DTI tractography of lumbosacral plexus and sciatic nerve: initial experience

Transcription

1 DTI tractography of lumbosacral plexus and sciatic nerve: initial experience Poster No.: C-2076 Congress: ECR 2012 Type: Scientific Paper Authors: J. Broncano, P. Gomez-Angulo, I. Simon Yarza, G. Viteri, J. M Bondia, J. L. Zubieta, J. D. Aquerreta ; Pamplona/ES, Murcia/ ES Keywords: Neuroradiology peripheral nerve, MR-Diffusion/Perfusion, Imaging sequences, Tissue characterisation DOI: /ecr2012/C-2076 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 21

2 Purpose Sciatic nerve is the largest nerve of the body. It is originated by the ventral rami of L4 to S3 spinal roots and exits the pelvis trough the greater sciatic foramen usually anterior to and accompanied by the piriformis muscle, as distinct tibial and peroneal divisions, enclosed in a common nerve sheath [1]. It is one of the major neural pathways of the lower extremity allowing the flexion of the knee by harmstring muscles and also provides all the sensory and motor functions below the knee to the muscle groups innervated by its tibial and peroneal divisions [1,2] (figure 2) Sciatica is one of the commonest variations of low back pain and is described as pain radiating to the leg, normally below the knee and into the foot and toes [3-6]. The most common location of sciatic nerve and lumbosacral plexus pathology is the spine, due to lumbosacral hernia [1]. The estimated prevalence, registered in the literature, rated sciatic symptoms between 1.2% and 43% [7]. Peripheral entrapment can occur more frequently in the pelvis. The main etiologies are trauma, iatrogenic injury -due to gynecological or joint replacement surgeries- and the piriformis muscle syndrome [1,8]. MRI is a useful method to evaluate neural entrapment, especially by using T2 weighted sequences like Short Time Inversion Recovery (STIR) T2 weighted images or fatsaturated heavily-t2 weighted sequences [9-13] Figure 3). But with the appropriate magnetic field gradients, MRI could assess intrinsic tissular characteristic as well, like the random motion of water molecules inside anatomic structures. This property, called diffusion, provides essential information about the ultrastructure of the anatomic regions under study. When water motion is homogeneous in all directions in the space we talk about isotropic diffusion. In contrast, when this movement is predominantly in one direction due to the intrinsic properties of the material, like in nervous pathways, we talk about anisotropic diffusion. Diffusion Tensor Imaging (DTI) is an MRI technique that using several weighted diffusion planes acquisitions is able to establish the three-dimensional mayor directions of water molecules movement (eigenvectors) and its magnitude (eigenvalues) owing to calculate the grade of anisotropy by the Fractional Anisotropy (FA) and the Apparent Diffusion Coefficient (ADC). Different specifically design software traduces the FA registered encoding the eigenvectors and its eigenvalues in a plane colour-coded map representative of the structures under study. Fiber tracking represents these values in a virtual 3D colour-coded map (Tractography) [14,15] (figure 4). Page 2 of 21

3 There are only one study evaluating the sciatic nerve DTI- tractography in the thigh but, to the best of our knowledge, there are no other works assessing the intrinsic properties of intrapelvic lumbosacral plexus and sciatic nerve [16]. The aim of our study was to evaluate the feasibility of using DTI-Tractography for the evaluation of lumbosacral plexus and sciatic nerve, in 1.5 T and 3 T MRI and to demonstrate the reproducibility of this imaging protocol among different radiologists. Page 3 of 21

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6 Methods and Materials Patients Ten consecutive patients were prospectively enrolled between February to April of 2010, for realising a pelvic MRI study done for other indication and adding the DTI-Tractography sequence to the standard protocol without complications. The global mean age was 44.18±20.84 years. Five patients were male (mean age of 48.23±25.41 years) and 5 were female (mean age 31.2±3.44 years). Five patients were scanned on 1,5 Tesla MRI (3 male, 41,66±19.35 years; 2 female, 30.33±5.13 years) and the other 5 on 3 Tesla MRI (2 male, 80.5±4,94 years; 3 female, 36.33±7.09 years). As a result 10 patients and 20 sciatic nerves were visualised with DTI and fiber tracking (figure 1). The presence of any pathologic condition interesting the lumbosacral hinge or pelvic waist, previous trauma or any neurological or systemic condition that could be cause of sciatic neuropathy was considered as exclusion criteria. The institutional review board approved the study protocol. All patients gave written informed consent before the exam (figure 5). Acquisition Protocol 5 patients were evaluated in 1,5 Tesla MRI equipment (Symphony Tim, Siemens Healthcare, Erlangen, Germany) and the other 5 patients in 3 Tesla MRI (Trio Tim, Siemens Healthcare, Erlangen, Germany). All patients were studied in supine position. The field gradient and slew rate of the 1.5 T MRI scan was 30mT/m2 and 125 T/m/s and 45 mt/m2 and 200 T/m/s for the 3 T MRI scan respectively. Eight elements body phased array coil combined with high-resolution pelvic surface coil were used for the acquisition. The diffusion study was made by a coronal fat suppressed single-shot fast-spin-echo echo-planar DTI sequence with 30 encoded directions, diffusion weighted b value of 1000 s/mm2 and two b0 images per DTI sequence. Image acquisition protocol is summarised in figure 6. The image acquisition parameters for the 1,5 T MRI scan were as follows: TE=97 ms, TR=9000 ms, Field of view (FoV)=350 x 100 mm. The transverse image resolution was 2,9 x 2,9 mm with a slice thickness of 2 mm without gap. Total acquisition sequence time was 9 min 47 s. For the 3 Tesla MRI scan, the image acquisition parameters were TE=95 ms, TR=8900 ms and FoV=230x100 mm. The voxel size was 1,9 x 1,9 x 2 mm and total sequence registration lasted 9 min 40 s. In both cases parallel imaging technique with a sense factor of 2, partial Fourier acquisition with a half-scan factor of 0,681 and 1322 Hz per pixel frequency direction bandwidth were applied. Right-to-left phase encoding and 122 x 122 matrix were also used (figure 6). Page 6 of 21

