User:Snake playing a saxaphone/magnetic resonance neurography

n/a

Mechanism
[problem with regular MRI is one of contrast - can't see nerves due to partial volume averaging effect & nearby signal from isotropic diffusion structures?] - SNR and CNR/conspicuity

MRN can be described as a form of tissue-selective imaging to identify and characterize nerve tissue. This includes the internal fascicular appearance, variations in signal intensity along the nerve's course, and the topology of branching. MRN takes advantage of nerve tissue's longer T2 relaxation time. By nullifying the signal of nearby tissue such as fat, blood vessels, and muscle, the nerve signal can be seen in isolation on T2W imaging.

MRN achieves tissue-selective imaging of nerves by nullifying the signal of nearby tissue such as fat and blood vessels  , and then relying on the nerve tissue's longer T2 relaxation time to visualize the nerve on T2-weighted (T2W) imaging.

Fat suppression
Homogenous fat suppression is a necessity for MRN. There are several ways to achieve this with varying tradeoffs, but the most commonly mentioned approaches are short-tau inversion recovery (STIR), spectral adiabatic inversion recovery (SPAIR) , and the Dixon method.

Examples of fat suppression include short-tau inversion recovery (STIR) and spectral adiabatic inversion recovery (SPAIR).

STIR is the most robust method for fat suppression but suffers from low SNR and pulsation artifacts.

SPAIR is an alternative to STIR and provides higher SNR.

The dixon method is also used for fat suppression with no penalty to SNR.

Vascular suppression
Two common vascular suppression techniques are 3D motion-sensitized driven equilibrium (MSDE) and 3D diffusion-weighted reversed fast imaging with steady state precession (3D DW PSIF).

Diffusion-weighted imaging
Water molecules normally move in random directions through brownian motion. However in biological tissues, the direction of diffusion may restricted by the shape and structure of cells. This concept of preferential water diffusion is called anisotropy. In nerve tissue, the diffusion of protons (Hydrogen atoms in water) is three times higher along the nerve due to the shape of the myelin sheath. This anisotropy in nerves is the basis for diffusion-weighted imaging, which can measure the level of anisotropy and the direction of diffusion to map out nerve tracts in nerve tissue.

Diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) are based on the preferential diffusion of water along nerve fibers and tracts. This concept is called anisotropy. This imaging technique is a form of functional imaging as it measures the movement of water. Normally, the diffusion of protons (hydrogen atoms in water) are 3 times higher along than across the nerve due to the shape of the myelin sheath.

DTI is considered a form of quantitative imaging as some of its outputs are pure numerical values, such as ADC and FA. There is no qualitative assessment of these values in the same way as radiologists interpret signal intensity and morphological characteristics as indicative of disease.

[need to discuss difference between DWI and DTI]

Visualization
[include pictures]

There are two main ways to image nerves: axial slicing and longitudinal slicing. With axial slicing, you can view the fascicular pattern, diameter, and shape of the nerve. With longitudinal imaging, you can better see variations in diameter and signal intensity along the course of the nerve than axial images show. Longitudinal views are also how surgeons typically see nerves when operating, and it can act as a surgical reference of the nerve or structures adjacent to the nerve for surgical planning.

MRN relies on several imaging features: nerve size/diameter/caliber, fascicular appearance, signal intensity, nerve course, effacement of perineural fat planes, and changes in regional muscle denervation.

[pretty sure DTI has visualization aspects to it]

2D imaging is obtained in the axial and the longitudinal direction.

2D imaging is considered the standard for MRN.

3D imaging is used to minimize the partial volume averaging effect and allows multi-planar reformation along the longitudinal axis of the nerves.

3D imaging can be reconstructed in any desired plane without loss of resolution.

[axial]

With an axial view, you can view the fascicular pattern, diameter, and shape of the nerve.

[longitudinal & MIP]

Longitudinal images are how surgeons typically see nerves when operating. It is valuable in surgical planning and can act as a surgical reference of the nerve or structures adjacent to the nerve.

Longitudinal imaging lends itself to identifying variations in diameter and signal intensity than axial images do.

MRN relies on several imaging features: nerve size/diameter/caliber, fascicular appearance, signal intensity, nerve course, effacement of perineural fat planes, and changes in regional muscle denervation.

