Optimisation of b -values for the accurate estimation of the apparent diffusion coefficient (ADC) in whole-body diffusion-weighted MRI in patients with metastatic melanoma

Objective To establish optimised diffusion weightings (‘ b -values’) for acquisition of whole-body diffusion-weighted MRI (WB-DWI) for estimation of the apparent diffusion coefficient (ADC) in patients with metastatic melanoma (MM). Existing recommendations for WB-DWI have not been optimised for the tumour properties in MM; therefore, evaluation of acquisition parameters is essential before embarking on larger studies. Methods Retrospective clinical data and phantom experiments were used. Clinical data comprised 125 lesions from 14 examinations in 11 patients with multifocal MM, imaged before and/or after treatment with immunotherapy at a single institution. ADC estimates from these data were applied to a model to estimate the optimum b -value. A large non-diffusing phantom was used to assess eddy current–induced geometric distortion. Results Considering all tumour sites from pre- and post-treatment examinations together, metastases exhibited a large range of mean ADC values, [0.67–1.49] × 10 −3 mm 2 /s, and the optimum high b -value ( b high ) for ADC estimation was 1100 (10th–90th percentile: 740–1790) s/mm 2 . At higher b -values, geometric distortion increased, and longer echo times were required, leading to reduced signal. Conclusions Theoretical optimisation gave an optimum b high of 1100 (10th–90th percentile: 740–1790) s/mm 2 for ADC estimation in MM, with the large range of optimum b -values reflecting the wide range of ADC values in these tumours. Geometric distortion and minimum echo time increase at higher b -values and are not included in the theoretical optimisation; b high in the range 750–1100 s/mm 2 should be adopted to maintain acceptable image quality but performance should be evaluated for a specific scanner. Key Points • Theoretical optimisation gave an optimum high b-value of 1100 (10th–90th percentile: 740–1790) s/mm 2 for ADC estimation in metastatic melanoma. • Considering geometric distortion and minimum echo time (TE), a b-value in the range 750–1100 s/mm 2 is recommended. • Sites should evaluate the performance of specific scanners to assess the effect of geometric distortion and minimum TE.