This prospective study was approved by the local institutional review board (Cantonal Ethics Committee, Zurich, Switzerland).
MRI data were acquired were performed on a 3-T system (MAGNETOM Prisma, Siemens Healthineers AG, Erlangen, Germany) using a dedicated 16-channel shoulder coil. MRI scans were performed between May and August 2023, and comprised a total of three scans of the same shoulder for each volunteer, with an interval of seven days between the scan sessions, respectively.
For the planning of diffusion-weighted imaging, the maximum visible length of the intramuscular tendon of the anterior SSP muscle bundle was identified on the water-only images of a coronal oblique proton-density weighted turbo-spin-echo DIXON sequence.
The diffusion-weighted imaging parallel to the SSP tendon orientation was performed along 64 directions with three b = 0 s/mm2 images as reference using a monopolar stimulated echo acquisition mode research sequence (mixing time: 200 ms) with a single-shot echo-planar imaging readout. The sequence parameters are listed in Table 1.
Images were then analyzed by three fellowship-trained radiologists who were blinded to all participant data and read images in random order on a dedicated workstation. Two freehand regions of interest (ROIs) were drawn by each reader on the relevant slice of maximum tendon visualization by the readers in the computed PA maps using ITK-SNAP in the anterior and posterior bundle of the SSP muscle (Figure 1).
PA measurements of one reader were repeated after a three-week interval to assess intra-reader agreement.
DTI tractography was performed solely for visualization purposes and was not a primary endpoint of this study. The motion-corrected STEAM dataset of an exemplary subject at a single time point was processed in MRtrix3 [7] .
PAs were averaged across examination time points and readers for further analysis. To assess the test-retest reliability, inter-reader agreement, and intra-reader agreement of PAs measured with DTI, intraclass correlation coefficients (ICCs) for absolute agreement were calculated using a two-way mixed model.