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Current MR Neurography Techniques and Whole-Body MR Neurography Avneesh Chhabra, MD1
John Carrino, MD, MPH2
1 Department of Radiology, UT Southwestern, Dallas, Texas 2 Department of Radiology and Imaging, Hospital for Special Surgery,
New York, New York
Address for correspondence Avneesh Chhabra, MD, Department of Musculoskeletal Radiology, Radiology and Orthopaedic Surgery, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9178 (e-mail: [email protected]).
Abstract Keywords
► ► ► ►
MR neurography technique whole body nerves
MR neurography (MRN) techniques continue to evolve, leading to a better demonstration of peripheral nerve anatomy and pathology. This article discusses various technical perspectives and provides recommendations to obtain the best possible high-resolution imaging of the peripheral nerves in various areas of the body. We also discuss technical perspectives of whole-body MRN and its potential utility.
Magnetic resonance neurography (MRN) techniques have evolved since their initial introduction in the early 1990s. At that time, MRN involved axial T1-weighted (T1W) and obliquely oriented two-dimensional (2D) inversion recovery imaging for better nerve visualization along the long axis. Only three-dimensional (3D) software was used to visualize the peripheral nerves in different planes, which were limited due to primarily 2D sequence acquisition.1,2 With improved hardware and software, as well as refined techniques, high-resolution 2D and 3D imaging can lead to marked improvement in demonstrating neural and perineural anatomy and pathology.3,4 We discuss current MRN techniques, the different options available on various scanners, and our recommendations to obtain a superior imaging examination. We also touch on the whole-body high-resolution MRN capability now available and its potential utility.
Two-Dimensional Imaging Multiplane 2D imaging is the workhorse for high-resolution demonstration of the peripheral nerve architecture. Imaging in axially matched non–fat-suppressed T1W and fatsuppressed T2W (fsT2W) contrast is essential for optimal neural and perineural assessment. The desired resolution depends on whether a screening or a diagnostic examination is needed. The radiologist should closely examine the clinical
Issue Theme Advanced Imaging of Peripheral Nerves; Guest Editor, Avneesh Chhabra, MD
history, findings, and available electrophysiology results before prescribing the examination. If the clinical history is vague, a screening examination using thick-slice (4–5 mm with 10–20% interslice gap) axial T1W and fsT2W imaging could initially be needed before focusing on a certain area where neural or perineural abnormality is eventually detected on screening. If an entrapment is suspected along the length of the nerve, high-resolution imaging of the joint and axial imaging of the adjacent extremity could be performed.5 For example, in the setting of pain or sensory symptoms along the radial nerve, elbow imaging and high-resolution axial imaging of the upper arm to encompass the spiral groove area could be performed. It is important to use different coils for both sites: a flex coil to image the extremity and an elbow coil to image the elbow separately. A torso coil could be wrapped tightly to image both areas, but the field of view must be adjusted to avoid > 20% blank air space around the imaged extremity. This will result in a superior demonstration of the relatively small peripheral nerves in the field of view, allowing the reader to avoid magnification blur while evaluating the internal architecture of the nerve. The elbow and upper arm images can be stitched or composed together by the technologist before transferring to the picture archiving and communication system (PACS). Stitched images in T1W and fsT2W techniques are viewed in tandem for clearer, more accurate identification of the pathology.6,7
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0035-1545074. ISSN 1089-7860.
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Semin Musculoskelet Radiol 2015;19:79–85.
MR Neurography Techniques
Chhabra, Carrino
Fig. 1 (a) Axial T1-weighted and (b) T2 SPAIR images show normal intermediate signal sciatic nerve (arrows).
Both T1W (TR/TE, 500–800/5–9; TF, 6–9) and fsT2W (TR/ TE, 4,000–4,800/60–65; TF, 15–22) imaging should be obtained in high resolution, keeping the matrix above 256 (inplane resolution of 0.3–0.4 mL) and tight echo spacing to avoid blur. Such T1W imaging is easy to obtain on most scanners, but fsT2W imaging can be a challenge, especially on low-field scanners. High-field 3-T imaging is preferred for smaller structures such as nerves and superior depiction of regional musculoskeletal structures in the same examination due to higher signal-to-noise ratio (SNR) and shorter imaging examination time.8,9 The slice thickness should be kept at 4 to 5 mm (thigh) (►Fig. 1), 3.5 to 4 mm (brachial plexus, lumbar plexus, upper arm, forearm, knee, and calf), 3 mm (ankle, elbow, wrist), and 2 to 3 mm (hand and foot). The interslice gap should not be > 10% of the slice thickness.7 There are a variety of ways to obtain fsT2W imaging, such as frequency-selective fat suppression, short tau inversion recovery (STIR), spectral adiabatic inversion recovery (SPAIR), and two- or three-point Dixon-based techniques. Frequencyselective fat suppression frequently fails in extremities due to curvatures and angled imaging, despite providing good soft tissue contrast and SNR. STIR is limited by longer imaging time, poorer SNR, and pulsation artifacts, but it might be the only way to obtain the best fat suppression in difficult areas, such as the brachial plexus, on many scanners. SPAIR offers superior SNR compared with STIR, with almost no pulsation artifacts, and it provides better fat suppression than frequency-selective fat-saturated imaging, especially in off-center areas. High-resolution SPAIR (matrix 256–384) is therefore
