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How to choose between a PET scan vs. MRI.

PET scan vs MRI: Smiling woman with her arms crossed against a grey background

Key takeaways:

  • A PET scan is a nuclear medicine procedure, while an MRI scan is a radiology imaging technique.
  • Unlike PET scans, MRIs do not use damaging ionizing radiation.
  • PET scans look at cellular activity, which are often the starting points for malignant disorders. 

In a study, researchers evaluated the accuracy of breast cancer recurrence with various screening mechanisms such as MRI (magnetic resonance imaging), CT (computed tomography), X-ray, mammography, and PET scans (positron emission tomography). They found PET scans were 90% accurate. The other screening procedures combined yielded an accuracy of only 75%.

When comparing PET scan vs. MRI scans, note that both can detect diseases and abnormalities. However, PET scans can show how your body performs at the cellular level. On the other hand, PET scans include radiation. MRI scans don’t expose you to harmful radiation, which can be preferable for certain patients and annual scans.

Here, we’ll look further into how else PET scans differ from MRIs to help you decide which one you need.

PET scan vs. MRI: Key differences

Before we dive into the details, let’s look at the key differences between how a PET scan and an MRI help with medical diagnoses:

  • MRI scans use magnets and radio waves to make still images of your body parts. 
  • PET scans show how an organ is working in real-time. 
  • Tissue and organ changes can appear on a PET scan before they appear on an MRI scan.
  • An MRI contrast scan uses a contrast agent such as gadolinium or iodine. 
  • A PET scan uses a radioactive tracer such as glucose (sugar) or fluorodeoxyglucose (FDG).

PET scan vs. MRI: Why do healthcare providers use them?

PET scan vs MRI: Smiling elderly man wearing a fedora

Your healthcare provider might order a PET scan or an MRI based on which of the two can better serve the diagnosis. 

PET scan helps to check for signs of:

  • Cancers such as thyroid cancer, breast cancer, and lung cancer
  • Brain disorders, including Alzheimer’s disease, brain tumors, dementia, and epilepsy
  • Heart problems such as coronary artery disease

MRI imaging test helps to check for signs of:

  • Brain and spinal injuries, including multiple sclerosis, aneurysms
  • Heart disorders, such as damage from heart attacks, pericarditis (inflammation of the tissue that surrounds the heart), and tumors inside heart chambers
  • Bone and joint diseases like fractures, bone tumors, and torn cartilages
  • Soft tissue diagnosis, including more than a dozen types of cancerous tumors

It’s important to note that there are two types of MRI—with contrast and without contrast. As the name suggests, an MRI with contrast uses a contrast dye, such as iodine or gadolinium, that is injected into your veins to provide much more detailed, high-clarity images that aren’t available without contrast. Since the density of breast tissue can make it difficult to detect the presence of cancerous cells, be sure to discuss your options with your physician when planning a breast cancer MRI scan.

PET scan vs. MRI: What are the risks?

While getting an MRI or a PET scan, your radiologist can inject you with a contrast dye or a radioactive tracer, respectively. These may cause side effects for some people. 

PET scan.

The radioactive tracer used in PET scans such as glucose contains a small amount of radiation, so the risk of negative effects is low. However, the tracer can cause the following:

  • Allergic reactions: Alert your physician if you are allergic to iodine, aspartame, or saccharin.
  • Radiation exposure to you or your child: Let your physician know if you’re pregnant or breastfeeding. 

MRI scan.

There is no radiation used here, but due to the usage of strong magnets, MRIs cannot be performed on patients with:

  • Implants
  • Prosthetic devices
  • Stimulators
  • Pacemakers
  • Metal IUDs

MRIs may not be right for pregnant patients. If you believe you may be pregnant, notify your physician. An MRI screening can cause a harmful increase in the temperature of the amniotic fluid and can be risky to the developing fetus. 

If you have an allergy to contrast dye, you might face some allergic reactions from a contrast MRI. Generally, these wear off in an hour or so. However, let your physician know about your allergy before your procedure.

Other side effects can include dizziness, nausea and vomiting, and pain at the site of contrast injection. 

PET scan vs. MRI: How do they work?

PET scan vs MRI: Parents playing Jenga with their kids

The imaging technology of PET and MRI differ in terms of radiation exposure and scanning mechanisms. 

PET scan.

PET is a type of nuclear medicine procedure. It uses a small amount of radioactive tracer— glucose, injected intravenously, inhaled in a gas, or swallowed in a drink. Your oncologist measures the metabolic activity of your cells based on how much tracer they absorb. 

Sometimes, instead of glucose, PET scanning can use an analog tracer called fluorodeoxyglucose (FDG). This helps identify malignant cells.

Once you are injected with either glucose or FDG, a whole-body PET scanner will take images to detect abnormalities. 

MRI scan.

An MRI uses radio waves and powerful magnets to show abnormalities in the body.

You can have an MRI with or without contrasts. In a contrast MRI, your radiologist gives you a contrast dye such as gadolinium or iodine, intravenously. 

Hybrid PET scan.

Generally, a PET scan is used alongside a CT scan (computed tomography) or MRI scan. The PET/CT scan or PET/MRI scan can help health professionals assess the degree of a disease’s impact. 

Certain scanning centers use these hybrid PET/MRI scanners to create high-contrast images. They are most often used for discovering and monitoring cancers of the soft tissues such as the brain, pelvis, liver, head, and neck.

PET scan vs. MRI: How should you prepare?

Woman smiling while working

You’ll need to take a few steps to prepare for your screening, regardless of which screening method you use. 

PET scan.

  • Twenty-four hours before the scan, avoid exercising. Six hours before the scan, don’t eat or drink anything, except water. 
  • During the scan, stay still and hold your breath when your radiologist asks you to. This will help you get clear images. 
  • The scan can take anywhere between 30 minutes to 1 hour
  • After the scan, you can resume your daily activities—including driving. Drink plenty of water, as it helps wash out the radioactive remnant from your system. 

MRI scan.

  • Before the imaging process, follow your normal diet unless your physician gives you different directions.
  • During the scan, which lasts about 15 to 90 minutes, ensure to remove all metallic objects—jewelry, watches, etc. They can interfere with the magnetic waves.
  • Also, notify your radiologist if you have any metal implants, hearing aids, or other prosthetic devices. The magnetic field induced by the MRI scan can disorient your implants causing injury or pain.
  • Since the MRI is painless, you don’t need to worry about pain medication. However, if you are claustrophobic, let your radiologist know so they can offer you a sedative if needed. 
  • After the scan, you can go about your daily chores. However, if you take a sedative, you might need a friend or family member to drive you home. 

Protect your health today. 

Both PET and MRI scans can help detect abnormalities in your body before they get worse. 

Since PET scans use radiation, MRI scans can be better options for annual screening. And upon your physician’s recommendation, you may go for a PET scan. 

Ezra is one of the few screening services that lets you book your appointment and get screened the following week. If interested,  you can find the Ezra partner facility nearest to you

First-time visitors can mention “EZRA100” to get $100 off on your first full-body scan