Magnetization Transfer Ratio in Peripheral Nerve Tissue: Does It Depend on Age or Location?

Abstract

Background and purpose: Magnetization transfer contrast imaging provides indirect information on the concentration of "bound" water protons and their interactions with "free" water molecules. The purpose of this study is to analyze location- and age-dependent changes in the magnetization transfer ratio (MTR) of lower extremity nerves.

Materials and methods: Ten younger (20-32 years) and 5 older (50-63 years) healthy volunteers underwent magnetization transfer contrast imaging at 3 Tesla Two 3-dimensional gradient echo sequences with and without an off-resonance saturation pulse (repetition time: 58 milliseconds; echo time: 2.46 milliseconds; band width: 530 Hz/Px; flip angle: α = 7°) were acquired at 3 different locations covering the proximal thigh to the distal lower leg in the group of younger volunteers and at 2 different locations covering the proximal to distal thigh in the group of older volunteers. Sciatic and tibial nerve regions of interest (ROIs) were manually drawn and additional ROIs were placed in predetermined muscles. Magnetization transfer ratios were extracted from respective ROIs and calculated for each individual and location.

Results: In young volunteers, mean values of nerve and muscle MTR were not different between the proximal thigh (nerve: 20.34 ± 0.91; muscle: 31.71 ± 0.29), distal thigh (nerve: 19.90 ± 0.98; P = 0.76; muscle: 31.53 ± 0.69; P = 0.87), and lower leg (nerve: 20.82 ± 1.07; P = 0.73; muscle: 32.44 ± 1.11; P = 0.51). An age-dependent decrease of sciatic nerve MTR was observed in the group of older volunteers (16.95 ± 1.2) compared with the group of younger volunteers (20.12 ± 0.65; P = 0.019). Differences in muscle MTR were not significant between older (31.01 ± 0.49) and younger (31.62 ± 0.37; P = 0.20) volunteers.

Conclusion: The MTR of lower extremity nerves shows no proximal-to-distal gradient in young healthy volunteers but decreases with age. For future studies using MTR in peripheral nerve disorders, these findings suggest that referencing magnetization transfer contrast values in terms of age, but not anatomical nerve location is required.