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

Abstract

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.

Keywords: Magnetic resonance imaging (MRI); diffusion MRI; diffusion tensor imaging (DTI); peripheral nerve; plexus.

Figure 1

Water diffusion within PN scheme. (A) Water diffusivity is facilitated along the main (longitudinal) axis of PN; (B) if motion probe gradients are applied perpendicular to the course of these PN, DWI will show restriction of water molecule movement in the short axis due to the presence of physiological barriers such as myelin sheaths. PN, peripheral nerves; DWI, diffusion weighted imaging.

Figure 2

DTI for PN evaluation. The use of motion probing gradients in multiple (at least 6) directions allows assessing the main direction of water diffusion that correlates with the main eigenvector (red arrows), as well as the other two perpendicular minor eigenvectors (green arrows). DTI, diffusion tensor imaging; PN, peripheral nerves.

Figure 3

DWI and DTI derived parametric maps and values in a healthy sciatic nerve (arrows). (A) Axial STIR of proximal right thigh shows normal sciatic nerve (white arrow); (B) ADC and (C) MD maps show values of 2.20×10⁻³ and 2.10×10⁻³ mm²/s, respectively; (D) FA demonstrates a value of 0.6 within normal limits for a healthy PN; (E) AD value corresponds to 3.01×10⁻³ mm²/s as water diffusivity is facilitated along the main axis of sciatic nerve; (F) however, RD shows a value of 1.03×10⁻³ mm²/s due to the restriction of water diffusion within the perpendicular axis; (G) DTI also permits to perform a 3D neurographic reconstruction using a ROI based model, which shows a well delimited fibrillar structure at the posterior aspect of the thigh, coded with blue color by consensus (head to toe main direction of water diffusion), consistent with sciatic nerve. STIR, short-tau inversion recovery; ADC, apparent diffusion coefficient; MD, mean diffusivity; FA, fractional anisotropy; PN, peripheral nerves; AD, axial diffusivity; RD, radial diffusivity; DTI, diffusion tensor imaging; ROI, region of interest.

Figure 4

Sciatic nerve compression by femoral osteochondroma. A 26-year-old female with paresthesia at right lower limb underwent a MRI with DWN sequence. (A) Coronal TSE T1-weighted image shows a pedunculated lesion in the posterior aspect of the distal femur, which shows normal bone marrow content and a cartilage cap (black arrows) consistent with osteochondroma. This tumor contacts and displaces the sciatic nerve (white arrows); (B,C) axial TSE T2-weighted and STIR images confirm the compression of the sciatic nerve that shows moderate increase of its signal intensity (white arrows); (D,E) DWI with a b value of 800 s/mm² and corresponding ADC map demonstrate focal hyperintensity of sciatic nerve (white arrow), more evident at the site of compression by the osteochondroma, where an increase of ADC value (2.0×10⁻³ mm²/s) is depicted consistent with edema; (F) coronal MPR of DWI with b value of 800 s/mm² allows obtaining a neurographic view of the sciatic nerve, demonstrating focal hyperintensity at the compression site (white arrow) and confirms its relation with the cartilaginous cap (black arrows); (G) fusion image of coronal T1-weighted sequence and DWI better shows all the described findings. MRI, magnetic resonance imaging; DWN, DW-based neurography; TSE, turbo-spin-echo; STIR, short-tau inversion recovery; DWI, diffusion weighted imaging; ADC, apparent diffusion coefficient; MPR, multiplanar reformation.

Figure 5

Pathophysiology of DTI findings in normal and injured PN. (A) Normal PN shows preserved axonal integrity with high FA values thanks to proper fiber organization with one main direction of water molecules movement (red arrows) and low RD values (green arrow) due to the presence of physiological barriers that impede the diffusion in the perpendicular plane; (B) an injured PN demonstrates decrease of FA, related to loss of fiber organization and increase of the extracellular space with no clear directionality of water molecules movement (red arrows). Also, there is an increase of RD (green arrows) reflecting loss of myelin sheath integrity. DTI, diffusion tensor imaging; PN, peripheral nerve; FA, fractional anisotropy; RD, radial diffusivity.

