What is a brain scan?
A specialized barrier between the brain and its blood vessels prevents foreign materials and chemicals from entering the brain. This “blood-brain barrier” prevents specifically water-soluble molecules from passing through (as are found in the blood). Even fat-soluble molecules are made water-soluble and prevented from entering the brain by binding to various proteins in the blood.
Of course, when trying to image the brain’s cells using radioactive “tracers”, this is not ideal since we need the chemical to enter the brain cells. Two fat-soluble tracers have allowed us to bypass this problem. Once they have passed through the blood-brain barrier, they enter the cells similarly through the fat-soluble cell walls.
Once inside the cell, the molecules either change the cells internal environment. This effectively traps the tracer inside the cell. This trapping of the tracer is not possible if the cell is dead (e.g. stroke), damaged (e.g. Alzheimer’s disease), or inactive (e.g. after an epileptic attack). These areas will show no uptake on scanning.
We can scan the brain because we have tagged these chemicals with a low dose radioactive substance that emits gamma rays (similar to Xrays). These rays are detected by a specialized machine called a gamma camera.
Another camera called a PET-CT is also just a type of radiation detection device to form a picture of the brain. The chemical used is like sugar and will take up in any living metabolically active cell and not in inactive or damaged tissue. In this case, however, the type of radiation molecule coupled to the sugar analogue emits a different kind of radiation (positron) and, in turn, requires another kind of detector to detect.
The tracers are known as perfusion tracers (gamma) and metabolic tracers (PET-CT). There are also numerous other tracers with chemicals taken up by particular cells related to specific diseases. These tend to be more expensive and are not always available. Some are imaged on the gamma system and others on the PET-CT system. The specifics of these targeted tracers will be discussed on an individual basis.
Generally, however, acquiring these images is more straightforward than the conventional tracers.
What can I expect to happen?
Since the amount of tracer taken up depends on the cellular activity of the brain, it is essential to keep brain stimulation to a minimum when injecting the tracer.
As such, an IV line will be inserted long before the tracer is injected. For at least 10 minutes, you will be kept in a quiet, dimly lit room with minimal sensory stimulation. The tracer will be injected, and you will similarly be kept in a calm environment for another period.
After giving enough time for the tracer to accumulate in the brain, you will be placed on the camera bed, and the image acquisition will commence. This lasts approximately 20 minutes in gamma imaging as the camera rotates around your head to recreate a three-dimensional image. In the PET-CT, this process lasts about 5-10 minutes, depending on the system used.
Afterwards, you are given a chance to “cool down” from the radiation while the Nuclear Physician reports the scan. This report will allow your doctor to make an informed decision about further management of your case.
How long will the whole scan take?
Approximately 1 ½ hours (including reporting time). Repeat studies may be required to compare the brain before and after an intervention or condition.
