P-017: Repeatability and reproducibility of apparent diffusion coefficient measurements of bone marrow in magnetic resonance imaging of multiple myeloma patients

Introduciton Diffusion weighed imaging (DWI) within whole-body magnetic resonance imaging (wb-MRI) is now recommended for imaging of patients with monoclonal plasma cell disorders, and quantitative ADC measurements are of high relevance for reading and reporting of wb-MRIs. However, the evidence on repeatability and especially on multicentric reproducibility of ADC measurements of the bone marrow (BM) is very limited. The goal of this study was to quantify interrater-variability, repeatability and reproducibility of ADC measurements under variation of MRI scanners and MRI sequences. Methods Fifty-five patients were prospectively included in this IRB-approved prospective study and underwent multiple scans in different settings within a maximum of 3 days between the scans. A standard scan (clinical 1.5T-MRI scanner, standard MRI-protocol) of the pelvic bone marrow was performed. Additionally, several measurements were performed: First, a retest after new positioning of the patient (same 1.5T scanner, same protocol, i.e., repeatability; n=37 paired scans). Second, retest at the same scanner but with different MRI protocol (n=37 paired scans). Third, retest at another 1.5T scanner with a harmonized MRI protocol to the initial setting (n=34 paired scans). Fourth, retest at another scanner with different field strength of 3T (n=40 paired scans). For measurement of ADC, one radiologist manually placed regions of interest (ROI) in a representative area of BM at the posterior iliac crest and a ROI in representative muscle tissue in the same slice. A second rater independently performed a second read of the standard scan. The Bland-Altman-approach was used to assess agreement of the different settings and mean bias, limits of agreement (LoA) and coefficients of variation (CoV) were calculated. Results Absolute bias / relative bias to mean / CoV of ADC measurements for the different retest measurements compared to the standard-scan were as follows: interrater setting -37.9mm2 s-1/-0.077/15.8%, repeatability -13.0mm2 s-1/-0.027/14.5%, second protocol -205.1mm2 s-1/-0.357/33.5%, second 1.5T scanner 5.8mm2 s-1/0.013/17.0%, 3T scanner 146.8mm2 s-1/0.371/41.3%. Normalization to muscle tissue did not improve the high relative bias of the second protocol or the 3T scanner to the standard setting. Conclusion In this study we quantified interrater-variability, repeatability and reproducibility of BM ADC measurements in repeated scans after patient repositioning, different protocols, different MRI scanners and different radiologists. Bias and CoV between two different 1.5T MRI scanners with harmonized protocols were in the range of interrater-variability and repeatability. On the contrary, variation of MRI protocols and field strength of the MRI scanner led to marked bias between ADC-measurements which could not be solved by normalization to muscle tissue. These findings are of high importance for planning of multicentric imaging studies in myeloma.