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NI FEATURE: THE EDITORIAL DEBATE I-- PROS AND CONS
Year : 2017  |  Volume : 65  |  Issue : 5  |  Page : 964-965

Diffusion tensor imaging in spinal pathology: A robust investigative tool in clinical practice


Department of Orthopaedics and Spine Surgery, Ganga Medical Centre and Hospitals, Coimbatore, Tamil Nadu, India

Date of Web Publication6-Sep-2017

Correspondence Address:
S Rajasekaran
Department of Orthopaedics and Spine Surgery, Ganga Medical Centre and Hospitals, Coimbatore - 641 043, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/neuroindia.NI_735_17

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How to cite this article:
Kanna RM, Rajasekaran S. Diffusion tensor imaging in spinal pathology: A robust investigative tool in clinical practice. Neurol India 2017;65:964-5

How to cite this URL:
Kanna RM, Rajasekaran S. Diffusion tensor imaging in spinal pathology: A robust investigative tool in clinical practice. Neurol India [serial online] 2017 [cited 2019 Dec 15];65:964-5. Available from: http://www.neurologyindia.com/text.asp?2017/65/5/964/214077


Predicting the actual severity of neuronal damage is a very critical element in the diagnosis and prognosis of spinal cord pathology. Though magnetic resonance imaging (MRI) remains the gold standard investigation for evaluating spinal pathologies, it still falls short of predicting the functional status of the spinal cord. Diffusion tensor imaging (DTI) is an extended arm of MRI, which can provide quantitative changes in the status of neurons before one can see gross MRI changes. DTI is extensively used in brain imaging but the efficacy of DTI in spinal cord imaging remains short of day-to-day clinical practice.[1] The authors of the paper, “The role of diffusion tensor imaging in spinal pathology: A review", have given a comprehensive review of studies that have focused on the role of DTI in various spinal pathologies including injuries, spondylotic myelopathy, spinal tumors, amyotropic lateral sclerosis and multiple sclerosis.[2]

DTI of the spinal cord – principles and clinical applications

DTI evaluates longitudinal anatomical structures through the analysis of the directionality of random proton movement (diffusion) that occurs in all tissues. Intact walls of the structures keep the proton movements in a linear axis which is recognised by DTI and this helps in tracking spinal cord white matter tracts (known as diffusion tensor tractography). Colour coded tracts are thus helpful in demonstrating actual interruption of fibre tracts in conditions like spinal cord injury, spinal tumors and syringomyelia.[3]

Disruption of diffusion occurs whenever there is injury to the myelin sheath and this can be quantified, thus enabling us to determine the severity of neuronal damage. Various diffusion anisotropy indices (termed as DTI datametrics) provide a numerical value to the quantity of diffusion, which include the fractional anisotropy (FA), apparent diffusion co-efficient (ADC), volume ratio (VR), relative anisotropy (RA) and Eigen vectors. Multiple studies have shown that among these, the FA and the ADC are the most sensitive and specific indices in evaluating spinal disorders.[4],[5] Usually in intact neurons, the FA value is closer to 1 because of the high degree of anisotropy. When there is any damage to the axonal membrane, the diffusion at that level becomes unrestricted and consequently the FA value decreases. ADC indicates the magnitude of diffusion. A low value for ADC would indicate that the imaged structure (e.g. nerve fibres) is organized with an intact myelin sheath while a high value for ADC indicates that these fibres are disrupted resulting in unrestricted diffusion.

Challenges in DTI of the spinal cord

Though DTI evaluation of the spinal cord continues to entice and offers significant hope, its clinical utility is far from being achieved to its maximum potential until the present moment. The execution of DTI of the spinal cord offers many practical challenges. The spinal cord surrounded by the vertebral body and its bone marrow causes magnetic susceptibility artefacts resulting in image distortions. DTI is highly sensitive to motion and several physiological events like carotid pulsations, cardiac and respiratory movements and even cerebrospinal fluid (CSF) pulsations can cause ghosting artefacts. Swallowing and other subtle patient movements especially in the acutely injured patients, also result in image artefacts. The surrounding CSF can also affect the diffusion anisotropy indices due to partial volume effects. Unlike the brain, the neurons in the spinal cord are arranged in a compact manner in the smaller spinal canal mandating the need for very high resolution imaging. To overcome some of these inherent problems, high strength magnetic fields and different sequencing techniques like interleaved echo planar imaging, fast spin echo diffusion weighted sequences, single shot echo planar imaging, use of parallel imaging techniques and line scan diffusion imaging have been developed to improve image acquisition.[6]

