Helical-Path, Half-Cone-Beam Acquisition for SPECT Brain Imaging

Convergent beam collimation can provide improved sensitivity and/or spatial resolution for single photon emission computed tomography (SPECT) brain imaging. A half-cone-beam (HCB) collimator is particularly useful because shoulder interference is mitigated and the base of the brain is within the collimator´s field of view. However, HCB SPECT acquisitions using a single circular orbit (SCO) result in insufficiently sampled projection data and consequently axial distortion. To reduce axial-distortion artifacts, we investigated HCB acquisitions using helical paths (HPs). A single HCB collimator with the focal point shifted towards the base of the brain was mounted on a Triad-XLT SPECT system. HP-HCB SPECT was evaluated with a Hoffman 3D brain phantom and a Defrise-type disk phantom. While the camera revolved in a 360 deg. circular orbit, the bed and phantom were translated at a slow speed. This combined motion resulted in a helical path of the focal point relative to the phantom. Two HP acquisitions, each with 360 deg. rotation, were combined to provide a single 720 deg. HP acquisition. An Ordered-Subset, Expectation-Maximization reconstruction algorithm was used. The HCB-HP SPECT images were compared with full-cone-beam (FCB) SCO, HCB-SCO SPECT, and parallel beam (PB) images. The FCB-SCO and HCB-SCO images of the disk phantom demonstrated strong axial artifacts. These artifacts were eliminated in HCB-HP images. HCB-HP SPECT provides markedly improved sampling of SPECT data than is possible with FCB-SCO and HCB-SCO SPECT, and offers the potential for improved SPECT imaging of the human brain.