Magic angle effect: a relevant artifact in MR neurography at 3T?

Quantitative double echo steady state T2 mapping of upper extremity peripheral nerves and muscles

Introduction

T2 mapping can characterize peripheral neuropathy and muscle denervation due to axonal damage. Three-dimensional double echo steady-state (DESS) can simultaneously provide 3D qualitative information and T2 maps with equivalent spatial resolution. However, insufficient signal-to-noise ratio may bias DESS-T2 values. Deep learning reconstruction (DLR) techniques can reduce noise, and hence may improve quantitation of high-resolution DESS-T2. This study aims to (i) evaluate the effect of DLR methods on DESS-T2 values, and (ii) to evaluate the feasibility of using DESS-T2 maps to differentiate abnormal from normal nerves and muscles in the upper extremities, with abnormality as determined by electromyography.

Methods and results

Analysis of images from 25 subjects found that DLR decreased DESS-T2 values in abnormal muscles (DLR = 37.71 ± 9.11 msec, standard reconstruction = 38.56 ± 9.44 msec, p = 0.005) and normal muscles (DLR: 27.18 ± 6.34 msec, standard reconstruction: 27.58 ± 6.34 msec, p < 0.001) consistent with a noise reduction bias. Mean DESS-T2, both with and without DLR, was higher in abnormal nerves (abnormal = 75.99 ± 38.21 msec, normal = 35.10 ± 9.78 msec, p < 0.001) and muscles (abnormal = 37.71 ± 9.11 msec, normal = 27.18 ± 6.34 msec, p < 0.001). A higher DESS-T2 in muscle was associated with electromyography motor unit recruitment (p < 0.001).

Discussion

These results suggest that quantitative DESS-T2 is improved by DLR and can differentiate the nerves and muscles involved in peripheral neuropathies from those uninvolved.

Magnetic Resonance Neurography of the Foot and Ankle

Peripheral neuropathies of the foot and ankle can be challenging to diagnose clinically due to concomitant traumatic and nontraumatic or degenerative orthopedic conditions. Although clinical history, physical examination, and electrodiagnostic testing comprised of nerve conduction velocities and electromyography are used primarily for the identification and classification of peripheral nerve disorders, MR neurography (MRN) can be used to visualize the peripheral nerves as well as the skeletal muscles of the foot and ankle for primary neurogenic pathology and skeletal muscle denervation effect. Proper knowledge of the anatomy and pathophysiology of peripheral nerves is important for an MRN interpretation.

Orientation dependent proton transverse relaxation in the human brain white matter: The magic angle effect on a cylindrical helix

PURPOSE

To overcome some limitations of previous proton orientation-dependent transverse relaxation formalisms in human brain white matter (WM) by a generalized magic angle effect function.

METHODS

A cylindrical helix model was developed embracing anisotropic rotational and translational diffusion of restricted molecules in WM, with the former characterized by an axially symmetric system. Transverse relaxation rates R₂ and R₂^∗ were divided into isotropic R₂ⁱ and anisotropic parts, R₂^a ∗ f(α,Φ - ε₀), with α denoting an open angle and ε₀ an orientation (Φ) offset from DTI-derived primary diffusivity direction. The proposed framework (Fit A) was compared to prior models without ε₀ on previously published water and methylene proton transverse relaxation rates from developing, healthy, and pathological WM at 3 T. Goodness of fit was represented by root-mean-square error (RMSE). F-test and linear correlation were used with statistical significance set to P ≤ 0.05.

RESULTS

Fit A significantly (P < 0.01) outperformed prior models as demonstrated by reduced RMSEs, e.g., 0.349 vs. 0.724 in myelin water. Fitted ε₀ was in good agreement with calculated ε₀ from directional diffusivities. Compared with those from healthy adult, the fitted R₂ⁱ, R₂^a, and α from neonates were substantially reduced but ε₀ increased, consistent with developing myelination. Significant positive (R₂ⁱ) and negative (α and R₂^a) correlations were found with aging (demyelination) in elderly.

CONCLUSION

The developed framework can better characterize orientation dependences from a wide range of proton transverse relaxation measurements in the human brain WM, thus shedding new light on myelin microstructural alterations at the molecular level.

Phase-shifted transverse relaxation orientation dependences in human brain white matter

This work aimed to demonstrate an essential phase shift ε 0 $$ {\varepsilon}_0 $$ for better quantifying R 2 $$ {R}_2 $$ and R 2 * $$ {R}_2^{\ast } $$ in human brain white matter (WM), and to further elucidate its origin related to the directional diffusivities from standard diffusion tensor imaging (DTI). ε 0 $$ {\varepsilon}_0 $$ was integrated into a proposed generalized transverse relaxation model for characterizing previously published R 2 $$ {R}_2 $$ and R 2 * $$ {R}_2^{\ast } $$ orientation dependence profiles in brain WM, and then comparisons were made with those without ε 0 $$ {\varepsilon}_0 $$ . It was theorized that anisotropic diffusivity direction ε $$ \varepsilon $$ was collinear with an axon fiber subject to all eigenvalues and eigenvectors from an apparent diffusion tensor. To corroborate the origin of ε 0 $$ {\varepsilon}_0 $$ , R 2 $$ {R}_2 $$ orientation dependences referenced by ε $$ \varepsilon $$ were compared with those referenced by the standard principal diffusivity direction Φ $$ \Phi $$ at b-values of 1000 and 2500 (s/mm² ). These R 2 $$ {R}_2 $$ orientation dependences were obtained from T 2 $$ {T}_2 $$ -weighted images (b = 0) of ultrahigh-resolution Connectome DTI datasets in the public domain. A normalized root-mean-square error ( NRMSE % $$ NRMSE\% $$ ) and an F $$ F $$ -test were used for evaluating curve-fittings, and statistical significance was considered to be a p of 0.05 or less. A phase-shifted model resulted in significantly reduced NRMSE % $$ NRMSE\% $$ compared with that without ε 0 $$ {\varepsilon}_0 $$ in quantifying various R 2 $$ {R}_2 $$ and R 2 * $$ {R}_2^{\ast } $$ profiles, both in vivo and ex vivo at multiple B 0 $$ {B}_0 $$ fields. The R 2 $$ {R}_2 $$ profiles based on Φ $$ \Phi $$ manifested a right-shifted phase ( ε 0 > 0 $$ {\varepsilon}_0>0 $$ ) at two b-values, while those based on ε $$ \varepsilon $$ became free from ε 0 $$ {\varepsilon}_0 $$ . For all phase-shifted R 2 $$ {R}_2 $$ and R 2 * $$ {R}_2^{\ast } $$ profiles, ε 0 $$ {\varepsilon}_0 $$ generally depended on the directional diffusivities by tan - 1 D ⊥ / D ∥ $$ {\tan}^{-1}\left({D}_{\perp }/{D}_{\parallel}\right) $$ , as predicted. In summary, a ubiquitous phase shift ε 0 $$ {\varepsilon}_0 $$ has been demonstrated as a prerequisite for better quantifying transverse relaxation orientation dependences in human brain WM. Furthermore, the origin of ε 0 $$ {\varepsilon}_0 $$ associated with the directional diffusivities from DTI has been elucidated. These findings could have a significant impact on interpretations of prior R 2 $$ {R}_2 $$ and R 2 * $$ {R}_2^{\ast } $$ datasets and on future research.

