Instrumentation and reconstruction aspects of imaging single photon emitters with electronic collimation

To overcome limitations of mechanical collimation in single photon imaging, a prototype electronically collimated gamma camera has been implemented by replacing the lead collimator with a unique 4x4 germanium detector. Coincident counts are recorded between the germanium detector and the uncollimated scintillation camera (Model Pho/gamma HP), stored in ''list'' mode and sorted as a function of position and energy within the germanium array. Each coincident count multiplexes activity on a conical surface through the object. As a first step to SPECT, demultiplexing is carried out to produce multiple-view pin-hole images of the object. In a new approach to demultiplexing, the planar distribution of coincident counts is mapped onto a spherical surface, thereby producing data which represent a convolution of the pin-hole projection with a circular point spread function on a spherical surface. Deconvolution is subsequently achieved either by iterative methods or a Fourier-filtering approach. Using these methods, pin-hole images have been obtained with point, line and uniformly distributed planar (8.8 cm diam) sources of Tc-99m, and a point source of Cs-137 (662 keV). The pin-hole image of a centrally located point source of Tc-99m produced a spatial resolution of 8.9 +- 1.2mm at a distance of 6cm from the germaniummore » detector. Also, using the prototype system, the sensitivity of a 32x32 germanium detector based camera was deduced to be a factor of 70 higher than a conventionally collimated camera for imaging Tc-99m distributed in a 5cm diam cylinder at equivalent resolution. Thus, electronic collimation increases sensitivity, enables imaging at high energies and facilitates dynamic SPECT.« less