Challenges of image generation for tight ring, long bore PET scanners

We investigate the challenges associated with calibration and corrections in tight ring, long bore geometry PET scanners. The reduced bore diameter is mainly driven by compatibility for a combined PET/MR system where the PET detector ring is inserted inside a MR scanner. The longer axial extent is driven by a desire for additional axial coverage. While the system sensitivity is increased due to the greater solid angle, these geometries present several challenges to different aspects of the image generation chain. Many of the approximations made on the current generation of clinical PET/CT scanners, where the aspect ratio of bore length to bore diameter is small, are no longer valid for the PET/MR compatible geometries. Further, due to the lack of space inside tight ring PET scanners, the side shields have to be very short providing little protection from out of field activity. We identify, present, and show solutions to four of these challenges: (a) the geometric and crystal efficiency measurements are different for direct and oblique projections, (b) projections along oblique angles are no longer on a plane but on curved surfaces, (c) increased parallax effect on the radial and axial point spread functions, and (d) the absence of side-shields greatly increases the out-of-field scatter contribution to the end slices. We perform Monte Carlo simulations for three PET scanner geometries: (a) bore diameter = 82 cm, axial length = 15 cm, (b) bore diameter = 60 cm, axial length = 15 cm, and (c) bore diameter = 60 cm, axial length = 30 cm. We have developed calibration and correction algorithms that account for all of the above factors. Reconstructing with these new algorithms demonstrates a significant improvement in image quality.