Magnetic Resonance Imaging (MRI)

Overview

Magnetic resonance imaging (MRI) produces three-dimensional, detailed anatomical images based on sophisticated technology that excites and detects changes in the properties of fluids in and around cells (consisting mostly of water) that make up living tissues. Doctors often use it to detect disease, make diagnoses, and monitor treatment.

MRI scanners use powerful magnets that produce a strong magnetic field, forcing a small number of protons in the body to align with that field. When a low-energy radiofrequency signal is sent into the patient, the protons are stimulated out of their equilibrium state. When the MRI system's radiofrequency field is turned off, MRI sensors detect the radiofrequency energy released as the protons realign. The time it takes for protons to realign, and the amount of energy released, changes depending on the environment and the chemical nature of the molecules — allowing physicians to tell the difference between tissues and disease markers. You can watch a video to learn more: How Does an MRI Scan Work?

MRI scanners are particularly well suited to image soft tissues of the body. The brain, spinal cord, nerves, muscles, ligaments, and tendons appear much more clearly with MRI than with X-rays or computed tomography (CT). MRI does not use ionizing radiation, so it is useful when frequent imaging is required for diagnosis or therapy, especially in the brain. However, MRI equipment is more expensive than X-ray or CT.

Related terms: Other types of MRI not mentioned in this article include:

• Magnetic Resonance Angiography (MRA) (when used to image blood vessels)
• Magnetic Resonance Urography (MRU) (when used to evaluate the urinary tract)

What to Expect During an MRI

An MRI scanner has a tube surrounded by a very large cylinder-shaped magnet. The patient lies on a special table that slides into the tube. They will be asked to remove jewelry, eyeglasses, removable dental devices, clothing with metal, and other items that might interfere with the magnetic imaging. Because patients must remain still during the MRI, very young children or those with certain medical conditions may be secured by cushions, or in certain cases, need to be sedated.

For MRI scans, a specialized detector is placed over the target body area. The patient may hear grating or knocking noises as part of the imaging process. Earphones or earplugs can help block out the sounds.

A contrast dye may be injected into the vein to better show certain areas or tissues. If intravenous contrast is required, patients may first need a blood test to check kidney function, because the contrast agent gadolinium can increase the risk of a rare disease in people with advanced kidney disease.

Types of Diagnostic MRI

MRI is used to diagnose and monitor a wide range of conditions throughout the body including the brain and nervous system, the heart, the musculoskeletal system, and the urinary tract.

Structural MRI

To obtain an MRI image, a patient is first placed inside a large magnet. Contrast agents may be given intravenously before or during the MRI to improve contrast between normal and pathological tissues.

MRI can image differences between bone, soft tissues, and fluid-filled spaces because of differences in water content and tissue properties. In the brain, MRI can differentiate between white matter and gray matter and can also diagnose aneurysms and tumors. Depending on the part of the body being scanned, MRI can take up to an hour to complete.

Cardiac MRI

Cardiac MRI is a painless, noninvasive imaging technique. It creates detailed pictures of the heart and can provide information on the type and seriousness of heart disease. Cardiac MRI can provide an accurate look at the heart muscle, heart chamber sizes and function, and connecting blood vessels. It is an excellent tool to look for scarring of the heart muscle as might be seen in a heart attack, or inflammation of the heart as can be seen with heart infection. Cardiac MRI may be performed as a resting study or used in combination with a stress medicine or exercise to look for low blood flow to the heart muscle. It is also used to evaluate tumors or clots in the heart and to help monitor congenital heart disease or problems with heart valves or aorta. Cardiac MRI may be used when results from other tests, such as an echocardiogram or chest CT scan, are not clear.

Functional MRI (fMRI)

Functional MRI (fMRI) uses the blood’s magnetic properties to create real-time images of blood flow to specific areas of the brain. fMRI can pinpoint areas of the brain where blood oxygenation changes, giving insights into brain activity and function. This imaging process can help identify brain regions that are involved in cognitive functions such as language, motor function, or sensation. This can help doctors improve patient outcomes by identifying critical brain areas before surgery. Researchers use fMRI to study healthy brain function, head injuries, epilepsy, and neurodegenerative disorders such as Alzheimer’s disease.

Magnetic Resonance Spectroscopy (MRS)

Magnetic Resonance Spectroscopy (MRS) is a noninvasive analytic imaging technique used to study metabolic changes. MRS is particularly useful in identifying diseases affecting the brain, including tumors, strokes, and seizures. MRS complements MRI as a noninvasive means for the characterization of tissue, by providing a measure of the concentration of different chemical components within the tissue.

Monitoring Disease and Treatment With MRI

MRI is used not only for diagnosis but also to monitor treatment and guide care across many conditions. Because MRI does not use X-rays or other ionizing radiation, it is the imaging modality of choice when frequent imaging is required for diagnosis or therapy. Cardiac MRI can provide information on the type and seriousness of heart disease to help a doctor decide the best way to treat a condition. Brain MRI can provide information on the progression of neurological disorders and guide their treatment.

