application.

Abstract

This dissertation reports investigations into improvements of the performance of "modular" gamma-ray cameras. Each modular camera has a 10 cm x 10 cm NaI(Tl) scintillation crystal and four 5 cm x 5 cm photomultiplier tubes (PMTs). When the gamma-ray photons interact with the crystal, scintillation flashes are emitted from the crystal and detected by the PMTs. The PMTs then convert the light flashes to current pulses. A digital computing circuitry processes the PMT outputs and assigns an estimated (x,y) coordinate corresponding to the location of interaction of each gamma-ray photon in the crystal and thus an image is formed. This dissertation is concerned with improvements and clinical applications of the modular cameras. There are some areas in which we improve the camera performance. These areas include position and energy estimation, and scatter rejection. We use look-up tables (LUTs) with different windowing methods to estimate the scintillation positions and reject scattered photons. We study several possible position estimators and compare the results by the bias and the variance of the position estimates. There are two types of events that occur in our image: photoelectric absorption and Compton scattering. When gamma rays are emitted from the organ, scattered photons will be generated which are not the desired events. Milster et al. introduced a discrimination method called the likelihood window (LW) to reject scattered events. We also extend the maximum-likelihood estimation rule to include energy estimation of different gamma-ray photons. Then we use the LUTs calculated from both position and energy estimators to study scatter rejection by the energy window (EW) when there are scattered photons. We introduce a Bayesian window (BW) for scatter rejection and compare the results with our usual LW and EW through receiver operating characteristic studies. We measure the short-term and long-term stability of PMT gain. We describe and compare some uniformity correction methods used in the research group. We do the camera characterization by measuring spatial resolution and sensitivity of the modular camera and compare these characteristics to those of the commercial gamma camera. We design and construct a stand-alone planar imaging system based on the modular camera. This imager is examined with a set of phantom and clinical tests to demonstrate the clinical feasibility of the stand-alone system.