To introduce high-resolution 3D steady-state imaging for examining microstructural nerve injury with 7T MRI.In chronic and neuropathic pain management, MRI examination is frequently ordered to search the nerve injury causing pain. Unfortunately, anatomical information captured with conventional MR imaging approaches has had limited efficacy in the accurate identification of nerve injury¹. Recently, simultaneous [18F] FDG PET-MRI has shown great promise for enhanced detection of the nerve injury by combining [18F] FDG PET’s sensitivity to inflammation with MRI’s high-resolution anatomic information of the nerve injury². However, our experience with simultaneous PET-MRI so far suggests that there still remains a lack of anatomic visualization and characterization of the actual nerve injury or exact appearance of neuroinflammation since the causative nerve injury might be too subtle or small. Here, we present ultra-high-resolution 3D steady-state imaging sequences at 7T that can visualize microstructures of peripheral nerves in the extremities to enable advanced diagnosis for subtle structural nerve damage causing pain.

We performed 3D steady-state imaging with FISP and PSIF^3,4 sequences. In FISP, the image contrast is dominated by FID while that of PSIF is dominated by echo pathways, resulting in (roughly) T1 weighting for FISP and T2/diffusion weighting for PSIF. FISP images capture much greater anatomic details with higher resolution than PSIF due to their SNR advantage, but limited contrast between nerves and blood vessels can cause confusion. This ambiguity can be resolved with PSIF images because the diffusion weighting of PSIF can be set in a direction parallel to the blood vessel to suppress the signal from flowing blood. Also, the SNR boost from 3D imaging, 7T, and use of a surface coil for signal reception enable the in-plane resolution in the range of 100um.

We conducted an initial survey to compare anatomic nerve details between 3T and 7T FISP images to demonstrate the advantage of 7T MRI for imaging micro nerve structures. We also performed FISP and PSIF imaging on different human extremities including foot, ankle, and finger. For high resolution FISP imaging, the in-plane image resolution was about 0.12 x 0.12mm2 over 8.0 x 6.4 cm FOV while the slice thickness was 1.5-2mm. For PSIF imaging, the in-plane resolution was 0.25 x 0.25mm2 over 8.0 x 6.4 cm FOV with 2mm slice thickness. The imaging experiments were conducted on a GE 7T Discovery MR950 scanner and a GE 3T Discovery MR750 scanner (GE Healthcare, Milwaukee, U.S.). A single channel receive surface coil was used at 7T and a 16 channel GEM flex coil was used in 3T. All images except Fig. 2A are water images obtained by 2-point Dixon fat-water separation.

Simplified pulse sequence diagrams of FISP and PSIF sequences are plotted in Fig. 1. In FISP, signal acquisition occurs before the spoiler, while In PSIF, signal acquisition takes place after the spoiler, and this acquisition timing difference leads to the aforementioned contrast difference. Fig. 2 compares axial FISP images of a human ankle collected at different field strengths with different resolution. Red arrows in Fig. 2A point to tibial nerves, which are magnified in Fig. 2B. Fig. 2B demonstrates that more details of the fascicular pattern of the tibial nerve can be captured as resolution improves (0.4mm x 0.4mm to 0.12mm x 0.12mm) and 7T enables more clear delineation of the fascicular pattern. Fig. 3 compares axial FISP and PSIF images of a human ankle. The red arrows point to blood vessels and the green arrow points to a tibial nerve. While it is very difficult to differentiate between blood vessels and the tibial nerve in the FISP image, blood-flow nulling with diffusion weighting in PSIF led to clear distinction between them. Fig. 4 shows an axial FISP image of human second and third fingers. The third finger shows normal course of the vessels (red arrows) and nerves (yellow arrows) at the level of the DIP joint whereas the second finger demonstrates distortion of the subcutaneous tissue by scar (short green arrow) and edema formation (orange asterisk) with associated altered course of the vessels (red arrows) and nerves (yellow arrows) along the radial aspect of the distal phalanx. Fig. 5 shows an axial FISP image of a human foot, and nerve swelling from Morton’s neuroma (red arrow) is clearly distinguished from normal interplantar neurovascular bundles (green arrows).We presented ultra-high-resolution 3D steady-state imaging at 7T to enable in-vivo examination of microstructures in peripheral nerves. We believe our approach has great promise for enhancing diagnosis and treatment planning for peripheral nerve injury and pain management.