Quantifiable Imaging Biomarkers for Evaluation of the Posterior Cruciate Ligament Using 3-T Magnetic Resonance Imaging: A Feasibility Study

Abstract

Background: Quantitative magnetic resonance imaging (MRI) techniques, such as T2 and T2 star (T2*) mapping, have been used to evaluate ligamentous tissue in vitro and to identify significant changes in structural integrity of a healing ligament. These studies lay the foundation for a clinical study that uses quantitative mapping to evaluate ligaments in vivo, particularly the posterior cruciate ligament (PCL). To establish quantitative mapping as a clinical tool for identifying and evaluating chronic or acute PCL injuries, T2 and T2* values first must be determined for an asymptomatic population.

Purpose: To quantify T2 and T2* mapping properties, including texture variables (entropy, variance, contrast, homogeneity), of the PCL in an asymptomatic population. It was hypothesized that biomarker values would be consistent throughout the ligament, as measured across 3 clinically relevant subregions (proximal, middle, and distal thirds) in the asymptomatic cohort.

Study design: Cross-sectional study; Level of evidence, 4.

Methods: Unilateral knee MRI scans were acquired for 25 asymptomatic subjects with a 3.0-T MRI system using T2 and T2* mapping sequences in the sagittal plane. The PCL was manually segmented and divided into thirds (proximal, middle, and distal). Summary statistics for T2 and T2* values were calculated. Intra- and interrater reliability was assessed across 3 raters to 2 time points.

Results: The asymptomatic PCL cohort had mean T2 values of 36.7, 29.2, and 29.6 ms in the distal, middle, and proximal regions, respectively. The distal PCL exhibited significantly higher mean, variance, and contrast and lower homogeneity of T2 values than the middle and proximal subregions (P < .05). T2* results exhibited substantial positive skew and were therefore presented as median and quartile (Q) values. Median T2* values were 7.3 ms (Q1-Q3, 6.8-8.9 ms), 7.3 ms (Q1-Q3, 7.0-8.5 ms), and 7.3 ms (Q1-Q3, 6.4-8.2 ms) in the distal, middle, and proximal subregions, respectively.

Conclusion: This is the first study to identify T2 and T2* mapping values, and their texture variables, for the asymptomatic PCL. The distal third of the PCL had significantly greater T2 values than the proximal or middle thirds.

Clinical relevance: T2 and T2* values of the asymptomatic PCL can provide a baseline for comparison with acute and chronic PCL injuries in future studies.

Keywords: MRI; T2 mapping; T2* mapping; asymptomatic; posterior cruciate ligament.

Figure 1.

Posterior cruciate ligament (PCL) volumes were created from the binary image series for each subject. For each volume, (A) a 3-dimensional centerline that followed the length of the ligament was extracted, and (B) the centerline was divided to create proximal, middle, and distal thirds of the PCL.

Figure 2.

Example asymptomatic posterior cruciate ligament (PCL) showing (A) T2 mapping image slice, (B) histogram of T2 values for the entire PCL volume with a kernel distribution fit line, (C) T2* mapping image slice, and (D) histogram of T2* values for the whole PCL volume with a kernel distribution fit line.

Figure 3.

Examples of subjects in which the distal posterior cruciate ligament (PCL) exhibited significantly higher mean, standard deviation, variance, and contrast of T2 mapping values compared with both the middle and proximal subregions (P < .05). The upper row displays the 3-dimensional PCL volume with voxels color-mapped to the T2 values (ms), and the lower row shows the corresponding T2 mapping image slice with the T2 values overlaid.