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MR neurography of the brachial plexus

Nerves run throughout the body and are responsible for our sense of touch. When nerves are injured or seem to be working incorrectly, physicians may want to look into the possible cause through medical imaging exams. However, nerves can be difficult to image, because they often run alongside blood vessels and can be difficult to identify from surrounding soft tissues. Within the field of Magnetic Resonance Imaging (MRI) is a specialty called Neurography (MRN), also known as peripheral nerve imaging, for imaging of nerves.1 This can be done by using certain techniques that will suppress the surrounding tissue and vessels. MRN is usually conducted within the musculoskeletal and neurological radiology practices. Ultimately, MRN can help surgeons identify the precise location of the associated nerve damage and avoid unnecessary incisions to the patient, allowing for a quicker recovery.

Advanced techniques used in MRN

Magnetic resonance neurography may be influenced by recent advances in magnetic resonance imaging techniques. Often times, the advanced techniques that are used in other MR studies, such as diffusion imaging, 3-D fast spin echo, and the Dixon method, may be used in MRN. These techniques could provide additional information about the nerves in the region of interest.

Diffusion-weighted imaging (DWI) relies on the Brownian (random) motion of water molecules within the body. This provides qualitative and quantitative information about water diffusivity within nerves.2 Accelerated diffusion imaging combined with post-processing may provide a similar signal-to-noise ratio to conventional exams.

An alternative or supplement to conventional MRI, 3-D imaging, combined with fast spin echo sequences could provide high-quality MRN.3 This method may also provide better spatial localisation of nerve lesions along nerve segments than conventional MRI.

The Dixon method is a technique that uses different pulse sequences to create multiple images with fat-water separation. The Dixon method produces multiple images, including fat separated and water separated images. The goal of using this technique within MRN is to highlight the nerves in the area being imaged.

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MRN of the brachial plexus

Motor vehicle accidents may lead to injury in a number of areas. One of the more common areas for nerve injury resulting from MVAs is the brachial plexus. This may be especially true for motorcycle accidents. The brachial plexus is the network of nerves from the spinal cord to the shoulder, and it is densely populated. Injuries to these nerves can cause problems that affect different areas, including the arm.

Additionally, respiratory motion can cause artifacts. Technologists conducting an MRN scan may find that prospective respiratory gating, a technique using bellows.1 The bellows get wrapped around the patient's torso to detect the end of the respiratory cycle.

Another challenge to obtaining adequate images of the brachial plexus stems from the coils used. Unfortunately, the brachial plexus may not be well covered with traditional surface coils.Traditional MR coils are rigid and designed for the average person. Because patients do not all have the same shape and size, these coils can cause issues with signal intensity. Typically, a higher signal-to-noise ratio could be correlated with better images. The proximity of the coil to the patient's region of interest may lead to a higher signal.

With newer, more flexible and durable coils, the coil may be able to be placed closer to the patient's body. These coils are sometimes blanket-like, which helps the technologist wrap the coil around the region of interest. Using positioners, the coil can stay that close to the patient throughout the exam.

Recent advancements in MR technology and techniques may influence MRN in the future.1 This is because innovations in other fields, such as cardiac MR, may transfer to additional information for physicians using magnetic resonance neurography.

MRN could help physicians determine the underlying cause of neuropathy, whether it is due to injury or some other abnormality. Advanced techniques, such as DWI, 3-D imaging, and Dixon sequences, may help enhance the acquired images to highlight the nerves further. This is particularly useful since nerves and vessels often run together. Physicians may find that combining conventional MRI with these methods could influence imaging of the brachial plexus in the future.

References:

Technical Advancements in Magnetic Resonance Neurography. Current Neurology and Neuroscience Reports. https://doi.org/10.1007/s11910-019-0996-x. Last accessed September 17, 2019.

Getting Quantitative Diffusion-Weighted MR Neurography and Tractography Ready for Clinical Practice. Journal of Magnetic Resonance Imaging. doi: 10.1002/jmri.26930. Last accessed September 17, 2019.

Brachial plexus assessment with three-dimensional isotropic resolution fast spin echo MRI: comparison with conventional MRI at 3.0 T. Br J Radiol. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3474100/. Last accessed September 18, 2019.