Diffusion tensor imaging .pptx

DTI & Tractography are special MR procedures to evaluate white matter tracts

1. Diffusion Tensor Imaging Dr . Yasna Kibria Resident, Phase B Department of Radiology & Imaging BSMMU

2. Diffusion tensor imaging (DTI)   is an MRI technique that estimates the  macroscopic axonal organization of the brain by using anisotropic diffusion of water molecules to generate contrast in MR images.

3. MR Tractography is a 3D reconstruction technique to assess  white matter tracts  using the data collected by diffusion tensor imaging

4. Diffusion Diffusion means random movement of the molecules (water protons) from a region of higher concentration to a region of lower concentration until they are equally distributed The process by which molecules (water protons) diffuse randomly in the space is called BROWNIAN MOTION

5. ISOTROPIC DIFFUSION Possibility of water protons moving in any one particular direction is equal to the probability that it will move in any other direction Isotropy = uniformity in all direction In an unrestricted environment, water molecules move randomly- I sotropy Isotropic diffusion forms the basis for routine DWI

6. ANISOTROPIC DIFFUSION In anisotropic diffusion , water molecules have preferred direction of movement Water protons move more easily in some direction than other In a constrained environment, water molecules move more easily in one axis- Anisotropy Anisotropic diffusion forms the basis for Diffusion Tensor Imaging

7. Tensor T ensor  is a rather abstract mathematic entity having specific properties that enable complex physical phenomena to be quantified In the present context, 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 diffusivity The direction of maximum diffusivity has been shown to coincide with the WM fiber tract orientation

8. Various maps are used to indicate orientation of fibres : FA ( Fractional Anisotropy) RA (Regional Anisotropy ) VA (Volume ratio )

9. Anisotropic Map Basic colors can tell the observer how the fibers are oriented in a 3D coordinate system, this is termed an "anisotropic map" . The brightness of the color is controlled by Fractional anisotropy. The software could encode the colors in this way: Red indicates directions in the  X  axis: right to left or left to right. Green indicates directions in the  Y  axis:  posterior  to anterior or from anterior to posterior. Blue indicates directions in the  Z  axis : cranio -caudal direction or vice versa. A) FA map B) FA-Color map

10. A) FA map B)FA- color coded orientation map FA map- without directional information FA Color coded map- color hues indicate direction Brightness is proportional to FA

11. Classification of White matter tracts Association fibers : Interconnect cortical areas in each hemisphere Commissural fibers : interconnect cortical areas between opposite hemisphere Projection fibers : interconnect cortical areas with deep nuclei, brainstem, cerebellum and spinal cord

12. Typically identified fibers in DTI

13. Occasionally identified fibers Optic tract Fornix Tapetum F ibers from brainstem and cerebellum

14. Association fibers Short association fibres which interconnect the adjacent gyri by hooking around the sulcus Long association fibres  travel for long distances and interconnect the widely separated gyri of different lobes. The long association fibres are grouped into bundles. 

15. Association fiber: Cingulum An association fiber that allows the communication between components of the limbic system . Interconnect portion of frontal, parietal and temporal lobe C –shaped structure located above the corpus callosum and beneath the cingulate gyrus within the medial surface of the brain Begins in the par-olfactory area of the cortex below the rostrum of corpus callosum, then courses within the cingulate gyrus, arching around the corpus callosum, extends forward into the para-hippocampal gyrus and uncus

16. Superior occipito -frontal fasciculus It lies beneath the corpus callosum It connects occipital and frontal lobes, extending posteriorly along the dorsal border of caudate nucleus. Some part of it parallel to superior longitudinal fasciculus but separated from it by corona radiata and internal capsule It also connects the occipital and frontal lobe but is far inferior than superior occipitofrontal fasciculus Extend along the inferolateral edge of the claustrum, below the insula Inferior occipito -frontal fasciculus

17. U ncinate fasciculus It connects parts of the limbic system such as temporal pole, anterior para-hippocampus and amygdala Hook shaped bundle which links anterior temporal lobe with orbital & inferior frontal gyrus of the frontal lobe

18. Superior longitudinal fasciculus Largest association fiber Lateral to centrum semiovale C onnects part of frontal, parietal, temporal and occipital lobe Sweeps along the superior margin of the insula in a great arc

19. SLF III

20. Inferior longitudinal fasciculus (Inferior occipito -temporal fasciculus) It connects temporal and occipital lobe . This tract traverses the length of the temporal lobe Joins with the inferior occipito -frontal fasciculus, inferior aspect of superior longitudinal fasciculus and optic radiation to form much of the sagittal stratum traversing the occipital lobe.

