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Home , Arcuate fasciculus, Cingulum (brain), Corticopontine fibers, Lentiform nucleus, Nucleus (neuroanatomy), Uncinate fasciculus

What Is So Hard about Connectional ? The Easy “Tract” via Diffusion-Tensor Imaging and Krystle Barhaghi, MD; Sarah Castillo MD; Jeremy B. Nguyen, MD, MS; Quan Nguyen, MS; Enrique Palacios, MD Tulane University Hospital and Clinics , New Orleans, LA

Background Anatomy and Connectivity Connectional Neuroanatomy Commissural Fibers Diffusion tensor imaging (DTI) is an advanced form of diffusion weighted The 3 descriptive neuroanatomical classifications: The fiber tracts have been classified into 3 groups based on the imaging in magnetic resonance imaging with useful clinical general course of the fiber tracts: The largest fiber bundle applications. However, understanding DTI can be challenging, since the SURFACE ANATOMY (IMAGE A) * The corpus callosum consists of fibers technology is dependent on highly complex mathematics and Describes the appearance and topographic organization * Association fibers interconnect cortical areas in each connecting corresponding areas of cortex physics. Learning the basic concepts of tensor analysis is key to of gyral convolutions and the interposed sulci. A hemisphere. These fibers include , superior and inferior between the hemispheres. interpreting DTI and tractography. SECTIONAL ANATOMY (IMAGE B) occipitofrontal fasciculi, , superior Fibers traversing the callosal body are Diffusion imaging is based on the inherently random motion of water Describes the spatial organization of superficial (e.g. longitudinal fasciculus, and inferior longitudinal fasciculus. transversely oriented, whereas the fibers molecules known as Brownian motion. DTI exploits Brownian motion of cortex), subcortical and deep (e.g. ) Projection fibers interconnect cortical areas with deep nuclei, traversing the genu and splenium arch water molecules in tissues allowing characterization of molecular structures conventionally within orthogonal planes. * brain stem, , and . There are both efferent anteriorly and posteriorly to reach the diffusion in three dimensions of space. CONNECTIONAL ANATOMY (IMAGE C) B C and afferent fibers. Projection fibers include the corticospinal, anterior and posterior poles of the Right–sided central sulcus and perirolandic primary corticobulbar, and corticopontine tracts, as well as the optic Diffusion anisotropy effects can be fully characterized and utilized to Describes the course from start to finish of connecting motor and sensory gyri denoted by asterisk (*). hemispheres provide exquisite detail on tissue microstructure. The two most pathways. denoted by arrows. radiations common scalar metrics are fractional anisotropy (FA) and mean Commissural fibers interconnect similar cortical areas between diffusivity (MD), which are used to generate images of the diffusion Association Fibers opposite hemispheres. Fibers of the corpus callosum and The anterior commissure crosses through the . data. anterior commissure Its anterior fibers connect the olfactory bulbs and nuclei. The Tractography can also be performed using data from diffusion tensor Cingulum posterior fibers connect middle and inferior temporal gyri imaging to allow the mapping of the white matter fiber tracts in the The cingulum begins in the parolfactory area of the cortex below Transverse Fibers of brain. Transverse Fibers of the the Corpus Callosum the rostrum of the corpus callosum, then courses within the Corpus Callosum Diffusion is predominantly anisotropic in the white matter fiber tracts. cingulate gyrus, and, arching around the entire corpus The direction of maximum diffusivity coincides with fiber tract callosum, extends forward into the and orientation and is contained within a 3 x 3 matrix of diffusivity . measurements known as a diffusion tensor, which can be graphically depicted as an ellipsoid. The ellipsoid is characterized by an eigenvector and its eigenvalues. Cingulum Eigenvectors (ν) – direction of the ellipsoid (orientation) Tractography Eigenvalues (λ) – shape of the ellipsoid (diffusivities) DEC image

