Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is a powerful medical test that creates detailed pictures of the inside of your body without using radiation, helping doctors diagnose and monitor many different health conditions.

Table of contents

• What is magnetic resonance imaging?
• How MRI technology works
• Types of MRI machines
• What an MRI can show
• MRI with contrast material
• How MRI differs from CT scans
• Preparing for your MRI
• What to expect during the exam
• Safety considerations

What is magnetic resonance imaging?

Magnetic resonance imaging (MRI) is a painless medical test that produces very clear images of the organs and structures inside your body^[2]. It is one of the most commonly used tests in medicine, particularly for examining the brain and spinal cord^[4].

MRI is a noninvasive way for medical professionals to examine your organs, tissues, and skeletal system. It produces high-resolution images of the inside of the body that help diagnose a variety of conditions^[3]. The test creates detailed pictures of the body’s internal structures that are clearer, more detailed, and more likely in some instances to identify and accurately characterize disease than other imaging methods^[5].

Because MRI doesn’t use X-rays or other radiation, it’s often the imaging test of choice when people need frequent imaging for diagnosis or treatment monitoring, especially of their brain^[2]. This makes it safer overall compared to tests that use ionizing radiation^[7].

How MRI technology works

MRI uses a large magnet, radio waves, and a computer to produce detailed images^[2]. Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field inside works with radio waves and hydrogen atoms in your body to create cross-sectional images, like slices in a loaf of bread. The MRI machine can also produce 3D images that can be viewed from different angles^[3].

The technology is based on the magnetization properties of atomic nuclei. A powerful, uniform external magnetic field is used to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined. This alignment is then disrupted by introducing external radio frequency (RF) energy. The nuclei return to their resting alignment through various relaxation processes and, in doing so, emit RF energy. After a certain period, these emitted signals are measured and converted by a computer into images^[4].

Hydrogen atoms are naturally abundant in humans and other biological organisms, which is why they are most often used to generate the images detected by the machine^[7]. The magnetic field generated is so powerful that it can attract and align hydrogen atoms inside the body. Radio wave pulses are then focused on the aligned atoms in a specific organ or body part, and these signals are returned to a computer, which translates them into three-dimensional images^[8].

Types of MRI machines

There are two main types of MRI machines: closed bore and open bore^[2].

Closed-bore MRI machines have a ring of magnets that forms an open hole or tube in the middle where you lie to get the images. These machines are narrow with tight head-to-ceiling space. This can cause anxiety and discomfort for some people, but these MRI machines take the best quality images^[2].

An open (or “open bore”) MRI typically has two flat magnets positioned over and under you with a large space between them for you to lie. This allows for open space on two sides and alleviates much of the claustrophobia (fear of enclosed spaces) many people experience with closed-bore MRI machines. However, open MRIs don’t take as clear images as closed-bore MRI machines^[2].

Modern MRI scanners have improved significantly. Today’s scanners feature wider openings, better lighting, and improved ventilation. Many facilities now offer wide-bore machines that are 27.5 inches in diameter, compared to the traditional 23.5 inches, giving you crucial extra breathing room^[17].

If you’re nervous about your MRI scan or have a fear of closed spaces, talk to your healthcare provider. If needed, your provider will discuss options for sedatives (medicines to make you feel relaxed) or even anesthesia if necessary^[2].

What an MRI can show

Healthcare providers can use MRI to look at and evaluate several different structures inside your body, producing detailed images that help diagnose a variety of conditions^[2].

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, conditions of the eye and inner ear, multiple sclerosis, spinal cord conditions, stroke, tumors, and brain injury from trauma^[3].

A special type of MRI called functional MRI (fMRI) produces images of blood flow to certain areas of the brain. It can be used to examine the brain’s anatomy and show which parts of the brain are handling critical functions, language, and movements. This information can help guide decisions when considering someone for brain surgery. Functional MRI can also check for damage from a head injury or from conditions such as Alzheimer’s disease^[3].

MRI that focuses on the heart or blood vessels can check the size and function of the heart’s chambers, the thickness and movement of the walls of the heart, and the extent of damage caused by heart attacks or heart disease^[3].

MRI can be used to evaluate the body for a variety of conditions, including tumors and diseases of the liver, heart, and bowel. It may also be used to monitor an unborn child in the womb^[5].

MRI with contrast material

Some MRI exams use an injection of contrast material. The contrast agent contains gadolinium, which is a rare earth metal. When this substance is present in your body, it alters the magnetic properties of nearby water molecules, which enhances the quality of the images. This improves the sensitivity and specificity of the diagnostic images^[2].

Contrast material enhances the visibility of tumors, inflammation, infection, blood supply to certain organs, and blood vessels^[2].

If your MRI requires a contrast material, a healthcare provider will insert an intravenous catheter (IV line) into a vein in your hand or arm. They’ll use this IV to inject the contrast material^[2].

