What is an MRI?
Magnetic resonance imaging (MRI) is a medical imaging technique that uses a magnetic field and computer-generated radio waves to create detailed images of the organs and tissues in the body.
Most MRI machines are large, tube-shaped magnets. When the patient lies inside an MRI machine, the magnetic field temporarily realigns water molecules in the body. Radio waves cause these aligned atoms to produce faint signals, which are used to create cross-sectional MRI images — like slices in a loaf of bread.
The MRI machine can also produce 3D images that can be viewed from different angles.
Why get one?
An MRI helps a doctor diagnose a disease or injury, and it can monitor how well a patient is doing with treatment. MRIs can be done on different parts of the body. It’s especially useful for looking at soft tissues and the nervous system.
MRI is the most frequently used imaging test of the brain and spinal cord. It’s often performed to help diagnose:
- Aneurysms of cerebral vessels
- Disorders of the eye and inner ear
- Multiple sclerosis
- Spinal cord disorders
- Brain injury from trauma
A special type of MRI is the functional MRI of the brain (fMRI). It produces images of blood flow to certain areas of the brain. It can be used to examine the brain’s anatomy and determine which parts of the brain are handling critical functions.
This helps identify important language and movement control areas in the brains of people being considered for brain surgery. Functional MRI can also be used to assess damage from a head injury or from disorders such as Alzheimer’s disease.
MRI of the heart and blood vessels
MRI that focuses on the heart or blood vessels can assess:
- Size and function of the heart’s chambers
- Thickness and movement of the walls of the heart
- Extent of damage caused by heart attacks or heart disease
- Structural problems in the aorta, such as aneurysms or dissections
- Inflammation or blockages in the blood vessels
MRI of other internal organs
MRI can check for tumors or other abnormalities of many organs in the body, including the following:
- Liver and bile ducts
MRI of bones and joints
MRI can help evaluate:
- Joint abnormalities caused by traumatic or repetitive injuries, such as torn cartilage or ligaments
- Disk abnormalities in the spine
- Bone infections
- Tumors of the bones and soft tissues
MRI of the breasts
MRI can be used with mammography to detect breast cancer, particularly in women who have dense breast tissue or who might be at high risk of the disease.
Risks of MRI
Because MRI uses powerful magnets, the presence of metal in a patient’s body can be a safety hazard if attracted to the magnet. Even if not attracted to the magnet, metal objects can distort the MRI image. Hazardous devices include:
- Metallic joint prostheses
- Artificial heart valves
- An implantable heart defibrillator
- Implanted drug infusion pumps
- Implanted nerve stimulators
- A pacemaker
- Metal clips
- Metal pins, screws, plates, stents, or surgical staples
- Cochlear implants
- A bullet, shrapnel, or any other type of metal fragment
- Intrauterine device
What to expect?
The MRI machine looks like a long narrow tube that has both ends open. The patient lies down on a movable table that slides into the opening of the tube. A technologist monitors them from another room. They can talk with the person by microphone.
The MRI machine creates a strong magnetic field around the patient, and radio waves are directed at their body. The procedure is painless. Patients don’t feel the magnetic field or radio waves, and there are no moving parts around them.
During the MRI scan, the internal part of the magnet produces repetitive tapping, thumping, and other noises. In some cases, a contrast material, typically gadolinium, will be injected through an intravenous (IV) line into a vein in the patient’s hand or arm. The contrast material enhances certain details.
An MRI can last anywhere from 15 minutes to more than an hour. The patient must hold still the entire time because the movement can blur the resulting images.
During a functional MRI, patients often are asked to perform a number of small tasks — such as tapping a thumb against their fingers or answering simple questions. This helps pinpoint the portions of the brain that control these actions.
If the patient hasn’t been sedated, they can resume their usual activities immediately after the scan.
MRI results may take a few days to come back, but this varies from facility to facility. The doctor or the MRI technologist will inform the patient about how long they should expect to wait and what they might need to think about in terms of potential results.
A radiologist will look at and interpret the MRI scans. He or she will then write up and send a radiology report detailing the results to the doctor, who will then share the main findings of the MRI with the patient and talk about their next steps.
It may be difficult to make sense of the MRI without some advanced medical knowledge. A doctor or radiologist can answer any questions the patient may have.
A typical radiology report includes a number of sections (exam type, clinical history, etc.), one of which is the radiologist’s findings of each of the areas in the body that were scanned in the MRI. Each area is classified as normal, abnormal, or potentially abnormal.
In the impression section, the most important part of the report, the radiologist combines a patient’s medical history with the findings of the MRI and reason for the test and gives a diagnosis based on these factors. If there isn’t enough information for a specific diagnosis, the radiologist lists possible diagnoses (differential diagnoses) that may fit the situation.
Patients may need to follow-up with their doctor if the MRI results are not normal.
If there is an abnormal or potentially abnormal finding, depending on the circumstances, the radiologist may recommend steps such as:
- Additional imaging, such as a repeat MRI, a CT scan, ultrasound, X-ray, or nuclear medicine imaging, such as positron-emission tomography (PET)
- Comparing the MRI finding with lab results and/or the patient’s symptoms
- Comparing the MRI to past imaging scans, if possible
Inconclusive: If the MRI didn’t find what the doctor was looking for, the patient will probably have a repeat MRI scan that uses different views or with a special imaging technique, such as a magnetic resonance angiography (MRA) to look at the patient’s blood vessels, an fMRI, or MRI with contrast to look more in-depth for what the doctor is trying to find. Patients may also have one of the imaging tests mentioned above instead of or in addition to MRI.
A potentially abnormal finding on the MRI may also warrant a follow-up MRI to see if the area has changed. In either of these situations, the doctor may schedule these as soon as possible.
In cases where an MRI helped to diagnose a specific medical condition, a doctor will talk to the patient about a treatment plan. The patient may also have another MRI (or more than one) so that the doctor can monitor the abnormality for changes and see if the treatment is working. This may be scheduled for a later time.