Investigation of a Technique for Providing a Pseudo-Continuous Detector Ring for Positron Tomography

The use of small bismuth germanate (BGO) detectors (2.5-4.5 mm wide) and a simple coding technique for identification of the crystal of interaction with the impinging gamma-ray was investigated in terms of a technique for providing a positron tomography system of the highest possible intrinsic resolution with the additional capability of providing an artifact free image in a stationary system. The technique employs one photomultiplier per two BGO crystals in a scheme of alternating single PMT/crystal and shared PMT/crystal combinations to form a pseudo-continuous ring. Problems of crosstalk between detectors, nonuniform sensitivities and variable resolution are shown to be minimized once discriminator thresholds are clearly above the energy of the bismuth x-rays induced in the BGO crystals by the 511 keV gammas from the positron annihilations. A stationary circular system employing 4.5 mm wide BGO detectors can provide transverse sampling equal to 1/2 the center to center crystal distance or 2.5 mm sampling. The measured intrinsic resolution of a bench top version of this system is 5 to 6 mm FWHH for a 30cm diameter field of view for a 100 cm system diameter. The 2.5 mm sampling with reconstruction filter of appropriate frequency response should provide a high resolution image on the order of 7 to 8 mm FWHM with good signal to noise characteristics which is free of significant aliasing artifacts.