MRI: What You Need to Know

Medically Reviewed by Zilpah Sheikh, MD on March 26, 2024
Written by Kathryn Whitbourne
10 min read

Magnetic resonance imaging (MRI) is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures of the inside of your body.

MRI scan vs. CT

Both tests let a doctor see inside your body to diagnose a condition or how you're responding to a treatment. Both procedures require you to lie on a table and go inside a circular scanner while a technician goes into another room to take the images. But the tests have different purposes.

CT (computed tomography) scans combine X-rays with computer technology. They're used mainly to look at structures deep inside your body like tumors or bone fractures, or for when doctors need to make an emergency diagnosis, as results can be had in as little as 10 minutes. CT scans are cheaper than MRIs, too.

MRIs provide more details than a CT scan but results take longer -- maybe 1 hour. MRIs are used to diagnose problems like torn ligaments, inflammation, or spinal issues. It's especially useful for looking at soft tissues (like organs and muscles) and the nervous system. Unlike X-rays and CT scans, MRIs don’t use damaging radiation.

The MRI is basically a tube-shaped magnet. While you're inside the machine, protons from water in your body get pulled into the magnetic field around you and start to line up. The machine also sends out radio waves that cause these protons to spin. When the waves are turned off, the atoms lineup again. As they line up, they send out radio signals. A computer receives these and converts them into an image of the body part being looked at. This image shows up on a screen.

MRI with contrast vs. without

Before some MRIs, you'll get contrast dye injected into a vein in your arm or hand. This dye helps the doctor more clearly see structures inside your body or subtle changes between tissue that is healthy and tissue that is cancerous. The dye often used in MRIs is called gadolinium. It can leave a metallic taste in your mouth. 

Most MRIs are performed without a contrast dye because it isn't needed to get accurate results for the suspected problem. Also, certain patients (those who are pregnant as well as those with kidney disease) shouldn't get the contrast dye because of possible complications.

 

An MRI helps a doctor diagnose a disease or injury, and it can monitor how well you’re doing with a treatment. MRIs can be done on different parts of your body. 

Brain MRI

An MRI of the brain and spinal cord can help find many things, like:

Heart MRI

An MRI of the heart and blood vessels looks for:

Bone and joint MRI

An MRI of the bones and joints looks for:

Breast MRI

An MRI of the breast can:

  • Screen for breast cancer in people who have a high risk for developing the disease
  • See how large a tumor is and how far it has spread in people who've been diagnosed with breast cancer
  • Find out whether the cancer has come back after being treated with surgery or chemotherapy
  • See whether breast implants have ruptured

Liver MRI

A liver MRI might be used to check for 

  • Liver cancer
  • Hepatitis 
  • Cirrhosis 
  • Other liver diseases

Kidney MRI

A kidney MRI can look for:

  • Kidney cancer
  • Chronic kidney disease
  • Renal vein thrombosis
  • Kidney stones or tumors

Although the risk is still low, people with chronic kidney disease (CKD) do have a higher chance of developing problems if a contrast dye is used for their MRI. This is because people with CKD can't filter out waste products (like a contrast dye) as quickly as others can. In some cases they can get diseases from the dye like contrast induced nephropathy and nephrogenic systemic fibrosis.

Ovary MRI

This is mainly given to figure out whether a mass in your ovaries is cancerous or not (benign).

Pancreas MRI

Located behind the stomach, the pancreas helps with digestion and regulates blood sugar. An MRI of this organ is used to detect:

  • Pancreatic cancer
  • Pancreatitis
  • Whether cysts and lesions in this area are cancerous or benign

Prostate MRI

This is used to see whether there's cancer in your prostate and if it's spreading, as well as to evaluate other issues like an enlarged prostate.

fMRI

A special kind of MRI called a functional MRI (fMRI) maps brain activity.

This test looks at blood flow in your brain to see which areas become active when you do certain tasks. An fMRI can detect brain problems, such as the effects of a stroke, Alzheimer’s, or a brain injury. It can also be used for brain mapping if you need brain surgery for epilepsy or tumors. Your doctor can use this test to plan your treatment.

MRI contrast dye risks

Don't get contrast dye if you've had an allergic reaction to it in the past or you have severe kidney disease. If you are allergic to shellfish or iodine, or have conditions such as asthma, allergies, anemia, low blood pressure, or sickle cell disease, let your doctor know in advance.

Pregnancy and MRI

Pregnant women shouldn’t get an MRI during their first trimester unless they absolutely need the test. The first trimester is when the baby's organs develop. 

It's considered safe to have contrast dye while you're breastfeeding because there's no evidence that the tiny amount of gadolinium contrast that would find its way into breast milk would be toxic. If you're still concerned, you could stop breastfeeding for 24 hours after getting the test and "pump and dump" that milk. 

MRI and metal implants

Metal in your body can distort an MRI or make the test dangerous to you. But a lot depends on what kind of metal you have in your body and where it is. For instance, you should be able to get an MRI even if you have a knee replacement or other metal that's secured to a bone. Dental fillings or braces should be OK too. However, you normally can't get an MRI if you have:

  • Clips used to treat brain aneurysms
  • Certain metal coils placed in blood vessels

Bullet fragments may be all right depending on where they're located. The main thing is to alert your doctor in advance to any metal implant you have so they can know how to handle it. Metal can include:

  • Artificial heart valves
  • Body piercings
  • Cochlear implants
  • Drug pumps
  • Fillings and other dental work
  • Implanted nerve stimulator
  • Insulin pump
  • Metal fragments, such as a bullet or shrapnel
  • Metal joints or limbs
  • Pacemaker or implantable cardioverter-defibrillator (ICD)
  • Pins or screws

MRI and pacemakers

Many newer models of pacemakers are compatible with MRIs, but older ones may be damaged by an MRI's magnets. If you must have an MRI, let your doctor know about your pacemaker. Some hospitals are able to do this test safely even with older pacemakers.

