In vivo electrical conductivity measurement of muscle, cartilage, and peripheral nerve around knee joint using MR-electrical properties tomography

Abstract

This study aimed to investigate whether in vivo MR-electrical properties tomography (MR-EPT) is feasible in musculoskeletal tissues by evaluating the conductivity of muscle, cartilage, and peripheral nerve around the knee joint, and to explore whether these measurements change after exercise. This prospective study was approved by the institutional review board. On February 2020, ten healthy volunteers provided written informed consent and underwent MRI of the right knee using a three-dimensional balanced steady-state free precession (bSSFP) sequence. To test the effect of loading, the subjects performed 60 squatting exercises after baseline MRI, immediately followed by post-exercise MRI with the same sequences. After reconstruction of conductivity map based on the bSSFP sequence, conductivity of muscles, cartilages, and nerves were measured. Measurements between the baseline and post-exercise MRI were compared using the paired t-test. Test-retest reliability for baseline conductivity was evaluated using the intraclass correlation coefficient. The baseline and post-exercise conductivity values (mean ± standard deviation) [S/m] of muscles, cartilages, and nerves were 1.73 ± 0.40 and 1.82 ± 0.50 (p = 0.048), 2.29 ± 0.47 and 2.51 ± 0.37 (p = 0.006), and 2.35 ± 0.57 and 2.36 ± 0.57 (p = 0.927), respectively. Intraclass correlation coefficient for the baseline conductivity of muscles, cartilages, and nerves were 0.89, 0.67, and 0.89, respectively. In conclusion, in vivo conductivity measurement of musculoskeletal tissues is feasible using MR-EPT. Conductivity of muscles and cartilages significantly changed with an overall increase after exercise.

Figure 1

An example of conductivity analysis in the right knee of a 30-year old male volunteer. Regions of interest (ROIs) were placed in the vastus medialis (VM), semimembranosus (SM) muscles, biceps femoris muscle, including both long and short heads (BF), tibial nerve at the distal thigh level, and patellar and trochlear cartilage on the axial three-dimensional balanced fast-field echo sequence (a–c). ROIs with identical shapes, sizes, and positions were automatically generated on the conductivity map (d–f). The unit for the scale bars is S/m. Arrows, tibial nerve; dotted lines, ROIs.

Figure 2

Baseline (a, b) and the post-exercise conductivity map (c, d) in a 28-year old male volunteer. Compared to the baseline, conductivity of the vastus medialis muscles and patellar cartilage increased from 1.033 to 1.188 S/m, and from 1.119 to 1.450 S/m, respectively. B₁ inhomogeneity is noted across field of view, shown as demarcated areas with increased conductivity in red-colored zones (arrowheads). The unit for the scale bars is S/m. BF, biceps femoris muscle; SM, semimembranosus muscle; VM, vastus medialis muscle; arrows, tibial nerve; dotted lines, ROIs.

Figure 3

Baseline (a) and the post-exercise conductivity map (b) in a 28-year old male volunteer. Compared to the baseline, conductivity of the trochlear cartilage increased from 1.887 to 2.539 S/m. The unit for the scale bars is S/m. dotted lines, ROIs.

Figure 4

Graphs showing change of conductivity of vastus medialis muscle (a), semimembranosus muscle (b), biceps femoris muscle (c), patellar cartilage (d), trochlear cartilage (e), and tibial nerve (f) for each volunteer. The mean values are represented by red dotted lines, 95% confidence intervals of mean values by blue green bars, respectively.