A clinical evaluation of a prototype positron camera for longitudinal emission tomography

Abstract

A multiwire proportional-chamber positron camera, developed at the Rutherford Laboratory, has been evaluated at The Royal Marsden Hospital. The prototype camera consists of two opposing 30 × 30 cm² chambers. Longitudinal tomograms of a positron-emitting radioactive distribution placed between the detectors are obtained via back-projection and 2Ddeconvolution. Due to the limited stereoscopic angle achieved with stationary detectors, only five planes parallel to the detector faces are reconstructed. A selection of images is presented of phantoms using ⁶⁸Ga and of patients using ¹⁸F, ¹⁸F fluorodeoxyglucose and Na ¹²⁴I, to illustrate the tomographic performance of the positron camera. A comparison between back-projected and deconvoluted images shows that the 2D-deconvolution process, which includes filtration of image noise, successfully removes the background due to scattered photons. The spatial resolution achieved depends on the half-height frequency cut-off used in the filtering process, and this parameter was chosen according to the count density in the back-projected images. A qualitative visual comparison was made between the positron images and equivalent single photon studies on the same patient. The results justify further development of this new detector, especially for use with generator-produced positronemitting radionuclides. Three-dimensional deconvolution resulted in improved tomographic performance for phantom data, but was less successful for patient data. These problems associated with limited-angle tomography will be overcome by multi-view acquisition. This study has shown that a relatively low-cost positron imaging system can be used for routine organ imaging. Further developments in hardware and software should yield images which are superior to those from single-photon planar or tomographic studies.