Why Is a b-value Range of 1500-2000 s/mm2 Optimal for Evaluating Prostatic Index Lesions on Synthetic Diffusion-Weighted Imaging?

Objective: It is uncertain why a b-value range of 1500-2000 s/mm(2) is optimal. This study was aimed at qualitatively and quantitatively analyzing the optimal b-value range of synthetic diffusion-weighted imaging (sDWI) for evaluating prostatic index lesions. Materials and Methods: This retrospective study included 92 patients who underwent DWI and targeted biopsy for magnetic resonance imaging (MRI)-suggested index lesions. We generated sDWI at a b-value range of 1000-3000 s/mm(2) using dedicated software and true DWI data at b-values of 0, 100, and 1000 s/mm(2). We hypothesized that lesion conspicuity would be best when the background (i.e., MRI-suggested benign prostatic [bP] and periprostatic [pP] regions) signal intensity (SI) is suppressed and becomes homogeneous. To prove this hypothesis, we performed both qualitative and quantitative analyses. For qualitative analysis, two independent readers analyzed the b-value showing the best visual conspicuity of an MRI-suggested index lesion. For quantitative analysis, the readers assessed the b-value showing the same bP and pP region SI. The 95% confidence interval (CI) or interquartile range of qualitatively and quantitatively selected optimal b-values was assessed, and the mean difference between qualitatively and quantitatively selected b-values was investigated. Results: The 95% CIs of optimal b-values from qualitative and quantitative analyses were 1761-1805 s/mm(2) and 1640-1771 s/mm(2) (median, 1790 s/mm(2) vs. 1705 s/mm(2); p = 0.003) for reader 1, and 1835-1895 s/mm(2) and 1705-1841 s/mm(2) (median, 1872 s/mm(2) vs. 1763 s/mm(2); p = 0.022) for reader 2, respectively. Interquartile ranges of qualitatively and quantitatively selected optimal b-values were 1735-1873 s/mm(2) and 1573-1867 s/mm(2) for reader 1, and 1775-1945 s/mm(2) and 1591-1955 s/mm(2) for reader 2, respectively. Bland-Altman plots consistently demonstrated a mean difference of less than 100 s/mm(2) between qualitatively and quantitatively selected optimal b-values. Conclusion: b-value range showing a homogeneous background signal may be optimal for evaluating prostatic index lesions on sDWI. Our qualitative and quantitative data consistently recommend b-values of 1500-2000 s/mm(2).