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

Dual-Echo Turbo Spin Echo and 12-Echo Multi Spin Echo Sequences as Equivalent Techniques for Obtaining T2-Relaxometry Data: Application in Symptomatic and Asymptomatic Hereditary Transthyretin Amyloidosis as a Surrogate Disease

OBJECTIVES

Multi spin echo (MSE) sequences are often used for obtaining T2-relaxometry data as they provide defined echo times (TEs). Due to their time-consuming acquisition, they are frequently replaced by turbo spin echo (TSE) sequences that in turn bear the risk of systematic errors when analyzing small structures or lesions. With this study, we aim to test whether T2-relaxometry data derived from either dual-echo TSE or 12-echo MSE sequences are equivalent for quantifying peripheral nerve lesions. Hereditary transthyretin (ATTRv) amyloidosis was chosen as a surrogate disease, as it allows the inclusion of both asymptomatic carriers of the underlying variant transthyretin gene (varTTR) and symptomatic ATTRv amyloidosis patients.

MATERIALS AND METHODS

Overall, 50 participants with genetically confirmed varTTR (20 clinically symptomatic ATTRv amyloidosis; 4 females, 16 males; mean age, 61.8 years; range, 33-76 years; and 30 asymptomatic varTTR-carriers; 18 females, 12 males; mean age, 43.1 years; range, 21-62 years), and 30 healthy volunteers (13 females, 17 males, mean age 41.3 years, range 22-73) were prospectively included and underwent magnetic resonance neurography at 3 T. T2-relaxometry was performed by acquiring an axial 2-dimensional dual-echo TSE sequence with spectral fat saturation (TE1/TE2, 12/73 milliseconds; TR, 5210 milliseconds; acquisition time, 7 minutes, 30 seconds), and an axial 2-dimensional MSE sequence with spectral fat saturation and with 12 different TE (TE1, 10 milliseconds to TE12, 120 milliseconds; ΔTE, 10 milliseconds; TR, 3000 milliseconds; acquisition time, 11 minutes, 23 seconds) at the right mid to lower thigh. Sciatic nerve regions of interest were manually drawn in ImageJ on 10 central slices per participant and sequence, and the apparent T2-relaxation time (T2app) and proton spin density (ρ) were calculated individually from TSE and MSE relaxometry data.

RESULTS

Linear regression showed that T2app values obtained from the dual-echo TSE (T2appTSE), and those calculated from the 12-echo MSE (T2appMSE) were mathematically connected by a factor of 1.3 throughout all groups (controls: 1.26 ± 0.02; varTTR-carriers: 1.25 ± 0.02; symptomatic ATTRv amyloidosis: 1.28 ± 0.02), whereas a factor of 0.5 was identified between respective ρ values (controls: 0.47 ± 0.01; varTTR-carriers: 0.47 ± 0.01; symptomatic ATTRv amyloidosis: 0.50 ± 0.02). T2app calculated from both TSE and MSE, distinguished between symptomatic ATTRv (T2appTSE 66.38 ± 2.6; T2appMSE 84.6 ± 3.3) and controls (T2appTSE 58.1 ± 1.0, P = 0.0028; T2appMSE 72.8 ± 0.7, P < 0.0001), whereas differences between varTTR-carriers (T2appTSE 61.8 ± 1.5; T2appMSE 76.7 ± 1.3) and ATTRv amyloidosis were observed only for T2appMSE (P = 0.0082). The ρ value differentiated well between healthy controls (ρTSE 365.1 ± 7.2; ρMSE 170.4 ± 3.8) versus varTTR-carriers (ρTSE 415.7 ± 9.8, P = 0.0027; ρMSE 193.7 ± 5.3, P = 0.0398) and versus symptomatic ATTRv amyloidosis (ρTSE 487.8 ± 17.9; ρMSE 244.7 ± 13.1, P < 0.0001, respectively), but also between varTTR-carriers and ATTRv amyloidosis (ρTSEP = 0.0001; ρMSEP < 0.0001).

CONCLUSIONS

Dual-echo TSE and 12-echo MSE sequences provide equally robust and reliable T2-relaxometry data when calculating T2app and ρ. Due to their shorter acquisition time and higher resolution, TSE sequences may be preferred in future magnetic resonance imaging protocols. As a secondary result, ρ can be confirmed as a sensitive biomarker to detect early nerve lesions as it differentiated best among healthy controls, asymptomatic varTTR-carriers, and symptomatic ATTRv amyloidosis, whereas T2app might be beneficial in already manifest ATTRv amyloidosis.

