Quantitative MRI biomarker for classification of clinically significant prostate cancer: calibration for reproducibility across echo times

Abstract

BackgroundRestriction Spectrum Imaging restriction score (RSIrs) is a quantitative biomarker for detecting clinically significant prostate cancer (csPCa). However, the quantitative value of the RSIrs is affected by imaging parameters such as echo time (TE).PurposeThe purpose of the present study is to develop a calibration method to account for differences in echo times and facilitate use of RSIrs as a quantitative biomarker for the detection of csPCa.MethodsThis 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∼75ms and once at TE=90ms (TEmin1, TEmin2, and TE90, respectively). A proposed calibration method, trained on patients without csPCa, estimated a linear scaling factor (f) for each of the four diffusion compartments (C) of the RSI signal model. A linear regression model was determined to match C-maps of TE90 to the reference C-maps of TEmin1within the interval ranging from 95thto 99thpercentile of signal intensity within the prostate. RSIrs comparisons were made at 98thpercentile within each patient’s prostate.We compared RSIrs from calibrated TE90 (RSIrsTE90corr) and uncorrected TE90 (RSIrsTE90) to RSIrs from reference TEmin1(RSIrsTEmin1) and repeated TEmin2(RSIrsTEmin2). Calibration performance was evaluated with sensitivity, specificity, area under the ROC curve, positive predicted value, negative predicted value, and F1-score.ResultsScaling factors for C1, C2, C3, and C4were estimated as 1.70, 1.38, 1.03, and 1.19, respectively. In non-csPCa cases, the 98thpercentile of RSIrsTEmin2and RSIrsTEmin1differed by 0.27±0.86SI (mean±standard deviation), whereas RSIrsTE90differed from RSIrsTEmin1by 1.81±1.20SI. After calibration, this bias was reduced to -0.41±1.20SI, representing a 78% reduction in absolute error. For patients with csPCa, the difference was 0.54±1.98SI between RSIrsTEmin2and RSIrsTEmin1and 2.28±2.06SI between RSIrsTE90and RSIrsTEmin1. After calibration, the mean difference decreased to -0.86SI, a 38% reduction in absolute error. At the Youden index for patient-level classification of csPCa (8.94SI), RSIrsTEmin1has a sensitivity of 66% and a specificity of 72%. Prior to calibration, RSIrsTE90at the same threshold tended to over-diagnose benign cases (sensitivity 44%, specificity 88%). Post-calibration, RSIrsTE90corrperforms more similarly to the reference (sensitivity 71%, specificity 62%).ConclusionThe proposed linear calibration method produces similar quantitative biomarker values for acquisitions with different TE, reducing TE-induced error by 78% and 38% for non-csPCa and csPCa, respectively.