Technical Update on MR Neurography

Imaging evaluation of peripheral nerves (PNs) is challenging. Magnetic resonance imaging (MRI) and ultrasonography are the modalities of choice in the imaging assessment of PNs. Both conventional MRI pulse sequences and advanced techniques have important roles. Routine MR sequences are the workhorse, with the main goal to provide superb anatomical definition and identify focal or diffuse nerve T2 signal abnormalities. Selective techniques, such as three-dimensional (3D) cranial nerve imaging (CRANI) or 3D NerveVIEW, allow for a more detailed evaluation of normal and pathologic states. These conventional pulse sequences have a limited role in the comprehensive assessment of pathophysiologic and ultrastructural abnormalities of PNs. Advanced functional MR neurography sequences, such as diffusion tensor imaging tractography or T2 mapping, provide useful and robust quantitative parameters that can be useful in the assessment of PNs on a microscopic level. This article offers an overview of various technical parameters, pulse sequences, and protocols available in the imaging of PNs and provides tips on avoiding potential pitfalls.

Nerve Imaging in the Wrist

Neuropathic symptoms involving the wrist are a common clinical presentation that can be due to a variety of causes. Imaging plays a key role in differentiating distal nerve lesions in the wrist from more proximal nerve abnormalities such as a cervical radiculopathy or brachial plexopathy. Imaging complements electrodiagnostic testing by helping define the specific lesion site and by providing anatomical information to guide surgical planning. This article reviews nerve anatomy, normal and abnormal findings on ultrasonography and magnetic resonance imaging, and common and uncommon causes of neuropathy.

Intravenous contrast does not improve detection of nerve lesions or active muscle denervation changes in MR neurography of the common peroneal nerve

OBJECTIVE

To evaluate the effect of intravenous (IV) contrast on sensitivity, specificity, and accuracy of magnetic resonance (MR) neurography of the knee with attention to the common peroneal nerve (CPN) in identifying nerve lesions and active muscle denervation changes.

MATERIALS AND METHODS

A retrospective search for contrast-enhanced MR neurography cases evaluating the CPN at the knee was performed. Patients with electrodiagnostic testing (EDX) within 3 months of imaging were included and those with relevant prior surgery were excluded. Two radiologists independently reviewed non-contrast sequences and then 4 weeks later evaluated non-contrast and contrast sequences. McNemar's tests were performed to detect a difference between non-contrast only and combined non-contrast and contrast sequences in identifying nerve lesions and active muscle denervation changes using EDX as the reference standard.

RESULTS

Forty-four exams in 42 patients (2 bilateral) were included. Twenty-eight cases had common peroneal neuropathy and 29, 21, and 9 cases had active denervation changes in the anterior, lateral, and posterior compartment/proximal muscles respectively on EDX. Sensitivity, specificity, and accuracy of non-contrast versus combined non-contrast and contrast sequences for common peroneal neuropathy were 50.0%, 56.2%, and 52.3% versus 50.0%, 56.2%, and 52.3% for reader 1 and 57.1%, 50.0%, and 54.5% versus 64.3%, 56.2%, and 61.4% for reader 2. Sensitivity, specificity, and accuracy of non-contrast and combined non-contrast and contrast sequences in identifying active denervation changes for anterior, lateral, and posterior compartment muscles were not significantly different. McNemar's tests were all negative.

CONCLUSION

IV contrast does not improve the ability of MR neurography to detect CPN lesions or active muscle denervation changes.

Magnetic Resonance Neurography: Improved Diagnosis of Peripheral Neuropathies

Peripheral neuropathies account for the most frequent disorders seen by neurologists, and causes are manifold. The traditional diagnostic gold-standard consists of clinical neurologic examinations supplemented by nerve conduction studies. Due to well-known limitations of standard diagnostics and atypical clinical presentations, establishing the correct diagnosis can be challenging but is critical for appropriate therapies. Magnetic resonance neurography (MRN) is a relatively novel technique that was developed for the high-resolution imaging of the peripheral nervous system. In focal neuropathies, whether traumatic or due to nerve entrapment, MRN has improved the diagnostic accuracy by directly visualizing underlying nerve lesions and providing information on the exact lesion localization, extension, and spatial distribution, thereby assisting surgical planning. Notably, the differentiation between distally located, complete cross-sectional nerve lesions, and more proximally located lesions involving only certain fascicles within a nerve can hold difficulties that MRN can overcome, when basic technical requirements to achieve sufficient spatial resolution are implemented. Typical MRN-specific pitfalls are essential to understand in order to prevent overdiagnosing neuropathies. Heavily T2-weighted sequences with fat saturation are the most established sequences for MRN. Newer techniques, such as T2-relaxometry, magnetization transfer contrast imaging, and diffusion tensor imaging, allow the quantification of nerve lesions and have become increasingly important, especially when evaluating diffuse, non-focal neuropathies. Innovative studies in hereditary, metabolic or inflammatory polyneuropathies, and motor neuron diseases have contributed to a better understanding of the underlying pathomechanism. New imaging biomarkers might be used for an earlier diagnosis and monitoring of structural nerve injury under causative treatments in the future.

