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).