frequently used for an excellent 2D imaging portion of the MRN examination.10,11 Recently, 2D T2 Dixon (chemical shift type) imaging has become popular because it provides both “fat-only” and “water-only” imaging in the same setting that allows nerve and muscle evaluation in different soft tissue contrasts. However, 2D Dixon imaging is frequently a longer sequence, and breathing, as well as bowel pulsation artifacts, could mar the abdominopelvic imaging, especially along the more mobile anterior abdominal muscles (►Fig. 2). In the future, use of parallel imaging with T2 Dixon might circumvent some of these artifacts. The Dixon technique also comes with its own set of summation and swap artifacts due to patient motion and metal in the field of view (►Fig. 3). In general, MRN imaging should be performed on 1.5 T if there is metal in the region of interest to minimize the hardwarerelated susceptibility artifacts. High-bandwidth STIR imaging is preferred in such circumstances. Diffusion imaging (TR/TE 6,000/50–60; b-value, 0, 800, 1,000 s/mm2, directions, 12–15) should be performed in the axial plane to avoid ghosting artifacts. The b-value could be dropped to 600 to 800 s/mm2 on 3 T in plexuses to obtain better SNR. There is a better chance of obtaining superior diffusion imaging by keeping echo spacing tight, performing manual shim before the examination, asking the patient to remain still during the acquisition, and adding frequency selective or inversion recovery fat suppression techniques (►Fig. 4). A medium number of directions of interrogation11–19 is usually sufficient to obtain good tractography while
Fig. 2 (a) Axial T1-weighted and (b, c) T2 Dixon images depict normal plexus nerves. Notice breathing artifacts over the anterior abdomen that may make it difficult to assess anteriorly located nerves, for example, the lateral femoral cutaneous nerve in this case (arrow). Seminars in Musculoskeletal Radiology
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preserving enough SNR. Keeping zero interslice spacing allows multiplanar reformats to be obtained along the nerve axis without stairstep artifacts. Diffusion-weighted imaging (DWI) allows quantitative nerve imaging, and diffusion tensor imaging (DTI) further allows tractography to provide evaluation of internal nerve and intralesional architecture12–15 (►Fig. 5). However, DTI is more demanding than simple DWI and, of all these techniques, fsT2W imaging is the most essential. Therefore, it should be performed first to get the most from the examination, in case the study gets motion degraded.
Three-Dimensional Imaging Three-dimensional imaging is technically demanding because it involves an additional phase-encoding gradient and almost doubles the acquisition time. The two types of 3D imaging are nerve nonselective and nerve selective. Nerve nonselective imaging is currently available on most commercial scanners. Since the inception of MRN in the early 1990s, this type of 3D imaging was limited to faster acquisition gradient-echo (GRE) sequences, but such techniques resulted in inferior resolution and produced mostly T1 contrast (►Fig. 6). GRE imaging was also marred by frequent susceptibility artifacts from nearby metal, field inhomogeneity, and air. Three-dimensional imaging with spin-echo contrast is now available on an array of scanners including SPACE (Siemens), CUBE (GE), and VISTA (Philips). Different types of weighting (T1W, proton-density weighted [PDW], T2W) and contrasts (STIR, SPAIR) are available on such sequences.
Although T1W contrast is useful for postgadolinium imaging (fsT1W GRE, T1W mDixon, and T1W SPACE), T2W imaging is more desirable because of its superior depiction of nerve course, continuity, signal, and architectural alterations, as well as its relationship to the surrounding lesions and musculoskeletal structures. STIR SPACE (TR/TE 1,800–2,200/70–80; TF, 48–60; 1.3–1.5 mL isotropic) is useful in plexuses, due to its superior fat suppression and the variable size of a patient’s neck or body habitus10,11,16 (►Fig. 7). SPAIR SPACE (1.1 mm isotropic) provides higher resolution for the extremities due to lower demands of fat suppression. Both techniques allow multiplanar isotropic reconstructions to obtain variably prescribed oblique angled images for ideal nerve depiction along the orthogonal axes. Three-dimensional imaging also results in some diffusion weighting and vascular signal suppression due to the increased number of acquisition steps. Although it is effective in arterial signal suppression, 3D imaging still poses a problem with venous signal contamination. Additionally, these techniques generate a certain amount of blur, despite 1.1- to 1.5-mm voxel size, because of higher turbo factors and volume acquisition. Creating thick slab maximum intensity projections in various angles along the nerve reduces image graininess and results in a smooth, uniform appearance to the nerves and the surrounding soft tissues. Applying saturation bands to reduce venous signal contamination is ineffective due to various obliquities of small veins. One can apply minor diffusion weighting with anatomical technique to successfully accomplish such a goal. Three-dimensional diffusionweighted reversed fast imaging with steady state precession
Fig. 4 Axial inversion recovery–based diffusion tensor imaging. (a) Axial b0, (b) b800, and (c) apparent diffusion coefficient images show normal nerves and no ghosting artifact. Notice effective vascular suppression at b600 (arrows). Seminars in Musculoskeletal Radiology
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Fig. 3 T2 Dixon fat-water swap artifact. (a) Axial T1-weighted shows metal in the field of view (arrow). (b) Corresponding water-only T2 Dixon image shows summation artifact due to perturbed frequencies (arrows).
MR Neurography Techniques
Chhabra, Carrino rest-tissue RARE imaging, Philips). This technique involves a motion-sensitized driven-equilibrium pulse for blood signal suppression and a SPAIR pulse for muscle and fat signal suppression, followed by a variable turbo spin-echo sequence. The result is a vessel-suppressed nerve selective imaging of the plexus. This sequence is also isotropic (TR/TE, 1,800–2,000/TE, 150–160; voxel, 1.2–1.5s) and can be reconstructed in similar thick slab maximum-intensity projection (MIP) fashion for excellent selective peripheral nerve display (►Fig. 9). The minor issue of susceptibility to B0 and B1 inhomogeneity remains but can be tackled with further technical developments in this domain. Finally, a 3D balanced fast field-echo (3D bFFE) or a 3D constructive interference in steady state (3D CISS) imaging sequence is added to the plexus imaging if there is suspected arachnoiditis (►Fig. 10), nerve root avulsion, or perineural and leptomeningeal metastasis. This allows excellent nerve to cerebrospinal fluid contrast and aids in detection of tiny lesions around the nerves in the thecal sac, individual rootlets of preganglionic nerve segments, and fibrous bands causing entanglements of the intrathecal nerves.7,19–21
Fig. 5 Axial inversion recovery–based diffusion tensor imaging. Normal tracts in the lumbosacral plexus with bilateral symmetry.