Figure 6

Evaluation of PN transection with DTI. A MRI is carried out in a 23-year-old male with previous reconstruction surgery of posterior cruciate ligament, refers weakness and impossibility for active feet dorsal flexion. (A-C) Axial SPAIR T2-weighted images demonstrate denervation signs within the anterior leg compartment (black arrow) with thickening of the distal common peroneal nerve (straight white arrow) and interruption of its visualization where it shows a nodular well-delimited appearance in vicinity of a surgical scar area (curved white arrow); (D) axial postcontrast fat suppressed T1-weighted image demonstrates peripheral enhancement of the PN lesion (curved arrow), consistent with terminal neuroma, and diffuse pseudonodular enhancement of distal biceps femoris muscle that suggest scarring tissue (arrowhead); (E,F) AD parametric maps under and above the lesion, respectively, show lower AD values of peroneal nerve below the neuroma (1.2×10⁻³ mm²/s) in comparison to the ones above the injury (1.9×10⁻³ mm²/s)), suggesting decrease of the normal axonal flow; (G) 3D DTI neurographic reconstruction demonstrates the absence of common peroneal nerve fibers below the neuroma (arrow) probably related to low distal FA values which conditions an underrepresentation of those nerve fibers under the same threshold value set. PN, peripheral nerves; DTI, diffusion tensor imaging; MRI, magnetic resonance imaging; SPAIR, spectral attenuated inversion recovery; AD, axial diffusivity; FA, fractional anisotropy.

Figure 7

Evaluation with DTI of PN acute trauma. A 21-year-old male with a stab wound at forearm, who refers paresthesias and weakness at 4th and 5th fingers is studied with MRI. (A) Axial SPAIR proton-density images located below, at the level and distal to the injury site show diffuse increase of signal intensity within forearm flexor muscles (black arrow) as well as thickening of ulnar nerve (white arrows); (B) axial TSE T1-weighted image demonstrates a hemorrhagic collection close to the flexor carpi ulnaris (curved arrow); (C) axial DWI with a b value of 1,000 s/mm² shows an increase of signal intensity within the ulnar nerve (white arrow), representing nerve edema and increase of signal intensity within the hematoma, with peripheral low signal rim, due to susceptibility artifact (curved arrow); (D) 3D DTI neurography demonstrates absence of continuity between both segments (white arrow). DTI, diffusion tensor imaging; PN, peripheral nerves; MRI, magnetic resonance imaging; SPAIR, spectral attenuated inversion recovery; TSE, turbo-spin-echo; DWI, diffusion weighted imaging.

Figure 8

Brachial plexus post-radiation neuropathy. A MRI is performed in a 64-year-old female with history of radiotherapy 5 months before, as part of her treatment for left breast carcinoma, refers left arm numbness and pain. (A) Coronal TSE T2-weighted image does not reveal significant anomalies at brachial plexus trunks; (B) sagittal STIR demonstrates mild thickening of brachial plexus nerve trunks above left subclavian artery (arrow); (C,D) coronal MPR of high b value DWI with conventional and inverted gray scale windowing allow obtaining a neurographic view of brachial plexus, which depicts an increase of signal intensity and asymmetric thickening of left brachial plexus trunks. MRI, magnetic resonance imaging; STIR, short-tau inversion recovery; MPR, multiplanar reformation; DWI, diffusion weighted imaging.

Figure 9

Assessment by DWI neurography of brachial plexus neuropraxia. A MRI is carried out in a 42-year-old female with right arm weakness and cervical pain. (A) Sagittal STIR of cervical spine shows multiple disk-osteophyte complexes (white arrows) compressing the spinal cord; (B) axial STIR at C6-C7 level demonstrates denervation signs within right splenius capitis muscle (black arrow); (C) no clear asymmetries are identified between both brachial plexus at coronal STIR neurography sequence; (D) axial DWI with a b value of 800 s/mm² shows higher signal intensity within right nerve root and ganglionic segment (white arrow) compared with left one; (E) ADC map demonstrated higher ADC values at right nerve root (1.55×10⁻³ mm²/s) compared with the contralateral one (1.41×10⁻³ mm²/s) consistent with edema; (F) fusion of morphological and functional (DWI) neurography demonstrated focal asymmetric increase of signal intensity within right C6 nerve root (arrow). DWI, diffusion weighted imaging; MRI, magnetic resonance imaging; STIR, short-tau inversion recovery; ADC, apparent diffusion coefficient.

