Update in the evaluation of peripheral nerves by MRI, from morphological to functional neurography

A scoping review of current and emerging techniques for evaluation of peripheral nerve health, degeneration and regeneration: part 2, non-invasive imaging

Peripheral neuroregenerative research and therapeutic options are expanding exponentially. With this expansion comes an increasing need to reliably evaluate and quantify nerve health. Valid and responsive measures of the nerve status are essential for both clinical and research purposes for diagnosis, longitudinal follow-up, and monitoring the impact of any intervention. Furthermore, novel biomarkers can elucidate regenerative mechanisms and open new avenues for research. Without such measures, clinical decision-making is impaired, and research becomes more costly, time-consuming, and sometimes infeasible. Part 1 of this two-part scoping review focused on neurophysiology. In part 2, we identify and critically examine many current and emerging non-invasive imaging techniques that have the potential to evaluate peripheral nerve health, particularly from the perspective of regenerative therapies and research.

Evaluation of muscle atrophy after sciatic nerve defect repair – experimental model

Peripheral nerve injuries are one of the most important causes for disability generating considerable costs around the world. Appropriate assessment of the extent of peripheral nerve lesions and the choice of the right therapeutic protocol remain some of the biggest challenges. Non-invasive neuroimaging approaches are suitable in managing peripheral nerve repairs, providing in the same time accuracy details in structural neural detection, with minimal discomfort at a low cost. Medical imaging technology development has led to progress in examination of peripheral nervous system, using a series of tools and methods, such as: ultrasonography (US), positron emission tomography (PET), magnetic resonance imaging (MRI) and magnetic diffusion tensor imaging (DTI). In this study, we evaluated the regeneration process in Wistar rats after sciatic nerve defects repair with 4 different techniques (i.e. nerve graft, rat aorta used as a nerve conduit, rat aorta filled with platelet rich plasma (PRP) and rat aorta filled with stem cells) by using MRI investigation alongside the clinical evaluation. Our results showed that among the 4 batches, the PRP batch had the best results in muscle atrophy condition (both on MRI scan and on gastrocnemius index); on the second place the stem cell batch, then the nerve graft batch and finally the aortic conduit batch. MRI proved to be a reliable non-invasive monitoring method and showed good result in correlation with the footprint test (sciatic functional index) and the gastrocnemius index

Diffusion-weighted MR is useful to assess peripheral nerve invasion of soft tissue tumor

To investigate the feasibility of readout-segmented diffusion-weighted (rsDW) magnetic resonance (MR) in assessing nerve invasion of soft tissue tumor. Forty-four patients with soft tissue mass in upper leg suspected of nerve invasion underwent rsDW MR. Nerve invasion by tumor was rated by 2 radiologists, respectively. Sensitivity and specificity of rsDW MR in identifying nerve invasion were calculated, with operation findings as reference of standard. Apparent diffusion coefficient and fraction of anisotropy of nerve were obtained using DW MR, and then were compared between invaded nerves and noninvasion cases. Inter-reader agreement in using rsDW MR to rate nerve invasion was excellent (kappa = 0.891 ± 0.043, P < 0.001). Sensitivity and specificity of rsDW MR in identifying nerve invasion were 89% and 88%, respectively. Apparent diffusion coefficient was significantly higher in invaded nerves versus normal nerves (1.45 ± 0.67 × 10-3 mm2/s vs 1.39 ± 0.46 × 10-3 mm2/s, P < 0.05). Fraction of anisotropy was significantly lower in invaded nerves versus normal nerves (0.22 ± 0.11 vs 0.37 ± 0.13, P < .05). Readout-segmented DW MR was feasible in assessing sciatic nerve invasion by soft tissue tumor in selected patients.

New insights into the evaluation of peripheral nerves lesions: a survival guide for beginners

PURPOSE

To perform a review of the physical basis of DTI and DCE-MRI applied to Peripheral Nerves (PNs) evaluation with the aim of providing readers the main concepts and tools to acquire these types of sequences for PNs assessment. The potential added value of these advanced techniques for pre-and post-surgical PN assessment is also reviewed in diverse clinical scenarios. Finally, a brief introduction to the promising applications of Artificial Intelligence (AI) for PNs evaluation is presented.

METHODS

We review the existing literature and analyze the latest evidence regarding DTI, DCE-MRI and AI for PNs assessment. This review is focused on a practical approach to these advanced sequences providing tips and tricks for implementing them into real clinical practice focused on imaging postprocessing and their current clinical applicability. A summary of the potential applications of AI algorithms for PNs assessment is also included.

RESULTS

DTI, successfully used in central nervous system, can also be applied for PNs assessment. DCE-MRI can help evaluate PN's vascularization and integrity of Blood Nerve Barrier beyond the conventional gadolinium-enhanced MRI sequences approach. Both approaches have been tested for PN assessment including pre- and post-surgical evaluation of PNs and tumoral conditions. AI algorithms may help radiologists for PN detection, segmentation and characterization with promising initial results.

CONCLUSION

DTI, DCE-MRI are feasible tools for the assessment of PN lesions. This manuscript emphasizes the technical adjustments necessary to acquire and post-process these images. AI algorithms can also be considered as an alternative and promising choice for PN evaluation with promising results.

State of the Art and Advances in Peripheral Nerve Surgery

This review is intended to describe and actualize the basic knowledge of the three basic entities that affect the peripheral nerve system and can be treated by surgery: nerve trauma, chronic nerve compressions, and tumors.Regarding trauma, emphasis is given on the timing of surgery, given the fact that the moment in which the surgery is performed and the employed microsurgical reconstruction technique are the most important factors in the final result. Open lesions with associated nerve injury should be managed with an early exploration carried out before 7 days. Closed injuries are usually deferred, with few exceptions, from 3 to 6 months after the trauma.In turn, chronic compressions require an appropriate clinical, neurophysiological, and imaging diagnosis. Isolated sensory symptoms can be treated actively though without surgery: motor signs like atrophy should be regarded as a sign for immediate surgery, as a deferred treatment might cause an irreversible nerve and muscular damage. Endoscopic approaches are a valuable tool for treatment in selected neuropathies.Finally, nerve tumors demand a thorough preoperative evaluation, as benign tumors are treated in a very different way when compared to malignant lesions. Benign tumors can usually be safely and completely resected without sacrificing the nerve of origin. When malignancy is confirmed, extensive resection to optimize patient survival is the main objective, potentially at the expense of neurological function. This may then be followed by adjuvant radiation and/or chemotherapy, depending on the nature of the tumor and the completeness of resection attained. The role of nerve biopsy remains controversial, and several modern diagnostic techniques might be helpful.