Developing an efficient phase-matched attenuation correction method for quiescent period PET in abdominal PET/MRI

Respiratory motion causes misalignments between positron emission tomography (PET) and magnetic resonance (MR)-derived attenuation maps (μ-maps) in addition to artifacts on both PET and MR images in simultaneous PET/MRI for organs such as liver that can experience motion of several centimeters. To address this problem, we developed an efficient MR-based attenuation correction (MRAC) method to generate phase-matched μ-maps for quiescent period PET (PET_Q) in abdominal PET/MRI. MRAC data was acquired with CIRcular Cartesian UnderSampling (CIRCUS) sampling during 100 s in free-breathing as an accelerated data acquisition strategy for phase-matched MRAC (MRAC_PM-CIRCUS). For comparison, MRAC data with raster (Default) k-space sampling was also acquired during 100 s in free-breathing (MRAC_PM-Default), and used to evaluate MRAC_PM-CIRCUS as well as un-matched MRAC (MRAC_UM) that was un-gated. We purposefully oversampled the MRAC_PM data to ensure we had enough information to capture all respiratory phases to make this comparison as robust as possible. The proposed MRAC_PM-CIRCUS was evaluated in 17 patients with ⁶⁸Ga-DOTA-TOC PET/MRI exams, suspected of having neuroendocrine tumors or liver metastases. Effects of CIRCUS sampling for accelerating a data acquisition were evaluated by simulating the data acquisition time retrospectively in increments of 5 s. Effects of MRAC_PM-CIRCUS on PET_Q were evaluated using uptake differences in the liver lesions (n = 35), compared to PET_Q with MRAC_PM-Default and MRAC_UM. A Wilcoxon signed-rank test was performed to compare lesion uptakes between the MRAC methods. MRAC_PM-CIRCUS showed higher image quality compared to MRAC_PM-Default for the same acquisition times, demonstrating that a data acquisition time of 30 s was reasonable to achieve phase-matched μ-maps. Lesion update differences between MRAC_PM-CIRCUS (30 s) versus MRAC_PM-Default (reference, 100 s) were 0.1% ± 1.4% (range of -2.7% to 3.2%) and not significant (P > .05); while, the differences between MRAC_UM versus MRAC_PM-Default were 0.6% ± 11.4% with a large variation (range of -37% to 20%) and significant (P < .05). In conclusion, we demonstrated that a data acquisition of 30 s achieved phase-matched μ-maps when using specialized CIRCUS data sampling and phase-matched μ-maps improved PET_Q quantification significantly.