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Diffusion Tensor Imaging
Theory and Practice
Noam Eshkar, M.D. Chairman Director of Neuroradiology Department of Radiology JFK Medical Center Edison Radiology Group Edison, N.J.
Diffusion Tensor Imaging
• Method of Magnetic Resonance Imaging
• MRI sensitized to mobility of protons
– Differences in rate of motion mapped
– Each voxel represents rate of water diffusion at that location
• Derives from Brownian Motion
Brownian Motion Random motion of particles in a fluid as a result of continuous collisions with molecules of the surrounding medium
Restricted Diffusion
Isotropy / Anisotropy
Isotropy: Movement uniform in all directions
Anisotropy: Movement not uniform in all directions –directionally dependent
Diffusion - Normal Anatomy
CSF
White matter
Grey matter
Diffusion - Pathology
• Sensitive to changes in tissue microstructure
– Axonal damage
– Axonal loss
– Edema
– Demyelination
– Inflammation
– Necrosis
Diffusion - Pathology
• Increased diffusion restriction – Acute infarct – Abscess – Epidermoid
• Variable diffusion restriction – Hematoma – Tumor – Thrombus – Demyelination – Encephalitis – MS – CJD – Hypertensive encephalopathy – CPM
Increased Diffusion Restriction
Acute Infarct Abscess Epidermoid
Increased Diffusion Restriction
MS PRES CJD
Decreased Diffusion Restriction
Chronic infarct Arachnoid cyst GBM
Diffusion Tensor Imaging
• Adds directional information
Diffusion Tensor Imaging
DTI Fractional Anisotropy (FA) color map
Fractional Anisotropy color map
0.7 0.3 0.14
Tractography
• Linear assembly of voxels with similar maximum diffusion direction.
Intensity and direction
Tractography
Seed tracking
• Selection of voxels from which to build tracts – user selected
Tracts – Projection fibers
• Interconnect cortical areas to deep nuclei, brainstem, cerebellum, spinal cord
– Primary motor/premotor/supplementary
• Corticospinal
• Corticobulbar
• Corticopontine
– Geniculocalcarine
• (Optic radiations)
Corticospinal tracts
Non fused
Tracts – Commissural fibers
• Interconnect similar cortical areas between opposite hemispheres
– Corpus callosum
• Interhemispheric sensorimotor /auditory
– Anterior commissure
• Nociception/pain
Corpus callosum
Tracts – Association fibers
• Interconnect cortical areas within a hemisphere. – Cingulum
• Visceromotor/visuaspatial/memory
– Superior / Inferior occipitofrontal fasciculus • Spatial awareness/auditory/visual association
– Uncinate fasciculus • Auditory/verbal memory
– Arcuate fasciculus (part of superior longitudinal) • Auditory/Speech
– Occipitotemporal fasciculus (inferior longitudinal) • Visual emotion/memory
Arcuate fasciculus
Broca/Wernicke
Uncinate fasciculus
• Inferior frontal to anterior temporal
• Auditory verbal and declarative memory
Optic radiations
Tumor – Surgical Planning
• Minimizing damage to eloquent cortex and white matter tracts while maximizing tumor resection. – Motor, Sensory, Language
• Color maps/tractography as presurgical and intraoperative guidance in regions adjacent to functional tracts – Correlate with conventional MR .
– Intraoperative electrical stimulation as needed.
Tumor – Surgical Planning
• Accuracy - tracts can be over or underrepresented
– Preop registration accuracy 1-2 mm
• Vs gold standard intraop stimulation
– Decreased accuracy w/ intraop shift/swelling
– Artifacts - susceptibility, blood, edema, air
– Non standard language of tumor/tract interaction
• Disrupted, displaced, deviated, deformed, destroyed, degenerated, interrupted, infiltrated, splayed…
Low grade tumor
Tractography BOLD fusion (anatomic + functional)
finger toe
Tractography BOLD fusion
High grade tumor
Tumor progression - GBM
Mixed displacement /disruption
Trauma
• Diffuse Axonal Injury
– Shear strain injury with disruption of axonal membranes and cytoskeletal network.
– Detection of microstructural injury.
– Persistent injury after mild TBI.
