P-150: Rapid whole-body PET/MRI with fat-suppressed T2-weighted imaging is superior to whole-body PET/MRI with diffusion-weighted imaging (DWI) in plasma cell disorders

Introduction: Accurate diagnostic imaging is essential for the proper classification and management of plasma cell disorders (PCD), including multiple myeloma (MM). Whole-body (wb) imaging with low-dose CT and/or MRI with or without PET are widely considered to be the gold standard for imaging, with MRI showing superior sensitivity of early focal lesions, particularly when diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) are applied. However, DWI has limitations in wbMRI, including long scan times, image distortion, and incomplete fat suppression. T2-weighted fast-spin-echo imaging with uniform Dixon-based fat-suppression (T2-Dixon) is an alternative to DWI and may overcome some of these limitations.

Methods: We retrospectively analyzed imaging and clinical decision making for all patients (pts) at our center with PCD who underwent wbMRI/PET between August 2019 and April 2022. 60 minutes after FDG administration, the following sequences were acquired: two-point Dixon for MR-based attenuation correction, 3D T1-weighted gradient-echo with Dixon, and T2-weighed fast spin-echo with Dixon. Once PET was completed from skull to feet, DWI was acquired. Two trained radiologists interpreted each scan independently and identified suspicious lesions and/or diffuse disease by T2-Dixon, followed by DWI and then PET. These results were then reviewed with a clinician trained in the management of PCD alongside clinical data to determine if the imaging findings would impact clinical management.

Results: 34 PET/MRI scans were reviewed in this analysis. 25 scans (73.5%) were for pts with MM, 2 (5.9%) for MGUS, 3 (8.8%) for smoldering myeloma, and 4 (11.8%) for solitary plasmacytoma. Median age was 69 (range 31-84). DWI was not obtained in 3 pts who requested exam termination due to discomfort. Among cases with all available imaging modalities, 139 focal lesions were identified. T2-Dixon had a significantly higher sensitivity than DWI and PET, respectively 93%, 66% and 47% (p< 0.001). Adding DWI to T2-Dixon resulted in detection of 3 additional lesions, but none impacted clinical management. Adding PET to T2-Dixon and DWI detected 7 additional bone lesions and identified suspicious uptake in an otherwise normal-appearing lymph node. There were 5 false positive lesions for T2-Dixon, 9 for DWI, and 2 for PET. Among 11 cases with diffuse bone marrow disease, T2-Dixon identified all cases, DWI identified 10/11 (91%) and PET only detected 6/11 (55%) (p=0.06).

Conclusions: Whole-body T2 with uniform Dixon fat-suppression detected more lesions than DWI in pts with PCD. Adding DWI to T2-Dixon did not impact disease stage or patient management. However, while PET demonstrated lower sensitivity than T2-Dixon in detection of focal and diffuse disease, it impacted management in one case by detecting extraosseous disease. Whole-body PET/MRI with T2-Dixon demonstrates promise as a rapid imaging modality in assessment of MM and PCD.