High Density Avalanche Chamber (HIDAC) Positron Camera

A prototype positron camera has been constructed consisting of two high density avalanche chamber (HIDAC) detectors operated in coincidence with a resolving time (2 tau) of 40 nsec. The detectors are multiwire chambers, with specially constructed lead converters added to improve the photon detection efficiency at 511 keV. The current HIDAC detectors have a singles efficiency of approximately 12%, a sensitive area of 31 X 31 cm and an intrinsic spatial resolution of less than 2 mm full width at half maximum (FWHM). During data acquisition, the detectors are rotated around the patient in order to collect a complete angular data set. A three-dimensional image of the positron distribution is reconstructed from a single scan by weighted backprojection of the data into a matrix of either 64 X 64 X 64 or 128 X 128 X 16 voxels. The camera point response function is deconvolved by frequency-space filtering. Corrections are made during backprojection both for photon attenuation and for spatial variations in point source sensitivity. The reconstructed image is further corrected for contributions from accidental and scattered coincidences and displayed as a sequence of two-dimensional transverse, sagittal, or coronal sections. In addition, three-dimensional display is available using shaded graphics techniques. The prototype camera is currently undergoing clinical evaluation.