Evaluation of cortical uptake of amyloid PET tracer with hybrid PET/MRI; comparison with PET/CT

Objectives Hybrid PET/MRI is now commercially available and is promising modality for neuroradiology. However, there still remain some challenges to be solved to use PET/MRI compared to conventional PET/CT. One of them is the problem associated with MR based attenuation correction (MR-AC). MR-AC ignores the presence of bone, therefore it may affect the evaluation of tracer uptake. However, the actual effect is still unknown. In this study, effect of MR-AC on brain uptake was evaluated using amyloid PET tracer.

Methods A total of 8 (55-72 average 67yo.) patients diagnosed as mild cognitive disorder were incorporated in this study. Serial PET/CT (biograph mCT) and PET/MRI (Ingenuity TF) scans were performed in all patients after intravenous injection of 700Mq of ¹¹C-Pittuburg Compound B. SUVmax of white matter, grey matter and SUVr (normalized by cerebellar cortex) were evaluated using 20 regions of interest per patient and compared them between PET/CT and PET/MRI. Positivity or negativity of amyloid deposition was also evaluated by both scanners.

Results Averaged SUVmax of white matter, gray matter and SUVr of negative patient were (1.93±0.4, 1.52±0.6, 0.95±0.3) in PET/CT and (1.76±0.4, 1.48±0.5, 1.15±0.4) in PET/MRI. Averaged SUVmax of white matter, gray matter, SUVr of positive patient were (1.56±0.43, 1.82±0.98, 2.41±0.8) in PET/CT and (1.46±0.4, 1.44±0.4, 2.96±1.2) in PET/MRI. SUVr calculated by PT/MRI was significantly higher than those calculated by PET/CT in both patient groups. This is thought to be effect of underestimation of cerebellar uptake caused by MR-AC. Judgment of positivity and negativity of amyloid deposition was completely consistent between PET/CT and PET/MRI.

Conclusions Hybrid PET/MRI is feasible to assess the deposit of amyloid plaque with the same accuracy with PET/CT. However, attention should be paid that calculated SUVr would be higher up to 22% than those calculated by conventional PET/CT.