퐴퐷퐶 퐴퐷퐶 퐴퐷퐶 푥푥 푥푦 푥푧 Tractography 퐴퐷퐶 퐴퐷퐶 퐴퐷퐶 eigenvectors Cingulum Cingulum 푦푥 푦푦 푦푧 푣1 퐴퐷퐶푧푥 퐴퐷퐶푧푦 퐴퐷퐶푧푧 푣2 complex 9 non-zero element Directional encoded color 푣3 Tractography Tractography matrix with 6 distinct elements λ2v2 (DEC) image green: anterior to posterior red: right to left Uncinate Fasciculus blue: cranial to caudal Note: Use of the Transformation different colors is not From the Latin uncus meaning “hook”, hooking around the lateral fissure to connect the orbital and inferior arbitrary and denotes Transverse Fibers of the frontal gyri to the anterior . direction! Corpus Callosum Anteriorly parallels and lies inferomedial to the inferior occipitofrontal fasciculus. Mid portion adjoins middle 휆1 0 0 part of inferior occipitofrontal fasciculus before heading inferolaterally into anterior temporal lobe 0 휆2 0 λ1v1 λ3v3 Inferior Occipitofrontal Fasciculus 0 0 휆3 Projection Fibers The inferior occipitofrontal fasciculus connects the occipital and frontal lobes but is far inferior compared simple matrix with 3 nonzero Corticospinal, Corticopontine, & Corticobulbar Tracts diagonal elements, eigenvalues with the superior occipitofrontal fasciculus. It extends below the insula. Posteriorly, the inferior occipitofrontal fasciculus joins the inferior longitudinal The corticospinal, corticobulbar and corticopontine tracts are A large and compact fiber bundle that serves as a major conduit of The transformation is a change of frame of reference relative to MR fasciculus, the descending portion of the superior longitudinal fasciculus, and portions of the major efferent projection fibers that connect to the fibers to and from the and is readily identified on scanner to the local of region of interest. The mathematical equivalence tract to form most of the sagittal stratum, a large and complex bundle that connects the occipital lobe to the brain stem and spinal cord. directional DTI color maps. of the transformation is ‘diagonalization’ of the diffusion tensor. This rest of the brain Corticospinal fibers converge into the corona radiate and continue The anterior limb lies between the head of the caudate and the through the posterior limb of the internal capsule to the cerebral rostral aspect of the lentiform . The anterior limb passes operation simplifies the matrix representation of diffusion tensor as Inferior Longitudinal Fasciculus shown above. Scalar (invariant) diffusion metrics can be calculated from peduncle on their way to the lateral . projection fibers to and from the (thalamocortical This fiber tract connects temporal and occipital lobe cortices. This tract traverses the length of the temporal the eigenvalues. Fractional Anisotropy (FA) reflects the anisotropic Corticobulbar fibers converge into the and continue projections) as well as frontopontine tracts, all of which are lobe fraction of the magnitude of the diffusion tensor. FA varies between 0 through the genu of the internal capsule to the primarily anteroposteriorly oriented It joins with the inferior occipitofrontal fasciculus, the inferior aspect of the superior longitudinal fasciculus, (isotropic diffusion) and 1 (infinite anisotropy). The degree of where they lie medial and dorsal to the corticospinal fibers. The posterior limb lies between the thalamus the posterior aspect and the optic radiations to form much of the sagittal stratum brightness indicates the degree of anisotropy on a gray scale FA Corticobulbar fibers predominantly terminate at the cranial motor of the . The fibers are the superior-inferiorly nuclei map. Alternatively, a color scale can be used to represent the degree of Uncinate oriented fibers of the corticospinal, corticobulbar, and anisotropy. fasciculus Inferior fronto- converge into the corona radiata and travel corticopontine tracts Occipital fasciculus along side the corticospinal fibers. The tract is named according Volume Ratio Inferior fronto-occipital the origination: frontopontine, parietopontine, temporopontine or 휆 휆 휆 1 2 3 fasciculus occipitopontine tract. These fibers terminate at the cranial nuclei Internal capsule 푉푅 = 3 Fractional Anistrophy 휆 in the Posterior limb 2 3 Median Diffusivity 3 휆1−휆 + 휆2−휆 + 휆3−휆 퐹퐴 = 휆1+휆2+휆3 2 2 2 2 푀퐷 = Inferior longitudinal 휆1 +휆2 +휆3 3 Relative Anisotrophy fasciculus 휆1+휆2+휆3 with the trace 휆 = 2 3 DEC image DEC image Tractography Tractography Posterior limb 3 휆1−휆 + 휆2−휆 + 휆3−휆 푅퐴 = Internal capsule 3휆 Superior Longitudinal Fasciculus This tract sweeps along the superior margin of the insula in a great arc, gathering and shedding fibers along DEC image the way to connect cortex to parietal, temporal, and occipital lobe cortices. The superior Optic Radiation longitudinal fasciculus is the largest association bundle The optic radiation connects the lateral geniculate nucleus to Tractography Superior Occipitofrontal Fasciculus occipital (primary visual) cortex. The more inferior fibers of the optic radiation sweep around the posterior horns of the lateral The superior occipitofrontal fasciculus lies beneath the cingulum. It connects occipital and frontal lobes, Transverse Tractography uses various mathematical algorithms to bidirectionally ventricles and terminate in the calcarine cortex; the more superior extending posteriorly along the dorsal border of the Tractography pontine track the course of white matter fiber tracts passing through a selected fibers fibers take a straighter, more direct path. region of interest. The most commonly used tracking algorithm follows Portions of the superior occipitofrontal fasciculus parallel the superior longitudinal fasciculus but they are the principle directions of diffusion (the principle eigenvectors) of separated from the superior longitudinal fasciculus by the corona radiata and internal capsule Corona Radiata adjacent voxels (tensors) so long as the fractional anisotropy is above a A white matter sheet that continues Superior Longitudinal Fasciculus ventrally as the internal capsule and set threshold and the principle direction of diffusion is within a given sweeps along superior margin of the insula range (cone of probability). dorsally as the . It contains both descending and The squares are 2-D representations of voxels ascending that carry nearly all of Optic Radiation within a given plane. Optic Radiation the neural signals from and to the The red arrows represent the principle eigenvector, Uncinate the primary direction of diffusion for each tensor. fasciculus cerebral cortex. The corona radiata is Inferior fronto-occipital fasciculus associated with the corticospinal tract, Optic Radiation The green spheres represent isotropic tensors which by definition have no principle eigenvector the corticopontine tract, and the and are below the threshold of fractional anisotropy. They terminate the tracking algorithm. The yellow ellipsoids represent tensors with connecting Authors and Affliations fractional anisotropy below threshold and primary Superior Broca’s and Wernicke’s area Corona radiata direction of diffusion beyond the acceptable range Fronto-occipital of angular variance. They also terminate the fasciculus Dr. Krystle Barhaghi is a radiology resident in training at Tulane tracking algorithm. University School of Medicine in New Orleans, LA. Dr. Enrique The blue ellipsoids represent tensors with fractional Superior Palacios and Dr. Jeremy B. Nguyen are faculty members at the anisotropy and direction within the given Longitudinal Department of Radiology at Tulane University Medical Center. thresholds described above. fasciculus Quan Nguyen is a first year medical student at Louisiana State Start! The translucent disc represents a region of interest, University Health Sciences Center in New Orleans, LA. the starting point at which the algorithm begins Fiber tracking of superior Special thanks to Donald Olivares, Digital Imaging Specialist, for tracking the principle direction of diffusion from Corona radiata tractography voxel to voxel. DEC image longitudinal fasciculus DEC image assistance with poster design and printing.