Acquisition
Both sides of the body should be imaged for a side-by-side comparison of nerve shape/size and signal intensity, especially in the event of suspected mononeuropathy.

Axial T1W and fluid-sensitive fat-suppressed T2W images are the core aspects of MRN interpretation.

3T scanners are preferred over 1.5T due to the increased SNR and faster imaging.

[T1W - view structures around nerves]

T1W imaging is used to capture anatomic markers of surrounding structures or landmarks around the peripheral nerve. It can also be used to detect secondary abnormalities like fatty infiltration or scarring.

[T2W - view nerve signal itself]

T2-weighted (T2W) images are used to visualize the nerve fibres itself. Increased signal changes are thought to reflect edema and it correlates with clinical and electrodiagnostic evidence of nerve damage.

T2W signal with fat suppression is the most useful diagnostic imaging sign for nerve injury.

[DWI / DTI - functional imaging based on anisotropy]

[2D imaging]

2D imaging is considered the standard for MRN.

[3D imaging - very good for plexuses]

In 2D MR imaging, the smallest unit of resolution (the voxel) is like an elongated rectangle. If the plane isn't oriented exactly perpendicular to the axis of the nerve (which tend to be small), the voxel will can contain nerve tissue and non-nervous tissue. The signal of the non-nervous tissue will dominate the signal, decreasing the contrast to noise ratio. This effect is called the partial volume averaging effect. As nerves can course in irregular patterns, and perhaps the radiologist may want to image many nerves which occupy the same volume, 3D imaging can reduce the partial volume averaging effect when the course of the nerve is hard to capture with 2D slicing or the course of the nerve is hard to predict. The voxels in 3D imaging are cubes, which if small enough may better capture a pure nerve signal. Then the radiologist can post-process the data into arbitrary planes without loss of resolution. One downside of 3D imaging is that the in-plane resolution isn't as good as 2D imaging, so 2D imaging is still used.

[coils]

Clinical uses
The most common imaged conditions can be classified as entrapment or injury.

MRN can add clinically useful diagnostic information when existing diagnostics like electrophysiological testing, clinical exam, and conventional imaging are inconclusive.

The current gold standard in peripheral neuropathy diagnostics is a detailed medical history followed by a clinical examination and potentially followed by electrodiagnostic testing (e.g. NCV, DML, SNAP, CMAP, MVC).

MRN has unique capabilities such as assessing the continuity of a nerve, localizing a lesion, identifying the lesion extension, identifying the spatial distribution of lesions, and identifying secondary muscle denervation.

EMG cannot localize the nerve injury, assess the cause of nerve injury (e.g. fibrosis, mass lesion, neuroma), or be used in deeply situated nerves. There is also no way to distinguish between a proximal entrapment of several fascicles and distal entrapment of the entire nerve innervated by those proximal fascicles. However such information is accessible via MRN, permitting appropriate spatial resolution of the nerve fascicles in question. Such information would almost certainly alter the course of diagnosis and treatment planning.

EMG isn't useful for detecting sensory damage.

EMG gives non-specific results in up to 1/3 of cases.

Failure to address nerve injury early can lead to long-term deficits through treatment delay.

EMG isn't useful for diagnosing nerve pathologies like lumbosacral plexus entrapment due to the deep location and variable innervation of the regional muscles.

Peripheral nerve injury
[todo]

There's a 5% incidence of peripheral nerve injuries to a level I trauma center.

MRN can help detect and grade nerve injuries.

MRN can assess nerve continuity immediately after injury.



Delay in diagnosis can lead to progressive muscle enervation, which can lead to poor functional recovery.

Nerve entrapment diagnosis
[MRN provides additional info that MRI & EMG/NCV cannot]

Standard MRI is used to assess for mass lesions that might alter the course or caliber of nerves.

[you can look at morphological characteristics & enhanced signal]

[MRN influences diagnosis]

In nerve entrapment, the abnormality is greatest just proximal to the entrapment site.

Nerve hyperintensity is maximum just adjacent to the site of entrapment which aids in lesion localization.

