An MRI scanner can be used to take images of any part of the body (e.g., head, joints, abdomen, legs, etc.), in any imaging direction. MRI provides better soft tissue contrast than CT and can differentiate better between fat, water, muscle, and other soft tissue than CT (CT is usually better at imaging bones). These images provide information to physicians and can be useful in diagnosing a wide variety of diseases and conditions.
MR images are made without using any ionizing radiation, so patients are not exposed to the harmful effects of ionizing radiation. But while there are no known health hazards from temporary exposure to the MR environment, the MR environment involves a strong, static magnetic field, a magnetic field that changes with time (pulsed gradient field), and radiofrequency energy, each of which carry specific safety concerns:
- The strong, static magnetic field will attract magnetic objects (from small items such as keys and cell phones, to large, heavy items such as oxygen tanks and floor buffers) and may cause damage to the scanner or injury to the patient or medical professionals if those objects become projectiles. Careful screening of people and objects entering the MR environment is critical to ensure nothing enters the magnet area that may become a projectile.
- The magnetic fields that change with time create loud knocking noises which may harm hearing if adequate ear protection is not used. They may also cause peripheral muscle or nerve stimulation that may feel like a twitching sensation.
- The radiofrequency energy used during the MRI scan could lead to heating of the body. The potential for heating is greater during long MRI examinations.
The use of gadolinium-based contrast agents (GBCAs) also carries some risk, including side effects such as allergic reactions to the contrast agent. See GBCAs for more information.
Some patients find the inside of the MRI scanner to be uncomfortably small and may experience claustrophobia. Imaging in an open MRI scanner may be an option for some patients, but not all MRI systems can perform all examinations, so you should discuss these options with your doctor. Your doctor may also be able to prescribe medication to make the experience easier for you.
To produce good quality images, patients must generally remain very still throughout the entire MRI procedure. Infants, small children, and other patients who are unable to lay still may need to be sedated or anesthetized for the procedure. Sedation and anesthesia carry risks not specific to the MRI procedure, such as slowed or difficult breathing, and low blood pressure.
Patients with implants, external and accessory devices
The MR environment presents unique safety hazards for patients with implants, external devices and accessory medical devices. Examples of implanted devices include artificial joints, stents, cochlear implants, and pacemakers. An external device is a device that may touch the patient like an external insulin pump, a leg brace, or a wound dressing. An accessory device is a non-implanted medical device (such as a ventilator, patient monitor) that is used to monitor or support the patient.
- The strong, static magnetic field of the MRI scanner will pull on magnetic materials and may cause unwanted movement of the medical device.
- The radiofrequency energy and magnetic fields that change with time may cause heating of the implanted medical device and the surrounding tissue, which could lead to burns.
- The magnetic fields and radiofrequency energy produced by an MRI scanner may also cause electrically active medical devices to malfunction, which can result in a failure of the device to deliver the intended therapy.
- The presence of the medical device will degrade the quality of the MR image, which may make the MRI scan uninformative or may lead to an inaccurate clinical diagnosis, potentially resulting in inappropriate medical treatment.
Therefore patients with implanted medical devices should not receive an MRI exam unless the implanted medical device has been positively identified as MR Safe or MR Conditional. An MR Safe device is nonmagnetic, contains no metal, does not conduct electricity and poses no known hazards in all MR environments. An MR Conditional device may be used safely only within an MR environment that matches its conditions of safe use. Any device with an unknown MRI safety status should be assumed to be MR Unsafe.
Adverse events for MRI scans are very rare. Millions of MRI scans are performed in the US every year, and the FDA receives around 300 adverse event reports for MRI scanners and coils each year from manufacturers, distributors, user facilities, and patients. The majority of these reports describe heating and/or burns (thermal injuries). Second degree burns are the most commonly reported patient problem. Other reported problems include injuries from projectile events (objects being drawn toward the MRI scanner), crushed and pinched fingers from the patient table, patient falls, and hearing loss or a ringing in the ear (tinnitus). The FDA has also received reports concerning the inadequate display or quality of the MR images.