7 Study evaluation After DTI tractography acquisition, data were transferred to a workstation with specificdesigned software (Neuro3D, Siemens Healthcare, Erlangen, Germany) for quantitative assessment of DTI sequence and fiber tracking. Colour-coded coronal maps were used to locate the spinal roots and sciatic nerve. Circular ROIs smaller than the nerves, for avoiding partial volume effects, were placed in the sciatic nerve and spinal roots at 5 different levels: L4, L5 and S1 roots, intrapelvic and extrapelvic portions of sciatic nerve. ROI location on the colour-coded coronal planes were confirmed using a T2 weighted Fast Spin Echo coronal sequence included in the standard protocol. Mean FA and ADC values were automatically calculated (figure 7). DTI tractography was performed by placing multiple seed points in a region larger than the nerve interested to visualise it at different locations, using an FA threshold of 0.1 and allowing fiber angulation of up to 30 degrees. For qualitative evaluation a visual scale range from 1 to 3 (1=equal codification; 2=slightly different codification and 3=different codification) was used to assess differences in the colour codification between different portions of sciatic nerve. These patterns were assessed on both coronal plane colour-coded maps and virtual 3D tractography. Two radiology in-training readers (4th year) reviewed all DTI tractography studies and collected those data blindly. Because FA and ADC values depend on signal-to-noise ratio (SNR) of the DTI acquisition, we have determined the SNR for all DTI sequence (figure 8). Where SInerve represents the mean signal intensity within a ROI (mean size of 0,1 cm2) placed on the intrapelvic portion of the sciatic nerve, and SDbackground_noise denote the average standard deviation of the background noise measured by four standardised ROIs rating 3 cm2 placed in four reproducible image locations outside the pelvis. A correction factor was used to account for the systematic error in noise measurements in magnitude images. These SNR values were determined on 1,5 T and 3 T studies to verify the possible extrapolation of the quantitative diffusion and anisotropy parameters between the two types of MRI scans [17-19]. Statistical analysis A two tailed alpha evidence level of 0.05 was set previous to the realisation of statistical studies using a commercially available statistical program (SPSS version 17.0, SPSS Page 7 of 21

8 Inc.). Also, exact p value calculation was applied. The kolmogorov-smirnov test was used to corroborate the normality distribution of the study sample but due to the small sample size comparison of the SNR, FA and ADC values between patient genre and between the type of MRI scan; U of Mann-Withney non-parametric test was applied. In the evaluation of the intrasubject side-to-side variability of the FA and ADC values T of Wilcoxon test was done. Intraclass correlation coefficient and Bland-Altmann curves were assessed in order to evaluate the interobserver variability and interpreted as follows: 0,20 slight or poor agreement, fair agreement, good agreement and >0.80 excellent agreement (figure 8). Page 8 of 21

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12 Results DTI tractography and MRI type of scan Centering on the evaluation of the differences in SNR, ADC and FA no significant differences were obtained between the different types of MRI in the evaluation of diffusion and anisotropy of the sacral plexus and sciatic nerve. The global mean anisotropy of sciatic nerve was 0.395±0.081 (1.5 T MRI ± and 3 T MRI ± 0.071; p>0.05). Also the global mean diffusivity was ± mm2/s (1.5 T MRI ± mm2/s and 3 T MRI ± mm2/s; p>0.05) (figure 9). Patient sex and side-to-side variability In the evaluation of the FA and ADC of the sciatic nerve variations between genders, no significant differences were observed. The mean fractional anisotropy in male patients was 0.398±0.059 and for female patients was 0.393±0.089 (p>0.05, see figure 3). When assessing diffusivity parameters, ADC of the sacral plexus and sciatic nerve in male patients was ± mm2/s and ± mm2/s in female patients (p>0.05, see figure 10). For the evaluation of the intrasubject side-to-side variability of the sciatic nerve diffusion parameters, T of Wilcoxon tests were done to compare the FA and ADC values between the 20 sciatic nerves under evaluation. These differences are represented on table 3 and no significant variations were observed (p>0.05; figure 11). Interobserver agreement Finally, in the comparison of the two-observer evaluation of the quantitative and qualitative assessment of the lumbosacral plexus and sciatic nerve tractography, intraclass correlation coefficient (ICC) and Bland-Altmann plots were applied. An excellent correlation was obtained in the quantitative evaluation of ADC and FA (ICC=0.803). But in the use of qualitative criteria, this agreement was fair (ICC=0.324) (figure 12). Page 12 of 21

