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

Restriction 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).

The 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.

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∼75ms and once at TE=90ms (TEmin ₁ , TEmin ₂ , 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 TEmin ₁ within the interval ranging from 95 ^th to 99 ^th percentile of signal intensity within the prostate. RSIrs comparisons were made at 98 ^th percentile within each patient's prostate. We compared RSIrs from calibrated TE90 (RSIrs _TE90corr ) and uncorrected TE90 (RSIrs _TE90 ) to RSIrs from reference TEmin ₁ (RSIrs _TEmin1 ) and repeated TEmin ₂ (RSIrs _TEmin2 ). Calibration performance was evaluated with sensitivity, specificity, area under the ROC curve, positive predicted value, negative predicted value, and F1-score.

Scaling factors for C ₁ , C ₂ , C ₃ , and C ₄ were estimated as 1.70, 1.38, 1.03, and 1.19, respectively. In non-csPCa cases, the 98 ^th percentile of RSIrs _TEmin2 and RSIrs _TEmin1 differed by 0.27±0.86SI (mean±standard deviation), whereas RSIrs _TE90 differed from RSIrs _TEmin1 by 1.81±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 RSIrs _TEmin2 and RSIrs _TEmin1 and 2.28±2.06SI between RSIrs _TE90 and RSIrs _TEmin1 . 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), RSIrs _TEmin1 has a sensitivity of 66% and a specificity of 72%. Prior to calibration, RSIrs _TE90 at the same threshold tended to over-diagnose benign cases (sensitivity 44%, specificity 88%). Post-calibration, RSIrs _TE90corr performs more similarly to the reference (sensitivity 71%, specificity 62%).

The 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.