Pictures of the brain: what are they and what do they tell us?
CT, CTA, MRI, MRA, fMRI, PET are an alphabet soup of brain tests that might be used in evaluation of a person suspected of possibly having Alzheimer's disease. I would like to define and describe each of these tests and then briefly explain how each one might be used.
CT stands for computerized tomography and it, as with the other acronyms, is usually followed by the word scan. This terminology represents the same thing as the older term CAT, which stood for computerized axial tomography. This test is a computerized X-ray that measures density of tissue. This measurement plus computerized analysis allows the production of pictures of the brain. Typically, these pictures are oriented as slices from the top of the head through the bottom of the skull.
CTA stands for computerized tomographic angiography. It uses the same principles as CT, but with the addition of an injection it allows the production of pictures of the arteries (Angio loosely means artery.) of the head.
MRI stands for magnetic resonance imaging.
These pictures are produced using a technology that is far more complicated than that used to produce CT images. A very high powered magnet produces brief electronic changes in certain molecules in the brain.
These brief changes produce electronic signals that are measured and manipulated, allowing the production of pictures of the brain.
These pictures have better resolution (We can see smaller brain parts.) than the scans produced by CT.
Tthe orientation of the pictures can be from top to bottom as with CT, but also side to side and front to back.
MRA stands for magnetic resonance angiography. As with CTA, it allows the production of pictures of the arteries of the head.
fMRI stands for functional MRI. Using certain types of stimulation and measurement, it allows the production of pictures that show the level of function of specific brain regions.
PET stands for positron emission tomography. This system measures low levels of radioactivity from compounds that are injected into the blood and are processed or used by the brain. Like fMRI, PET measures function. It differs from fMRI in that it does not typically use sophisticated stimulation methods and it also differs from fMRI in that the resolution is much less.
As you probably have determined, these various tests examine three different things: brain structure (CT and MRI), arteries (CTA and MRA), and function (fMRI and PET). These distinctions govern how they are used in evaluating patients with possible Alzheimer's disease. Tests of structure are used to make certain that there are no structural abnormalities that might be contributing to the behavioral difficulties (such as memory, language, personality, etc) from which the patient is suffering. Structural abnormalities include, but are not limited to tumors and strokes. The other use that structural tests may have is measurement of atrophy (brain shrinkage), a common consequence of Alzheimer's disease. Tests of the arteries are used to examine the condition of the arteries that supply blood to the brain. Abnormalities of the blood vessels can cause strokes. Strokes can cause some of the same impairments of thinking and memory that occur with Alzheimer's disease. Tests of function can be helpful in certain circumstances, such as helping distinguish what type of dementia a patient may have or why is a certain ability such as naming impaired but another ability, such as reading is not impaired.
Since the first CT scanner was first used in the United States in 1976, there have been huge increases in our ability to asses normal and abnormal brain structure and function.
However, although these tests have improved our insights into Alzheimer's and associated diseases, they do not substitute for the evaluation and insight of a good clinician. It is the clinician who will ultimately make the diagnosis of whether or not someone has Alzheimer's disease.