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Neurology India, Vol. 58, No. 6, November-December, 2010, pp. 877-878 Topic of the Issue-Editorial Diffusion tensor imaging: A colorful collage or a clinical tool? Bejoy Thomas Department of Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India Code Number: ni10251 PMID: 21150053 Detection of anisotropic Brownian motion of water molecules along structurally oriented tissue forms the basis of diffusion tensor imaging (DTI). [1],[2] Over the past two decades, it has evolved as one of the most powerful magnetic resonance imaging (MRI) tools to study microstructural alterations occurring in the brain. These techniques have been perfected by Pierpaoli and Baser [2] and several other investigators since then. [3],[4] In the initial years of its development, DTI remained mainly as a research tool and its clinical utility was very much limited. Availability of this sequence and post-processing software on clinical scanners and increased awareness among clinical colleagues have led to development of many novel applications for DTI. The greatest clinical impact was thought to be in demonstrating eloquent white matter fiber tracts preoperatively, in epilepsy and brain tumor surgeries. White matter pathways of the human brain can be demonstrated either as a DTI-based color orientation map or as 3D fiber tracts superimposed on anatomical images for orientation. Various projection, association and commissural pathways can be demonstrated by DTI [4] [Figure - 1]. In a review of nine cases, Cao et al. [5] demonstrated the usefulness of DTI in providing extremely valuable information regarding the relationship between the principal fiber tracts and brainstem lesions, which was useful in neurosurgical planning of resection of these tumors. Similar results have been reported earlier by other investigators also. [6] However, one should be careful in inferring the relative positions of the thickly packed fibers in the brainstem, especially adjacent to lesions like cavernous angiomas which can induce significant susceptibility artifacts and hence spuriously distort the DT images. Carvi et al.[7] have shown utility of DTI in demonstrating the alterations of posterior fossa white matter tracts caused by the tumors in the region. Integration of the tract information to the neuronavigational systems and the availability of per-operative fiber tracking may further help the surgeons to reduce morbidity in resection of complex brain lesions. In addition to demonstrating the fiber orientation adjacent to the tumors, directional averaged mean diffusivity (Dav) is considered to be useful in predicting tumor consistency. Fractional anisotropy (FA), a measure of the degree of anisotropic diffusion, was generally considered to be less useful in differentiating tumor subtypes. [8] However, recent reports have shown that in addition to looking at FA, a more elaborate evaluation of the diffusion metrics can characterize and grade various brain tumors. [9],[10],[11] Detection of microstructural changes early in the course of a disease process is considered to be an important application of DTI. Multiple sclerosis, motor neuron disease, normal and abnormal aging processes and detecting hitherto unknown substrates of focal epilepsy are examples of such applications. However, most of these applications still remain to be confined to the neuroscience research domain. Wang et al., in this issue, have shown the utility of DTI in demonstrating possible pathophysiological mechanisms underlying cognitive impairment in temporal lobe epilepsy attributable to microstructural changes in the frontal lobe. [12] The horizon of applications of DTI is unlimited and potentially it could be extended to study any disease process involving brain or even other organs. However, DTI has several limitations too. The most important among them is the lack of specificity, with most disease processes showing an increase in Dav and a reduction in FA. The inferences from DTI based studies should not overstate its utility. A DTI fiber tract has, if at all, only indirect relationship to the real number of axons, and is highly dependent on the resolution of the DTI scan and the chosen FA and angular deflection thresholds. Neurosurgeons should be aware of such limitations as spurious visualization or non-visualization of tracts could be a practical limitation of the technique. Another important difficulty is mapping of crossing and/or branching fibers like in the case of optic chiasm. High angular diffusion imaging and diffusion spectrum imaging aim to overcome some of these limitations by reconstructing the diffusion displacement profile by sampling diffusion over a wide range of angles. [13] With advancements in MR imaging and post-processing capabilities, these limitations could be potentially reduced or eliminated in the future. [14] References
Copyright 2010 - Neurology India The following images related to this document are available:Photo images[ni10251f1.jpg] |
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