Tissue‐Specific T(2)* Biomarkers in Patellar Tendinopathy by Subregional Quantification Using 3D Ultrashort Echo Time MRI

BACKGROUND: Quantitative MRI of patellar tendinopathy (PT) can be challenging due to spatial variation of T(2)* relaxation times. PURPOSE: 1) To compare T(2)* quantification using a standard approach with analysis in specific tissue compartments of the patellar tendon. 2) To evaluate test–retest reliability of different methods for fitting ultrashort echo time (UTE)‐relaxometry data. STUDY TYPE: Prospective. SUBJECTS: Sixty‐five athletes with PT. FIELD STRENGTH/SEQUENCE: 3D UTE scans covering the patellar tendon were acquired using a 3.0T scanner and a 16‐channel surface coil. ASSESSMENT: Voxelwise median T(2)* was quantified with monoexponential, fractional‐order, and biexponential fitting. We applied two methods for T(2)* analysis: first, a standard approach by analyzing all voxels covering the proximal patellar tendon. Second, within subregions of the patellar tendon, by using thresholds on biexponential fitting parameter percentage short T(2)* (0–30% for mostly long T(2)*, 30–60% for mixed T(2)*, and 60–100% for mostly short T(2)*). STATISTICAL TESTS: Average test–retest reliability was assessed in three athletes using coefficients‐of‐variation (CV) and coefficients‐of‐repeatability (CR). RESULTS: With standard image analysis, we found a median [interquartile range, IQR] monoexponential T(2)* of 6.43 msec [4.32–8.55] and fractional order T(2)* 4.39 msec [3.06–5.78]. The percentage of short T(2)* components was 52.9% [35.5–69.6]. Subregional monoexponential T(2)* was 13.78 msec [12.11–16.46], 7.65 msec [6.49–8.61], and 3.05 msec [2.52–3.60] and fractional order T(2)* 11.82 msec [10.09–14.44], 5.14 msec [4.25–5.96], and 2.19 msec [1.82–2.64] for 0–30%, 30–60%, and 60–100% short T(2)*, respectively. Biexponential component short T(2)* was 1.693 msec [1.417–2.003] for tissue with mostly short T(2)* and long T(2)* of 15.79 msec [13.47–18.61] for mostly long T(2)*. The average CR (CV) was 2 msec (15%), 2 msec (19%) and 10% (22%) for monoexponential, fractional order and percentage short T(2)*, respectively. DATA CONCLUSION: Patellar tendinopathy is characterized by regional variability in binding states of water. Quantitative multicompartment T(2)* analysis in PT can be facilitated using a voxel selection method based on using biexponential fitting parameters. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1 J. Magn. Reson. Imaging 2020;52:420–430.