(3D DW-PSIF) is such a technique that encompasses 3D isotropic imaging (TR/TE 12–13/2.5–3.5; voxel, 0.8–0.9 mm) and fat suppression (water-selective type or principle of selective excitation technique [ProSet]), allowing excellent vascular signal suppression (broken hyperintense venous signal) while maintaining continuous endoneurial nerve signal (►Fig. 8). The peripheral nerves show uniform intermediate signal on this sequence, and pathologic signal alterations are nicely observed in multiple planes on 3D DW PSIF similar to nerve nonselective 3D imaging techniques.17,18 However, it should be noted that this sequence is quite sensitive to motion and susceptibility artifacts. Loss of fat suppression due for any reason will mar the sequence tremendously. It is better limited to extremity work and small field-ofview facial (e.g., the inferior alveolar nerve) or neck imaging. For nerve selective plexus imaging, a new technique is available, referred to as SHINKEI (nerve-sheath signal increased with inked
Recommendation for MRN of Different Body Regions The following are our recommendations for optimal MRN of different body areas. These techniques could be modified depending on the scanner and technical availability. If metal is in the regional field of view, use 1.5-T imaging. Thick slab MIP reconstructions are desirable for longitudinal nerve and lesion depiction, and they can vary based on generated SNR (from 4 to 8 mm for 3D density-weighted PSIF (8–40 mm for 3D STIR SPACE, 3D SPAIR SPACE ,and 3D SHINKEI; 20–80 mm for DTI tensor images). Intravenous gadolinium should be added with pre- and post 3D T1W imaging and subtraction manipulation, if there is suspected acute inflammation, or local or remote malignancy. 1. Extremity or tunnel MRN: Use axial T1W, axial T2 SPAIR or mDixon, coronal fsPDW, 3D coronal or sagittal SPAIR SPACE, 3D DW PSIF, axial DTI 2. LS plexus MRN: Use axial T1W, axial T2 SPAIR or mDixon, 3D coronal STIR SPACE or 3D SHINKEI, axial DTI. Add spine imaging: 3D sagittal T2 SPACE or 2D axial and sagittal T2W.
Fig. 6 Three-dimensional T1-weighted gradient-recalled echo. Multiplanar isotropic depiction of the median nerve (arrows). Notice good depiction of the nerve; however, signal alterations cannot be judged as they would be on T2-weightedW images. Seminars in Musculoskeletal Radiology
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Fig. 8 (a) Coronal three-dimensional (3D) inversion recovery turbo spin echo, (b) 3D DW PSIF, and (c) maximum intensity projection 3D diffusionweighted (DW) reverse fast imaging with steady-state free precession (PSIF) images show the inferior alveolar nerves (arrows). Notice selective depiction of the nerves on the vessel signal-suppressed 3D DW PSIF images.
Add 3D coronal CISS or bFFE if suspected arachnoiditis, leptomeningeal or perineural metastasis. 3. Brachial plexus MRN: Use axial T1W, sagittal STIR, 3D coronal STIR SPACE, axial DTI. Add spine imaging: 3D sagittal T2 SPACE or axial and sagittal T2W. Add 3D coronal CISS or bFFE if suspected nerve root avulsion, leptomeningeal or perineural metastasis. 4. Facial MRN: Use axial T1W, axial T2 SPAIR, 3D coronal 3D SPAIR SPACE, 3D coronal DW PSIF, axial DTI, axial 3D CISS or bFFE through the posterior fossa of the brain.
Whole-Body MRN
Fig. 9 Coronal three-dimensional nerve-sheath signal increased with inked rest-tissue RARE imaging (SHINKEI) images shows selective bilateral femoral nerve depiction (arrows).
Whole-body MRN is also available on some scanners. It is an extension of whole-body MR imaging that has been available for many years. The advancement lies in full-body 3D imaging in the same setting without having to move the patient using total imaging matrix technology, superior and isotropic resolution, relative vascular signal suppression due to 3D imaging, and additional diffusion imaging for nerves. It has potential noninvasive utility in diffuse polyneuropathy conditions such as chronic inflammatory demyelinating polyneuropathy (CIDP), Charcot-Marie-Tooth disease, leprosy, neurofibromatosis, and schwannomatosis because it can allow assessment of total disease burden (►Fig. 11), identification Seminars in Musculoskeletal Radiology
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Fig. 7 Three-dimensional (3D) inversion recovery turbo spin echo. (a) 3D isotropic depiction of bilateral brachial plexuses (large arrows). Note the good depiction of the nerves. (b) Venous contamination is still seen, especially on maximum intensity-projection image (small arrows).
MR Neurography Techniques
Chhabra, Carrino
Fig. 10 (a–c) Three-dimensional balanced fast field echo images show fibrous tissue (arrows) entangling the cauda equina nerves in a case of arachnoiditis from prior spine surgery. Previous L spine MRI (outside) was evidently reported as unremarkable.
Fig. 11 Whole-body MR neurography images in a patient with hereditary neuropathy show diffusely thickened peripheral nerves in the (a) brachial plexus and arms, (b) intercostal spaces, and (c) lumbosacral plexus. Diffusion tensor imaging of the (d) brachial and (e) LS plexuses confirm symmetrical bilateral nerve thickening with selective nerve demonstration.
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Conclusion High-resolution 2D and 3D MRN imaging techniques are currently available on most scanners for direct neuromuscular demonstration. The provided recommendations should be followed to conduct optimal MRN examinations while performing these examinations wisely and in light of available clinical information.
