Feasibility of quantitative ultrashort echo time (UTE)-based methods for MRI of peripheral nerve.

Abstract

Peripheral nerves are a composite tissue consisting of neurovascular elements packaged within a well-organized extracellular matrix. Their composition, size, and anatomy render nerves a challenging medical imaging target. In contrast to morphological MRI, which represents the predominant approach to nerve imaging, quantitative MRI sequences can provide information regarding tissue composition. Here, we applied standard clinical Carr-Purcell-Meiboom-Gill (CPMG) and experimental three-dimensional (3D) ultrashort echo time (UTE) Cones sequences for quantitative nerve imaging including T2 measurement with single-component analysis, T2 * measurement with single-component and bi-component analyses, and magnetization transfer ratio (MTR) analysis. We demonstrated the feasibility and the high quality of single-component T2 *, bi-component T2 *, and MTR approaches to analyze nerves imaged with clinically deployed 3D UTE Cones pulse sequences. For 24 single fascicles from eight nerves, we measured a mean single-component T2 * of 22.6 ±8.9ms, and a short T2 * component (STC) with a mean T2 * of 1.7 ±1.0ms and a mean fraction of (6.74 ±4.31)% in bi-component analysis. For eight whole nerves, we measured a mean single-component T2 * of 16.7 ±2.2ms, and an STC with a mean T2 * of 3.0 ±1.0ms and a mean fraction of (15.56 ±7.07)% in bi-component analysis. For nine fascicles from three healthy nerves, we measured a mean MTR of (25.2 ±1.9)% for single fascicles and a mean MTR of (23.6 ±0.9)% for whole nerves. No statistically significant correlation was observed between any MRI parameter and routine histological outcomes, perhaps due to the small sample size and lack of apparent sample pathology. Overall, we have successfully demonstrated the feasibility of measuring quantitative MR outcomes ex vivo, which might reflect features of nerve structure and macromolecular content. These methods should be validated comprehensively on a larger and more diverse set of nerve samples, towards the interpretation of in vivo outcomes. These approaches have new and broad implications for the management of nerve disease, injury, and repair.