[Peripheral nerve reconstruction - diagnostics as a basis for decision-making: report of the Consensus Workshop at the 35th Meeting of the DAM]

In the early stage of nerve lesions, the clinical differentiation between neurapraxia, axonotmesis and neurotmesis often presents a big challenge. Especially in the early stage, however, it is crucial to correctly classify the type of damage because this is what essentially determines the therapeutic concept, in particular the surgical approach and, therefore, the prognosis. A precise diagnosis not only requires detailed clinical assessment and medical history taking, but also the use of additional electrophysiological (functional) and/or imaging examinations. Electrophysiological diagnostic tests may provide information ion localization, severity, course, type of damage and incipient or past reinnervation. Preoperative functional diagnostic measures should include neurography, needle electromyography (EMG) and, if needed, evoked potentials (EP), while imaging procedures should include neural sonography and magnetic resonance imaging (MRI). As a complimentary procedure, EMG may also be performed during surgery.

[Imaging of the hand : What should be considered regarding the nerves?]

BACKGROUND

Peripheral nerve disorders of the hand and wrist are most commonly caused by entrapment neuropathies, while traumatic nerve injuries and neoplasms are less common.

OBJECTIVES

The indication for additional imaging methods and different imaging options, especially in patients with atypical symptoms or remaining unclear etiology of symptoms after completion of standard diagnostics, are presented.

MATERIALS AND METHODS

The imaging methods magnetic resonance (MR) neurography and neurosonography are introduced, and typical findings as well as diagnostic pitfalls are presented.

RESULTS

The diagnostic gold standard, which comprises a past medical history, neurologic examination and electrophysiology, can often establish the diagnosis. Imaging methods, especially MR neurography and neurosonography, are gaining increasing importance in the diagnostic workup of atypical neuropathies, as well as in the determination of the exact lesion location and spatial lesion extension, especially for surgical planning. Recent technical advances allow high-resolution depiction of small distal terminal nerve branches.

CONCLUSIONS

MR neurography allows for the high-resolution depiction of peripheral nerves of the hand and wrist. It can confirm the diagnosis of neuropathy, identify the exact lesion location, and rule out any differential diagnoses. Neurosonography is a time- and cost-efficient alternative diagnostic method.

Peripheral nervous system alterations in infant and adult neurofibromatosis type 2

OBJECTIVE

To examine the involvement of dorsal root ganglia and peripheral nerves in children with neurofibromatosis type 2 compared to healthy controls and symptomatic adults by in vivo high-resolution magnetic resonance neurography.

METHODS

In this prospective multicenter study, the lumbosacral dorsal root ganglia and sciatic, tibial, and peroneal nerves were examined in 9 polyneuropathy-negative children diagnosed with neurofibromatosis type 2 by a standardized magnetic resonance neurography protocol at 3T. Volumes of dorsal root ganglia L3 to S2 and peripheral nerve lesions were assessed and compared to those of 29 healthy children. Moreover, dorsal root ganglia volumes and peripheral nerve lesions were compared to those of 14 adults with neurofibromatosis type 2.

RESULTS

Compared to healthy controls, dorsal root ganglia hypertrophy was a consistent finding in children with neurofibromatosis type 2 (L3 +255%, L4 +289%, L5 +250%, S1 +257%, and S2 +218%, p < 0.001) with an excellent diagnostic accuracy. Moreover, peripheral nerve lesions occurred with a high frequency in those children compared to healthy controls (18.89 ± 11.11 vs 0.90 ± 1.08, p < 0.001). Children and adults with neurofibromatosis type 2 showed nonsignificant differences in relative dorsal root ganglia hypertrophy rates (p = 0.85) and peripheral nerve lesions (p = 0.28).

CONCLUSIONS

Alterations of peripheral nerve segments occur early in the course of neurofibromatosis type 2 and are evident even in children not clinically affected by peripheral polyneuropathy. While those early alterations show similar characteristics compared to adults with neurofibromatosis type 2, the findings of this study suggest that secondary processes might be responsible for the development and severity of associated polyneuropathy.

Association of Serum Cholesterol Levels With Peripheral Nerve Damage in Patients With Type 2 Diabetes

IMPORTANCE

Lowering serum cholesterol levels is a well-established treatment for dyslipidemia in patients with type 2 diabetes (T2D). However, nerve lesions in patients with T2D increase with lower serum cholesterol levels, suggesting that lowering serum cholesterol levels is associated with diabetic polyneuropathy (DPN) in patients with T2D.

OBJECTIVE

To investigate whether there is an association between serum cholesterol levels and peripheral nerve lesions in patients with T2D with and without DPN.

DESIGN, SETTING, AND PARTICIPANTS

This single-center, cross-sectional, prospective cohort study was performed from June 1, 2015, to March 31, 2018. Observers were blinded to clinical data. A total of 256 participants were approached, of whom 156 were excluded. A total of 100 participants consented to undergo magnetic resonance neurography of the right leg at the Department of Neuroradiology and clinical, serologic, and electrophysiologic assessment at the Department of Endocrinology, Heidelberg University Hospital, Heidelberg, Germany.

EXPOSURES

Quantification of the nerve's diameter and lipid equivalent lesion (LEL) load with a subsequent analysis of all acquired clinical and serologic data with use of 3.0-T magnetic resonance neurography of the right leg with 3-dimensional reconstruction of the sciatic nerve.

MAIN OUTCOMES AND MEASURES

The primary outcome was lesion load and extension. Secondary outcomes were clinical, serologic, and electrophysiologic findings.

RESULTS

A total of 100 participants with T2D (mean [SD] age, 64.6 [0.9] years; 68 [68.0%] male) participated in the study. The LEL load correlated positively with the nerve's mean cross-sectional area (r = 0.44; P < .001) and the maximum length of a lesion (r = 0.71; P < .001). The LEL load was negatively associated with total serum cholesterol level (r = -0.41; P < .001), high-density lipoprotein cholesterol level (r = -0.30; P = .006), low-density lipoprotein cholesterol level (r = -0.33; P = .003), nerve conduction velocities of the tibial (r = -0.33; P = .01) and peroneal (r = -0.51; P < .001) nerves, and nerve conduction amplitudes of the tibial (r = -0.31; P = .02) and peroneal (r = -0.28; P = .03) nerves.

