Quantitative Sodium MRI of the Human Patellar Tendon

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Abstract
Sodium MRI can be used in musculoskeletal imaging to assess tissue composition because sodium concentration is related to glycosaminoglycan (GAG) content. In tendinopathy, the GAG content is elevated, resulting in a high signal in sodium MRI. Because biochemical changes typically precede structural alterations, sodium MRI may be a useful indicator for detecting early tendon matrix changes associated with tendinopathy. Quantitative sodium concentration determination has only been applied to Achilles tendons but not yet to smaller tendons, such as the patellar tendon. Due to its smaller diameter, the patellar tendon is more susceptible to partial volume artifacts, making accurate sodium concentration quantification more challenging. However, partial volume correction (PVC) may help overcome these difficulties. In this study, we determined the apparent tissue sodium concentration (aTSC) in seven patients with patellar tendinopathy and 20 healthy controls using clinical 3 T MRI with a density‐adapted 3D radial MR‐sequence. Sodium images were corrected for partial volume effects using an in‐house developed PVC, called the estimated single target correction (eSTC). Additionally, 1 H T 2 * values were calculated for comparison, as they are sensitive to structural alterations. Sodium relaxation times were determined in three healthy controls, resulting in a T 1 of 30.2 ± 0.4 ms and a biexponential T 2s * of 0.7 ± 0.2 ms and T 2l * of 13.0 ± 0.1 ms. The short component p s accounted for 35.7% ± 2.1% of the transversal relaxation. aTSC values were 35.4 ± 10.1 mM in tendinopathy patients and 21.6 ± 3.9 mM in healthy controls. 1 H T 2 * values were 5.4 ± 1.9 ms and 2.8 ± 0.3 ms, respectively. Differences between patients and controls were significant for both parameters. These findings support the suitability of sodium MRI for quantitative assessment of the patellar tendon and demonstrate its potential for visualizing biochemical changes associated with patellar tendinopathy.

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