Variable refocusing flip angle single-shot fast spin echo imaging of liver lesions: increased speed and lesion contrast.

Purpose: To evaluate acquisition time and clinical image quality of a variable refocusing flip angle (vrf) single-shot fast spin echo (SSFSE) sequence in comparison with a conventional SSFSE sequence for imaging of liver lesions in patients undergoing whole-body PET/MRI for oncologic staging.
Methods: A vrfSSFSE sequence was acquired in 43 patients with known pancreatic neuroendocrine tumors undergoing ⁶⁸ Ga-DOTA-TOC PET on a simultaneous time-of-flight 3.0T PET/MRI. Liver lesions ≥1.5 cm with radionucleotide uptake were analyzed. Contrast-to-noise ratios (CNRs) were measured, and four blinded radiologists assessed overall image quality. Differences in repetition time and CNR were assessed using a paired Student's t test with p < 0.05 considered statistically significant. Inter-reader variability was assessed with Fleiss' kappa statistic.
Results: 53 eligible lesions in 27 patients were included for analysis. vrfSSFSE demonstrated higher mean lesion CNR compared to SSFSE (9.9 ± 4.1 vs. 6.7 ± 4.1, p < 0.001). Mean repetition time (TR) was 679 ± 97 ms for the vrfSSFSE sequence compared to 1139 ± 106 ms for SSFSE (p < 0.0001), corresponding to a 1.7-fold decrease in acquisition time. Overall quality of liver lesion and common bile duct images with the vrfSSFSE sequence was graded as superior than or equivalent to the SSFSE sequence for 59% and 67% of patients, respectively.
Conclusions: Compared to conventional SSFSE, vrfSSFSE resulted in improved lesion contrast on simultaneous PET/MRI in patients with liver metastases. Due to decreased SAR demands, vrfSSFSE significantly decreased TR, allowing coverage of the entire liver in a single twenty-second breath hold. This may have important clinical implications in the setting of PET/MRI, where scan time is limited by the necessity of whole-body image acquisition in addition to bed specific imaging.