CONCLUSIONS AND RELEVANCE

The findings suggest that lowering serum cholesterol levels in patients with T2D and DPN is associated with a higher amount of nerve lesions and declining nerve conduction velocities and amplitudes. These findings may be relevant to emerging therapies that promote an aggressive lowering of serum cholesterol levels in patients with T2D.

T2 mapping of lumbosacral nerves in patients suffering from unilateral radicular pain due to degenerative disc disease

OBJECTIVE

Lumbosacral radicular syndrome (LRS) is a very common condition, often requiring diagnostic imaging with the aim of elucidating a structural cause when symptoms are longer lasting. However, findings on conventional anatomical MRI do not necessarily correlate with clinical symptoms, and it is primarily performed for the qualitative evaluation of surrounding compressive structures, such as herniated discs, instead of to evaluate the nerves directly. The present study investigated the performance of quantitative imaging by using magnetic resonance neurography (MRN) in patients with LRS.

METHODS

Eighteen patients (55.6% males, mean age 64.4 ± 10.2 years), with strict unilateral LRS matching at least one dermatome and suspected disc herniation, underwent high-resolution 3-T MRN using T2 mapping. On T2 maps, the presumably affected and contralateral unaffected nerves were identified; subsequent regions of interest (ROIs) were placed at preganglionic, ganglionic, and postganglionic sites; and T2 values were extracted. Patients then underwent an epidural steroid injection (ESI) with local anesthetic agents at the site of suspected nerve affection. T2 values of the affected nerves were compared against the contralateral nerves. Furthermore, receiver operating characteristics were calculated based on the measured T2 values and the responsiveness to ESI.

RESULTS

The mean T2 value was 77.3 ± 1.9 msec for affected nerves and 74.8 ± 1.4 msec for contralateral nerves (p < 0.0001). In relation to ESI performed at the site of suspected nerve affection, MRN with T2 mapping had a sensitivity/specificity of 76.9%/60.0% and a positive/negative predictive value of 83.3%/50.0%. Signal alterations in affected nerves according to qualitative visual inspection were present in only 22.2% of patients.

CONCLUSIONS

As one of the first of its kind, this study revealed elevated T2 values in patients suffering from LRS. T2 values of lumbosacral nerves might be used as more objective parameters to directly detect nerve affection in such patients.

Diffusion tensor imaging in anterior interosseous nerve syndrome - functional MR Neurography on a fascicular level

Purpose

By applying diffusor tensor imaging (DTI) in patients with anterior interosseous nerve syndrome (AINS), this proof of principle study aims to quantify the extent of structural damage of a peripheral nerve at the anatomical level of individual fascicles.

Methods

In this institutional review board approved prospective study 13 patients with spontaneous AINS were examined at 3 Tesla including a transversal T2-weighted turbo-spin-echo and a spin-echo echo-planar-imaging pulse sequence of the upper arm level. Calculations of quantitative DTI parameters including fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) for median nerve lesion and non-lesion fascicles as well as ulnar and radial nerve were obtained. DTI values were compared to each other and to a previously published dataset of 58 healthy controls using one-way Analysis of Variance with Bonferroni correction and p-values <.05 were considered significant. Receiver operating characteristic (ROC) curves were performed to assess diagnostic accuracy.

Results

FA of median nerve lesion fascicles was decreased compared to median nerve non-lesion fascicles, ulnar nerve and radial nerve while MD, RD, and AD was increased (p < .001 for all parameters). Compared to median nerve values of healthy controls, lesion fascicles showed a significant decrease in FA while MD, RD, and AD was increased (p < .001 for all parameters). FA of median nerve non-lesion fascicles showed a weak significant decrease compared to healthy controls (p < .01) while there was no difference in MD, RD, and AD. ROC analyses revealed an excellent diagnostic accuracy of FA, MD and RD in the discrimination of median nerve lesion and non-lesion fascicles in AINS patients as well as in the discrimination of lesion fascicles and normative median nerve values of healthy controls.

Conclusion

By applying this functional MR Neurography technique in patients with AINS, this proof of principle study demonstrates that diffusion tensor imaging is feasible to quantify structural nerve injury at the anatomical level of individual fascicles.

•

DTI is capable to quantify structural nerve injury on a fascicular level.

•

Lesion- and non-lesion fascicles can be discriminated at high diagnostic accuracy.

•

FA seems to be to most sensitive parameter in quantitative DTI.

Diffusion Tensor Imaging of Tibial and Common Peroneal Nerves in Patients With Guillain-Barre Syndrome: A Feasibility Study

BACKGROUND

The development of a noninvasive, objective, and accurate method to assess peripheral nerve disorders in Guillain-Barre syndrome (GBS) is of clinical significance. Diffusion tensor imaging (DTI) has been used to evaluate some peripheral nerve disorders.

PURPOSE

To investigate the feasibility of DTI in evaluating the peripheral nerve disorders in patients with GBS.

STUDY TYPE

Case control.

SUBJECTS

Twenty GBS patients and 16 healthy volunteers.

FIELD STRENGTH/SEQUENCE

3.0T, T₁ WI-SE, T₂ WI-SPAIR, DTI; electrophysiology.

ASSESSMENT

MRI data were analyzed by two radiologists blindly and independently. Fractional anisotropy (FA), apparent diffusion coefficient (ADC), axial diffusion coefficient (AD), and radial diffusion coefficient (RD) values of tibial nerve (TN) and common peroneal nerve (CPN) were recorded. Motor nerve conduction velocity (MCV) and motor nerve conduction amplitude of TN and CPN were recorded.

STATISTICAL TESTS

Intraclass correlation coefficient (ICC), t-test, receiver-operating characteristic (ROC), and area under the curve (AUC) analysis, Pearson correlation coefficient.

RESULTS

The FA and AD values of TN and CPN in the GBS group were significantly lower and the ADC and RD values were higher than those in the controls (P <0.05). The AUC of the FA values (0.970 for TN and 0.927 for CPN) were higher than that of the ADC, AD, and RD values. FA and AD values were positively correlated and ADC, RD values were negatively correlated with MCV and motor nerve conduction amplitude, respectively (P <0.05). The correlations between FA value and electrophysiology parameters were the highest.

