Diffusion tensor imaging (DTI) has been widely used to investigate peripheral nerves^1,2. It is non-invasive and allows for the evaluation of microstructural tissue properties of these nervous structures. As such, DTI could be beneficial to study peripheral nervous disorders such as multifocal motor neuropathy (MMN). This disease is characterized by weakness in the muscle without any sensory involvement, where distal limbs are usually first affected³. One of the main drawbacks of DTI applied to peripheral nerve imaging is that due to the generally low spatial resolution, partial volume effects occur⁴ confounding the estimation of diffusion parameters. With 7T MRI higher resolutions can be achieved without any major loss in signal to noise ratio (SNR) compared with 3T MRI. However, a higher field strength also has its drawback, since it suffers from more pronounced susceptibility induced deformation⁵. Reducing susceptibility deformation can be achieved by using higher parallel imaging factors, which benefit from using a dedicated high density receive coil array. In this study, we explore the use of 7T to investigate the nerves in the forearm using DTI and fiber tractography (FT) and evaluate the results from two acquisition schemes with different image resolution.

Written informed consent was given prior to the MRI examination. 4 healthy volunteers underwent MRI for the right forearm on a 7 Tesla MR system (Achieva; Philips Healthcare, Best, The Netherlands) using a dedicated 32 channel wrist coil. DTI data was acquired with a voxel size of 0.75×0.75×2.0 mm³ (high-resolution protocol) and a voxel size of 1.0×1.0×2.0 mm³ (low-resolution protocol), using a diffusion-weighted spin echo single-shot echo planar imaging (EPI) protocol (for all acquisition parameters see Table 1). An axial T1 TSE was obtained for anatomical reference. The DTI data were processed with ExploreDTI⁶. First, DTI scans were corrected for subject motion, eddy current distortions, and EPI deformations^7,8. Diffusion tensors were fitted using the REKINDLE procedure⁹. Second, whole volume tractography was performed with a seeding density of 2x2x2 mm³, a fractional anisotropy (FA) threshold of 0.35 and a curvature threshold of 30°. Last, 2 SEED ROIs and 1 AND ROI were used to select a segment of 5 cm of three nerves of the forearm (i.e. median, ulnar, and superficial radial nerve). From these segments the FA, mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were computed.Figure 1 shows data and diffusion parameters maps derived from both DTI protocols. From the DTI data it was possible to reconstruct the median and ulnar nerves in the forearm of all datasets using FT (Fig. 2). In all but one low-resolution dataset, it was possible to reconstruct the superficial radial nerve. Table 2 and Figure 3 show the diffusion values for both low- and high-resolution protocols. The MD, AD, and RD showed a tendency (not significant) toward lower values in the low-resolution protocol for the ulnar and median nerve compared to the high-resolution protocol.Although higher resolutions in DTI typically lead to a reduction in partial volume effects⁴, MD and RD values in the median, ulnar, and radial nerve were not significantly different between both DTI protocols in this study. However, FA values are higher and MD, and RD values are lower, compared to an earlier study visualizing the nerves in the forearm at 3T¹⁰. Another study at 3T with a reconstructed resolution of 1.0×1.0×3.0 mm³, shows higher FA values, and lower MD, AD, and RD². A study in the upper arm at 3T with a resolution of 1.14×1.14×4 mm³ showed similar results, but decreased RD values¹¹. All these differences might be SNR related¹². With the high-resolution data the radial nerve could be reconstructed in all subjects, which was not the case with the low-resolution protocol and in an earlier study at 3T¹⁰. This DTI study shows that the median, ulnar, and superficial radial nerves in healthy volunteer subjects can be investigated reliably at 7T. Although these are preliminary results with a small sample size, we have clearly demonstrated the potential of 7T to identify these nerves and quantify their diffusion characteristics in a reliable way. No significant differences in diffusion values between the protocols were found. However, compared to earlier studies at 3T, partial volume effects are likely to be reduced with increased resolution of DTI at 7T. We expect that the use of high resolution DTI can be beneficial in the investigation of peripheral nervous tissue in the forearm and might aid in identifying changes due to pathology.