Type:
Educational Exhibit
Keywords:
Oncology, Neuroradiology brain, Paediatric, MR, MR-Functional imaging, Neural networks, Physics, Technical aspects, Localisation, Neoplasia, Multidisciplinary cancer care, Tissue characterisation
Authors:
T. Chandra1, K. Gupta2; 1Orlando/US, 2Chennai/IN
DOI:
10.26044/ecr2019/C-2977
Findings and procedure details
DTI Metrics
Mean Diffusivity (MD) = (λ1+λ2+λ3)/3
Axial Diffusivity (Da) = λ1
Radial Diffusivity (Dr) = (λ2+λ3)/2 (Fig 5)
Fractional Anisotropy –FA
- Measures the degree of anisotropic (unequal) diffusion in a voxel
- Reflects the degree of alignment of tissue microstructure
- Ranges from 0 to 1 (no units)
0 – isotropic (sphere-like)
1 – Purely anisotropic (straight line)
- Can characterize demyelinating lesions,
eg.,
breakdown of myelin and axonal loss can reduce FA and remyelination can increase FA
- FA value of CSF is 0
Mean Diffusivity –MD
- Measure of directionally averaged magnitude of diffusion (λ1+λ2+λ3)/3
- Higher MD values mean that the tissue is more isotropic
- MD is an inverse measure of membrane density and tumor cellularity
- Sensitive to cellularity,
edema and necrosis
Axial Diffusivity – Da
- Da is the apparent diffusion parallel to white matter tracts
- Da = Principal Eigen value = λ1
- Da is variable in white matter pathologies
- Da decreases in axonal degeneration
Radial Diffusivity –Dr
- Apparent diffusion perpendicular to the white matter tracts
- Dr = (λ2+λ3)/2
- Dr generally increases in white matter demyelination and dysmyelination
- Change in axonal diameter and density also affect Dr
Fiber Tractography
- Technique to assess direction of white matter tracts within the brain
- Directional information from neighboring voxels is combined to estimate 3D structure of major white-matter pathways
- Voxels are connected together taking into consideration both the direction of principle Eigen vector and FA value (Fig 6,7)
White Matter Pathways:
The major white matter pathways in brain are as follows:
1.
Projection Fibers: These connect cortical areas with deep nuclei,
brainstem,cerebellum,
spinal cord.
These include:
- Corticospinal and corticobulbar tracts
- Corticopontine tracts
- Corona Radiata
- Internal Capsule
- Thalamic Radiations
- Optic Radiations
2.
Asssociation Fibers: These interconnect cortical areas in each hemisphere.
These include:
- Superior Longitudnal Fasciculus
- Middle Longitudnal Fasciculus
- Inferior Longitudnal Fasciculus
- Superior Occipitofrontal Fasciculus
- Inferior Occipitofrobntal Fasciculus
- Uncinate Fasciculus
3.
Commisural Fibers: These interconnect similar cortical areas in opposite hemispheres.
These include:
- Corpus Callosum
- Anterior Commisure
- Cingulum
- Fornix
- Stria Terminalis
Clinical Applications:
Normal Brain
- Fiber tracking provides critical information about white matter anatomy and connections
- Regions with similar tractographic features tend to be functionally co-activated - “neurons that fire together,
wire together” (Fig 8)
- IQ has been positively correlated with anisotropy in white matter association areas
- Reading ability has been correlated with anisotropy of left temporoparietal areas
- In the visual pathway,
DTI has shown the retinotropic organization of fibers
Tumours
- MD decreases as tumor cellularity increases,
due to decreased ECF volume (Fig 9)
- Atypical and malignant meningiomas - lower MD than typical meningiomas
- Primary CNS lymphoma and Medulloblastoma also have low MD
- MD increases with tumor response with treatment and can be used as a biomarker
- Relationship of FA with tumor cellularity and treatment response is unclear
- In the peritumoral zone,
DTI metrics do not reliably differentiate edema from tumor infiltration
Surgical Planning
- This is the major clinical application of DTI in current clinical practice.
- Progression free survival is directly related to the extent of resection
- However,
benefits of cytoreduction must be weighed against risk of damage to eloquent structures and white matter tracts
- Pre-surgical mapping with DTI and fMRI results in more informed pre-surgical planning and decreases the risk of post-operative neurological deficits (Fig 10-15)
Demyelination
- MS lesions have higher ADC and lower FA values than Normal Appearing White Matter (NAWM) (Fig 16)
- Significantly increased ADC and lower FA values are seen in acute (enhancing) MS lesions than chronic (non-enhancing) lesions
- Animal models – Dr surrogate marker for active demyelination
Epilepsy
- Increased MD and lower FA values are seen in hippocampi of patients with mesial temporal sclerosis
- In patients with malformations of cortical development,
increased MD and lower FA values are seen in abnormal areas within MCD and also in the normal appearing areas on MR
- Increased MD and low FA can be used to localize lesions in MR negative cases of epilepsy
Congenital Anomalies
- White matter abnormalities in congenital brain malformations can be assessed with DTI
- Pertinent applications include callosal agenesis,
cortical dysplasia,
holoproscencephaly,
schizencephaly,
Chiari II malformation etc (Fig 17,18)
- Improved understanding of white matter abnormalities in developmental lesions
Traumatic Brain Injury
- DTI is a useful technique to evaluate microstructural injury to the white matter fiber tracts in patients with TBI
- Decreased FA and increased MD in areas afflicted by TBI that are occult on conventional MRI
- Studies suggest some correlation between findings on DTI with EEG and neuropsychological testing
- In the future,
DTI may serve as a surrogate marker for closed head injury
Interpretative Challenges
- Tumor,
edema and radiation-induced decrease in anisotropy
- Tumor-induced geometric distortions of fiber tracts
- DTI Metrics are sensitive but non-specific
- Anatomic constraints
- Distinguishing functionally different pathways in the same white matter bundle
- Acute angulations and blending of white matter pathways