DATA CONCLUSION

DTI quantitative parameters could evaluate the disorders of peripheral nerves in patients with GBS. A moderate correlation was observed between DTI and electrophysiology parameters.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1356-1364.

Demystifying MR Neurography of the Lumbosacral Plexus: From Protocols to Pathologies

Magnetic resonance neurography is a high-resolution imaging technique that allows evaluating different neurological pathologies in correlation to clinical and the electrophysiological data. The aim of this article is to present a review on the anatomy of the lumbosacral plexus nerves, along with imaging protocols, interpretation pitfalls, and most common pathologies that should be recognized by the radiologist: traumatic, iatrogenic, entrapment, tumoral, infectious, and inflammatory conditions. An extensive series of clinical and imaging cases is presented to illustrate key-points throughout the article.

High Isotropic Resolution T2 Mapping of the Lumbosacral Plexus with T2-Prepared 3D Turbo Spin Echo

PURPOSE

Isotropic high-resolution three-dimensional (3D) magnetic resonance neurography (MRN) is increasingly used to depict even small and highly oblique nerves of the lumbosacral plexus (LSP). The present study introduces a T2 mapping sequence (T2-prepared 3D turbo spin echo) that is B1-insensitive and enables quantitative assessment of LSP nerves.

METHODS

In this study 15 healthy subjects (mean age 28.5 ± 3.8 years) underwent 3 T MRN of the LSP area three times. The T2 values were calculated offline on a voxel-by-voxel basis and measured at three segments (preganglionic, ganglionic, postganglionic) of three LSP nerves (S1, L5, L4) by two independent investigators (experienced and novice). Normative data for the different nerves were extracted and intraclass correlation coefficients (ICCs) were calculated to assess reproducibility and interobserver reliability of T2 measurements.

RESULTS

The T2 mapping showed excellent reproducibility with ICCs ranging between 0.99 (S1 preganglionic) and 0.89 (L5 postganglionic). Interobserver reliability was less robust with ICCs ranging between 0.78 (S1 preganglionic) and 0.44 (L5 postganglionic) for S1 and L5. A mean T2 value of 74.6 ± 4.7 ms was registered for preganglionic segments, 84.7 ± 4.1 ms for ganglionic and 65.4 ± 2.5 ms for postganglionic segments, respectively. There was a statistically significant variation of T2 values across the nerve (preganglionic vs ganglionic vs postganglionic) for S1, L5, and L4.

CONCLUSION

Our approach enables isotropic high-resolution and B1-insensitive T2 mapping of LSP nerves with excellent reproducibility. It might reflect a robust and clinically useful method for future diagnostics of LSP pathologies.

Sciatic neurosteatosis: Relationship with age, gender, obesity and height

AIM

To evaluate inter-reader performance for cross-sectional area and fat quantification of bilateral sciatic nerves on MRI and assess correlations with anthropometrics.

METHODS

In this IRB-approved, HIPPA-compliant study, three readers performed a cross-sectional analysis of 3T lumbosacral plexus MRIs over an 18-month period. Image slices were evaluated at two levels (A and B). The sciatic nerve was outlined using a free hand region of interest tool on PACS. Proton-density fat fraction (FF) and cross-sectional areas were recorded. Inter-reader agreement was assessed using intra-class correlation coefficient (ICC). Spearman correlation coefficients were used for correlations with age, BMI and height and Wilcoxon rank sum test was used to assess gender differences.

RESULTS

A total of 67 patients were included in this study with male to female ratio of 1:1. Inter-reader agreement was good to excellent for FF measurements at both levels (ICC=0.71-0.90) and poor for sciatic nerve areas (ICC=0.08-0.27). Positive correlations of sciatic FF and area were seen with age (p value<0.05). Males had significantly higher sciatic intraneural fat than females (p<0.05).

CONCLUSION

Fat quantification MRI is highly reproducible with significant positive correlations of sciatic FF and area with age, which may have implications for MRI diagnosis of sciatic neuropathy.

KEY POINTS

• MR proton density fat fraction is highly reproducible at multiple levels. • Sciatic intraneural fat is positively correlated with increasing age (p < 0.05). • Positive correlations exist between bilateral sciatic nerve areas and age (p < 0.05). • Males had significantly higher sciatic intraneural fat than females (p < 0.05).

Quantitative magnetic resonance (MR) neurography for evaluation of peripheral nerves and plexus injuries

Traumatic conditions of peripheral nerves and plexus have been classically evaluated by morphological imaging techniques and electrophysiological tests. New magnetic resonance imaging (MRI) studies based on 3D fat-suppressed techniques are providing high accuracy for peripheral nerve injury evaluation from a qualitative point of view. However, these techniques do not provide quantitative information. Diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI) are functional MRI techniques that are able to evaluate and quantify the movement of water molecules within different biological structures. These techniques have been successfully applied in other anatomical areas, especially in the assessment of central nervous system, and now are being imported, with promising results for peripheral nerve and plexus evaluation. DWI and DTI allow performing a qualitative and quantitative peripheral nerve analysis, providing valuable pathophysiological information about functional integrity of these structures. In the field of trauma and peripheral nerve or plexus injury, several derived parameters from DWI and DTI studies such as apparent diffusion coefficient (ADC) or fractional anisotropy (FA) among others, can be used as potential biomarkers of neural damage providing information about fiber organization, axonal flow or myelin integrity. A proper knowledge of physical basis of these techniques and their limitations is important for an optimal interpretation of the imaging findings and derived data. In this paper, a comprehensive review of the potential applications of DWI and DTI neurographic studies is performed with a focus on traumatic conditions, including main nerve entrapment syndromes in both peripheral nerves and brachial or lumbar plexus.

Anatomical and MR correlative study of the proximal sciatic nerve vasculature

OBJECTIVE

The aim of our study is to demonstrate that increased T₂ signal on MRI could be due to intraneural vessels in asymptomatic individuals, and may therefore be a normal finding.

METHODS

An initial anatomic cadaveric study was undertaken to gain a better understanding of the vascular supply of the proximal sciatic nerve. Secondly, a retrospective study of MR imaging of patients without sciatic symptoms was performed to assess the prevalence of intraneural vessels, defined as hyperintensity on at least three consecutive slices on both T₂ and gadolinium enhanced T₁ weighted imaging, visible on routine MSK pelvic imaging.