DTI in cervical myelopathy

Among several spinal disorders, cervical spondylotic myelopathy is one entity, where the utility of DTI has been extensively studied and proven to be of relatively consistent clinical benefit. While tractography remains largely as a visual representation of the compressed fibre tracts, the anisotropy indices have shown significant efficacy in identifying early myelin disruption and also in prognostication following surgery in cervical myelopathy. In a prospective study, we analysed 35 patients with CSM who required surgical decompression and 40 controls.[4],[5] All patients underwent surgical decompression and DTI datametrics were obtained at 12 months post-operatively. We observed that there was significant difference in DTI datametrics between patients and control with decrease in FA (0.49 ± 0.081 vs. 0.53 ± 0.07) and increase in ADC (1.8 ± 0.315 vs. 1.44 ± 0.145). There was also a significant difference between increasing grades of myelopathy – controls versus self-ambulant myelopathic patients and dependent myelopathic patients. On follow-up evaluation, we observed that DTI indices showed significant improvements in postoperative DTI indices for ADC, E1 and E2 values in patients who showed neurological recovery while in neurologically static/worsened individuals, the indices remained similar or showed insignificant changes. Apart from surgical intervention, 30 patients with CSM were evaluated with DTI to analyse the efficacy of riluzole in early CSM and we observed that there were no significant differences between the two groups of patients (placebo versus riluzole groups.[7]


  Conclusion Top


In summary, the role of DTI in spinal cord evaluation is still in the preliminary stage. The development of DTI acquisition techniques in the spinal cord and applications of these techniques towards potential clinical usage are still developing. Currently, we understand that this technique is robust and it can provide useful information about fibre direction and the diffusion anisotropy properties of neural tissue; it can provide complementary knowledge to the information obtained through conventional MR imaging. With further refinements in acquisition technology, fiber tracking and DTI indices could possibly help the surgeon to determine the timing and type of surgical approach, in prognostication and in monitoring the response to treatment in various spinal disorders.

 
  References Top

1.
Thomas B. Diffusion tensor imaging: A colorful collage or a clinical tool? Neurol India 2010;58;877-8.  Back to cited text no. 1
    
2.
Li DC, Malcolm JG, Rindler RS, Baum GR, Rao A, Kurpad SN, Ahmad FU. The role of diffusion tensor imaging in spinal pathology: A review. Neurol India 2017;65:982-92.  Back to cited text no. 2
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3.
Loy DN, Kim JH, Xie M, Schmidt RE, Trinkaus K, Song SK. Diffusion tensor imaging predicts hyperacute spinal cord injury severity. J Neurotrauma 2007;24:979-90.  Back to cited text no. 3
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4.
Rajasekaran S, Yerramshetty JS, Chittode VS, Kanna RM, Balamurali G, Shetty AP. The assessment of neuronal status in normal and cervical spondylotic myelopathy using diffusion tensor imaging. Spine (Phila Pa 1976) 2014;39:1183-9.  Back to cited text no. 4
    
5.
Rajasekaran S, Kanna R, Chittode VS, Maheswaran A, Aiyer SN, Shetty AP. Efficacy of diffusion tensor imaging indices in assessing postoperative neural recovery in cervical spondylotic myelopathy. Spine (Phila Pa 1976) 2017;42:8-13.  Back to cited text no. 5
    
6.
Tsuchiya K, Katase S, Fujikawa A. Diffusion-weighted MRI of the cervical spinal cord using a single-shot fast spin-echo technique: Findings in normal subjects and in myelomalacia. Neuroradiology 2003;45:90-4.  Back to cited text no. 6
    
7.
Rajasekaran S, Aiyer SN, Shetty AP, Kanna RM, Maheswaran A, Shetty JY. Effectiveness of riluzole as a pharmacotherapeutic treatment option for early cervical myelopathy: A double-blinded, placebo-controlled randomised controlled trial. Eur Spine J 2016;25:1830-5.  Back to cited text no. 7
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