Figure 10

Partum related neuropraxia of sciatic nerve. A MRI is performed in a 32-year-old female with severe limitation of movement and deep tendon reflexes at right lower limb after prolonged partum. (A) Coronal SPAIR T2-weighted sequence shows a diffuse increase of signal intensity especially at the right gluteus medium muscle (curved arrow) without other relevant findings; (B) axial DWI study with a high b value of 800 s/mm² demonstrates severe thickening and hyperintensity of right sciatic nerve (white arrow) compared with left sciatic nerve (black arrow); (C,D) coronal MPR and sagittal MIP of high b value image confirm this asymmetry between both sciatic nerves and points out the relationship between right sciatic nerve (white arrow in D) and the sacral wing, as the prolonged compression of this nerve by uterus against this bone structures is the main cause of neuropraxia in this case. MRI, magnetic resonance imaging; SPAIR, spectral attenuated inversion recovery; DWI, diffusion weighted imaging; MPR, multiplanar reformation; MIP, maximum intensity projection.

Figure 11

Lumbar root disk compression. A 56-year-old male with left sciatica is submitted to perform a MRI. (A) Sagittal STIR shows a left subarticular disk protrusion at L4–L5 level (curved arrow) that contacts with left L5 nerve root at its intracanal segment; (B) axial postcontrast THRIVE demonstrates moderate thickening and enhancement of left L5 nerve root (white arrow); (C) ADC map shows a moderate increase of ADC value at left L5 nerve root (white arrow, 1.7×10⁻³mm²/s) in comparison to the contralateral root (black arrow, 1.4×10⁻³ mm²/s); (D) FA map demonstrates also a decrease of FA values at left nerve root (white arrow, 0.3) compared with right one (black arrow, 0.5). All these findings are consistent with edema and fiber disorganization due to disk compression at left L5 nerve root; (E) coronal MPR of high b value allows obtaining a neurographic view of lumbosacral plexus, which shows an increase of signal intensity of the preganglionic segment of left L5 nerve root with moderate deviation of its normal path (white arrow). MRI, magnetic resonance imaging; STIR, short-tau inversion recovery; THRIVE, T1 High-Res Isotropic Vol Excitation; ADC, apparent diffusion coefficient; FA, fractional anisotropy; MPR, multiplanar reformation.

Figure 12

Carpal tunnel syndrome evaluation with DTN. Carpal tunnel syndrome in a 56-year-old female with numbness of 1st, 2nd and 3rd right hand fingers is studied with MRI. (A) Axial STIR at the flexor retinaculum level does not demonstrate any relevant alteration of neither the median nerve (white arrow) nor the ulnar nerve (black arrow); (B) fusion of axial STIR and MD color parametric map shows a moderate increase of MD values at median nerve (white arrow) within extensor retinaculum compared with ulnar nerve (black arrow) consistent with edema due to compression; (C) 3D DTI neurography depicts a slight change in angulation of median nerve path and a decrease of FA values (white arrows) suggesting focal loss of fiber integrity. DTN, DTI-neurography; MRI, magnetic resonance imaging; STIR, short-tau inversion recovery; MD, mean diffusivity; DTI, diffusion tensor imaging; FA, fractional anisotropy.

Figure 13

Piriformis syndrome assessment by DWN. (A,B) Axial and coronal SPAIR T2-weighted sequences show increase of signal intensity within right sciatic nerve (white arrow) as it crosses above piriformis muscle compared with contralateral sciatic nerve (black arrow). Note also the presence of denervation changes within the right piriformis muscle (curved arrow); (C,D) DWI study with a high b value of 800 s/mm² and corresponding ADC map demonstrate both hyperintensity of sciatic nerve (white arrows) with regard to contralateral nerve (black arrows), and also an increased ADC value of 1.8×10⁻³ mm²/s) at the right sciatic nerve in comparison to the ADC value of left sciatic nerve (1.3×10⁻³ mm²/s) consistent with edema and increase of extracellular space at right sciatic nerve. Besides, a moderate restriction of diffusion is also seen at right piriformis muscle (curved arrows); (E) coronal MPR of high b value shows a neurographic view of lumbosacral plexus and lumbar roots confirming the asymmetry between both sciatic nerves. DWN, DW-based neurography; SPAIR, spectral attenuated inversion recovery; DWI, diffusion weighted imaging; ADC, apparent diffusion coefficient; MPR, multiplanar reformation.