MR imaging of venous malformations: sciatic nerve infiltration patterns and involved muscle groups

The aim of this retrospective cross-sectional study was to provide an MRI-based examination framework of venous malformations (VMs) infiltrating the sciatic nerve and determine the frequency of nerve infiltration patterns and muscle involvement in correlation to the patients’ quality of life. Pelvic and lower limb MR images of 378 patients with vascular malformations were examined retrospectively. Pain levels and restriction of motion were evaluated with a questionnaire. Cross-sectional areas of affected nerves were compared at standardized anatomical landmarks. Intraneural infiltration patterns and involvement of muscles surrounding the sciatic nerve were documented. Sciatic nerve infiltration occurred in 23/299 patients (7.7%) with VM. In all cases (23/23; 100%), gluteal or hamstring muscles surrounding the nerve were affected by the VM. Infiltrated nerves were enlarged and showed signal alterations (T2-hyperintensity) compared to the unaffected side. Enlarged nerve cross-sectional areas were associated with elevated pain levels. Three nerve infiltration patterns were observed: subepineurial (12/23; 52.2%), subparaneurial (6/23; 26.1%) and combined (5/23; 21.7%) infiltration. This study provides a clinically relevant assessment for sciatic nerve infiltration patterns and muscle involvement of VMs, while suggesting that VMs in gluteal and hamstring muscles require closer investigation of the sciatic nerve by the radiologist.

Comparison between optical coherence tomography imaging and histological sections of peripheral nerves

OBJECTIVE

Optical coherence tomography (OCT) is an imaging technique that uses the light-backscattering properties of different tissue types to generate an image. In an earlier feasibility study the authors showed that it can be applied to visualize human peripheral nerves. As a follow-up, this paper focuses on the interpretation of the images obtained.

METHODS

Ten different short peripheral nerve specimens were retained following surgery. In a first step they were examined by OCT during, or directly after, surgery. In a second step the nerve specimens were subjected to histological examination. Various steps of image processing were applied to the OCT raw data acquired. The improved OCT images were compared with the sections stained by H & E. The authors assigned the structures in the images to the various nerve components including perineurium, fascicles, and intrafascicular microstructures.

RESULTS

The results show that OCT is able to resolve the myelinated axons. A weighted averaging filter helps in identifying the borders of structural features and reduces artifacts at the same time. Tissue-remodeling processes due to injury (perineural fibrosis or neuroma) led to more homogeneous light backscattering. Anterograde axonal degeneration due to sharp injury led to a loss of visible axons and to an increase of light-backscattering tissue as well. However, the depth of light penetration is too small to allow generation of a complete picture of the nerve.

CONCLUSIONS

OCT is the first in vivo imaging technique that is able to resolve a nerve's structures down to the level of myelinated axons. It can yield information about focal and segmental pathologies.

[Entrapment and non-entrapment syndromes at the elbow]

CLINICAL/METHODICAL ISSUE

Entrapment syndromes of peripheral nerves at the elbow are common and are often diagnostically challenging disorders. Difficulties consist in lesion localization and recognition of complex spatial lesion patterns as well as in differentiation of focal and multifocal disorders.

STANDARD DIAGNOSTIC METHODS

Medical history taking, neurological examination and neurophysiological tests represent the gold standard in the diagnosis of peripheral nerve lesions at the elbow, but have known methodical limitations.

METHODICAL INNOVATIONS

Additional diagnostic imaging tools recently developed for high-resolution visualization of extended peripheral nerve segments include 3 T magnetic resonance neurography (MRN) and neurosonography.

PERFORMANCE

MRN and neurosonography can directly visualize and thus precisely localize focal and nonfocal peripheral nerve lesions of various origins with high spatial resolution at the anatomical level of nerve fascicles.

ACHIEVEMENTS

MRN can cover peripheral nerve structures at the elbow, evaluate spatial nerve lesion patterns and partly disclose underlying causes.

PRACTICAL RECOMMENDATIONS

Imaging of peripheral nerves is a valuable addition in the diagnostic work-up of entrapment syndromes at the elbow and provides important assistance in the differentiation of nonfocal differential diagnoses, especially in cases that cannot be clarified using standard diagnostic methods. The evaluation of spatial nerve lesion pattern may give additional information on the origin of the underlying disease, which is essential for further treatment.

High-resolution in vivo imaging of peripheral nerves using optical coherence tomography: a feasibility study

OBJECTIVE

Because of their complex topography, long courses, and small diameters, peripheral nerves are challenging structures for radiological diagnostics. However, imaging techniques in the area of peripheral nerve diseases have undergone unexpected development in recent decades. They include MRI and high-resolution sonography (HRS). Yet none of those imaging techniques reaches a resolution comparable to that of histological sections. Fascicles are the smallest discernable structure. Optical coherence tomography (OCT) is the first imaging technique that is able to depict a nerve's ultrastructure at micrometer resolution. In the current study, the authors present an in vivo assessment of human peripheral nerves using OCT.

