The impact of the number of projections on image quality in Compton scatter tomography

The availability of high resolution, energy discriminating photon counting detectors should make it possible to use Compton scattered photons to improve the diagnostic capability of computed tomography (CT). With high, spatial and energy resolution detectors Compton scatter tomography (CST) images of adequate quality can be obtained with a single projection. In practice, the limitations of realistic detectors require multiple projections for good quality images. The relationship between the number of projections used for reconstruction and the reconstructed image quality obtained for conventional CT does not necessarily apply to multi-projection Compton scatter tomography (MPCST). The purpose of this work was to investigate the dependence of the reconstructed image quality on the number of projections for MPCST. Analytical simulations and reconstructions were used to evaluate the contrast and spatial resolution for images reconstructed with one to 720 projections. Contrast-to-noise ratios (CNR) and the modulation transfer functions (MTF) demonstrated that the contrast increases monotonically with the number of projections while spatial resolution was independent of the number of projections. The contrast initially increases rapidly with projection number, becoming more gradual as the number of projections increase, with the rate of change being a function of fluence. The number of projections required to asymptotically approach the maximum contrast decreases as the fluence increases, with no indication of an optimal value for the range of fluences and projections investigated. For the projections considered, an increase in the number of projections increases the CNR even though the number of photons per projection decreases.