21. Commissural fiber : corpus callosum The corpus callosum is the  largest commissure of the brain  connecting the cerebral cortex of the two cerebral hemispheres Concave inferior aspect of corpus callosum is attached with the convex superior aspect of the fornix by the  septum pellucidum   Convex superior aspect is covered by a thin layer of grey matter, the  indusium griseum , embedded in which are the fibre bundles of bilateral  medial and lateral longitudinal striae . Superior aspect of corpus callosum is covered on each side by cingulate gyrus from which it is separated by a  callosal sulcus .

22. Parts of the corpus callosum Rostrum, Genu,   Trunk &   Splenium

23. **The   tapetum  is the thin lamina of white fibres which forms the roof and lateral wall of the posterior horn; and lateral wall of the inferior horn of the lateral ventricle . **The tapetum is formed by those fibres of the trunk and splenium of corpus callosum which are not intersected by the fibres of corona radiata .

24. Commissural fiber: Anterior commissure This tract connects two temporal lobes of both hemispheres & placed in front of the columns of the fornix It crosses through the lamina terminalis Its anterior fibers connect the olfactory bulb and nuclei, its posterior fibers connect middle and inferior temporal gyri

25. Projection fibers: corticospinal and corticobulbar tract They connect the motor cortex with brainstem and spinal cord Both converge in corona radiata In the internal capsule- corticospinal tract continues through the posterior limb and corticobulbar tract continues through the genu Corticobulbar fibers predominantly terminate at the cranial motor nuclei They can not be distinguished by directional DTI maps, but corticobulbar tract is situated medial and dorsal to the corticospinal tract

26. Corona Radiata Though not a specific tract per se, the corona radiata is one of the most easily identified structures on directional DTI color maps. Its coronally oriented fibers tend to give it a color quite distinct from that of surrounding tracts Fibers to and from virtually all cortical areas fan out superolaterally from the internal capsule to form the corona radiata

27. Projection fiber: Internal Capsule It is situated in the inferomedial part of each cerebral hemisphere of the brain The internal capsule is a large and compact fiber bundle that serves as a major conduit of fibers to and from the cerebral cortex and is readily identified on directional DTI color maps The anterior limb lies between the head of the caudate and the rostral aspect of the lentiform nucleus the posterior limb lies between the thalamus and the posterior aspect of the lentiform nucleus. The anterior limb passes projection fibers to and from the thalamus ( thalamocortical projections) as well as frontopontine tracts, all of which are primarily antero -posteriorly oriented In contradistinction to the posterior limb , which passes the superior-inferiorly oriented fibers of the corticospinal and corticopontine tracts. This gives the anterior and posterior limbs distinctly different colors on directional DTI maps.

28. Projection fiber: Optic Radiation It connects Lateral Geniculate body to primary visual cortex Inferior fibers of optic radiation sweep around the posterior horns of lateral ventricle and terminates in the calcarine cortex The more superior fibers are more straighter The optic radiation mingles with the inferior occipitofrontal fasciculus, inferior longitudinal fasciculus, and inferior aspect of the superior longitudinal fasciculus to form much of the sagittal stratum in the occipital lobe.

29. Brain stem fibers mlf

30. Altered DTI pattern by tumor and our goals Goals- Maximize the extent of tumor resection Minimizing post-operative neurological deficit Intraoperative mapping of tumor Relationship of tumor with functional structures

31. Altered patterns Intact but deviated: have normal anisotropy. Deviated by the tumor. Identified in their new location in DTI color map Edematous : might loose some anisotropy but retain enough directional organization to remain identified on DTI maps Infiltrated: loose anisotropy. Seen in small infiltrative glioma where mass effect appeared to be insufficient to account for abnormal hues on DTI map Destroyed: consists of isotropic diffusion . So tract can not be identified on DTI color map

32. Take home messages In DTI, anisotropy of the water protons are used to estimate the axonal organization of brain The tensor model is used to identify the orientation of the fibres Direction of the diffusion is represented as coloured Fractional Anisotropy map