Monte Carlo modelling of the performance of a rotating slit-collimator for improved planar gamma-camera imaging

Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has 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 is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result 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 are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.

This paper presents an analysis of two cone beam configurations (having focal lengths of 40 and 60 cm) for the acquisition of single photon emission computed tomography (SPECT) projection data. A three-dimensional filtered backprojection algorithm is used to reconstruct SPECT images of cone beam projection data obtained using Monte Carlo simulations. The mathematical analysis resulted in on-axis point source sensitivities (calculated for a distance of 15 cm from the collimator surface) for cone beam configurations that were 1.4-3 times the sensitivities of parallel-hole and fan beam geometries having similar geometric resolutions. Cone beam collimation offers the potential for improved sensitivity for SPECT devices using large-field-of-view scintillation cameras.