AUTOMATIC HIGH-RESOLUTION CARTILAGE THICKNESS MAPPING AND SCORING: EXTENSION TO STANDARD CLINICAL MRI - DATA FROM THE...

A fully automated MRI-based framework was recently developed to assess femorotibial cartilage damage severity using anatomically standardized cartilage thickness maps (CTh-Maps) and an associated pattern-based severity score (CTh-Score, ranging from 0, indicating healthy cartilage, to 100, indicating end-stage osteoarthritis). The framework showed excellent reproducibility, strong agreement with expert MRI cartilage-loss assessment, and sensitivity to structural osteoarthritis progression. It was developed and validated using high-resolution DESS MRI, which is not routinely acquired in clinical practice. Standard clinical knee MRI more commonly uses tri-planar 2D fat-suppressed intermediate-weighted fast spin echo (FSE FS IW) sequences, which have lower through-plane resolution and reduced cartilage-bone contrast (Figure 1). Translating CTh-Maps and CTh-Score computation to standard clinical MRI would support routine use.

To adapt the framework to routine clinical tri-planar 2D FSE FS IW MRI and assess whether clinical MRI-derived CTh-Maps and CTh-Scores agree with DESS-based reference measurements.

This study included 430 participants of the Lausanne Knee Study (LKS), with one randomly selected knee per participant. Each underwent 3D DESS MRI, with 0.63 mm isotropic voxels, and tri-planar 2D FSE FS IW MRI, with through-plane voxel dimensions 3.3 mm. DESS cartilage segmentations generated by expert musculoskeletal radiologists served as reference standards. A 3D U-Net implemented with nnU-Net was trained to segment cartilage from interpolated and concatenated sagittal, coronal, and axial FSE FS IW images using DESS-derived segmentations as ground truth. Training included 352 subjects, with 78 held out for testing. CTh-Map accuracy, precision, and agreement were assessed using mean thickness difference, standard deviation of thickness difference, and voxel-wise ICC, respectively. CTh-Scores from FSE FS IW- and DESS-based maps were compared using ICC and Bland-Altman limits of agreement. DESS test-retest reproducibility was assessed in 11 independent knees.

The cohort included 192 males and 238 females, with a mean age of 47.8 ± 16.2 years old . In the DESS test-retest analysis, CTh-Maps showed an accuracy of -0.01 ± 0.02 mm, precision of 0.21 ± 0.02 mm, and excellent reliability, with an ICC of 0.96 ± 0.02. In the 78-subject test set, FSE FS IW-based CTh-Maps closely agreed with DESS-based reference maps, with an accuracy of 0.04 ± 0.08 mm, precision of 0.21 ± 0.11 mm, and ICC of 0.95 ± 0.02. CTh-Scores computed from FSE FS IW- and DESS-based maps also showed excellent agreement, with an ICC of 0.91. Bland-Altman analysis showed a bias of -1.8 points on the 0-100 CTh-Score scale, with 95% limits of agreement from -7.5 to 5.0.

CTh-Maps and CTh-Scores can be reliably derived from routine clinical tri-planar 2D MRI. Despite the lower through-plane resolution and reduced cartilage-bone contrast of clinical sequences, FSE FS IW-based CTh-Maps and CTh-Scores showed excellent agreement with DESS-based references (Figure 2), comparable to the DESS test-retest results. These findings support the clinical translation of automated cartilage thickness mapping and scoring to standard knee MRI protocols.