DTI Basics

Diffusion Tensor Imaging (DTI) is a type of magnetic resonance imaging (MRI) technique that is able to detect brain injuries that affect the integrity of white matter tracts, which are the lines of communication in your brain. Without white matter, the brain would consist of four different sections, or lobes, that are responsible for various functions, but could not work together. Imagine you’re a remote employee working on an important team project, but your internet is down and you are unable to communicate with anyone else in your office. You have all the files necessary to complete your part of the project, but you can’t share them with the rest of your team.  White matter tracts are like your brain’s internet connectivity – allowing all the sections of the brain to communicate with each other. When these are damaged, the different lobes of your brain cannot talk to each other, leading to common symptoms such as memory problems, mood changes, and balance issues.

DTI works by measuring the diffusion of water molecules within the white matter tracts, which means it looks towards the molecules’ movements. In normal white matter, water diffuses in the direction of a specific white matter tract. When the molecules encounter barriers, such as cell membranes, DTI can map the directionality of white matter tracts. It can detect when these water molecules move in a more chaotic manner which would indicate damage caused by a brain injury. These disruptions to the normal organization of white matter tracts can indicate issues such as traumatic brain injury, stroke, or multiple sclerosis.

For example, in a TBI, the impact can cause damage to axons (the long, thin projections of nerve cells that form the white matter tracts). This damage can disrupt the directional diffusion of water molecules (what we touched on above), leading to changes in the DTI signal that can be visualized on a DTI map. These changes can help identify the location and extent of the injury, as well as the severity of the damage.

Think of marbles rolling down a track. A stick is placed in front of them and does not allow the marbles to flow down as they should. Now they start bouncing all over the place. DTI can locate that bouncing and tell us that there was likely an injury to the track. The severity of that bouncing tells us how severe the injury is. Where the bouncing and chaos starts tells us where the injury is.

Overall, DTI has become a valuable new tool to not only to detect brain injuries and their severity, but also monitor their progression.

DTI in Legal Cases

In legal cases, DTI can be used as a tool to provide evidence of brain injury or damage, particularly in cases of traumatic brain injury (TBI). What sets DTI apart from, say, MRI, is that it can be used to demonstrate how an injury has affected an individual’s cognitive or motor functions, which is extremely relevant in personal injury cases. For example, suppose an individual has suffered a TBI as a result of a car accident. In that case, DTI could be used to demonstrate how the injury has affected their ability to drive or perform other tasks.

When it comes to minor brain injuries, often an MRI will not reveal any noticeable structural damage. On the other hand, DTI’s intricate dive further into the inner workings of the brain can detect such minor brain injuries. And remember, just because a brain injury is minor, does not mean it does not affect one’s everyday life and potentially come with lifelong side effects.

Admissibility of DTI Evidence in Personal Injury Cases

The State of Colorado follows what is called the Daubert standard in determining the admissibility of scientific evidence into trial. The case, Daubert v. Merrell Dow Pharmaceuticals Inc., 509 U.S. 579 (1993), was a United States Supreme Court Case that held that the admissibility of scientific evidence is based on determining whether there is valid methodology based on: (1) whether the theory or technique in question can be and has been tested; (2) whether it has been subjected to peer review and publication; (3) its known or potential error rate; (4) the existence and maintenance of standards controlling its operation; and (5) whether it has attracted widespread acceptance within a relevant scientific community. In order for the scientific evidence to be admitted, all five of those elements must be met.

In personal injury cases, defense attorneys will spend a lot of energy on fighting the admissibility of scientific evidence because it can hold a whole lot of weight with a jury, especially if it can prove that an “invisible injury” is much more extensive than it seems on its surface. That is why these relatively new technologies such as DTI are so integral to the legal field  – it gives the injured party’s trial team more ammo to present to the jury. So, it is almost inevitable that a defense attorney will attack DTI because of the damning evidence that it can present. They may go after its reliability by trying to present various outlier studies to bar its admission. Or they may attack how it is relatively new and there is not enough evidence to understand DTI’s true effectiveness.