• Most brain injury mild
• 30% with residual deficits/disability
• Large public health issue
• CT/MRI underestimate axonal injury
DTI – Axonal Injury
• Acute - Reduced Fractional Anisotropy (FA) – In vivo detection – early stages TBI < 24 hours.
• Most common: Internal capsule and corpus callosum
• Chronic - Marker for long term neurocognitive deficits – Reduced FA - poorer performance on cognitive tasks
• Currently limited utility for individual cases. – No universal threshold for standard for abnormality – Spurious low FA: crossing tracts, edema – Research limitations: Heterogeneity of trauma date, ratings,
mechanisms/ ages/ control groups/ cohorts/ magnets/sequences/ lesion localization/ study design/ data analysis/ outcome measures, etc.
– Longitudinal studies limited. Acute – subacute – chronic.
Axonal Injury
FLAIR Gradient Echo
< FA -Normal vs abnormal -Technical vs anatomic -?Significance -Group vs individual
Epilepsy
• Preoperative planning - avoiding eloquent structures
– Temporal lobectomy
– other resective surgeries
• Augmenting search for epileptogenic focus
– Conventional MRI sensitivity approx 50%
• Epilepsy protocol
– PET/SPECT/MEG
Epilepsy - preoperative
Meyer’s loop
Epilepsy – Diffusion - Acute
• Periictal / immediate postictal
– Restricted diffusion > normalization
• Similar to ischemia
• Normalizes in approximately 14 days
– Mostly grey matter changes
– May spread beyond epileptogenic focus
– Rare to capture clinically
• Differential diagnosis
Epilepsy – Diffusion - Chronic
• Chronic – interictal – Decreased FA. – May be more sensitive than conventional MR
• “Nonlesional” – negative MRI
– Subtle hippocamal sclerosis / cortical dysplasia • Decreased FA in normal appearing ipsilateral white matter
– Multilesional cases – epileptogenic lesion • Tuberous Sclerosis
– Corpus callosum - decreased FA in new epilepsy in children.
– Chronic refractory – synaptic reorganization/ altered connectivity
Multiple Sclerosis - DTI
• Decreased FA –normal appearing white matter – Most pronounced with chronic lesions – Peri -plaque FA abnormalities – May precede an acute lesion
• < FA lesion burden correlating w /disability – Global histogram vs regional ROI
• Decreased fiber tracts, local vs. distant. – Wallerian degeneration
• FA can be unreliable – fiber crossings – Edema – Anatomic variation – Technical
Multiple Sclerosis – DTI - FA
0.15
0.24
0.30
0.30
0.75
Dementia – cognitive impairment
• Mild Cognitive Impairment – early AD
– < FA in normal appearing white matter
– Decreased hippocampal FA may precede atrophy
– < FA in cingular bundle and other association tracts.
– FA abnormalities overlap with other dementias – nonspecific.
Psychiatric
• Schizophrenia
– Disconnectivity theory
• Alteration in fiber bundle connectivity
– < FA abnormalities in multiple areas – inconsistent
• Major Depression
– < FA in sup long/inf long/ inf front-occ fasciculi
Development
• Increasing FA in increasing age – Infancy to adulthoood – Conventional MR stabilizes at age 2
• Evaluating injuries that affect development • Hypoperfusion/hypoxia
• Autism – Prefrontal and temporal < FA – Connectivity disorder?
• Congenital anomalies – Corpus callosum and other tract malformations – Cortical dysplasias
• FA abnormalities beyond MR visible lesions • Decreased tracts serving cortical anomalies
Corpus Callosum Agenesis
Ischemia/Infarct
• DWI critical in acute stroke
• DTI progressively < FA with infarct age
• DTI abnormalities in Wallerian degeneration distant from infarct precedes visible MR abnormalities
– Role of local and distant FA abnormalities in prediction of clinical outcomes?
Brain stem/Spinal Cord
• Utility limited – Small size of cord – Breathing – Swallowing – CSF pulsation
• Multiple Sclerosis – < FA in lesions and in adjacent normal appearing cord
• Tumors – Ependymoma vs astrocytoma
• Areas of investigation – Spondylotic myelopathy/Transverse
myelitis/Trauma/Ischemia
Medulla tumor
Thank you!