MRI Scan (Magnetic Resonance Imaging)

An MRI scan (magnetic resonance imaging) uses magnetism and radio frequencies to create images from within the body. It is a much different technology than X-ray or CT scan because no radiation that penetrates the body is used.

An MRI scan (magnetic resonance imaging) uses magnetism and radio frequencies to create images from within the body. It is a much different technology than X-ray or CT scan because no radiation that penetrates the body is used.

The MRI requires a two-step process to generate an image once the patient is placed into the scanner.

First, a powerful magnet is used to align all the protons in the body in the same direction. Protons are part of the core of every atom in nature.

Second, radiofrequency waves are pulsed through the patient’s body. This causes the protons to try to move out of alignment in the magnetic field. When the radiofrequency is turned off, the machine detects the time it takes for protons to return to their previous position and how much energy is released. Different cells, tissue, and organs have different responses to the magnet and the radiofrequencies. These differences can be measured and the MRI image generated.

In some patient situations, gadolinium, an intravenous dye, may be injected during the MRI scan, to help produce clearer images. The gadolinium affects the timing of protons as they move back into positions, and is often given in patients with tumors or abscesses. It can also be used to measure blood flow in arteries. The dye is usually not needed for common orthopedic imaging such as knee or shoulder MRIs.

MRI is able to image the soft tissues of the body. It is better able to see the differences between water, fat and muscle, and therefore can evaluate ligaments and tendons. It is very useful to “see” damage within a knee, shoulder, or hip.

The MRI allows the brain and spinal cord to be better imaged with greater clarity than a CT scan, and may be used in patients with tumors, stroke, or other neurologic disease.

Because the MRI scan requires a significant amount of time to complete, a CT scan is often used as a first-line test in an acute stroke patient to decide whether emergent intervention is an option.

What is an fMRI scan?

An MRI scan is used to show images of the body anatomy. Functional MRI (fMRI) is a specific brain scan to measure brain function. It is used to determine what parts of the brain are working during a scan. Because different parts of the brain control each body function, blood flow can be mapped as the patient is asked to do a specific task, like tap a finger or talk.

The ability to measure blood flow to the brain as it is asked to control the body, allows your provider (usually a radiologist or neurologist or neurosurgeon) to evaluate diseases like tumors or epilepsy. It can also be used to monitor patient recovery after surgery or radiation treatments. This is an evolving technology with new applications and uses expected over the coming years.

MRI scan vs. CT scan

The MRI and CT scans are completely different technologies. The machines may appear similar, but how they work to produce an image are not the same.

Computerized tomography (CT) scan uses an X-ray that penetrates the body to produce images based on how effectively different tissues block those X-rays. For example, bones block X-rays effectively so they show up as white on the image, while air does not block X-rays and shows up as black. Each tissue blocks the X-ray differently and that is how images are produced.

An MRI scan uses no radiation and its images are produced based on how protons in the body are affected by magnetism and radiofrequency.

The different technologies are also not alike when it comes to how long the tests take. CT scans may be completed within seconds and therefore are referred in emergent evaluation of trauma or acute stroke; MRI scans may take an hour or longer and are not often emergently ordered, though there may be special situations when that is the case.

The patient's condition and why the test is ordered will determine if the CT scan or MRI is a better modality.

An MRI has no X-ray radiation exposure. There are no known side effects of an MRI scan.

There are some patients who cannot have an MRI scan. The scan uses a powerful magnet and therefore no metallic objects are allowed in the room where the MRI machine is located. It means that patients with metal inside their body may not be able to be scanned.

When the magnet is turned on, any metal objects inside the room can become projectiles and fly toward the magnet. If there are metal objects within the patient, they may move or heat up because of the magnetic energy.

Some implanted medical devices include heart pacemakers or defibrillators, insulin pumps, medication patches, cochlear implants, aneurysm clips, artificial joints, programmable shunts, and nerve stimulators.

Some medical devices are MRI safe. The patient, their health care provider, and the medial record that contains the device serial number and description, should be able to determine whether the device can qualify for a scan.

Patients may also have retained metallic foreign bodies from previous injuries (metal shavings, bullets, shrapnel), or piercings.

Some tattoo inks may contain flecks of metal. Most often, tattoos do not prevent an MRI, but they can start to heat up during the scan and the patient must tell the MRI technologist if this occurs. Skin burns are a potential complication.

Pregnant patients may undergo an MRI at any time during their pregnancy. There is no risk to the patient or the fetus.

Prior to scheduling the MRI, the patient is asked to complete a questionnaire to determine whether they qualify for the scan, specifically addressing medical devices and other retained metal fragments from previous injury.

Before entering the room that contains the MRI scanner, the patient will be asked to undress completely and put on a gown. This will ensure that no metal is brought into the room.

MRI scanning requires that the patient lie still to get the best images. The patient has to lie still within the machine for a prolonged time. The MRI may take 30 minutes to an hour or more. Occasionally, the patient will be given a sedative medication prior to the scan to decrease anxiety and allow them to relax for the duration of the scan.

What not to do before an MRI?

There is little preparation needed for an MRI.

Depending upon the type of scan, you may be asked to not eat or drink for a few hours beforehand.

Sometimes you are asked to not urinate prior to the scan.

You must tell the MRI technologist if you have metal objects on or in your body.

How long does an MRI scan take?

Depending upon the diagnosis, the area of the body being scanned, and what information is needed from the scan, the MRI may take between 30 and 90 minutes to complete.

There may be hundreds of images produced from a single MRI scan. They are interpreted by a radiologist and the results provided to the health care provider who ordered the test. Often the results are available within hours.

In emergent situations, the results are reviewed immediately or the care providers may even watch the images in real time.