8 Anderson ML, Skinner JA, Felmlee JP, Berger RA, Amrami KK.
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References 1 Du R, Auguste KI, Chin CT, Engstrom JW, Weinstein PR. Magnetic
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resonance neurography for the evaluation of peripheral nerve, brachial plexus, and nerve root disorders. J Neurosurg 2010; 112(2):362–371 Filler AG, Maravilla KR, Tsuruda JS. MR neurography and muscle MR imaging for image diagnosis of disorders affecting the peripheral nerves and musculature. Neurol Clin 2004;22(3):643–682, vi–vii Chhabra A, Lee PP, Bizzell C, Soldatos T. 3 Tesla MR neurography— technique, interpretation, and pitfalls. Skeletal Radiol 2011; 40(10):1249–1260 Bäumer P, Pham M, Ruetters M, et al. Peripheral neuropathy: detection with diffusion-tensor imaging. Radiology 2014;273(1):185–193 Chhabra A, Andreisek G, Soldatos T, et al. MR neurography: past, present, and future. AJR Am J Roentgenol 2011;197(3):583–591 Zhang H, Xiao B, Zou T. Clinical application of magnetic resonance neurography in peripheral nerve disorders. Neurosci Bull 2006; 22(6):361–367 Chhabra A, Flammang A, Padua A Jr, Carrino JA, Andreisek G. Magnetic resonance neurography: technical considerations. Neuroimaging Clin N Am 2014;24(1):67–78
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Diagnostic comparison of 1.5 Tesla and 3.0 Tesla preoperative MRI of the wrist in patients with ulnar-sided wrist pain. J Hand Surg Am 2008;33(7):1153–1159 Soher BJ, Dale BM, Merkle EM. A review of MR physics: 3T versus 1.5T. Magn Reson Imaging Clin N Am 2007;15(3):277–290, vv Thawait SK, Wang K, Subhawong TK, et al. Peripheral nerve surgery: the role of high-resolution MR neurography. AJNR Am J Neuroradiol 2012;33(2):203–210 Chhabra A, Williams EH, Wang KC, Dellon AL, Carrino JA. MR neurography of neuromas related to nerve injury and entrapment with surgical correlation. AJNR Am J Neuroradiol 2010;31(8): 1363–1368 Zhao L, Wang G, Yang L, Wu L, Lin X, Chhabra A. Diffusion-weighted MR neurography of extremity nerves with unidirectional motionprobing gradients at 3 T: feasibility study. AJR Am J Roentgenol 2013;200(5):1106–1114 Gasparotti R, Lodoli G, Meoded A, Carletti F, Garozzo D, Ferraresi S. Feasibility of diffusion tensor tractography of brachial plexus injuries at 1.5 T. Invest Radiol 2013;48(2):104–112 Guggenberger R, Eppenberger P, Markovic D, et al. MR neurography of the median nerve at 3.0T: optimization of diffusion tensor imaging and fiber tractography. Eur J Radiol 2012;81(7): e775–e782 Eguchi Y, Ohtori S, Yamashita M, et al. Diffusion-weighted magnetic resonance imaging of symptomatic nerve root of patients with lumbar disk herniation. Neuroradiology 2011;53(9): 633–641 Vargas MI, Viallon M, Nguyen D, Beaulieu JY, Delavelle J, Becker M. New approaches in imaging of the brachial plexus. Eur J Radiol 2010;74(2):403–410 Chhabra A, Subhawong TK, Bizzell C, Flammang A, Soldatos T. 3T MR neurography using three-dimensional diffusion-weighted PSIF: technical issues and advantages. Skeletal Radiol 2011; 40(10):1355–1360 Zhang Z, Meng Q, Chen Y, et al. 3-T imaging of the cranial nerves using three-dimensional reversed FISP with diffusion-weighted MR sequence. J Magn Reson Imaging 2008;27(3):454–458 Yoneyama M, Takahara T, Kwee TC, Nakamura M, Tabuchi T. Rapid high resolution MR neurography with a diffusion-weighted prepulse. Magn Reson Med Sci 2013;12(2):111–119 Kanoto M, Toyoguchi Y, Hosoya T, Oda A, Sugai Y. Visualization of the trochlear nerve in the cistern with use of high-resolution turbo spin-echo multisection motion-sensitized driven equilibrium. AJNR Am J Neuroradiol 2013;34(7):1434–1437 Blitz AM, Macedo LL, Chonka ZD, et al. High-resolution CISS MR imaging with and without contrast for evaluation of the upper cranial nerves: segmental anatomy and selected pathologic conditions of the cisternal through extraforaminal segments. Neuroimaging Clin N Am 2014;24(1):17–34 Yamashita T, Kwee TC, Takahara T. Whole-body magnetic resonance neurography. N Engl J Med 2009;361(5):538–539 Chhabra A, Zhao L, Carrino JA, et al. MR neurography: advances. Radiol Res Pract 2013;2013:809568
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of incipient or growing malignancy, regular surveillance of such conditions, and pre- and posttreatment response in these conditions, such as CIDP on plasma exchange therapy and schwannomatosis on mitotic inhibitors or chemotherapy. The technique involves multiple, two to four sets of 3D STIR SPACE (1.5 mm isotropic, integrated parallel acquisition technique [iPAT] factor 3–4), matched 3D T1W imaging (3D GRE or mDixon, 1.5 mm isotropic), and axial DTI (two to three sets, especially including brachial and lumbar plexuses). One can variably add postcontrast 3D T1W imaging with subtraction, sagittal STIR imaging of brachial plexus (for subtle lesions), and axial fsT2W imaging (for the detection of superimposed entrapments), depending on time constraints and clinical indication.22,23 This modality should be chosen carefully and only with expert imaging support because it is not only costly but also time consuming to obtain full-body high-resolution imaging.
