The use of offset cone-beam collimators in a dual head system for combined emission transmission brain SPECT: a feasibility study

Cone-beam (CB) collimators lead to higher sensitivity and resolution in SPECT brain images than parallel beam (PB) collimators. Point sources can be placed in the focal points of the CB collimators in order to obtain additional transmission CT projection data. In the design of a dual-head system with opposing detectors (Opposing Heads System, OHS), it is not possible to position the point sources in the focal points without truncating the patient´s head in the projections, even when large crystals are used. Space for the point sources can be created by positioning the cameras at a right angle (Right Angle System, RAS), but such CB systems require collimators with long focal distances or strong offsets for the focal points. It is therefore important to study the effects of focal point positions on the emission image quality. To this end, the following cases were compared in a simulation study: half cone-beams (HCB) in OHS and RAS and quarter cone-beam (QCB) collimators in RAS, each CB with different focal distances (ED). Projections of a disc phantom and a brain phantom acquired with circular orbits were simulated. The size of the crystals was 51×38 cm. Images were reconstructed iteratively with an Ordered Subset Expectation Maximization algorithm. The disc phantom experiment showed slice-to-slice crosstalk in the CB reconstructions which decreases with increasing focal distance. The CB reconstructions of the brain phantom did not show these artifacts. This study indicates that (i) An OHS with large crystals and HCB collimators provides superior emission image quality compared to RAS with HCB collimators and RAS with QCB collimators. (ii) Dual head systems with large crystals have potential for very high resolution brain imaging