Gallium-67 Imaging With Low Energy Collimators And Energy Weighted Acquisition

Abstract

The medium and high energy collimators used in 67Ga imaging have poorer resolution than low-energy collimators, such as the LEAP. The low energy collimators could be used for gallium imaging if the background under the 93 and 185 keV peaks could be reduced without degrading the signal-tonoise ratio unacceptably. Energy weighted acquisition provides a means of accomplishing this background reduction. We have developed weighting functions for gallium imaging through LEAP and high resolution collimators. The resolution of the low energy collimators is realized while the background is comparable to, or better than, the background in normal, energy-window imaging with the medium energy collimator. The pixel noise is somewhat greater than the Poisson noise in normal gallium imaging, and some noise correlations, or noise texture, is introduced. I. INTRODUCTION Gallium-67, a nuclide important in nuclear medicine, emits photons at 93, 185,300, and 394 keV, along with less abundant ones up to 888 keV. Gallium images from collimated gamma cameras are corrupted by a diffuse background that is mainly due to two processes: penetration of photons through the collimator's walls, or septa, causing events that are not properly collimated; and scatter of photons in the patient before they pass through the collimator, causing a scatter fraction in clinical gallium imaging of about 50%. For reducing the septum penetration, the collimators of choice in clinical gallium imaging [1,2] are medium-energy WED-ENG) and high-energy ones, whose thick septa are designed to shield against the high-energy photons of 67Ga and 1311, respectively. These collimators have disadvantages, however: the large holes are discernible in images, and the resolution and utilization are inferior to those of the thin-walled collimators used to image low-energy isotopes with photons well below 200 keV, for instance the low-energy all-purpose and highresolution collimators (LEAP and HI-RES). The thick septa do not reduce the scattered radiation component of the background, for many scattered photons pass through the collimator holes along with the primary radiation. Since collimator design cannot reduce the scatter, one uses different approaches to control this component of the background. This paper asks whether one can effectivcly image 67Ga with a low-energy collimator, and remove the background mathematically. Various workers have removed background in WmTc and 1231 imaging by using two cncrgy windows at a time and, in a post-processing operation, subtracting one image from the other.[3,4] Here, we consider energyweighted acquisition [5,6] (EWA) to reduce the background in 67Ga imaging. The effect of EWA is to collect a low-statistics image in each 1-keV energy interval between 0 and 511 keV, filter each image with a spatial filter designed for that energy (a weighting function), and sum the filtered images to form an energy-weighted image; or, the energy-weighted images can be acquired on the fly with the WAM hardware described in references [5,61. In the case of 67Ga imaging with low-energy collimators, EWA allows us to remove much of the background under the well-collimated 93 keV peak, while controlling the scatter background and enhancing the spatial resolution of the 185 keV peak.