Diffusion Tensor Imaging: A Pictorial Review of Physics and Major Fiber Tract Anatomy of Cerebral White

Matter

Abstract ID No.: IRIA - 1153

Introduction• Diffusion tensor imaging (DTI) is a magnetic resonance

imaging technique used to characterize the orientational properties of diffusion of water molecules.

• Application of this technique to the brain has been demonstrated to provide exceptional information on white matter architecture.

• DTI is the only technique available at present for in-vivo study of white matter tracts.

Physics of DT imaging• Anisotropic diffusion (directionally dependent) is the basic

principle of DTI.• The direction of maximum diffusivity has shown to coincide

with white matter fiber tract orientation.• Fractional anisotrophy (FA) is a common metric used to

describe the degree of directional diffusivity.• The 3D diffusivity is modeled as an ellipsoid whose orientation

is characterized by 3 eigenvectors whose shape is characterized by 3 eigenvalues.

X

Z

Y

Ɛ1, λ1Ɛ3, λ3

Ɛ2, λ2

Aims and ObjectivesIn this pictorial review, the following are illustrated based on a study conducted on 20 healthy volunteers:

Anatomical course of 8 major cerebral white matter tracts. Their orientation and conventional colour coding in DTI. Average normal range of FA and ADC values. Their clinical significance.

Materials and Methods

• DTI images for this study were obtained with the 1.5T system: Philips Multiva 16 channel; Coil: Head spine coil; Matrix: 112 × 110; FOV: 224 mm; Voxel size: 2 × 2 × 2 mm; Slice thickness: 2 mm; No. of slices: 64; Isotrophic diffusion encoding in 16 directions; Flip angle: 90°.

• DT imaging data were acquired by single shot echo planar imaging sequence. Diffusion registration is done after finishing DTI scanning, for distortion correction.

• Post processing done in a offline workstation where fiber tracking and FA value calculations were done using free hand region of interest (ROI) method.

• The convention used for directional colour mapping is red for left to right, green for anteroposterior and blue for superior-inferior.

Corticospinal, corticopontine & corticobulbar tracts

• Type: Projection fibers.• Anatomical course: corticospinal fibers converge into corona radiata posterior limb of internal capsule cerebral peduncle. Corticobulbar fibers

converge into corona radiata genu of internal capsule cerebral peduncle predominantly terminate in cranial motor nuclei. These bundles run together and are not discriminated on directional DTI maps.

• Orientation: Superoinferior.

• Colour coding: Blue.• FA value: 0.50 – 0.60• ADC value (103 mm2/sec): 0.85 – 0.99• Average number of lines in fiber bundle: 150 – 200• Average length of fiber tract (mm): 40 – 60

• Clinical significance: Major efferent projection fibers that connect motor cortex to the brain stem and spinal

cord. Corticospinal tracts are involved in movement of muscles of body while corticobulbar

tracts are involved in movement of muscles of head. Damage of these fibers result in upper motor neuron syndrome characterized by

spasticity, hyperactive reflexes, loss of fine movements and Babinske’s sign.

Corticospinal, corticopontine & corticobulbar tracts

Geniculocalcarine tract (optic radiation)

• Type: projection fibers.• Anatomical course: The optic radiation connects the lateral geniculate nucleus

to occipital cortex. The more inferior fibers sweep around posterior horns of lateral ventricles and terminate in calcarine cortex, more superior fibers take a strighter, more direct path.

• Orientation: Anteroposterior.

• Colour coding: Green.

• FA value: 0.45 – 0.59

• ADC value (103 mm2/sec): 0.85 – 0.99

• Average number of lines in fiber bundle: 120 – 180

• Average length of fiber tract (mm): 35 – 70

• Clinical significance:

Transmits visual information from retina of eye to the visual cortex.

Lesion affecting unilateral optic radiation results in quandrantanopia with

respective superior (or) inferior quandrant of visual field being affected.

Geniculocalcarine tract (optic radiation)

Internal capsule (Anterior limb)

• Type: projection fibers.• Anatomical course: Anterior limb - lies between head of candate and rostral

aspect of lentiform nucleus, posterior limb – lies between thalamus and posterior aspect of lentiform nucleus, Anterior limb – passes thalamocortical projection fibers to & from thalamus and frontopontine tracts, Posterior limb – Corticospinal tracts.

• Orientation: Anterior limb – Anteroposterior, Posterior limb - Superoinferior.

• Colour coding: Anterior limb – green, Posterior limb - blue.• FA value: 0.45 – 0.59• ADC value (103 mm2/sec): 0.75 – 0.89

• Average number of lines in fiber bundle: 20 – 40

• Average length of fiber tract (mm): 20 – 40

• Clinical significance: Spike and wave activity within the thalamocortical network can cause absence seizures

and other forms of epileptic behaviour. Thalamocortical dysrhythmia is associated with impulse control disorders such as OCD,

Parkinsons disease, ADHD and other form of chronic psychosis. Damage to these fibers can cause loss of consciousness. Primary motor cortex sends its axons through posterior limb of internal capsule and

therefore lesions result in contralateral hemiparesis (or) hemiplegia.

Internal capsule (Anterior limb)

Cingulum• Type: Association fibers.• Anatomical course: begins in parolfactory area of cortex below rostrum of

corpus collosum, courses within the cingulate gyrus and arching around the entire corpus collosum, extends forward into parahippocampal gyrus and uncus.

