Emission Computer Assisted Tomography with Single-Photon and Positron Annihilation Photon Emitters

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Abstract
Computed transverse section emission tomography using 99mTc with the Anger camera was compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system had a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gave a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring had a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination was essential in both techniques. The signal-to-noise ratio in a reconstruction was diminished by a factor of 1.2 .times. (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation caused more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attentuation resulted in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20 cm cylinder imaged in transverse section, only 20% of the positron annihilation events were not scattered; however, at 140 keV, 40% of the photons were not scattered. The relative crystal efficiency gave single-photon imaging an advantage of 5. The solid angle advantage of positron photon coincidence imaging was about 100. Taking these factors into account, positron-computed section imaging had a 10 fold increase in sensitivity over multiple-view imaging with the scintillation camera, which gave multiple sections but required camera or patient rotation.

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