Atypical cases of nerve entrapment are difficult to diagnose and can easily lead to surgery on the wrong area without accurate localization.

Chronic low back pain and pelvic pain






Failed back surgery syndrome
MR lumbar spine is considered the gold standard for imaging in failed back surgery syndrome, but it can't grade nerve compression.

[mention findings from https://pubmed.ncbi.nlm.nih.gov/29846365/]

Image-guided injections
[MRN has the best soft tissue resolution]

MRN can be used for image-guided injections.

Image-guided nerve blocks are the most robust way to localize a nerve.

Presurgical planning
[MRN influences treatment decisions]

MRN can avoid the need for exploratory surgery to locate a lesion and reduce the area that needs to be explored.

One study found MRN significantly influenced the diagnosis and therapeutic recommendations of peripheral nerve surgeons.

There are approximately 100,000 peripheral nerve surgeries performed per year.

MRN can distinguish from intraneural and perineural masses which each require separate surgical techniques.

MRN can provide anatomical location related to nerves for surgery.

Postsurgical follow up
[Can show healing and potential reentrapment]

Current limitations
[nerves below 2mm hard to visualize]

Most peripheral nerves in the human body are less than 1cm and many are less than 1mm, making image resolution an important component of MRN.

Nerves that are greater than 2-3mm in diameter are easily distinguished from their surrounding vessels.

Fascicles can be consistently depicted for nerves greater than 3mm diameter.

[partial volume averaging effect given the very tiny size of nerves]

2D imaging is susceptible to the partial volume averaging effect. The partial volume averaging effect is when the voxels (3 dimensional pixels which are visualized in the planar imaging) encompass multiple tissue types and the signal is averaged across them. The irregular shape of a voxel in 2D imaging is like a long rectangle, and since the nerve is like a long tube, it's improbable that the voxels and target nerve will be perfectly aligned.

[sensitivity - true negative - needs to be increased]

[qualitative except for DTI]

Studies about the utility of MRN have largely been from retrospective studies (the study is done after the experiment) which tend to be more biased towards the positive side. Prospective studies are necessary to evaluate how useful MRN really is.

Future research directions
[UHF MRI like 7T or 9T for better resolution]

Limiting factors to MRN include signal to noise ratio (SNR) and contrast to noise ratio (CNR). While SNR is not a function of magnetic field strength, the magnetic field is one of the core determinants to the resolution of a scan. A higher magnetic field can be used in several ways, such as higher resolution of in-plane slicing, thinner slicing, or shorter acquisition times.

[Phased array surface coils for enhanced signal]

Multi-channel phased array coil improvements are necessary for increasing SNR.

[Parallel imaging which allows faster scan times or more k-space sampling]

Parallel imaging with an acceleration factor of 2-3 is frequently used for MRN.

[Contrast enhancement]

Contrast seems to have limited ability to help better visualize the nerve unless the blood-nerve barrier is compromised.

Contrast is currently not used in MRN due to the blood-nerve barrier preventing infiltration of the nerve tissue.

[DTI]

History
Due to limitations of resolution and conspicuity, CT, ultrasound, and MRI have largely been useful only in the detection of mass lesions around nerves due to the nerves themselves. The x-ray absorptive properties of nerves do not allow them to be distinguished from surrounding issue, making x-rays, fluoroscopy, and CT unable to directly visualize the nerve. In the past few decades, MRI pulse sequences and techniques have emerged which allow directly visualizing nerve injury.

[it wasn't until 3T that MRN started being taken seriously]

In its earlier stages, the main limiting aspects of MRN were signal to noise ratio (SNR) and resolution. The use of 3T scanners and advances in coil technology, and advances in parallel imaging have made MRN more clinically feasible.

MRN was generally not accepted as a distinct technique until the use of 3T MRIs. [mention when 3T scanners came into clinical use]

The use of 3T scanners has allowed for 3D isotropic imaging and functional imaging (DWI/DTI).

[filler paper 1992]

[filler paper 2005 or so where he uses image guided injections]

Society and culture
[insurance company denials]

Most of the initial studies on MRN were retrospective, but prospective studies are necessary to unambiguously demonstrate the clinical utility of MRN.

Insurance companies often classify MRN as "experimental and investigational".