Chhabra, Carrino
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Current MR Neurography Techniques and Whole-Body MR Neurography Avneesh Chhabra, MD1
John Carrino, MD, MPH2
1 Department of Radiology, UT Southwestern, Dallas, Texas 2 Department of Radiology and Imaging, Hospital for Special Surgery,
New York, New York
Address for correspondence Avneesh Chhabra, MD, Department of Musculoskeletal Radiology, Radiology and Orthopaedic Surgery, UT Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9178 (e-mail: [email protected]).
Abstract Keywords
► ► ► ►
MR neurography technique whole body nerves
MR neurography (MRN) techniques continue to evolve, leading to a better demonstration of peripheral nerve anatomy and pathology. This article discusses various technical perspectives and provides recommendations to obtain the best possible high-resolution imaging of the peripheral nerves in various areas of the body. We also discuss technical perspectives of whole-body MRN and its potential utility.
Magnetic resonance neurography (MRN) techniques have evolved since their initial introduction in the early 1990s. At that time, MRN involved axial T1-weighted (T1W) and obliquely oriented two-dimensional (2D) inversion recovery imaging for better nerve visualization along the long axis. Only three-dimensional (3D) software was used to visualize the peripheral nerves in different planes, which were limited due to primarily 2D sequence acquisition.1,2 With improved hardware and software, as well as refined techniques, high-resolution 2D and 3D imaging can lead to marked improvement in demonstrating neural and perineural anatomy and pathology.3,4 We discuss current MRN techniques, the different options available on various scanners, and our recommendations to obtain a superior imaging examination. We also touch on the whole-body high-resolution MRN capability now available and its potential utility.
Two-Dimensional Imaging Multiplane 2D imaging is the workhorse for high-resolution demonstration of the peripheral nerve architecture. Imaging in axially matched non–fat-suppressed T1W and fatsuppressed T2W (fsT2W) contrast is essential for optimal neural and perineural assessment. The desired resolution depends on whether a screening or a diagnostic examination is needed. The radiologist should closely examine the clinical
Issue Theme Advanced Imaging of Peripheral Nerves; Guest Editor, Avneesh Chhabra, MD
history, findings, and available electrophysiology results before prescribing the examination. If the clinical history is vague, a screening examination using thick-slice (4–5 mm with 10–20% interslice gap) axial T1W and fsT2W imaging could initially be needed before focusing on a certain area where neural or perineural abnormality is eventually detected on screening. If an entrapment is suspected along the length of the nerve, high-resolution imaging of the joint and axial imaging of the adjacent extremity could be performed.5 For example, in the setting of pain or sensory symptoms along the radial nerve, elbow imaging and high-resolution axial imaging of the upper arm to encompass the spiral groove area could be performed. It is important to use different coils for both sites: a flex coil to image the extremity and an elbow coil to image the elbow separately. A torso coil could be wrapped tightly to image both areas, but the field of view must be adjusted to avoid > 20% blank air space around the imaged extremity. This will result in a superior demonstration of the relatively small peripheral nerves in the field of view, allowing the reader to avoid magnification blur while evaluating the internal architecture of the nerve. The elbow and upper arm images can be stitched or composed together by the technologist before transferring to the picture archiving and communication system (PACS). Stitched images in T1W and fsT2W techniques are viewed in tandem for clearer, more accurate identification of the pathology.6,7
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0035-1545074. ISSN 1089-7860.
This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited.
Semin Musculoskelet Radiol 2015;19:79–85.
MR Neurography Techniques
Chhabra, Carrino
Fig. 1 (a) Axial T1-weighted and (b) T2 SPAIR images show normal intermediate signal sciatic nerve (arrows).
Both T1W (TR/TE, 500–800/5–9; TF, 6–9) and fsT2W (TR/ TE, 4,000–4,800/60–65; TF, 15–22) imaging should be obtained in high resolution, keeping the matrix above 256 (inplane resolution of 0.3–0.4 mL) and tight echo spacing to avoid blur. Such T1W imaging is easy to obtain on most scanners, but fsT2W imaging can be a challenge, especially on low-field scanners. High-field 3-T imaging is preferred for smaller structures such as nerves and superior depiction of regional musculoskeletal structures in the same examination due to higher signal-to-noise ratio (SNR) and shorter imaging examination time.8,9 The slice thickness should be kept at 4 to 5 mm (thigh) (►Fig. 1), 3.5 to 4 mm (brachial plexus, lumbar plexus, upper arm, forearm, knee, and calf), 3 mm (ankle, elbow, wrist), and 2 to 3 mm (hand and foot). The interslice gap should not be > 10% of the slice thickness.7 There are a variety of ways to obtain fsT2W imaging, such as frequency-selective fat suppression, short tau inversion recovery (STIR), spectral adiabatic inversion recovery (SPAIR), and two- or three-point Dixon-based techniques. Frequencyselective fat suppression frequently fails in extremities due to curvatures and angled imaging, despite providing good soft tissue contrast and SNR. STIR is limited by longer imaging time, poorer SNR, and pulsation artifacts, but it might be the only way to obtain the best fat suppression in difficult areas, such as the brachial plexus, on many scanners. SPAIR offers superior SNR compared with STIR, with almost no pulsation artifacts, and it provides better fat suppression than frequency-selective fat-saturated imaging, especially in off-center areas. High-resolution SPAIR (matrix 256–384) is therefore
