PET Scan vs. MRI: What’s the Difference?

PET (positron emission tomography) scans and MRI (magnetic resonance imaging) scans are procedures that produce pictures of the inside of the body that a doctor can use to detect or track the development of medical conditions. In many cases, these scans are done in conjunction with one another to give the doctor both cellular-level and structural-level views of the area or organ in question.

What is a PET scan?

A PET (positron emission tomography) scan uses a radioactive chemical called a radiotracer (or tracer) and a PET scan machine to produce images that help a doctor understand how tissues or organs are functioning. PET scans are most often used to find cancer or track its progress, assess brain damage or disorders such as tumors, seizures or cognitive issues, evaluate damage to the heart following a heart attack, or assess the state of coronary artery disease. In many cases, a PET scan is used in conjunction with an MRI (magnetic resonance imaging) scan.

How Does a PET Scan Work?

In a PET scan, a tracer material is injected into your bloodstream and given time to circulate through the body. Then, you’re moved into a doughnut-shaped machine called a PET scanner. This PET scan machine detects energy given off by the tracer and uses it to create three-dimensional images. The doctor can then view cross-sectional pictures of the tissue or organ to look for signs of damage or disease.

PET scans are generally an outpatient procedure, meaning you go home after it’s completed. Your doctor will give you specific instructions on how to prepare for your scan. This will include what you are or aren’t allowed to eat or drink before the procedure. They’ll also ask you about any medications, herbal remedies or vitamins you’re taking and about whether you’re pregnant or might be pregnant. You should dress comfortably for your appointment, although the technician may have you change into a hospital gown.

PET Scan Risks

PET scans are safe. However, you’re exposed to radiation during the procedure, and that exposure comes with a small risk. But, the benefits of a PET scan typically outweigh any potential consequences.

PET Scan Results

After your PET scan is complete, the results will be interpreted by a radiologist, who’s a doctor that has specialized training in this area. The radiologist’s assessment is then shared with your doctor who will talk with you about the results. It typically takes approximately 24 hours to get test results back.

What is an MRI?

An MRI is an imaging technique that sends radio waves into the body, which are reflected by substances like water and fat. The waves are then captured and recorded by a device that turns this data into a detailed image of the area or organ.

Unlike a PET scan, which focuses on cellular-level activities, an MRI produces images of organs and structures. MRIs help diagnose problems in many areas, and are most commonly used to evaluate:

  • Joints (including wrists, ankles, knees, and back)
  • Blood vessels
  • Brain and spinal cord
  • Abdominal organs
  • Breasts

PET Scan vs. MRI

PET scans, CT (computerized tomography), and MRIs are similar in many ways.  The main difference in a PET scan vs. MRI or CT scan is that it can show cellular-level changes and issues with oxygen use, glucose metabolism and blood flow that reveal medical problems at a very early stage.

Your doctor may prescribe an MRI scan vs. a PET scan (or vice versa) based on a number of factors including their familiarity with the scans, the relative costs, the need for soft tissue visibility, convenience, desire to prevent radiation exposure, and others. They can talk with you about the PET vs. MRI decision.

PET Scan Machine vs. MRI Machine

You may find yourself wondering, is a PET scan machine like an MRI machine? While the tests are different, they are often performed on combination CT/PET or MRI/PET machines, making the process quite similar. Of these machines, CT/PET combination machines have been in use longer than MRI/PET machines, which are often more costly.

MRI vs. PET Scan Procedure

Because MRI scans and PET scans are often performed on combination machines, the procedure for each test is often very similar.

If your PET scan is performed on a combination machine, you will first receive a radioactive tracer. This substance can take up to an hour to absorb in the body. Next, you will lie on the table, which will slide into the machine. The machine will then begin imaging your body. This process can take up to an hour and a half, during which you must remain still. Once the imaging is complete, the table will slide back out of the machine.

Which Imaging Test Do You Need?

Your doctor will assess your medical condition and any concerns to determine which test is right for you. It is less a question of which test is better, and more about which test best aligns with your unique needs. For example, your doctor may recommend a PET scan to see how your body is functioning in regard to blood flow, oxygenation, and metabolism of organs and tissue. An MRI can be used to determine organ shape or to assess injured or unhealthy tissue.

If you need both an MRI and a PET scan, they can often be performed at the same time.

PET Scans and MRIs at Baptist Health

Learn more about our diagnostic radiology procedures, or contact a Baptist Health imaging location near you to get started.


Next Steps and Useful Resources: 

Learn more about MRIs
Learn more about PET scans

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