Diffusion Tensor Imaging: A Pictorial Review of Physics and Major Fiber Tract Anatomy of Cerebral White Matter Abstract ID No.: IRIA - 1153

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Diffusion Tensor Imaging A Pictorial Review of
Physics and Major Fiber Tract Anatomy of Cerebral
White MatterAbstract ID No. IRIA - 1153
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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.

Z
?2, ?2
?1, ?1
?3, ?3
Y
X
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Aims and Objectives

• In 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.

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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
  Babinskes sign.

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Corticospinal, corticopontine corticobulbar
tracts
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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.

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Geniculocalcarine tract (optic radiation)
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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.

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Internal capsule (Anterior limb)
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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.

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Cingulum
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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, alzheimers 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.

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Uncinate fasciculus
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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.

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Superior longitudinal (arcuate) fasciculus
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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.

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Inferior longitudinal fasiculus
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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.

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Corpus callosum
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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 Krabbes disease, cerebral
  adrenoleukodystrophy, AIDS, multiple sclerosis,
  hypertensive encephalopathy, age related changes,
  schizophrenia, alzheimers 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.
• Thank you