frequently used for an excellent 2D imaging portion of the MRN examination.10,11 Recently, 2D T2 Dixon (chemical shift type) imaging has become popular because it provides both “fat-only” and “water-only” imaging in the same setting that allows nerve and muscle evaluation in different soft tissue contrasts. However, 2D Dixon imaging is frequently a longer sequence, and breathing, as well as bowel pulsation artifacts, could mar the abdominopelvic imaging, especially along the more mobile anterior abdominal muscles (►Fig. 2). In the future, use of parallel imaging with T2 Dixon might circumvent some of these artifacts. The Dixon technique also comes with its own set of summation and swap artifacts due to patient motion and metal in the field of view (►Fig. 3). In general, MRN imaging should be performed on 1.5 T if there is metal in the region of interest to minimize the hardwarerelated susceptibility artifacts. High-bandwidth STIR imaging is preferred in such circumstances. Diffusion imaging (TR/TE 6,000/50–60; b-value, 0, 800, 1,000 s/mm2, directions, 12–15) should be performed in the axial plane to avoid ghosting artifacts. The b-value could be dropped to 600 to 800 s/mm2 on 3 T in plexuses to obtain better SNR. There is a better chance of obtaining superior diffusion imaging by keeping echo spacing tight, performing manual shim before the examination, asking the patient to remain still during the acquisition, and adding frequency selective or inversion recovery fat suppression techniques (►Fig. 4). A medium number of directions of interrogation11–19 is usually sufficient to obtain good tractography while
Fig. 2 (a) Axial T1-weighted and (b, c) T2 Dixon images depict normal plexus nerves. Notice breathing artifacts over the anterior abdomen that may make it difficult to assess anteriorly located nerves, for example, the lateral femoral cutaneous nerve in this case (arrow). Seminars in Musculoskeletal Radiology
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preserving enough SNR. Keeping zero interslice spacing allows multiplanar reformats to be obtained along the nerve axis without stairstep artifacts. Diffusion-weighted imaging (DWI) allows quantitative nerve imaging, and diffusion tensor imaging (DTI) further allows tractography to provide evaluation of internal nerve and intralesional architecture12–15 (►Fig. 5). However, DTI is more demanding than simple DWI and, of all these techniques, fsT2W imaging is the most essential. Therefore, it should be performed first to get the most from the examination, in case the study gets motion degraded.
Three-Dimensional Imaging Three-dimensional imaging is technically demanding because it involves an additional phase-encoding gradient and almost doubles the acquisition time. The two types of 3D imaging are nerve nonselective and nerve selective. Nerve nonselective imaging is currently available on most commercial scanners. Since the inception of MRN in the early 1990s, this type of 3D imaging was limited to faster acquisition gradient-echo (GRE) sequences, but such techniques resulted in inferior resolution and produced mostly T1 contrast (►Fig. 6). GRE imaging was also marred by frequent susceptibility artifacts from nearby metal, field inhomogeneity, and air. Three-dimensional imaging with spin-echo contrast is now available on an array of scanners including SPACE (Siemens), CUBE (GE), and VISTA (Philips). Different types of weighting (T1W, proton-density weighted [PDW], T2W) and contrasts (STIR, SPAIR) are available on such sequences.
Although T1W contrast is useful for postgadolinium imaging (fsT1W GRE, T1W mDixon, and T1W SPACE), T2W imaging is more desirable because of its superior depiction of nerve course, continuity, signal, and architectural alterations, as well as its relationship to the surrounding lesions and musculoskeletal structures. STIR SPACE (TR/TE 1,800–2,200/70–80; TF, 48–60; 1.3–1.5 mL isotropic) is useful in plexuses, due to its superior fat suppression and the variable size of a patient’s neck or body habitus10,11,16 (►Fig. 7). SPAIR SPACE (1.1 mm isotropic) provides higher resolution for the extremities due to lower demands of fat suppression. Both techniques allow multiplanar isotropic reconstructions to obtain variably prescribed oblique angled images for ideal nerve depiction along the orthogonal axes. Three-dimensional imaging also results in some diffusion weighting and vascular signal suppression due to the increased number of acquisition steps. Although it is effective in arterial signal suppression, 3D imaging still poses a problem with venous signal contamination. Additionally, these techniques generate a certain amount of blur, despite 1.1- to 1.5-mm voxel size, because of higher turbo factors and volume acquisition. Creating thick slab maximum intensity projections in various angles along the nerve reduces image graininess and results in a smooth, uniform appearance to the nerves and the surrounding soft tissues. Applying saturation bands to reduce venous signal contamination is ineffective due to various obliquities of small veins. One can apply minor diffusion weighting with anatomical technique to successfully accomplish such a goal. Three-dimensional diffusionweighted reversed fast imaging with steady state precession
Fig. 4 Axial inversion recovery–based diffusion tensor imaging. (a) Axial b0, (b) b800, and (c) apparent diffusion coefficient images show normal nerves and no ghosting artifact. Notice effective vascular suppression at b600 (arrows). Seminars in Musculoskeletal Radiology
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Fig. 3 T2 Dixon fat-water swap artifact. (a) Axial T1-weighted shows metal in the field of view (arrow). (b) Corresponding water-only T2 Dixon image shows summation artifact due to perturbed frequencies (arrows).
MR Neurography Techniques
Chhabra, Carrino rest-tissue RARE imaging, Philips). This technique involves a motion-sensitized driven-equilibrium pulse for blood signal suppression and a SPAIR pulse for muscle and fat signal suppression, followed by a variable turbo spin-echo sequence. The result is a vessel-suppressed nerve selective imaging of the plexus. This sequence is also isotropic (TR/TE, 1,800–2,000/TE, 150–160; voxel, 1.2–1.5s) and can be reconstructed in similar thick slab maximum-intensity projection (MIP) fashion for excellent selective peripheral nerve display (►Fig. 9). The minor issue of susceptibility to B0 and B1 inhomogeneity remains but can be tackled with further technical developments in this domain. Finally, a 3D balanced fast field-echo (3D bFFE) or a 3D constructive interference in steady state (3D CISS) imaging sequence is added to the plexus imaging if there is suspected arachnoiditis (►Fig. 10), nerve root avulsion, or perineural and leptomeningeal metastasis. This allows excellent nerve to cerebrospinal fluid contrast and aids in detection of tiny lesions around the nerves in the thecal sac, individual rootlets of preganglionic nerve segments, and fibrous bands causing entanglements of the intrathecal nerves.7,19–21
Fig. 5 Axial inversion recovery–based diffusion tensor imaging. Normal tracts in the lumbosacral plexus with bilateral symmetry.
