Simultaneous PET-SPECT with advanced photon transport modelling for high-precision image reconstruction

Objectives A newly developed Versatile Emission Computed Tomography system (VECTor) enables simultaneous imaging of SPECT at 0.5 mm resolution and PET at 0.75 mm resolution [Goorden, JNM 54 pp 1-7 (2013)]. VECTor uses dedicated collimators with a total of 162 clustered pinholes, mounted in a SPECT system with large-area stationary detectors. The high-energy (511 keV) photons resulting from PET tracers require advanced modelling of photon transport during image reconstruction. We evaluate effects of improved modeling on mouse and phantom images.

Methods Improvements to gamma photon transport models on which image reconstruction is based comprise (i) incorporation of gamma photon paths that go through multiple pinholes, (ii) effects of variable depth-of-interaction (DOI) in the scintillation crystal, and (iii) inclusion of larger portions of the tails of the point spread functions. Furthermore, the positron range was included in the forward projection step during reconstruction. Image reconstruction was performed with pixel-based OSEM [Branderhorst PMB 55 pp 2023-2034 (2010)].

Results With the optimized reconstruction software, rods in a micro-Jaszczak resolution phantom have improved contrast-noise characteristics compared to images reconstructed with the initial image reconstruction software. Also, the slight triangular deformations of the rods have disappeared and resolution slightly improved. Furthermore, we illustrate how these improvements affect an ECG-gated ¹⁸F-FDG cardiac mouse scan, a whole-body mouse and focused ¹⁸F-fluoride bone scans, and a mouse ¹⁸F-FDG brain scan.

Conclusions We conclude that improving gamma photon transport results in strongly improved VECTor images.

Research Support This research was co-funded by grant no. PID06015, forming part of the “Pieken in de Delta Zuidvleugel” Program of the Netherlands Ministry of Economic Affairs and the Province of Zuid-Holland.