Fortunately for the good guys, DTI is already gaining a lot of traction as an acceptable method to determine the extent of brain injuries. Courts across the country have admitted DTI into evidence leaving the defense to try and counter it at trial. However, a knowledgeable, properly trained, and well-spoken expert for the plaintiff will generally have minimal issues convincing a jury of DTI’s effectiveness in diagnosing and understanding the severity of brain injuries.

https://coloradoinjurylaw.com/blog/diffusion-tensor-imaging-in-personal-injury-cases/

Diffusion tensor imaging (DTI) is an MRI technique that uses anisotropic diffusion to estimate the axonal (white matter) organisation of the brain.

Fibre tractography (FT) is a 3D reconstruction technique to assess neural tracts using data collected by diffusion tensor imaging.

Diffusion-weighted imaging (DWI) is based on the measurement of thermal Brownian motion of water molecules. Within cerebral white matter, water molecules tend to diffuse more freely along the direction of axonal fascicles rather than across them. Such directional dependence of diffusivity is termed anisotropy. This direction of maximum diffusivity along the white-matter fibres is projected in the final image.

Clinical applications

For many, diffusion tensor imaging is synonymous with MRI of the CNS. However, its use for the assessment of highly-organised body systems outside the CNS, where anisotropy can facilitate early detection of pathology, has been gaining favour.

Applications include:

• assessment of the deformation of white matter by tumours - deviation, infiltration, destruction of white matter

• delineation of the anatomy of immature brains

• presurgical planning

• Alzheimer disease - detection of early disease

• schizophrenia

• focal cortical dysplasia

• multiple sclerosis - plaque assessment

• early identification of musculoskeletal and peripheral nerve pathology 7-10

Quantitative analysis methods

• ROI (region of interest) based

• voxel-based

• histogram analysis

• tractography/fibre tracking 

Physics of diffusion tensor imaging (DTI)

Some terms:

• diffusion tensor imaging (DTI) provides a quantitative analysis of the magnitude and directionality of water molecules

• the word tensor indicates the use of a 3x3 matrix with eigenvalues and eigenvectors as its constituents

• the two main parameters derived from DTI data are mean diffusivity (MD), also referred to as apparent diffusion coefficient (ADC), and fractional anisotropy (FA)

• FA reflects the directionality of molecular displacement by diffusion and varies between 0 (isotropic diffusion) and 1 (infinite anisotropic diffusion). FA value of CSF is 0

• MD reflects the average magnitude of molecular displacement by diffusion. The higher the MD value, the more isotropic the medium

• AD - axial diffusivity represents the longest eigenvector 

• RD - radial diffusivity represents the average of two shorter eigenvectors

• DTI data acquisition is done by SE-EPI with the application of diffusion gradients in multiple directions (SE-EPI = spin-echo echo-planar imaging, using pairs of 90-180 degree pulses, giving T2-weighting)

Colour coding

In fibre tractography imaging, the convention for colour coding is as follows:

• red: transverse fibres

• green: anteroposterior fibres

• blue: craniocaudal fibres

Fibres with oblique orientation are represented with colours originating from the combination of the three primary colours:

• red + blue = magenta

• green + red = yellow

• green + blue = cyan 15

Limitations of fibre tractography

The rationale of the tractographic algorithms lies in the presupposition that in each voxel, the direction of the main eigenvector coincides with the average direction of a single fibre bundle. The approach, therefore, leads to a correct result only if the region of interest is homogeneous and, the direction variations of the fasciculi are of the order of size of the voxels. It follows that where more bundles of fibres coexist or where they cross, approach, converge or diverge, the algorithm works poorly. These limits, in clinical practice, could lead to following paths that do not exist (false positive) or to not adequately follow paths that exist (false negative); therefore, the interpretation of tractographic reconstructions requires prior experience and knowledge 11-14.

Other limitations include, the inability of the algorithm to distinguish the direction of the neural pathway (afferent from efferent projections), to determine its function and to identify the presence of synapses along the course of the same neural pathway.

link:- https://radiopaedia.org/articles/diffusion-tensor-imaging-and-fibre-tractography-1