What is DTI (Diffusion Tensor Imaging) in brain MRI (Magnetic Resonance Imaging)?

What is DTI (Diffusion Tensor Imaging) in Brain MRI?

DTI is an advanced MRI technique that measures the direction and magnitude of water molecule diffusion in brain tissue to characterize white matter microstructure and visualize neuronal pathways in three dimensions. 1

Technical Principles

DTI works by applying diffusion-sensitizing gradients in at least 6 different directions to generate a second-order tensor that characterizes the directionality of water molecule diffusion. 1 This differs from standard diffusion-weighted imaging, which only generates a scalar coefficient representing average diffusivity for each voxel. 1

The technique capitalizes on the fact that water diffusion is:

• Anisotropic (directionally restricted) in white matter axons due to their organized fiber-tract architecture 2
• Isotropic (equal in all directions) in cerebrospinal fluid and gray matter cell bodies 2

Key Measurements

DTI generates several quantitative metrics that provide information about white matter integrity:

• Fractional Anisotropy (FA): Measures the degree of asymmetry between diffusion directions; higher in white matter than gray matter or CSF due to organized microstructure 1
• Mean Diffusivity (MD): Represents the average magnitude of water diffusion 3
• Axial Diffusivity: Diffusion along the long axis of the diffusion ellipsoid 1
• Radial Diffusivity: Diffusion perpendicular to the main fiber direction 1

Clinical Applications

Traumatic Brain Injury

DTI has been studied extensively as a potential marker of axonal integrity, particularly in persistently symptomatic mild TBI. 1 Multiple studies show regions of decreased fractional anisotropy and increased mean diffusivity in patients with mild, moderate, and severe TBI compared to healthy controls. 1 However, early subacute phase studies have shown paradoxically increased fractional anisotropy, attributed to cytotoxic edema or post-injury repair. 1

White Matter Diseases

DTI enables visualization and characterization of white matter fasciculi in normal and diseased brains, including multiple sclerosis, stroke, aging, dementia, and schizophrenia. 4 In multiple sclerosis, DTI indices including mean diffusivity and fractional anisotropy correlate with myelin content, tissue integrity, and axonal loss. 1

Cranial Neuropathy

DTI metrics suggest microstructural tissue changes in symptomatic nerves in trigeminal neuralgia compared with asymptomatic nerves and may be useful for making treatment decisions. 1

Fiber Tractography

DTI creates continuous three-dimensional white matter tracts by sequentially piecing together estimates of fiber orientation from the direction of diffusion within individual volume units. 2 This allows for identification and visualization of major white matter pathways and assessment of structural connectivity. 4

Important Limitations

Despite continuing improvements in scanner gradients and diffusion techniques, there is insufficient evidence to support the routine clinical use of DTI at the individual patient level. 1

Key limitations include:

• Significant heterogeneity in fractional anisotropy measurements among both diseased and healthy subjects, with published data based primarily on group-level analyses 1
• Partial volume effects when multiple fiber populations exist within a single voxel 4
• Inability to resolve crossing fibers with the standard tensor model 1, 4
• Non-specificity: FA is highly sensitive to microstructural changes but not specific to the type of changes (e.g., demyelination vs. axonal damage) 3

Optimal Acquisition Parameters

For pediatric brain tumor imaging, DTI should be acquired with:

• Slice thickness ≤4 mm 1
• No gap between slices 1
• In-plane resolution of 2.0 × 2.0 mm 1
• Can be acquired in axial plane 1

Clinical Interpretation Pitfalls

When interpreting DTI results, recognize that FA alone provides limited specificity. 3 To maximize diagnostic accuracy, combine multiple diffusion tensor measures (e.g., MD and FA, or axial and radial diffusivity) rather than relying on a single metric. 3 Additionally, DTI findings must be interpreted in the clinical context, as changes can reflect various pathological processes including ischemia, demyelination, axonal damage, inflammation, or edema. 3