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16 Conclusion In conclusion, employing DTI fiber tractography sequences is a feasible, objective and reproducible method in the evaluation of sacral plexus and sciatic nerve. Probably in near future it will become a useful tool in order to assess eventual repercussion on its structure secondary to spinal or extraspinal pathologies. Page 16 of 21

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18 References 1. Petchprapa CN, Rosenberg ZS, Sconfienza LM et al (2010) MR imaging of entrapment neuropathies of the lower extremity. Part 1. The pelvis and hip. Radiographics 30(4): Lanzieri CF, Hilal SK (1984) Computed tomography of the sacral ple xus and sciatic nerve in the greater sciatic foramen. Am J Roentgenol 143(1): Fairbank JC (2007) Sciatic: An archaic term. BMJ 335: Van Tulder M, Koes B, Bombardier C (2002) Low back pain. Best Pract Res Clin Rheumatol 16: Waddell G (2004) The Back Pain Revolution. London: Churchill Livingstone. 6. Konstantinou K, Dunn KM (2008) Sciatica: Review of epidemiological studies and prevalence estimates. Spine 33(22): Videman T, Nurminen T, Tola S, et al (1984) Low back pain in nurses and some loading factors at work. Spine 9(4): Benzon HT, Katz JA, Benzon HA et al (2003) Piriformis syndrome: Anatomic considerations, a new injection technique, and a review of the literature. Anesthesiology 98(6): Bendszus M, Stoll G (2005) Technology insight: Visualizing peripheral nerve injury using MRI. Nat Clin Pract Neurol 1(1): Stoll G (2009) Magnetic resonance imaging of the peripheral nervous system. J Neurol 256(7): Chappell KE, Robson MD, Stonebridge-Foster A et al (2004) Magic angle effects in MR neurography. Am J Neuroradiol 25(3): Zhang Z, Song L, Meng Q et al (2009) Morphological analysis in patients with sciatica: a magnetic resonance imaging study using three-dimensional high-resolution diffusionweighted magnetic resonance neurography techniques. Spine 34(7):E Howe FA, Saunders DE, Filler AG et al (1994) Magnetic resonance neurography of the median nerve. Br J Radiol 67(804): Mukherjee P, Chung SW, Berman JI et al (2008) Diffusion Tensor MR Imaging and Fiber Tractography: Theoretic Underpinnings. Am J Neuroradiol 29: Mukherjee P, Chung SW, Berman JI et al (2004) Diffusion Tensor MR Imaging and Fiber Tractography: Technical Considerations. Am J Neuroradiol 29: Skorpil M, Karlsson M, Nordell A (2004) Peripheral nerve diffusion tensor imaging. Magn Reson Imaging 22(5): Andreisek G, White LM, Kassner A et al (2010) Evaluation of diffusion tensor imaging and fiber tractography of the median nerve: Preliminary results on intrasubject variability and precision of measurements. Am J Roentgenol 194(1):W Dietrich O, Heiland S, Sartor K (2001) Noise correction for the exact determination of apparent diffusion coefficients at low SNR. Magn Reson Med 45(3): Henkelman RM (1985) Measurement of signal intensities in the presence of noise in MR images. Med Phys 12(2): Page 18 of 21

19 20. Hiltunen J, Suortti T, Arvela S et al (2005) Diffusion tensor imaging and tractography of distal peripheral nerves at 3 T. Clin Neurophysiol 116(10): Khalil C, Hancart C, Le Thuc V et al (2008) Diffusion tensor imaging and tractography of the median nerve in carpal tunnel syndrome: Preliminary results. Eur Radiol 18(10): Khalil C, Budzik JF, Kermarrec E et al (2010) Tractography of peripheral nerves and skeletal muscles. Eur J Radiol 76(3): Kabakci N, Gürses B, Firat Z et al (2007) Diffusion tensor imaging and tractography of median nerve: Normative diffusion values. Am J Roentgenol 189(4): Budzik JF, Le Thuc V, Demondion X et al (2007) In vivo MR tractography of thigh muscles using diffusion imaging: Initial results. Eur Radiol 17(12): García Santos JM, Ordóñez C, Torres del Río S (2008) ADC measurements at low and high b values: Insight into normal brain structure with clinical DWI. J Magn Reson Imaging 26: Page 19 of 21

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