METHODS

OCT measurement was performed in 34 patients with different peripheral nerve pathologies, i.e., nerve compression syndromes. The nerves were examined during surgery after their exposure. Only the sural nerve was twice examined ex vivo. The Thorlabs OCT systems Callisto and Ganymede were used. For intraoperative use, a hand probe was covered with a sterile foil. Different postprocessing imaging techniques were applied and evaluated. In order to highlight certain structures, five texture parameters based on gray-level co-occurrence matrices were calculated according to Haralick.

RESULTS

The intraoperative use of OCT is easy and intuitive. Image artifacts are mainly caused by motion and the sterile foil. If the artifacts are kept at a low level, the hyporeflecting bundles of nerve fascicles and their inner parts can be displayed. In the Haralick evaluation, the second angular moment is most suitable to depict the connective tissue.

CONCLUSIONS

OCT is a new imaging technique that has shown promise in peripheral nerve surgery for particular questions. Its resolution exceeds that provided by recent radiological possibilities such as MRI and HRS. Since its field of view is relatively small, faster acquisition times would be highly desirable and have already been demonstrated by other groups. Currently, the method resembles an optical biopsy and can be a supplement to intraoperative sonography, giving high-resolution insight into a suspect area that has been located by sonography in advance.

[Clinical indications for high-resolution MRI diagnostics of the peripheral nervous system]

CLINICAL/METHODICAL ISSUE

Peripheral neuropathies are common and diagnostically often challenging disorders. Difficulties particularly exist in lesion localization and recognition of complex spatial lesion patterns.

STANDARD DIAGNOSTIC METHODS

Medical history taking, neurological examination, neurophysiological tests and nerve ultrasonography represent the gold standard in the diagnosis of peripheral nerve lesions but have known methodical limitations.

METHODICAL INNOVATIONS

The use of 3 Tesla magnetic resonance neurography (MRN) is an additional diagnostic imaging tool recently developed for the high-resolution visualization of long segments of peripheral nerves. Reasonable clinical indications for MRN are exemplarily presented.

PERFORMANCE

Using MRN a direct visualization and thus precise localization of focal and non-focal peripheral nerve lesions of various origins can be achieved with high spatial resolution down to the anatomical level of nerve fascicles.

ACHIEVEMENTS

Using MRN large anatomical areas of the peripheral nervous system (PNS) can be covered in a single examination session, spatial nerve lesion patterns can be evaluated and the underlying causes can often be detected.

PRACTICAL RECOMMENDATIONS

The MRN is a valuable supplement to the diagnostic work-up of the PNS, especially in cases that cannot be clarified with standard diagnostic methods. Evaluation of the spatial nerve lesion pattern gives additional information on the origin of the underlying disease. Reasonable indications for MRN are the assessment of proximal nerve structures including the brachial and lumbosacral nerve plexi, the clarification of inconclusive diagnostic results, preoperative, postoperative and posttraumatic assessments, the identification of fascicular nerve lesions and the differential diagnosis of an alleged somatoform disorder.

Carpal and cubital tunnel and other, rarer nerve compression syndromes

BACKGROUND

Carpal tunnel syndrome is by far the most common peripheral nerve compression syndrome, affecting approximately one in every six adults to a greater or lesser extent. Splitting the flexor retinaculum to treat carpal tunnel syndrome is the second most common specialized surgical procedure in Germany. Cubital tunnel syndrome is rarer by a factor of 13, and the other compression syndromes are rarer still.

METHODS

This review is based on publications retrieved by a selective literature search of PubMed and the Cochrane Library, along with current guidelines and the authors' clinical and scientific experience.

RESULTS

Randomized controlled trials have shown, with a high level of evidence, that the surgical treatment of carpal tunnel syndrome yields very good results regardless of the particular technique used, as long as the diagnosis and the indication for surgery are well established by the electrophysiologic and radiological findings and the operation is properly performed. The success rates of open surgery, and the single-portal and dual-portal endoscopic methods are 91.6%, 93.4% and 92.5%, respectively. When performed by experienced hands, all these procedures have complication rates below 1%. The surgical treatment of cubital tunnel syndrome has a comparably low complication rate, but worse results overall. Neuro-ultrasonography and magnetic resonance imaging (neuro-MRI) are increasingly being used to complement the diagnostic findings of electrophysiologic studies.

CONCLUSION

Evidence-based diagnostic methods and treatment recommendations are now available for the two most common peripheral nerve compression syndromes. Further controlled trials are needed for most of the rarer syndromes, especially the controversial ones.