A pilot ambi-cranial PET system for GBM surgery guidance: Characterization and analysis

Complete resection of gioblastoma multiforme (GBM) is currently impossible due to the coordinate misalignment between pre-operative imaging and surgical operation caused by brain shifts. Intraoperative radioguidance has the potential to provide real-time feedback about residual tumor to assist surgery. The current study investigates an ambi-cranial PET system to detect annihilation activity with an intracranial detector array attached to a probe and an extracranial arc array. This pilot study characterizes the unique system response and analyzes image reconstruction behavior. Measurement were simulated to account for attenuation, detector efficiency variation, accidental coincidence, scattering and counting noise. For the pilot study, the intracranial probe and extracranial arc arrays were assumed to be 22cm apart radially, with 2mm and 6mm detector sizes respectively. Considering radially symmetric geometry, a 22cm*4cm 2D region of interest (ROI) was simulated with 2mm isotropic voxel resolution. MLEM was used for pilot image reconstruction. Point activities were simulated at various locations within the ROI and then estimated from noisy simulation. Direct inspection of the ideal system matrix suggested inhomogeneous and isotropic system response. Reconstruction results conformed such observation: in particular, directional resolution along the radial direction degraded as the point source moved from the probe side towards the extracranial arc detector, with the full-width-half-max values varying from 2mm, comparable to the ROI voxel size, to 16mm, suggesting deteriorating in localization accuracy. On the other hand, tangential resolution is almost constant at approximately 2mm throughout the ROI, in concordance with radial symmetry. This particular pattern of spatial variation and anisotropicity coincides with the clinical desire to accurately identify activity close to cavity for surgical adjustment and high tangential resolution for intracranial radiotherapy.