MRI and cochlear implants

This electronic device improves hearing for people who are deaf or hard of hearing. It also includes a surgically implanted magnet that could heat up during an MRI and cause pain, swelling, or infection. Some people with cochlear implants must have the magnet surgically removed before undergoing an MRI. For others, particularly those with newer implants, your doctor may be able to turn and reorient the magnet so as to lessen its impact on the device and your body.

 

Questions to ask your doctor before an MRI

It’s a good idea to fully understand the reasons for the MRI. You can start by asking your doctor questions like:

  • Why do I need an MRI?
  • Is an MRI the best way to check on my condition?
  • How will the results affect my treatment?
  • What are the risks?
  • Do the benefits of this test outweigh the risks to me?

What your doctor should know before an MRI

Before your MRI, let your doctor know if you:

  • Have any health problems, such as kidney or liver disease
  • Recently had surgery
  • Have any allergies to food or medicine, or if you have asthma
  • Are pregnant, or might be pregnant
  • Have any metal in your body
  • Have tattoos since some inks contain metal

What metal objects shouldn’t go into the MRI machine?

No metal is allowed in the MRI room, because the magnetic field in the machine is attracted to it. 

On the day of the test, wear loose, comfortable clothing that doesn't have snaps or other metal fasteners. You might need to take off your own clothes and wear a gown during the test.  

Remove all of these before you go into the MRI room:

  • Body piercing jewelry or metals
  • Cellphone
  • Coins
  • Credit cards or other cards with magnetic data
  • Dentures
  • Eyeglasses
  • Hairpins
  • Hearing aids
  • Jewelry
  • Keys
  • Underwire bra
  • Watch
  • Wig

If you don't like enclosed spaces or you're nervous about the test, tell your doctor. You may be able to have an open MRI or get a sedative to relax you before the test.

What not to do before an MRI

  • Eat and drink, unless the doctor says it's OK
  • Neglect to inform the doctor of any metal you have inside your body (knee replacement, pacemaker, bullet, etc.)
  • Neglect to tell the doctor if you're claustrophobic (have fear of enclosed spaces), pregnant or breastfeeding.

MRI scan cost

Costs vary widely depending on where you live, what body part is being scanned, whether you have a closed or open MRI and what your insurance covers. Costs can range between $400 and $12,000. 

A typical MRI machine is a large tube with a hole at both ends. A magnet surrounds the tube. You lie on a table that slides all the way into the tube.

In a short-bore system, you are not totally inside the MRI machine. Only the part of your body that's being scanned is inside. The rest of your body is outside the machine.

An open MRI is open on all sides. This type of machine may be best if you have claustrophobia or you're very overweight. The quality of images from some open MRI machines isn't as good as it is with a closed MRI.

You'll lie on a table that slides into the MRI machine. Straps might be used to hold you still during the test. Your body might be completely inside the machine. Or, part of your body may stay outside the machine.

If you're claustrophobic or have trouble staying still, you'll be given a mild sedative. If you're having a contrast MRI, you'll get an IV with the dye injected into your arm or hand. A family member can stay in the room with you if you wish.

The MRI scanner creates a strong magnetic field inside your body. A computer takes the signals from the MRI and uses them to make a series of pictures. Each picture shows a thin slice of your body. 

A technician sits behind a window during the scan to see the images on the screen. They'll tell you when to hold your breath while the images are being made. 

You might hear a loud thumping or tapping sound during the test. This is the machine creating energy to take pictures inside your body. You'll be given earplugs or ear muffs to muffle the sound.

You might feel a twitching sensation during the test. This happens as the MRI stimulates nerves in your body. It's normal, and nothing to worry about.

How long does an MRI take?

Each scan may take a few seconds to a few minutes. You usually need several scans from different angles. The whole procedure may take 15 to 90 minutes, including prep time. 

 

 

You can usually go home after an MRI and get back to your normal routine. If you had medicine to help you relax, you’ll stay in the imaging center until you’re fully awake. You’ll also need someone to drive you home. 

MRIs don't use radiation and don't generally cause any side effects. Rarely, some people feel warm or get a tingling sensation from being inside the MRI tube, but that goes away once the test is over. 

Others are allergic to contrast dye and may experience a headache, dizziness or nausea. This usually doesn't last long. 

People with chronic kidney disease will be given a blood test to see if the MRI can be performed safely.

There’s a chance some of the dye may stay in your body and build up in the brain and other organs. It’s not clear yet if this buildup is harmful. The FDA hasn’t restricted its use.

A specially trained doctor called a radiologist will read the results of your MRI and send the report to your doctor. This may take a few days.

Your doctor will explain the meaning of your test results and what to do next.

An MRI is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures of the inside of your body. It's helps a doctor diagnose a disease or injury. MRIs are safe and don't have side effects, but if you have metal inside your body (like an implant) or outside (like jewelry) either remove them or let your doctor know in advance of the MRI that you have them so you can keep safe. 

What exactly does an MRI show?

It takes pictures of soft tissue (like organs and muscles) that you can't see on an X-ray. You can use it to see whether there are any tumors in your body or injuries to your bones.

Why do you have to fast before an MRI with contrast?

You may be asked to fast because contrast dye has the rare side effect of vomiting. Vomiting while lying down could lead to food going into you airways which may stop your breathing.