(3D DW-PSIF) is such a technique that encompasses 3D isotropic imaging (TR/TE 12–13/2.5–3.5; voxel, 0.8–0.9 mm) and fat suppression (water-selective type or principle of selective excitation technique [ProSet]), allowing excellent vascular signal suppression (broken hyperintense venous signal) while maintaining continuous endoneurial nerve signal (►Fig. 8). The peripheral nerves show uniform intermediate signal on this sequence, and pathologic signal alterations are nicely observed in multiple planes on 3D DW PSIF similar to nerve nonselective 3D imaging techniques.17,18 However, it should be noted that this sequence is quite sensitive to motion and susceptibility artifacts. Loss of fat suppression due for any reason will mar the sequence tremendously. It is better limited to extremity work and small field-ofview facial (e.g., the inferior alveolar nerve) or neck imaging. For nerve selective plexus imaging, a new technique is available, referred to as SHINKEI (nerve-sheath signal increased with inked
Recommendation for MRN of Different Body Regions The following are our recommendations for optimal MRN of different body areas. These techniques could be modified depending on the scanner and technical availability. If metal is in the regional field of view, use 1.5-T imaging. Thick slab MIP reconstructions are desirable for longitudinal nerve and lesion depiction, and they can vary based on generated SNR (from 4 to 8 mm for 3D density-weighted PSIF (8–40 mm for 3D STIR SPACE, 3D SPAIR SPACE ,and 3D SHINKEI; 20–80 mm for DTI tensor images). Intravenous gadolinium should be added with pre- and post 3D T1W imaging and subtraction manipulation, if there is suspected acute inflammation, or local or remote malignancy. 1. Extremity or tunnel MRN: Use axial T1W, axial T2 SPAIR or mDixon, coronal fsPDW, 3D coronal or sagittal SPAIR SPACE, 3D DW PSIF, axial DTI 2. LS plexus MRN: Use axial T1W, axial T2 SPAIR or mDixon, 3D coronal STIR SPACE or 3D SHINKEI, axial DTI. Add spine imaging: 3D sagittal T2 SPACE or 2D axial and sagittal T2W.
Fig. 6 Three-dimensional T1-weighted gradient-recalled echo. Multiplanar isotropic depiction of the median nerve (arrows). Notice good depiction of the nerve; however, signal alterations cannot be judged as they would be on T2-weightedW images. Seminars in Musculoskeletal Radiology
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Fig. 8 (a) Coronal three-dimensional (3D) inversion recovery turbo spin echo, (b) 3D DW PSIF, and (c) maximum intensity projection 3D diffusionweighted (DW) reverse fast imaging with steady-state free precession (PSIF) images show the inferior alveolar nerves (arrows). Notice selective depiction of the nerves on the vessel signal-suppressed 3D DW PSIF images.
Add 3D coronal CISS or bFFE if suspected arachnoiditis, leptomeningeal or perineural metastasis. 3. Brachial plexus MRN: Use axial T1W, sagittal STIR, 3D coronal STIR SPACE, axial DTI. Add spine imaging: 3D sagittal T2 SPACE or axial and sagittal T2W. Add 3D coronal CISS or bFFE if suspected nerve root avulsion, leptomeningeal or perineural metastasis. 4. Facial MRN: Use axial T1W, axial T2 SPAIR, 3D coronal 3D SPAIR SPACE, 3D coronal DW PSIF, axial DTI, axial 3D CISS or bFFE through the posterior fossa of the brain.
Whole-Body MRN
Fig. 9 Coronal three-dimensional nerve-sheath signal increased with inked rest-tissue RARE imaging (SHINKEI) images shows selective bilateral femoral nerve depiction (arrows).
Whole-body MRN is also available on some scanners. It is an extension of whole-body MR imaging that has been available for many years. The advancement lies in full-body 3D imaging in the same setting without having to move the patient using total imaging matrix technology, superior and isotropic resolution, relative vascular signal suppression due to 3D imaging, and additional diffusion imaging for nerves. It has potential noninvasive utility in diffuse polyneuropathy conditions such as chronic inflammatory demyelinating polyneuropathy (CIDP), Charcot-Marie-Tooth disease, leprosy, neurofibromatosis, and schwannomatosis because it can allow assessment of total disease burden (►Fig. 11), identification Seminars in Musculoskeletal Radiology
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Fig. 7 Three-dimensional (3D) inversion recovery turbo spin echo. (a) 3D isotropic depiction of bilateral brachial plexuses (large arrows). Note the good depiction of the nerves. (b) Venous contamination is still seen, especially on maximum intensity-projection image (small arrows).
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Fig. 10 (a–c) Three-dimensional balanced fast field echo images show fibrous tissue (arrows) entangling the cauda equina nerves in a case of arachnoiditis from prior spine surgery. Previous L spine MRI (outside) was evidently reported as unremarkable.
Fig. 11 Whole-body MR neurography images in a patient with hereditary neuropathy show diffusely thickened peripheral nerves in the (a) brachial plexus and arms, (b) intercostal spaces, and (c) lumbosacral plexus. Diffusion tensor imaging of the (d) brachial and (e) LS plexuses confirm symmetrical bilateral nerve thickening with selective nerve demonstration.
