Positron Emission Tomography with Sparse Block Rings and Continuous Bed Motion

Clinical PET systems employ compact block rings to maximize sensitivity per axial field-of-view (AFOV) length, thereby resulting in considerable manufacturing costs. To reduce the cost per AFOV length, a sparse block rings configuration is modeled on a Siemens Biograph
^TM
mMR PET/MR scanner (sparse-mMR) by removing the coincidence counts from every other detector block ring. Moreover, Continuous Bed Motion (CBM) is performed along a limited distance to eliminate the axial sensitivity gaps. List-mode PET mMR data of the NEMA image quality phantom with 4:1 spheres-to-background ratio was acquired for 30min. The counts were binned assuming a sparse-mMR configuration and a CBM acquisition of constant speed along a distance of 2 blocks (16 detector rings, 6.4cm). The CBM scans were simulated by axially shifting the stationary mMR data to 16 different positions, including a reference, removing the counts associated with any even block rings, and shifting back to the reference position. The process was repeated with different input PET data of equal duration for all 16 positions and the corresponding output data were added. The CBM mode eliminated all axial sensitivity discontinuities by evenly acquiring data throughout the gaps. Contrast recovery (CR) and background variability (BV) were evaluated for PET images reconstructed from sparse-mMR CBM data against mMR and compact-½mMR stationary data of equal duration (~5min). The latter consisted of counts associated with only the 4 central mMR block rings. A similar CR performance was attained between the three configurations. Mean BV for sparse-mMR with CBM was 3.6% higher than mMR. Nevertheless, sparse-mMR with CBM attained a smoother axial variation in image noise, relative to compact-½mMR. CBM can restore continuity in the axial sensitivity profile of PET systems with sparse block rings to achieve similar contrast recovery and smoother noise variation, compared to compact systems, at half the cost or double the AFOV.