RESULTS

The anatomical study demonstrated a relatively abundant blood supply in the peri-ischiatic region. In the MR study, 20/76 (26%) patients showed visible intraneural vessels. More than one intraneural vessel was depicted in two of the sciatic nerves. Direct branching between the extrinsic and intrinsic systems was seen in only five cases.

CONCLUSION

Normal intraneural vessels can frequently be seen within the sciatic nerve on routine musculoskeletal pelvic imaging. Advances in knowledge: T₂ hyperintensity in the proximal sciatic nerve can be due to intraneural vessels and should not necessarily be reported as abnormal.

Magnetic resonance neurography: current perspectives and literature review

Magnetic resonance neurography (also called MRN or MR neurography) refers to MR imaging dedicated to the peripheral nerves. It is a technique that enhances selective multiplanar visualisation of the peripheral nerve and pathology by encompassing a combination of two-dimensional, three-dimensional and diffusion imaging pulse sequences. Referring physicians who seek imaging techniques that can depict and diagnose peripheral nerve pathologies superior to conventional MR imaging are driving the demand for MRN. This article reviews the pathophysiology of peripheral nerves in common practice scenarios, technical considerations of MRN, current indications of MRN, normal and abnormal neuromuscular appearances, and imaging pitfalls. Finally, the emerging utility of diffusion-weighted and diffusion tensor imaging is discussed and future directions are highlighted.

KEY POINTS

• Lesion relationship to neural architecture is more conspicuous on MRN than MRI. • 3D multiplanar imaging technique is essential for pre-surgical planning. • Nerve injuries can be classified on MRN using Sunderland's classification. • DTI provides quantitative information and insight into intraneural integrity and pathophysiology.

Ultrasound versus MRI in common fibular neuropathy

INTRODUCTION

We prospectively compared ultrasound (US) and MRI in patients with common fibular neuropathy.

METHODS

Forty adult patients with clinical suspicion of common fibular neuropathy and 40 healthy controls underwent both US and MRI. US and MRI datasets were randomized for prospective reading.

RESULTS

The overall sensitivity of US and MRI for diagnosing fibular neuropathy was 90% (95% confidence interval [CI], 79.7%-97.3%) and 87.5% (95% CI, 71.55%-93.1%), respectively. The overall specificity of US and MRI was 92% (95% CI, 77.45%-96.1%) and 85% (95% CI, 73.3%-94.4%), respectively. The overall sensitivity and specificity of US combined with MRI were 94% (95% CI, 0.80%-0.99%) and 84% (95% CI, 0.70%-0.91%), respectively. Overall intra- and inter-observer agreements among 3 readers were 0.76% (95% CI, 0.62%-0.85%) and 0.74% (95% CI, 0.65%-0.81%).

CONCLUSIONS

US diagnostic accuracy for common fibular neuropathy was slightly higher than that of MRI. Muscle Nerve 55: 849-857, 2017.

Magnetic Resonance Neurography of the Pelvic Nerves

Chronic pelvic pain syndrome is commonly caused by nerve injury, inflammation, or entrapment. Owing to the complex anatomy and branching patterns of pelvic nerves, pelvic neuropathies are often difficult to illustrate and diagnose. High-resolution 3-T magnetic resonance neurography is a promising technique for the evaluation of peripheral neuropathy. In this article, the authors discuss the normal anatomy of major pelvic nerves, technical considerations of high-resolution imaging, and normal and abnormal imaging appearances with relevant case examples.

Diffusion tensor imaging MR neurography for the detection of polyneuropathy in type 1 diabetes

PURPOSE

To evaluate if diffusion tensor imaging MR neurography (DTI-MRN) can detect lesions of peripheral nerves in patients with type 1 diabetes.

MATERIALS AND METHODS

Eleven type 1 diabetic patients with polyneuropathy (DPN), 10 type 1 diabetic patients without polyneuropathy (nDPN), and 10 healthy controls (HC) were investigated with a 3T MRI scanner. Clinical examinations, nerve-conduction studies, and vibratory-perception thresholds determined the presence of DPN. DTI-MRN (voxel size: 1.4 × 1.4 × 3 mm³ ; b-values: 0, 800 s/mm² ) covered proximal (sciatic nerve) and distal regions of the lower extremity (tibial nerve). Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were calculated and compared to T₂ -relaxometry and proton-spin density obtained from a multiecho turbo spin echo (TSE) sequence. Furthermore, we evaluated DTI reproducibility, repeatability, and diagnostic accuracy.

RESULTS

DTI-MRN could accurately discriminate between DPN, nDPN, and HC. The proximal FA was lowest in DPN (DPN 0.37 ± 0.06; nDPN 0.47 ± 0.03; HC 0.49 ± 0.06; P < 0.01). In addition, distal FA was lowest in DPN (DPN 0.31 ± 0.05; nDPN 0.41 ± 0.07; HC 0.43 ± 0.08; P < 0.01). Likewise, proximal ADC was highest in DPN (DPN 1.69 ± 0.25 × 10^-3 mm² /s; nDPN 1.50 ± 0.06 × 10^-3 mm² /s; HC 1.42 ± 0.12 × 10^-3 mm² /s; P < 0.01) as was distal ADC (DPN 1.87 ± 0.45 × 10^-3 mm² /s; nDPN 1.59 ± 0.19 × 10^-3 mm² /s; HC 1.57 ± 0.26 × 10^-3 mm² /s; P = 0.09). The combined interclass-correlation (ICC) coefficient of DTI reproducibility and repeatability was high in the sciatic nerve (ICC: FA = 0.86; ADC = 0.85) and the tibial nerve (ICC: FA = 0.78; ADC = 0.66). T₂ -relaxometry and proton-spin-density did not enable detection of neuropathy.

CONCLUSION

DTI-MRN accurately detects DPN by lower nerve FA and higher ADC. These alterations are likely to reflect proximal and distal nerve fiber pathology.

LEVEL OF EVIDENCE

1 J. Magn. Reson. Imaging 2017;45:1125-1134.

Three-Tesla MR imaging of the elbow in non-symptomatic professional baseball pitchers

OBJECTIVES

To retrospectively evaluate the qualitative and quantitative 3-T MR imaging features of the elbow in non-symptomatic professional baseball pitchers presenting as major league draft picks or trades.