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Conclusion High-resolution 2D and 3D MRN imaging techniques are currently available on most scanners for direct neuromuscular demonstration. The provided recommendations should be followed to conduct optimal MRN examinations while performing these examinations wisely and in light of available clinical information.
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resonance neurography for the evaluation of peripheral nerve, brachial plexus, and nerve root disorders. J Neurosurg 2010; 112(2):362–371 Filler AG, Maravilla KR, Tsuruda JS. MR neurography and muscle MR imaging for image diagnosis of disorders affecting the peripheral nerves and musculature. Neurol Clin 2004;22(3):643–682, vi–vii Chhabra A, Lee PP, Bizzell C, Soldatos T. 3 Tesla MR neurography— technique, interpretation, and pitfalls. Skeletal Radiol 2011; 40(10):1249–1260 Bäumer P, Pham M, Ruetters M, et al. Peripheral neuropathy: detection with diffusion-tensor imaging. Radiology 2014;273(1):185–193 Chhabra A, Andreisek G, Soldatos T, et al. MR neurography: past, present, and future. AJR Am J Roentgenol 2011;197(3):583–591 Zhang H, Xiao B, Zou T. Clinical application of magnetic resonance neurography in peripheral nerve disorders. Neurosci Bull 2006; 22(6):361–367 Chhabra A, Flammang A, Padua A Jr, Carrino JA, Andreisek G. Magnetic resonance neurography: technical considerations. Neuroimaging Clin N Am 2014;24(1):67–78
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Diagnostic comparison of 1.5 Tesla and 3.0 Tesla preoperative MRI of the wrist in patients with ulnar-sided wrist pain. J Hand Surg Am 2008;33(7):1153–1159 Soher BJ, Dale BM, Merkle EM. A review of MR physics: 3T versus 1.5T. Magn Reson Imaging Clin N Am 2007;15(3):277–290, vv Thawait SK, Wang K, Subhawong TK, et al. Peripheral nerve surgery: the role of high-resolution MR neurography. AJNR Am J Neuroradiol 2012;33(2):203–210 Chhabra A, Williams EH, Wang KC, Dellon AL, Carrino JA. MR neurography of neuromas related to nerve injury and entrapment with surgical correlation. AJNR Am J Neuroradiol 2010;31(8): 1363–1368 Zhao L, Wang G, Yang L, Wu L, Lin X, Chhabra A. Diffusion-weighted MR neurography of extremity nerves with unidirectional motionprobing gradients at 3 T: feasibility study. AJR Am J Roentgenol 2013;200(5):1106–1114 Gasparotti R, Lodoli G, Meoded A, Carletti F, Garozzo D, Ferraresi S. Feasibility of diffusion tensor tractography of brachial plexus injuries at 1.5 T. Invest Radiol 2013;48(2):104–112 Guggenberger R, Eppenberger P, Markovic D, et al. MR neurography of the median nerve at 3.0T: optimization of diffusion tensor imaging and fiber tractography. Eur J Radiol 2012;81(7): e775–e782 Eguchi Y, Ohtori S, Yamashita M, et al. Diffusion-weighted magnetic resonance imaging of symptomatic nerve root of patients with lumbar disk herniation. Neuroradiology 2011;53(9): 633–641 Vargas MI, Viallon M, Nguyen D, Beaulieu JY, Delavelle J, Becker M. New approaches in imaging of the brachial plexus. Eur J Radiol 2010;74(2):403–410 Chhabra A, Subhawong TK, Bizzell C, Flammang A, Soldatos T. 3T MR neurography using three-dimensional diffusion-weighted PSIF: technical issues and advantages. Skeletal Radiol 2011; 40(10):1355–1360 Zhang Z, Meng Q, Chen Y, et al. 3-T imaging of the cranial nerves using three-dimensional reversed FISP with diffusion-weighted MR sequence. J Magn Reson Imaging 2008;27(3):454–458 Yoneyama M, Takahara T, Kwee TC, Nakamura M, Tabuchi T. Rapid high resolution MR neurography with a diffusion-weighted prepulse. Magn Reson Med Sci 2013;12(2):111–119 Kanoto M, Toyoguchi Y, Hosoya T, Oda A, Sugai Y. Visualization of the trochlear nerve in the cistern with use of high-resolution turbo spin-echo multisection motion-sensitized driven equilibrium. AJNR Am J Neuroradiol 2013;34(7):1434–1437 Blitz AM, Macedo LL, Chonka ZD, et al. High-resolution CISS MR imaging with and without contrast for evaluation of the upper cranial nerves: segmental anatomy and selected pathologic conditions of the cisternal through extraforaminal segments. Neuroimaging Clin N Am 2014;24(1):17–34 Yamashita T, Kwee TC, Takahara T. Whole-body magnetic resonance neurography. N Engl J Med 2009;361(5):538–539 Chhabra A, Zhao L, Carrino JA, et al. MR neurography: advances. Radiol Res Pract 2013;2013:809568
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of incipient or growing malignancy, regular surveillance of such conditions, and pre- and posttreatment response in these conditions, such as CIDP on plasma exchange therapy and schwannomatosis on mitotic inhibitors or chemotherapy. The technique involves multiple, two to four sets of 3D STIR SPACE (1.5 mm isotropic, integrated parallel acquisition technique [iPAT] factor 3–4), matched 3D T1W imaging (3D GRE or mDixon, 1.5 mm isotropic), and axial DTI (two to three sets, especially including brachial and lumbar plexuses). One can variably add postcontrast 3D T1W imaging with subtraction, sagittal STIR imaging of brachial plexus (for subtle lesions), and axial fsT2W imaging (for the detection of superimposed entrapments), depending on time constraints and clinical indication.22,23 This modality should be chosen carefully and only with expert imaging support because it is not only costly but also time consuming to obtain full-body high-resolution imaging.
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