Contrast materials are safe drugs. Side effects ranging from mild to severe do occur, but severe reactions are very rare^[2]. MRI contrast is generally well tolerated. However, if you experience itching, throat swelling, or coughing after the injection, notify your technologist immediately^[6].

How MRI differs from CT scans

Magnetic resonance imaging (MRI) uses magnets, radio waves, and a computer to create images of the inside of your body, whereas computed tomography (CT) uses X-rays and computers^[2].

Healthcare providers often prefer to use MRI scans instead of CT scans to look at the non-bony parts or soft tissues inside your body. MRI scans are also safer since they don’t use the damaging ionizing radiation of X-rays^[2].

MRI scans take much clearer pictures of your brain, spinal cord, nerves, muscles, ligaments, and tendons than regular X-rays and CT scans^[2]. MRI provides exquisite detail of brain, spinal cord, and vascular anatomy, and has the advantage of being able to visualize anatomy in all three planes: axial, sagittal, and coronal^[4].

However, not everyone can undergo an MRI. The magnetic field of MRI can displace metal implants or affect the function of devices such as pacemakers and insulin pumps. If this is the case, a CT scan is the next best option^[2].

MRI scanning is usually more expensive than X-ray imaging or CT scanning^[2].

Preparing for your MRI

MRI usually requires no dietary changes or other special preparations. Unless indicated in your exam preparation instructions, you can eat and drink and take your regular medications as usual^[16]. In most cases, you can eat normally unless your doctor specifically instructs otherwise^[17].

Before your exam, you will need to complete an MRI screening questionnaire to make sure the MRI staff, technologist, and radiologist have essential information about any implants or other conditions they need to be aware of^[16].

Because of the powerful magnets, it’s essential to inform your healthcare team about any metal in or on your body before the scan. Some fabrics contain metal fibers, so you will need to change into a hospital gown to prevent a possible burn^[6]. You will be asked to change into a gown, pants, robe, and socks and place your clothes and belongings in a locker. Leave valuables at home, including jewelry, to prevent them from being lost. None can be worn during the scan^[16].

If you wear hearing aids, you will be asked to remove them before entering the MRI scan room. The magnetic field generated by the MRI can cause damage to hearing aids^[16].

If you wear a medication patch or glucose monitoring patch, it will need to be removed at the time of your exam for your safety. Before your exam, talk to your physician about removing and discarding your patch^[16].

What to expect during the exam

Plan on at least 1.5 to 2 hours for your MRI exam from check-in to checkout. If your doctor has ordered multiple exams, your appointment may take longer^[16]. MRI scans typically last between 20 minutes and 2 hours, depending on the type of exam^[6].

After you’ve been checked in for your MRI, radiology staff will verify your identification and the exam requested. You will lie on a table with a specialized coil placed over the area being scanned. The table will slide into a long tube, where you will remain for the duration of the exam^[6].

Remaining completely still is crucial for capturing clear images. You may feel mild warmth during the scan, which is normal and should not be uncomfortable^[6].

MRI machines produce loud noises during imaging. You will receive hearing protection, such as headphones or earplugs, to help muffle the noise^[6]. That noise you hear is electricity turning on and off through large wire cables inside the machine^[17]. You will also be given an alert button to signal if you need assistance. Your technologist will check on you via a two-way speaker between image captures^[6].

To help you feel comfortable, some facilities offer music through headphones during the scan. Many modern scanners are quieter and less enclosed than older models, which can ease feelings of anxiety^[15].

Safety considerations

MRI is noninvasive and does not use ionizing radiation^[5]. The procedure is completely painless, although you may hear loud sounds during imaging^[6].

The magnetic field generated by the MRI is so powerful that it can turn a small metal object into a dangerous projectile. You should complete screening forms with complete honesty about any metal in your body, including pacemakers, cochlear implants, neurostimulators, surgical clips, plates, screws, joint replacements, IUDs, dental implants, body piercings, tattoos with metallic ink, or even forgotten shrapnel from old injuries^[17].

Many people with modern versions of cardiac pacemakers and defibrillators can safely undergo an MRI with special precautions. At some specialized centers, patients with these devices are screened very carefully, and if they truly need the MRI and there’s no satisfactory alternative, the scans can often be done safely with careful monitoring^[18].

Metal that is well secured to the bone, such as hip and knee joint replacements, will not be affected by an MRI. The metal won’t heat up or move in response to the machine^[18].

Most metal tooth fillings or other permanent dental implants won’t cause a problem. If you have detachable metal braces or a retainer, you should take them out before you get an MRI^[18].

MRI is generally considered safe during pregnancy. However, contrast materials are typically avoided for expectant mothers. If you are pregnant or suspect you may be pregnant, inform your doctor immediately^[6].

Approximately 12.5% of people experience claustrophobia during MRI scans. If you feel anxious, you can ask your doctor about anti-anxiety medication, request an open MRI or wide-bore scanner, practice deep breathing beforehand, or keep your eyes closed during the scan to feel less cramped^[17].