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White Matter Anatomy: What the Radiologist Needs to Know

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White Matter Anatomy: What the Radiologist Needs to Know White Matter Anatomy What the Radiologist Needs to Know Victor Wycoco, MBBS, FRANZCRa, Manohar Shroff, MD, DABR, FRCPCa,*, Sniya Sudhakar, MBBS, DNB, MDb, Wayne Lee, MSca KEYWORDS Diffusion tensor imaging (DTI) White matter tracts Projection fibers Association Fibers Commissural fibers KEY POINTS Diffusion tensor imaging (DTI) has emerged as an excellent tool for in vivo demonstration of white matter microstructure and has revolutionized our understanding of the same. Information on normal connectivity and relations of different white matter networks and their role in different disease conditions is still evolving. Evidence is mounting on causal relations of abnormal white matter microstructure and connectivity in a wide range of pediatric neurocognitive and white matter diseases. Hence there is a pressing need for every neuroradiologist to acquire a strong basic knowledge of white matter anatomy and to make an effort to apply this knowledge in routine reporting. INTRODUCTION (Fig. 1). However, the use of specific DTI sequences provides far more detailed and clini- DTI has allowed in vivo demonstration of axonal cally useful information. architecture and connectivity. This technique has set the stage for numerous studies on normal and abnormal connectivity and their role in devel- DIFFUSION TENSOR IMAGING: THE BASICS opmental and acquired disorders. Referencing established white matter anatomy, DTI atlases, Using appropriate magnetic field gradients, and neuroanatomical descriptions, this article diffusion-weighted sequences can be used to summarizes the major white matter anatomy and detect the motion of the water molecules to and related structures relevant to the clinical neurora- from cells. This free movement of the water mole- diologist in daily practice. cules is random and thermally driven in neurons. In the axons, the axonal membranes and myelin MR IMAGING OF WHITE MATTER TRACTS sheaths act as barriers to the random motion of the water molecules, and this motion thus White matter is seen well on the T1, T2, and fluid- becomes directionally dependent or anisotropic, attenuated inversion recovery (FLAIR) sequences with the direction of maximum diffusivity aligning used in routine MR imaging. Certain white matter with the direction of white matter tract orientation.2 tracts are reasonably well demonstrated particu- With DTI, this degree of anisotropy and fiber direc- larly on T2 and FLAIR images1 because of their tion can be mapped voxel by voxel, allowing for location and in the pediatric age group, because the in vivo assessment of white matter tract of the differences in water content and myelination architecture. Financial disclosures: None. a Division of Neuroradiology, Department of Diagnostic Imaging, The Hospital for Sick Children, University of Toronto, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada; b Department of Radiology, Christian Medical College, Vellore 632002, Tamil Nadu, India * Corresponding author. E-mail address: [email protected] Neuroimag Clin N Am 23 (2013) 197–216 http://dx.doi.org/10.1016/j.nic.2012.12.002 1052-5149/13/$ – see front matter Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved. neuroimaging.theclinics.com 198 Wycoco et al Fig. 1. Axial FLAIR images in a normal 14-year-old boy show that the corticospinal tract is mildly hyperintense, as marked with white arrows. A diffusion tensor is a mathematical model con- 1. Projection fibers: These fibers connect the taining diffusion measurements from at least 6 cortical areas with the deep gray nuclei, brain- noncollinear directions, from which diffusivity in stem, cerebellum, and spinal cord or vice versa. any direction as well as the direction of maximum Corticospinal fibers, corticobulbar fibers, corti- diffusivity can be estimated.3 Using more than 6 copontine fibers, thalamic radiations, and gen- encoding directions will improve the accuracy of iculocalcarine fibers (optic radiations) are tracts the tensor measurements,4 and DTI is more often identifiable on DTI. obtained with 30 to 60 directions. The tensor 2. Association fibers: These fibers connect dif- matrix is ellipsoid, with its principal axis oriented ferent cortical areas within the same hemi- in the direction of maximum diffusivity. Via a linear sphere. The fibers can be long range or short algebraic procedure called matrix diagonalization, range, the latter including subcortical U fibers. 3 eigenvalues are obtained, which represent The major long tracts include cingulum, supe- apparent diffusivity in the 3 principal axes of the rior and inferior occipitofrontal fasciculus; unci- ellipsoid, namely the major, medium, and minor nate fasciculus; superior longitudinal fasciculus axes, also known as the eigenvectors.3 (SLF), including arcuate fasciculus; and inferior The x-, y-, and z-coordinate system to which the longitudinal fasciculus (occipitotemporal). scanner is oriented is rotated to a new coordinate 3. Commissural fibers: These fibers connect system, dictated by diffusivity information. Frac- similar cortical areas in the 2 hemispheres, tional anisotropy (FA) is derived from the standard including the corpus callosum and the anterior deviation of the 3 eigenvalues and ranges from commissure. 0 (isotropy) to 1 (maximum anisotropy). The orien- tation of maximum diffusivity may be mapped Other tracts and fibers, which can be seen in DTI using red, green, and blue color channels and maps, include the optic pathway, fornix, and many color brightness, modulated by FA, and this can fibers within the cerebellum and brainstem. These result in the formation of a color map demon- tracts and fibers are described separately. strating the degree of anisotropy and local fiber direction. PROJECTION FIBERS The conventional color coding is green for fibers Projection fibers are afferent and efferent tracts oriented anteroposterior (mainly the association that interconnect areas of the cortex with the fibers), red for right–left oriented fibers (mainly brainstem, deep nuclei and cerebellum, and spinal commissural fibers), and blue for superior–inferior cord. Of these, the main ones identifiable on DTI fibers (in particular as projection fibers). In 2D include the corticospinal, corticobulbar, cortico- images (Fig. 2), mixed color is seen when fibers pontine, and geniculocalcarine tracts (optic overlap, resulting in yellow (green and red), radiations). magenta (red and blue), and cyan (green and blue), and with changes in orientation (see Fig. 2).5 Corticospinal Tracts CLASSIFICATION OF WHITE MATTER TRACTS Corticospinal tracts are descending projection tracts connecting the motor area to the spinal The white matter tracts are broadly classified into cord (see Fig. 2; Fig. 3). Corticospinal tracts 3 groups according to their connectivity (Table 1): have long been believed to arise from the motor White Matter Anatomy 199 Fig. 2. Fractional anisotropy (A) and corresponding color maps (B and C) and Coronal T1 with schematic overlay of the corticospinal tract (D). Note that the horizontal fibers of the genu and splenium of the corpus callosum at this level are represented in red, whereas the vertically orientated corticospinal tracts in the posterior limb are represented in blue (arrow, B). Anteroposterior fibers such as the SLF are represented in green (B). Likewise because the corticospinal tracts enter the cerebral peduncles and take a medial course, their color value mixes red and blue into magenta (arrows, D). cortex of the precentral gyrus. In a study of 42 fibers represent those that connect to the lower healthy children using DTI, Kumar and colleagues6 limb with fibers in between representing the showed that the fibers originate in both precentral hand.7 Fibers then occupy the posterior aspect of and postcentral gyrus in 71% of older children, fol- the posterior limb of internal capsule (PLIC), begin- lowed by precentral gyrus, and least commonly ning more anteriorly at the middle of the PLIC from the postcentral gyrus. This pattern was not and then shifting more posteriorly as the tracts influenced by hand preference. descend. Fibers representing the hand are anterior From the cortices, the fibers converge into the to those of the feet.2 corona radiata. Here the more anterior fibers repre- At the level of the cerebral peduncles, the corti- sent those servicing the face and the posterior cospinal tracts occupy the middle third of the crus 200 Wycoco et al Table 1 White matter tracts, their connections and main functions White Matter Tracts Connection Function Cingulum Cingulate gyrus to the entorhinal Affect, visceromotor control; cortex response selection in skeletomotor control; visuospatial processing and memory access Fornix Hippocampus and the septal area to Part of the Papez circuit; critical in hypothalamus formation of memory; damage or disease resulting in anterograde amnesia Superior longitudinal Frontotemporal and frontoparietal Integration of auditory and speech fasciculus regions nuclei Inferior longitudinal Ipsilateral temporal and occipital Visual emotion and visual memory fasciculus lobes Superior fronto-occipital Frontal lobe to ipsilateral parietal Spatial awareness, symmetric fasciculus lobe—name being a misnomer processing Inferior fronto-occipital Ipsilateral frontal and occipital, Integration of auditory and visual fasciculus posterior parietal and temporal association cortices with lobes prefrontal cortex Uncinate fasciculus Frontal and temporal lobes Auditory verbal and declarative memory Thalamic radiations Lateral
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