QUANTITATIVE MRI [ back to Neuroimaging ]
Quantitative MRI uses the same technology as MRI but focuses on precise measurement of the volume of brain structures rather than qualitative aspect of their visual appearance, such as whether they look healthy or damaged. In the hands of experts, this technique can be used to show gradual, progressive shrinkage of the cerebral cortex (the thinking part of the brain) and gradual, progressive enlargement of other brain structures (e.g. the fluid filled ventricles) consequent to trauma, stroke, tumor or other neurologic insult.

Since baseline size of structures varies from individual to individual, the comparison is generally between the patient and the expected norm for a person of his gender and age, unless the patient is followed over time and scanned repeatedly. If the shrinkage is large enough (e.g. virtual complete loss of hippocampal structures in an elderly chronic alcoholic with global amnesia) there is no problem getting people to agree that it exists and what is signifies. When the postulated shrinkage is subtle and the post-traumatic behavioral changes are also subtle one has a much harder time in correlating shrinkage of brain structures to a traumatic event (rather than other factors). One must always view size measurements with clinical history, neuropsychological testing and other techniques, which is a complex and difficult endeavor.

Functional studies such as PET (which measures decreases in glucose consumption) and SPECT (which measures decreases in blood flow), may be an easier way, since gross reduction in expected brain metabolism following trauma is somewhat easier to correlate than a minute difference in the expected size of a brain structure. For example, suppose a university president who could very rapidly access and very efficiently manage 1000s of items of complex information before a head trauma, and afterwards had obvious difficulty recalling names, recalling associations between people and organizations or events or making decisions. Suppose also that on a post-injury PET scan his frontal lobes (the seat of working memory and executive function) were noticeably inactive, the correlation between injury and damage is clear. With Quantitative MRI the frontal lobes might appear the same size, and be misleading, and it might be more appropriate to try MEG. Combining techniques is also promising. Thus more could be learned about our head-injured professor by using all three.