By: Robert L. Shepherd, MS, Certified Medical Illustrator, President & CEO, MediVisuals Inc.
By: Robert L. Shepherd, MS, Certified Medical Illustrator, President & CEO, MediVisuals Inc.
In simple terms, traditional MRIs are far less able to image many brain injuries because they cannot detect physical injuries smaller than the head of a pin. Since the head of a pin is large enough for tens of thousands of axons to travel through it, it follows that a cluster of tens of thousands of axons would have to be injured in one location in order for those axonal injuries to be detected by traditional MRI. Unfortunately, in most instances, traumatic brain injuries are far more diffuse or widespread. These injuries are characteristically referred to as “Diffuse Axonal Injuries” and involve injuries to many tens of thousands of axons (or even millions), but because the injuries are not in a single location, traditional MRIs do not image them.
An analogy would be to compare focal vs. diffuse axonal injuries to tree destruction after the Mount St. Helens eruption (see Figure A) versus tree destruction after a hurricane as viewed from an airplane. When flying over Mount St. Helens after the eruption, with every tree over thousands of acres destroyed, the absence of those trees was easily visible (much like injury to tens of thousands of axons clustered together would be detectable by traditional MRI). By comparison, a hurricane may result in the destruction of more trees than the Mount St. Helens eruption, but because the destruction is far more wide-spread and interspersed with many trees that are still standing, the tree destruction from a hurricane is not visible from a plane (like thousands or millions of diffusely torn axons would not be visible by traditional MRI).
Figure A: Photo of the many acres of destruction of forest after eruption of Mt. St. Helens. ©Alamy Stock Photo, image license obtained
DTI (Diffusion Tensor Imaging) is a type of DW-MRI that involves the measurement of molecules in six or more directions (as opposed to typical DW-MRI that measures the movements in only three directions). Analysis of these molecular movements is then performed with “Fractional Anisotropy” (FA). FA measures the water molecule movements and then analyzes and characterizes those movements in different degrees of random (“isotrophic”) movements versus movements that have notable directional restrictions or preferences (“anisotrophic” movements).
Because of the highly organized, directional bundles of fiber tracts in the brain’s white matter, water movements in those areas are very anisotrophic (restricted directionally) because the water movements are channeled within and between the axons and myelin sheaths (see Figure B or click here to view a demonstrative animation).
Figure B: Image showing restricted or preferred flow of water molecules along uninjured parallel axon fiber tracts.
When axons are torn, water molecule movement is no longer restricted within and between the tracts of axonal fibers (see Figure C or click here to view a demonstrative animation).
Figure C: Image demonstrating diffuse axonal tears resulting in more random movement of water molecules.
Most areas of traumatic diffuse axonal injury are not truly “diffuse” throughout the entire brain; they are instead more concentrated in areas of the brain where most of the traumatic forces were concentrated or where the brain tissue was most vulnerable to the trauma. Imaging generated with DTI reveals these concentrations of diffuse axonal injury as absences of or decreases in the typical signal in those areas. For example, in the below image (Figure D), note the stark contrast between the signals in the right vs left frontal lobe tracts — indicating significant axonal injury in this area.
Figure D: DTI image demonstrating stark contrast between signals in the right vs. left frontal lobes – indicative of axonal injuries
One might expect a typical MRI to also reveal such a large injury (involving millions of axons). However, in this case and likely in most other cases of diffuse axonal injury, traditional MRI does not show the injuries because, instead of the injuries to the millions of axons being in one concentrated area, the injuries consist of multiple, microscopic and widespread islands of torn axons that are not visible on traditional MRI studies but still result in the same devastating neurocognitive deficits (as depicted in Figure E below and as shown in an animation that can be viewed here).
Figure E: Image demonstrating multiple widespread islands of axonal injuries that collectively represent injuries to millions of axons, but because they are interspersed through areas of intact axons, they are undetectable by traditional MRI imaging.
Please see the article at the below link for information about Daubert challenges associated with DTI:
www.braininjury.com/brain-injury-diffusion-tensor-imaging.shtml
