A Neurosurgeon s Perspectives of Diffusion Tensor Imaging(DTI) Diffusion Tensor MRI (DTI) Background and Relevant Physics.

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1 A Neurosurgeon s Perspectives of Diffusion Tensor Imaging(DTI) Kalai Arasu Muthusamy, D.Phil(Oxon) Senior Lecturer & Consultant Neurosurgeon. Division of Neurosurgery. University Malaya Medical Centre. Diffusion weighted MRI (DWI) Diffusion MR images measure water proton displacements at the cellular level. Diffusion weighted MRI (DWI) generated considerable enthusiasm because of its high sensitivity for detecting acute ischemia. Currently, DWI is becoming more widely used because it is highly sensitive to the microstructural properties of the tissue. The introduction of DTI made possible the estimation of fiber directionality in fibrous tissue such as white matter and subsequently the estimation of white matter connectivity using WMT. Diffusion Tensor MRI (DTI) The tensor is simply a matrix of numbers derived from diffusion measurements in several different directions, from which one can estimate the diffusivity in any arbitrary direction, or determine the direction of maximum diffusion. This tensor is called a diffusion tensor. In general, a tensor is a rather abstract mathematic entity having specific properties that enable complex physical phenomena to be quantified. Background and Relevant Physics Molecular diffusion, or brownian motion, was first formally described by Einstein in 1905 (1). The term molecular diffusion refers to the notion that any type of molecule in a fluid (eg, water) is randomly displaced as the molecule is agitated by thermal energy (Fig 1). In a glass of water, the motion of the water molecules is completely random and is limited only by the boundaries of the container. Relevant Physics DTI measures the diffusion of water in different regions of the brain and after subsequent processing, calculates a principal direction of diffusion for water in each imaging voxel. Diffusion direction varies with tissue environment, for example water in white matter tracts has anisotropic diffusion due to the orientational structure of cells. This anisotropic motion of water in white matter tracts allows determination of their anatomical course within the human brain (Conturo et al., 1999; Basser et al.,000). This enables us to see physical connections between functionally localised brain regions to improve our understanding of brain networks. DTI can therefore predict possible relationships between cortical and subcortical areas using diffusion weighted data Relevant Physics DT MRI measures the molecular diffusion of water in brain s WM and estimates a direction of the fastest diffusivity. The water diffusion reflects microstructural organization of the tissue. In WM the diffusion is fastest along fiber direction. Fiber tracking, also called White Matter Tractography (WMT) or diffusion tensor tractography (DTT), uses the directional information of diffusion tensor maps to estimate connection pathways in brain s WM

2 Diffusion in 3 D: White Matter Anisotropic Diffusion in WM Fibers X Z Y Water in an Oriented Tissue Diffusion Ellipse Water Motion Mean Diffusivitiy Diffusion Anisotropy λ λ 3 λ 1 The tensor matrix may be easily visualized as an ellipsoid whose diameter in any direction estimates the diffusion in that direction and whose major principle axis is oriented in the direction of maximum diffusion Mean Diffusivity is the average of the diffusion in the different directions λ1 DT = MD= 3 ( λ + λ + λ ) 1 0 λ λ 3 3 Anisotropy is normalized standard deviation of diffusion measurements in different directions FA and RA most common Range from 0 to 1 RA= ( λ MD) + ( λ MD) + ( λ MD) 1 3 6MD RA=0 RA<1 Color Diffusion Brain Connectivity DT data provides a directional tensor field in the brain, used to map neuronal fibers. Diffusion MR imaging techniques are increasingly varied, from the simplest and most commonly used technique the mapping of apparent diffusion coefficient values to the more complex, such as diffusion tensor imaging, q ball imaging, diffusion spectrum imaging, and tractography. The type of structural information obtained differs according to the technique used. This has found numerous research and clinical applications Previously could only be done using cadavers or invasive studies in primates

3 Streamline Tracking Streamline DTI Advantages: Conceptually and computationally simple Was the first to be developed Disadvantages: Limited to high anisotropy, high signal areas Can only produce one track Can t handle track splitting Has the greatest difficulty with crossing fibers Probabilistic DTT Behrens et al. MRM : Advantages: Better accounts for experimental errors More robust tracking results Better deals with crossing fibers, low SNR Disadvantages: Computationally intense Probabilities will be modified by crossing fibers Probabilistic Tracking Each pixel is independent in this model Example Probabilistic DTT End zone Start zone

4 Technique FMRIB's Diffusion Toolbox DWI Analysis and Tractography (FDT) using FSL software Medtronic stelt DTI software. Steps. Tracts threshold to 50% Summary of the tracts.

5 DTT application in neurosurgery DTT used mainly in : Pre surgical planning Post operative results Neuroscience interest in functional and development of networks of brain Diagnostic such as development abnormality, Aging and Neurodegenerative Disease, Psychiatric Disease, Demyelinating Disease, Ischemic Disease, Epilepsy & Neoplasms WHITE MATTER PATHOLOGY AND SURGICAL PLANNING White matter tracts displaced by a tumor can retain their anisotropy and remain identifiable in their new location or orientation on a fiber orientation color map. Edematous or tumor-infiltrated tracts may lose anisotropy, but still retain enough orientation organization to remain identifiable on a color map. Or white matter tracts might be destroyed or disrupted to the point where directional and anisotropy organization is lost completely. Jellison, AJNR, 004 Pre op Neurosurgical Application Applications: Anatomy Jellison AJNR 5:356 DTI of the newborn brain with pathology Follow up brain development DTI can reveal detailed anatomy of white matter development. Characterization of normal axonal growth of the white matter tracts. Understanding the extensive inhomogeneity of white matter injuries (e.g., hypoxic- ischemic regions) Reference standards for diagnostic radiology of premature newborns. Early detection can improve treatment How does brain fibers develop?

6 Pitfalls and Limitations of DTI First, it is not possible to differentiate afferent from efferent pathways, anterograde and retrograde pathways, inhibitory and excitatory connections, and direct versus indirect route in diffusion data. Second, tractography picks up mainly large fiber pathways; smaller pathways, or those through regions of fiber crossing or interrupted by synapses may not be detected. The probability of tracing a pathway between two points will be influenced by factors other than the true existence of an anatomical connection for example, longer or more tortuous paths are less likely to be traced. The DTI results will further need to be validated by neuroanatomical studies. Infant vs Adult brain Acquisition Difference with Adults: Fibers are less myelinated less anisotropy lower signal intensity. Motion artifacts can play a larger role (scan within 4 minutes full-term newborns) The size of the pre-term (and neonatal) brain is smaller than of an adult. Voxel contains more structures than in an adult. The signal strength decreases if the voxel size decreases. Thank You