Reproducibility analysis of diffusion tensor indices and fiber architecture of human calf muscles in vivo at 1.5 Tesla in neutral and plantarflexed ankle positions at rest

Abstract

Purpose: To investigate the reproducibility of diffusion tensor imaging (DTI) -derived indices and fiber architecture of calf muscles at 1.5 Tesla (T), to establish an imaging based method to confirm ankle position, and to compare fiber architecture at different ankle positions.

Materials and methods: Six subjects were imaged at 1.5T with the foot in neutral and plantarflexed positions. DTI indices were calculated in four muscle compartments (medial and lateral gastrocnemius [MG, LG], superficial and deep anterior tibialis [AT-S, AT-D]). Two subjects were scanned on 3 days to calculate the coefficient of variability (CV) and the repeatability coefficient (RC).

Results: DTI indices were close to the values obtained in earlier 3T and 1.5T studies. Fractional anisotropy decreased significantly in the MG and increased significantly in the AT-S and AT-D compartments while fiber orientation with respect to the magnet Z-axis increased significantly in the MG and decreased significantly in the AT-S compartment with plantarflexion. The CV and RC for the DTI indices and fiber orientations were comparable to 3T studies. Fiber lengths and orientation angles in the MG matched corresponding measures from ultrasound studies.

Conclusion: DTI at 1.5T provides reproducible measures of diffusion indices and fiber architecture of calf muscle at different muscle lengths.

Figure 1

Axial (top row) and coronal (bottom row) images of the color map of the lead eigenvector of the diffusion tensor from images acquired at plantarflexion Left column: prior to denoising and right column: corresponding after tensor denoising. The effect of tensor denoising on reducing noise in the lead eigenvector can be readily appreciated. Further, the smoothing algorithm preserves the boundaries between muscle regions. The color map is encoded as: Superior to Inferior: Blue; Right to Left: Red; and Anterior to Posterior: Green. Unsuppressed fat shows as regions around the muscle but was easily avoided in ROI measurements since the fat signal was very much reduced compared to muscle in the b0 images and could be readily identified. The fat was not thresholded during tensor fitting in order to ensure that small low intensity regions in the muscle were not inadvertently thresholded.

Figure 2

Anatomical images of the ankle acquired in the body coil at four ankle positions with ankle angle as the average of the measurements in two slices at each position. Positive ankle angle values shown in the images are the deviations from the neutral position (0⁰) in the plantarflexed direction. This imaging based assessment allowed accurate positioning and determination of the ankle angle.

Figure 3

Graphs of the variation of the DTI indices and fiber orientation in the four compartments with ankle angle. The lines are not fits to the underlying data but serve as a guide to the eye. The variation is clearly non-linear.

Figure 4

Fibers tracked from seed points in the MG at neutral (left) and at plantarflexion (right) positions of the ankle, shown here superposed on the corresponding 3D b0 volumes. The fiber statistics was derived for seed locations in the middle of the MG as it was possible to obtain fiber tracks in all subjects at this location. The 3D view is shown here with the volume at an angle so that the axial and coronal cross-sections can be seen. Since the 3D view was set interactively, it was not possible to reproduce the exact same view for images.