UTE-T2* versus conventional T2* mapping to assess posterior cruciate ligament ultrastructure and integrity-an in-situ study

Abstract

Background: Clinical-standard morphologic magnetic resonance imaging (MRI) is limited in the refined diagnosis of posterior cruciate ligament (PCL) injuries. Quantitative MRI sequences such as ultrashort echo-time (UTE)-T2* mapping or conventional T2* mapping have been theorized to quantify ligament (ultra-) structure and integrity beyond morphology. This study evaluates their diagnostic potential in identifying and differentiating partial and complete PCL injuries in a standardized graded injury model.

Methods: Ten human cadaveric knee joint specimens were imaged on a clinical 3.0 T MRI scanner using morphologic, conventional T2* mapping, and UTE-T2* mapping sequences before and after standardized arthroscopic partial and complete PCL transection. Following manual segmentation, quantitative T2* and underlying texture features (i.e., energy, homogeneity, and variance) were analyzed for each specimen and PCL condition, both for the entire PCL and its subregions. For statistical analysis, Friedman's test followed by Dunn's multiple comparison test was used against the level of significance of P≤0.01.

Results: For the entire PCL, T2* was significantly increased as a function of injury when acquired with the UTE-T2* sequence [entire PCL: 11.1±3.1 ms (intact); 10.9±4.6 ms (partial); 14.3±4.9 ms (complete); P<0.001], but not when acquired with the conventional T2* sequence [entire PCL: 10.0±3.2 ms (intact); 11.4±6.2 ms (partial); 15.5±7.8 ms (complete); P=0.046]. The PCL subregions and texture variables showed variable changes indicative of injury-associated disorganization.

Conclusions: In contrast to the conventional T2* mapping, UTE-T2* mapping is more receptive in the detection of structural damage of the PCL and allows quantitative assessment of ligament (ultra-)structure and integrity that may help to improve diagnostic differentiation of distinct injury states. Once further substantiated beyond the in-situ setting, UTE-T2* mapping may refine diagnostic evaluation of PCL injuries and -possibly- monitor ligament healing, ageing, degeneration, and inflammation.

Keywords: Magnetic resonance imaging (MRI); knee joint instability; posterior cruciate ligament (PCL); quantitative imaging; ultrashort echo-time (UTE)-T2*.

Figure 1

Exemplary 3D visualizations of the PCL and its subregions as a function of PCL condition. Displayed are the intact PCL (A) and the PCL after partial (B) and complete transection (C). Manually segmented outlines of the PCL were automatically divided into thirds, i.e., proximal (red), central (green), and distal (blue). In B, (*) indicates the site of partial transection and the excluded ligament portions, while in C, (#) indicates the completely transected ligament portions. PCL, posterior cruciate ligament; a, anterior; f, feet; h, head; p, posterior.

Figure 2

3D volumes of UTE-T2* and conventional T2* maps of the PCL as a function of increasing PCL injury. In this representative knee joint, 3D volumes of spatially resolved quantitative UTE-T2* (A) and conventional T2* (B) maps are displayed in the intact (A1, B1), partially PCL-injured (A2, B2), and completely PCL-injured (A3, B3) conditions. Voxel-wise, T2* relaxation times are color-coded as indicated on the scales on the right that range from 0 to 30 ms. PCL, posterior cruciate ligament; UTE, ultrashort echo-time.