Quantitative MRI biomarker for classification of clinically significant prostate cancer: Calibration for...

The purpose of the present study is to develop a calibration method to account for differences in echo times (TE) and facilitate the use of restriction spectrum imaging restriction score (RSIrs) as a quantitative biomarker for the detection of clinically significant prostate cancer (csPCa).

This study included 197 consecutive patients who underwent MRI and biopsy examination; 97 were diagnosed with csPCa (grade group ≥ 2). RSI data were acquired three times during the same session: twice at minimum TE ~75 ms and once at TE = 90 ms (TEmin₁, TEmin₂, and TE90, respectively). A linear regression model was determined to match the C-maps of TE90 to the reference C-maps of TEmin₁ within the interval ranging from 95th to 99th percentile of signal intensity within the prostate. RSIrs comparisons were made at the 98th percentile within each patient's prostate. We compared RSIrs from calibrated TE90 (RSIrs_TE90corr) and uncorrected TE90 (RSIrs_TE90) to RSIrs from reference TEmin₁ (RSIrs_TEmin1) and repeated TEmin₂ (RSIrs_TEmin2). Calibration performance was evaluated with sensitivity, specificity and area under the ROC curve (AUC).

Scaling factors for C₁, C₂, C₃, and C₄ were estimated as 1.68, 1.33, 1.02, and 1.13, respectively. In non-csPCa cases, the 98th percentile of RSIrs_TEmin2 and RSIrs_TEmin1 differed by 0.27 ± 0.86SI (mean ± standard deviation), whereas RSIrs_TE90 differed from RSIrs_TEmin1 by 1.82 ± 1.20SI. After calibration, this bias was reduced to -0.51 ± 1.21SI, representing a 72% reduction in absolute error. For patients with csPCa, the difference was 0.54 ± 1.98SI between RSIrs_TEmin2 and RSIrs_TEmin1 and 2.28 ± 2.06SI between RSIrs_TE90 and RSIrs_TEmin1. After calibration, the mean difference decreased to -1.03SI, a 55% reduction in absolute error. At the Youden index for patient-level classification of csPCa (8.94SI), RSIrs_TEmin1 has a sensitivity of 66% and a specificity of 72%.

The proposed linear calibration method produces similar quantitative biomarker values for acquisitions with different TE, reducing TE-induced error by 72% and 55% for non-csPCa and csPCa, respectively.