Magnetic Resonance Imaging (MRI) is a powerful method of looking inside the human body. The images obtained by MRI tend to be quite detailed, allowing physicians to see things they couldn't using other techniques like a CT scan or X-ray. Other than neurosurgery, an MRI provides the best look beneath the skull.
Because the MRI takes such compelling images, some people assume that the results must be easy to understand. Unfortunately, proper understanding of these images depends on a solid comprehension of the techniques and physics behind magnetic resonance imaging. For this reason, MRI scans are not only interpreted by the doctor who orders the scan, but also by radiologists, physicians specialized in interpretation of these images.
How an MRI works:
When a patient receives an MRI scan, they are placed in a narrow tube. Surrounding them is a very strong magnet. When this magnet is turned on, randomly spinning hydrogen atoms line up in the direction of the magnetic field. A radio pulse is applied to the area of the body to be examined. Atoms in this area absorb some of the pulse's energy, which leads them to spin in a specific frequency and direction. Smaller magnets are turned off and on in such a way to activate very precise regions known as slices. When the radio frequency pulse is turned off, the hydrogen atoms release absorbed energy, giving off a signal detected by the MRI machine in the brain. A computer analyzes these signals, and uses them to construct an image of the slice.
For more information regarding how an MRI works, try the following:
The good and bad of MRI
An MRI provides images of the human body as if it had been sliced like a loaf of bread. The slices can be in any direction, and can be a thin as a couple of millimeters. The MRI scan is the best way to look at soft tissues such as the brain and spinal cord. Also, MRI doesn't require exposure to radiation.
On the downside, an MRI is expensive and takes a long time to perform. The patient has to stay as still as possible during the scan, as even slight movement could distort the pictures. Pictures can also be distorted by any kind of metal. Finally, while MRI is good at looking at soft tissue, other techniques like a CT scan may be better at looking at bones for problems like fractures.
How an MRI can help with health-care decisions
A magnetic resonance image needs to be understood in the context of your story and your physical exam. Depending on the context, the same lesion depicted on MRI could be a sign of stroke, tumor, multiple sclerosis, or nothing important. Rather than giving a definitive answer, then, an MRI provides additional information that can help guide a physician to the correct diagnosis.
In addition to finding things on the MRI that could explain your problem, an MRI may uncover "incidental findings." These are abnormalities that may not have serious implications and are unrelated to the problem at hand. For example, many patients with neck pain are concerned about when an MRI shows mild disc bulges in the neck, until they are told that many patients without neck pain have similar disc bulges.
An MRI can be a very powerful way to assist in the diagnosis of disease, or to follow disease progression, but it is not appropriate in all situations. For more information about whether an MRI is right for you, read the following: Do I Need an MRI?
Understanding MRI reports
When most people read the MRI report provided by the radiologist, they may feel like it's written in another language. In many ways, they're right. Medical terminology is a combination of Greek and Latin. Furthermore, the report may use technical jargon to describe certain aspects of the MRI. For this reason, most physicians discuss results with their patients, rather than simply handing them a copy of the report.
When reading an MRI report, here are some common words that you may encounter--
Coronal- this is a slice that looks at the brain face first. It is like cutting a loaf of bread in the traditional fashion.
Axial- this slices the brain from the top down, like layers of a cake.
Sagittal- this slices the brain from the side.
Sequence- A sequence sets the MRI scan to certain parameters, and collects information under those parameters. Different sequences are best at showing different types of disease. Sequences can include T1, T2, and contrast.
Enhancing- sometimes MRI scans are done using contrast. This involves intravenous injection of a contrast agent, usually gadolinium. The contrast will increase, or enhance, the signal of certain types of lesions. For example, an active multiple sclerosis lesion will enhance under contrast, whereas inactive multiple sclerosis lesions will not enhance.
MRA- This stands for magnetic resonance angiogram. This is a type of MRI scan that focuses on the vessels of the brain. This is most helpful if there is concern for stroke or other vascular disease.
Hyperintensity/hypointensity- An MRI shows pixels from white to black, with many shades of gray in between. Lighter colors are said have higher signal or to be more intense. Hyperintensity, or hypointensity, signifies a region that is lighter or darker than expected-this may represent a lesion, but is sometimes due to artifact.
Artifact- Magnetic resonance images may be distorted by movement, metal, or scanner problems, among many others. In fact, there are over 60 types of artifact that can obscure MRI interpretation.
While we've discussed some common terms found in MRI reports, remember that even with a complete glossary of terms, interpretation of MRI still relies on having some understanding of the related anatomy, physics, physiology and pathology. These scans are best discussed with your doctor for a good understanding of what was found.
Robert I. Grossman and David M. Yousem. Neuroradiology: The Requisites 2nd ed. St. Louis, MO: Mosby; 2003.
Joseph P. Hornak, The Basics of MRI. Accessed November 17, 2011. http://www.cis.rit.edu/htbooks/mri/