DOI Estimation for a Clinical MRI-Compatible SPECT Insert

The estimation of the Depth-of-Interaction (DOI) of detected gamma photons is especially challenging for thick scintillators, yet relevant for moderately thin ones (8 mm CsI(Tl) in our case) employed in nuclear imaging. In SPECT, in particular using pinhole or slit-slat collimation, the parallax error produced by photons passing through the collimator holes, with trajectories non orthogonal to the crystal surface, can be reduced by including the DOI information into the tomographic reconstruction. We present an algorithm based on maximum likelihood that classifies into 4 layers the events absorbed in the scintillator. A novel initialization signal (a ring footprint surrounding the channel detecting the highest intensity) is proposed, only requiring a flood field irradiation as training. The algorithm is experimentally validated by means of a tilted collimated beam on a single camera. The DOI information will be applied in a stationary clinical MRI-compatible SPECT insert which is composed of 20 identical modules mounted in a partial ring.