MATERIALS AND METHODS

The Institutional Review Board (IRB) approved the HIPPA-compliant study. Informed consent was waived. Twenty-one professional non-symptomatic baseball pitchers (mean age 23, range 18 to 34 years old) underwent 3-T MR imaging of the pitching elbow. Two experienced readers independently performed qualitative (collateral ligaments, tendons, cartilage, bones, ulnar nerve, olecranon fossa, and joint fluid) and quantitative (collateral ligaments and posteromedial plica) evaluation. Descriptive statistics were calculated.

RESULTS

Collateral ligament thickening was seen in a high proportion, nearly half, however, without features of full thickness tearing. Tendinosis without tearing was seen in 19 % (4/21) of common extensors. Cartilage abnormalities were infrequent. Bone abnormalities manifested as edema in 24 % (5/21) and humeroulnar osteophytosis. Ulnar nerve signal and/or morphologic abnormalities were seen in a very high proportion, up to 81 % (17/21). The olceranon fat pad showed scarring features in about one third. The median ligament thicknesses in mm measured: 4.6 UCL anterior bundle, 1.8 UCL posterior bundle, 1.9 RCL, 2.5 LUCL, and 0.7 mm anular. The median plica dimensions were 5.3 by 2.2 by 2.7 mm.

CONCLUSION

High-resolution 3-T MR imaging frequently shows abnormalities involving the ligaments, tendons, nerves, olecranon fat pad, and bones in non-symptomatic baseball pitchers.

[MR neurography for lesion localization in the peripheral nervous system. Why, when and how?]

Peripheral neuropathies are frequent disorders which are often challenging in the diagnostic work-up. Diagnostic difficulties first and foremost arise with regard to lesion localization and the precise definition of spatial lesion patterns. Magnetic resonance (MR) neurography as a diagnostic imaging tool directly visualizes nerve lesions thereby facilitating lesion localization not only in traumatic nerve lesions but also in the large and heterogeneous group of intrinsic, spontaneously occurring non-focal neuropathies. The major diagnostic sign for lesion detection and localization is the T2 lesion which can be evaluated with high spatial resolution at the anatomical level of nerve fascicles. Lesion detection at the fascicular level by MR neurography advances the diagnostic work-up in the peripheral nervous system (PNS), because fascicular and partial nerve lesions of spontaneously occurring intrinsic neuropathies and polyneuropathies present a classical diagnostic pitfall for traditional localization by means of physical findings and electrophysiology. With the appropriate techniques and strategies MR neurography can now cover large anatomical areas of the PNS in a single examination session.

Peripheral neuropathy: detection with diffusion-tensor imaging

PURPOSE

To investigate the ability of diffusion-tensor imaging (DTI) and T2 to help detect the mildest nerve lesion conceivable, that is, subclinical ulnar neuropathy at the elbow.

MATERIALS AND METHODS

This prospective study was approved by the institutional ethics board. Written informed consent was obtained from all participants. Magnetic resonance neurography was performed at 3.0 T by using proton density- and T2-weighted relaxometry and DTI on elbows in 30 healthy subjects without clinical evidence of neuropathy. Quantitative analysis of ulnar nerve T2 and fractional anisotropy (FA) was performed, and T2 and FA values were correlated to electrical nerve conduction velocities (NCVs) with Pearson correlation analysis. Additional qualitative assessment of T2-weighted and FA images was performed by two readers, and sensitivity and specificity were calculated.

RESULTS

Ten of the 30 subjects (33%) had NCV slowing across the elbow segment. Compared with subjects without NCV slowing, subjects with slowing had decreased FA values (0.51 ± 0.09 vs 0.41 ± 0.07, respectively; P = .006) and increased T2 values (64.2 msec ± 10.9 vs 76.2 msec ± 13.7, respectively; P = .01) in the proximal ulnar sulcus. FA values showed a significant correlation (P = .01) with NCV slowing over the sulcus as an electrophysiologic indicator of myelin sheath damage. Qualitative assessment of FA maps and T2-weighted images helped identify subjects with conduction slowing with a sensitivity of 80% and 55%, respectively, and a specificity of 83% and 63%.

CONCLUSION

FA maps can accurately depict even mild peripheral neuropathy and perform better than the current standard of reference, T2-weighted images. DTI may therefore add diagnostic value as a highly sensitive technique for the detection of peripheral neuropathy.

Thoracic outlet syndrome in 3T MR neurography-fibrous bands causing discernible lesions of the lower brachial plexus

OBJECTIVES

To investigate whether targeted magnetic resonance neurography (MRN) of the brachial plexus can visualise fibrous bands compressing the brachial plexus and directly detect injury in plexus nerve fascicles.

METHODS

High-resolution MRN was employed in 30 patients with clinical suspicion of either true neurogenic thoracic outlet syndrome (TOS) or non-specific TOS. The protocol for the brachial plexus included a SPACE (3D turbo spin echo with variable flip angle) STIR (short tau inversion recovery), a sagittal-oblique T2-weighted (T2W) SPAIR (spectral adiabatic inversion recovery) and a 3D PDW (proton density weighted) SPACE. Images were evaluated for anatomical anomalies compressing the brachial plexus and for abnormal T2W signal within plexus elements. Patients with abnormal MR imaging findings underwent surgical exploration.

RESULTS

Seven out of 30 patients were identified with unambiguous morphological correlates of TOS. These were verified by surgical exploration. Correlates included fibrous bands (n = 5) and pseudarthrosis or synostosis of ribs (n = 2). Increased T2W signal was detected within compressed plexus portion (C8 spinal nerve, inferior trunk, or medial cord) and confirmed the diagnosis.

CONCLUSIONS

The clinical suspicion of TOS can be diagnostically confirmed by MRN. Entrapment of plexus structures by subtle anatomical anomalies such as fibrous bands can be visualised and relevant compression can be confirmed by increased T2W signal of compromised plexus elements.

KEY POINTS

• MR neurography (MRN) can aid the diagnosis of thoracic outlet syndrome (TOS). • Identifiable causes of TOS in MRN include fibrous bands and bony anomalies. • Increased T2W signal within brachial plexus elements indicate relevant nerve compression. • High positive predictive value allows confident and targeted indication for surgery.