• Orientation: predominant anteroposterior orientation

• Colour coding: green

• FA value: 0.50 - 0.60

• ADC value (103 mm2/sec): 0.80 - 0.89

• Average number of lines in fiber bundle: 70 – 150

• Average length of fiber tract (mm): 30 – 50

• Clinical significance:

one of the earliest identified brain structure.

Important brain structure involved in connectivity and integration of information.

Cognitive functions including attention, visual and spatial skills, working memory.

Linked to emotion especially apathy and depression. Cingulatomy, surgical severing of

anterior cingulum is a form of psychosurgery used to treat depression and OCD.

Cingulum

Uncinate fasciculus• Type: Association fibers.• Anatomical course: Hooks around the lateral fissure to connect the orbital and

inferior frontal gyri of frontal lobe to the anterior temporal lobe.

• Orientation: C shaped fibers.• Colour coding: colour changes from green to blue as fibers turn from

anteroposterior to superoinferior orientation.

• FA value: 0.45 – 0.55

• ADC value (103 mm2/sec): 0.80 – 0.95

• Average number of lines in fiber bundle: 50 – 100

• Average length of fiber tract (mm): 20 – 40

• Clinical significance: Uncinate fasciculus on left side shows greater FA value than on right side. This

relationship is altered in schizophrenia. Abnormality in this fasciculus is associated with social anxiety, alzheimer’s disease,

bipolar disorder, depression in elderly. Reduced FA in the right uncinate fasciculus is associated with personality traits and

those on left side are associated with general intelligence, verbal and visual memory and executive performance.

Uncinate fasciculus

Superior longitudinal (arcuate) fasciculus

• Type: Association fibers.• Anatomical course: Massive bundle of fibers that sweeps along superior margin

of insula with to and fro fibers connecting frontal lobe to parietal, temporal and occipital lobe cortices.

• Colour coding: colour changes from green to blue as fibers turn from anteroposterior to superoinferior orientation.

• FA value: 0.45 – 0.50

• ADC value (103 mm2/sec): 0.80 – 0.89

• Average number of lines in fiber bundle: 200 – 500

• Average length of fiber tract (mm): 20 – 30

• Clinical significance: Largest association bundle involved with regulating motor behaviour, provides

prefrontal cortex with parietal cortex information regarding perception of visual space. Transmits auditory information between superior temporal gyrus and dorsal prefrontal

cortex. Tranfers somatosensory information such as language articulation between Broadman

areas 44, 40 & 46.

Superior longitudinal (arcuate) fasciculus

Inferior longitudinal fasiculus• Type: Association fibers.

• Anatomical course: Traverses entire length of temporal lobe and connects

temporal and occipital cortices.

• Orientation: Anteroposterior.

• Colour coding: Green.

• FA value: 0.50 - 0.59

• ADC value (103 mm2/sec): 0.80 – 0.95

• Average number of lines in fiber bundle: 30 – 100

• Average length of fiber tract (mm): 50 – 70

• Clinical significance:

Appears to mediate fast transfer of visual signals to anterior temporal regions.

Functions of direct ILF pathway is unclear. Future tractography studies of patients with

occipito temporal disconnection syndrome may help define functional role of these

pathways.

Inferior longitudinal fasiculus

Corpus callosum• Type: commissural fibers.

• Anatomical course: massive accumulation of fibers connecting corresponding

areas of cortex between the hemispheres.

• Orientation: In Mid-sagittal plane left to right orientation.

• Colour coding: red.

• FA value: 0.60 – 0.65

• ADC value (103 mm2/sec): 0.85 – 0.99

• Average number of lines in fiber bundle: 130 – 200

• Average length of fiber tract (mm): 50 – 60

• Clinical significance: Largest white matter fiber bundle, facilitates interhemispheric

communication. Anterior callosal lesions results in akinetic mutism or tactile anomia. Posterior collosal lesions results in alexia without agraphia. Symptoms of refractory epilepsy can be reduced by corpus callosotomy. Research has been done on shape of corpus callosum in those with gender

identity disorder.

Corpus callosum

Applications DTI promises a wide range of applications in clinical medicine. Measures of diffusion tensor have been used to investigate brain

development. DTI is a useful tool in providing a road map which aids in neurosurgical

planning. Parameters derived from diffusion tensor such as anisotrophy indexes are

used to evaluate white matter diseases in Krabbe’s disease, cerebral adrenoleukodystrophy, AIDS, multiple sclerosis, hypertensive encephalopathy, age related changes, schizophrenia, alzheimer’s disease, ischaemic leukoaraiosis and epilepsy.

Properties derived from diffusion tensor like trace which reflects overall water content can be used to evaluate brain ischaemia.

Studies are also done on the potential of DTI in brain tumors, migraine, eclampsia, functioning of transplant kidney and diffuse axonal injury.

Summary: White matter tractography based on DTI is a rapidly evolving

technology in CNS imaging with many challenges and exiting new applications.

• This study has attempted to provide a concise pictorial review of major tract anatomy typically visualized on directional DTI colour maps and average range of normal FA, ADC values, fiber lines and tract length in our population though they may not be representative values.

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