Figure 14

Piriformis syndrome evaluation with DTN. A 29-year-old female with clinical suspicious of right piriformis syndrome underwent a MRI study. (A) Axial TSE T1-weighted image shows thickening of left piriformis muscle (white arrow) in comparison to the contralateral one (black arrow); (B) MPR of THRIVE acquisition parallel to the path of both sciatic nerves identifies the presence of a left accessory piriformis muscular fascicle (white arrow); (C) DTN demonstrates the split of left sciatic nerve in two bundles (white arrow) surrounding both sides of the previously referred accessory muscular fascicle, consistent with type B variation of the classification of Beaton and Anson. DTN, DTI-neurography; MRI, magnetic resonance imaging; TSE, turbo-spin-echo; MPR, multiplanar reformation; THRIVE, T1 High-Res Isotropic Vol Excitation; DTI, diffusion tensor imaging.

Figure 15

Common peroneal nerve neuropraxia evaluation with DTN. A 50-year-old male with right paresthesias at right lower limb after prolonged right knee flexion position was studied with a MRI. (A,B) Axial and sagittal STIR show mild increase of signal intensity of common peroneal nerve as it passes between the peroneal head and the insertion of the peroneus longus muscle; (C,D) DWI with a high b value of 800 s/mm² and corresponding ADC map demonstrate hyperintensity of common peroneal nerve (white arrows) with increased ADC values of 1.6×10⁻³ mm²/s consistent with edema; (E) FA map also reveals decrease of FA values (0.3) suggesting focal loss of fiber integrity (white arrow). DTN, DTI-neurography; MRI, magnetic resonance imaging; STIR, short-tau inversion recovery; DWI, diffusion weighted imaging; ADC, apparent diffusion coefficient; FA, fractional anisotropy.

Figure 16

Terminal neuroma. A 34-year-old male with soft tissue sarcoma at proximal thigh was studied with MRI. A nodular lesion is identified at morphological sequences. (A) Coronal TSE T1-weighted image and (B) axial STIR show a focal lesion in the vicinity of surgical scar tissue (white arrow) that forces to rule out malignancy recurrence; (C,D) DWN study with a high b value of 800 s/mm² and corresponding ADC map demonstrate moderate restriction of water diffusion within this lesion, with ADC value of 1.3×10⁻³ mm²/s consistent with a benign nature (white arrow); (E,F) coronal MPR of the high b value presented with conventional grey scale and inverted one, respectively, allow obtaining a neurographic view of the whole course of left sciatic nerve demonstrating both morphological and functional continuity and dependence of the suspicious lesion with the nerve (white arrows), being consistent, with amputation or terminal neuroma. MRI, magnetic resonance imaging; TSE, turbo-spin-echo; DWN, DWI-neurography; ADC, apparent diffusion coefficient; MPR, multiplanar reformation.

Figure 17

Median nerve continuity neuroma evaluation with DTN. A MRI is performed in a 45-year-old male with stab injury at the palmar surface of the distal forearm that refers paresthesias at 1st and 2nd fingers. (A) Axial SPAIR proton-density image shows focal thickening and a nodular lesion at median nerve (white arrow) at the same site of the previous stab injury; (B) ADC map demonstrates low ADC values (1.1×10⁻³ mm²/s) of this lesion (white arrow); (C) sagittal SPIR T2-weighted; (D) MPR in sagittal plane of high b value DWI and (E) 3D DTI neurographic reconstruction demonstrate morphological and functional continuity between the lesion and the median nerve (white arrows), findings consistent with neuroma-in-continuity. DTN, DTI-neurography; MRI, magnetic resonance imaging; SPAIR, spectral attenuated inversion recovery; ADC, apparent diffusion coefficient; MPR, multiplanar reformation; DWI, diffusion weighted imaging; DTI, diffusion tensor imaging.

Figure 18

Assessment with DTN of incomplete release of ulnar nerve. A 46-year-old female with sporadic numbness of 4th and 5th fingers after ulnar nerve surgery is submitted to a MRI. (A-C) Axial SPAIR proton-density images at the level of proximal ulnar tunnel, epitrochlear and distal ulnar tunnel, respectively, show moderate thickening and increase of signal intensity of ulnar nerve, which is more conspicuous at the inlet and epitrochlear levels; (D-F) consecutive FA maps at the same levels than (A-C) demonstrate decrease of FA values, particularly at the level of the epitrochlear (0.33) with normalization of FA values distally to ulnar tunnel (0.45); (G) 3D DTI neurographic reconstruction demonstrates continuity of ulnar nerve with partial loss of fiber integrity at the epitrochlear level (arrow). DTN, DTI-neurography; MRI, magnetic resonance imaging; SPAIR, spectral attenuated inversion recovery; FA, fractional anisotropy; DTI, diffusion tensor imaging.