Preparing for a Medical Imaging Test
Has a medical professional ever recommended that you undergo magnetic resonance imaging (MRI) and you found yourself thinking, “Why does the doctor want me to have that test done?” An MRI might sound intimidating to you. Although some people perceive the MRI as scary or uncomfortable, with advancement in technology have also come advances in brain imaging techniques that doctors commonly use to make assessments and diagnoses. If your doctor recommends one of these tests, you should, prior to the test, obtain some basic information on what to expect. This can be helpful to you, emotionally, because it will likely alleviate many of your fears. For example, you may be relieved to know that these imaging techniques are painless and do not involve radiation.
Various Imaging Modalities
There are many brain imaging techniques and advanced imaging modalities. Five of the most common techniques that doctors ask patients to undergo are as follows: structural MRI, functional MRI, magnetic resonance spectroscopy, and PET and SPECT scans.
Structural MRI (sMRI)
The resulting image or “picture” from structural magnetic resonance imaging informs the medical professional about the location, shape, size, and tissue composition of different structures in the brain. This technique records the differences that exist in the amount of water contained in different parts of the brain. For example, our gray matter is composed of approximately 80% water, whereas our white matter consists of about 70% water. These differences in the water content show up as different tones of gray, which the radiologist and your doctor use to differentiate the various areas of the brain.
Functional MRI (fMRI)
This type of imaging provides information on the brain’s activity patterns and on the regions that are activated during different situations, such as exercising, relaxing, smoking/using a drug, or seeing a loved one. The cells in the brain use oxygen as a fuel and when they increase their activity they demand more oxygen. Functional MRI records these differences in oxygenation and its effects on the brain’s arterial blood vessels, thereby shedding light on the brain’s activity patterns.
Magnetic Resonance Spectroscopy (MRS)
In addition to providing structural and functional imaging maps of the brain, the MRS imaging technique can measure cerebral metabolism and provide information on physiological processes involving specific brain chemicals that are working, or not working, in the brain. The output of an MRS scan might be a picture like those resulting from the more common types of imaging, a graph, and/or a color chart, showing density or flow of blood and brain chemicals. These results are used to evaluate brain growth and metabolism.
PET and SPECT Scans
Both the positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) scans are quite different from magnetic resonance imaging and are called nuclear medicine techniques because they require injecting radioactive molecules into an individual’s bloodstream before performing the test. PET and SPECT are advanced imaging techniques that produce 3-D images of the presence of specific molecules in the brain. The results provide real-time information about the functioning of the body’s different internal organs or, for example, the blood flow between organs; the state of tissue in the body, i.e., whether it is dead or alive; and the presence of malignant or benign disorders.
Deciding Which Test to Use
These brief descriptions are intended to describe some of the many imaging techniques that are currently available and how technology has advanced to the point where it is possible to obtain detailed information about the biological functioning of the body. The specific test that your physician orders will depend upon what he or she is interested in finding out.
Fowler, J. S., Volkow, N. D., Kassed, C. A., & Chang, L. (2007). Imaging the addicted human brain. Science and Practice Perspectives, 3, 4-16