Peripheral MR neurography: approach to interpretation

The magnetic resonance neurography (MRN) examination is rapidly becoming a part of the diagnostic algorithm of patients with peripheral neuropathy; however, because of the technical demands and the lack of required reading skills, the examination is relatively underutilized and is currently limited to a few tertiary care centers. The radiologists with interest in peripheral nerve imaging should be able to perform and interpret this examination to exploit its potential for widespread use. This article outlines the systematic, stepwise approach to its interpretation and a brief discussion of the imaging pitfalls.

Magnetic resonance neurography of the pelvis and lumbosacral plexus

Recent advances in magnetic resonance (MR) imaging have revolutionized peripheral nerve imaging and made high-resolution acquisitions a clinical reality. High-resolution dedicated MR neurography techniques can show pathologic changes within the peripheral nerves as well as elucidate the underlying disorder or cause. Neurogenic pain arising from the nerves of the pelvis and lumbosacral plexus poses a particular diagnostic challenge for the clinician and radiologist alike. This article reviews the advances in MR imaging that have allowed state-of-the-art high-resolution imaging to become a reality in clinical practice.

Effect of object orientation angle on t2* image and reconstructed magnetic susceptibility: numerical simulations

The magnetic field resulting from material magnetization in magnetic resonance imaging (MRI) has an object orientation effect, which produces an orientation dependence for acquired T2* images. On one hand, the orientation effect can be exploited for object anisotropy investigation (via multi-angle imaging); on the other hand, it is desirable to remove the orientation dependence using magnetic susceptibility reconstruction. In this report, we design a stick-star digital phantom to simulate multiple orientations of a stick-like object and use it to conduct various numerical simulations. Our simulations show that the object orientation effect is not propagated to the reconstructed magnetic susceptibility distribution. This suggests that accurate susceptibility reconstruction methods should be largely orientation independent.

MR neurography in ulnar neuropathy as surrogate parameter for the presence of disseminated neuropathy

Purpose

Patients with ulnar neuropathy of unclear etiology occasionally present with lesion extension from elbow to upper arm level on MRI. This study investigated whether MRI thereby distinguishes multifocal neuropathy from focal-compressive neuropathy at the elbow.

Methods

This prospective study was approved by the institutional ethics committee and written informed consent was obtained from all participants. 122 patients with ulnar mononeuropathy of undetermined localization and etiology by clinical and electrophysiological examination were assessed by MRI at upper arm and elbow level using T2-weighted fat-saturated sequences at 3T. Twenty-one patients were identified with proximal ulnar nerve lesions and evaluated for findings suggestive of disseminated neuropathy (i) subclinical lesions in other nerves, (ii) unfavorable outcome after previous decompressive elbow surgery, and (iii) subsequent diagnosis of inflammatory or other disseminated neuropathy. Two groups served as controls for quantitative analysis of nerve-to-muscle signal intensity ratios: 20 subjects with typical focal ulnar neuropathy at the elbow and 20 healthy subjects.

Results

In the group of 21 patients with proximal ulnar nerve lesion extension, T2-w ulnar nerve signal was significantly (p<0.001) higher at upper arm level than in both control groups. A cut-off value of 1.92 for maximum nerve-to-muscle signal intensity ratio was found to be sensitive (86%) and specific (100%) to discriminate this group. Ten patients (48%) exhibited additional T2-w lesions in the median and/or radial nerve. Another ten (48%) had previously undergone elbow surgery without satisfying outcome. Clinical follow-up was available in 15 (71%) and revealed definitive diagnoses of multifocal neuropathy of various etiologies in four patients. In another eight, diagnoses could not yet be considered definitive but were consistent with multifocal neuropathy.

Conclusion

Proximal ulnar nerve T2 lesions at upper arm level are detected by MRI and indicate the presence of a non-focal disseminated neuropathy instead of a focal compressive neuropathy.

T2-signal of ulnar nerve branches at the wrist in guyon's canal syndrome

OBJECTIVE

To evaluate T2-signal of high-resolution MRI in distal ulnar nerve branches at the wrist as diagnostic sign of guyon's-canal-syndrome (GCS).

MATERIALS AND METHODS

11 GCS patients confirmed by clinical/electrophysiological findings, and 20 wrists from 11 asymptomatic volunteers were prospectively included to undergo the following protocol: axial T2-weighted-fat-suppressed and T1-weighted-turbo-spin-echo-sequences (3T-MR-scanner, Magnetom/Verio/Siemens). Patients were examined in prone position with the arm extended and wrist placed in an 8-channel surface-array-coil. Nerve T2-signal was evaluated as contrast-to-noise-ratios (CNR) from proximal-to-distal in ulnar nerve trunk, its superficial/sensory and deep/motor branch. Distal motor-nerve-conduction (distal-motor-latency (dml)) to first dorsal-interosseus (IOD I) and abductor digiti minimi muscles was correlated with T2-signal. Approval by the institutional review-board and written informed consent was given by all participants.

RESULTS

In GCS, mean nerve T2-signal was strongly increased within the deep/motor branch (11.7±4.8 vs.controls:-5.3±2.4;p = 0.001) but clearly less and not significantly increased in ulnar nerve trunk (6.8±6.4vs.-7.4±2.5;p = 0.07) and superficial/sensory branch (-2.1±4.9vs.-9.7±2.9;p = 0.08). Median nerve T2-signal did not differ between patients and controls (-9.8±2.5vs.-6.7±4.2;p = 0.45). T2-signal of deep/motor branch correlated strongly with motor-conduction-velocity to IOD I in non-linear fashion (R(2) = -0.8;p<0.001). ROC-analysis revealed increased nerve T2-signal of the deep/motor branch to be a sign of excellent diagnostic performance (area-under-the-curve 0.94, 95% CI: 0.85-1.00; specificity 90%, sensitivity 89.5%).

CONCLUSIONS

Nerve T2-signal increase of distal ulnar nerve branches and in particular of the deep/motor branch is highly accurate for the diagnostic determination of GCS. Furthermore, for the first time it was found in nerve entrapment injury that T2-signal strongly correlates with electrical-conduction-velocity.