Safety Considerations and Contraindications

Although MRI does not emit ionizing radiation, it does use a strong magnetic field. The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects. It is strong enough to fling a wheelchair across the room. Patients should notify their physicians of any form of medical implant prior to an MRI scan.

The following considerations apply when having an MRI scan:

• People with implants, such as pacemakers, vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and capsules from capsule endoscopy should not enter an MRI machine or be in the same room. The MRI machine can damage these devices or cause a metallic implant to move.
• Prior surgeries: Metal inside the body from previous surgeries (for example, from clips or metal parts) can interfere with the MRI machine, cause the metal to move, cause artifacts in images, or cause local heating. Surgery-related metal may be safe in the MRI machine, but the imaging team must carefully screen patients ahead of time.
• Metal on the body: Piercings, jewelry, or some transdermal skin patches can interfere with the MRI machine or cause skin burns. Tattoos may cause a problem because older tattoo inks may contain small amounts of metal.
• Noise: Loud noise, commonly referred to as clicking and beeping, as well as sound intensity up to 120 decibels (similar to being at a loud rock concert or near a jet when it takes off) may occur in certain MRI scanners. In these cases, the patient may require special hearing protection.
• Nerve stimulation: A twitching sensation sometimes results from the rapidly switched fields in the MRI.
• Contrast agents: Patients with severe renal failure who require dialysis may risk a rare but serious illness called nephrogenic systemic fibrosis, which may be linked to the use of certain gadolinium-containing agents, such as gadodiamide and others. Although a causal link has not been established, current guidelines in the United States recommend that dialysis patients should only receive gadolinium agents when essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.
• Pregnancy: While no effects have been demonstrated on the fetus, it is recommended that MRI scans be avoided as a precaution, especially in the first trimester of pregnancy when the fetus' organs are being formed. Contrast agents, if used, could enter the fetal bloodstream. A fetal MRI may be ordered when prenatal ultrasound reveals a possible problem; fetal MRI is considered safe for the fetus because it does not use radiation or contrast dye.
• Breastfeeding: If a person who is breastfeeding needs to receive MRI contrast, they may be instructed to discard breastmilk for up to 2 days after the MRI study.

Coping With Claustrophobia During an MRI

People with even mild claustrophobia may find it difficult to tolerate long scan times inside the machine. The following approaches can help patients manage discomfort:

• Familiarization with the machine and process
• Visualization techniques
• Sedation and anesthesia
• Listening to music or watching a video or movie
• Closing or covering the eyes
• Holding a panic button

The open MRI is a machine that is open on the sides rather than a tube closed at one end, so it does not fully surround the patient. It was developed to accommodate the needs of patients who are uncomfortable with the narrow tunnel and noises of the traditional MRI, and for patients whose size or weight make the traditional MRI impractical. Newer open MRI technology provides high-quality images for many types of examinations.

Some centers use open MRI machines that do not completely surround the person being tested and are less confining. However, open MRI does not currently provide the same picture quality as standard MRI.

Preparing for Your Scan

Patients should talk to their doctor if they are uncomfortable in tight or closed spaces to see if they need medicine to help them relax during the test. Patients will be able to hear from and talk to the technician performing the test while inside the machine. The technician may ask patients to hold their breath for a few seconds multiple times during the test.

Find Clinical Trials

Clinical trials uncover better ways to prevent, diagnose, treat, and understand diseases and conditions. To ensure results apply to everyone, volunteers of all ages, sexes, and backgrounds, including both healthy individuals and those with specific medical conditions, are needed. Find clinical trials applying magnetic resonance imaging (MRI) on various disorders and conditions.

Research Information

The National Institutes of Health (NIH) is the largest public funder of biomedical research in the world. NIH invests most of its budget in medical research seeking to enhance life and to reduce illness and disability. NIH-funded research has led to breakthroughs and new treatments helping people live longer, healthier lives, and building the research foundation that drives discovery.

NIH supports research to improve MRI technology and usability for different audiences. It also supports research to apply MRI technologies to improve diagnosis and treatment of many disorders.

Making MRI more Usable and Accessible

NIH-funded research has supported the development of a high-performance, low magnetic-field MRI system that vastly improves image quality of the lungs and other internal structures of the human body. The new system is more compatible with interventional devices that could greatly enhance image-guided procedures that diagnose and treat disease, and the system makes medical imaging more affordable and accessible for patients.

The low-field MRI system may also be safer for patients with pacemakers or defibrillators, quieter, and easier to maintain and install. Low-field MRIs are more cost-effective and may be portable, which may eventually provide another way to bring diagnostic capabilities to people living in areas with limited healthcare facilities.

Find NIH-funded research projects using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies.