PET Scan vs. CT Scan

Both positron-emission tomography (PET) and computed tomography (CT) scans use energy to create images of the body and can diagnose many types of medical conditions. They differ in the type of energy they use and the reasons for use. Often, a PET scan is performed along with a CT scan to provide detailed images of the body.

This article will review the basics of PET and CT scanning, including their differences and their medical uses.

CAT scan patient

JohnnyGreig / Getty Images

CT Scans

A CT scan uses multiple X-rays to make a 3D image for healthcare providers to view organs.


CT scans are frequently used in many types of situations to diagnose and monitor health conditions throughout the body, including:

  • Evaluation of blood vessels and bleeding
  • Infection or abscess (pocket of pus and inflammation)
  • Injury due to trauma, such as bone fractures or spinal cord injury
  • Blood clots, like pulmonary embolism (blood clot in an artery in the lung)
  • Cancer
  • Obstruction to blood flow in blood vessels of the heart
  • Fluid accumulation in the lungs or other tissues

How It Works

A CT scanner sends out X-rays that penetrate your body and are detected on the opposite side. The scanner moves around your body and takes "slices" of images. How dense a tissue is will determine how X-rays will be absorbed, resulting in pictures with varying shades of brightness. For example, bones show up bright white on a CT scan, while blood and other tissues show up in shades of gray.

Preparing for a CT Scan: What to Expect

Your healthcare provider will discuss any specific instructions with you prior to a CT scan. Depending on what is being imaged, you may be asked to fast for a period of time before the test.

Depending on the part of the body that is being evaluated, you may be given oral contrast to drink, or have an intravenous line (IV) placed so that contrast dye can be injected into the bloodstream. Injection of the contrast may make your skin feel warm or flushed.

Contrast Allergy

Some people may have an allergy to contrast dye that can cause anaphylaxis, with symptoms such as hives, facial and throat swelling, and low blood pressure, which can be life-threatening. If you have ever experienced an allergy to contrast before, inform your healthcare team. Sometimes premedication with antihistamines and steroids can prevent an allergic reaction.

You will be asked to lie on a table where a donut-shaped scanner will rotate around you while releasing X-rays and collecting images of your body. This creates a whirring and clicking sound. You may also be asked to hold your breath for a moment, particularly if the heart or lungs are being imaged. These will be displayed on a computer so that a radiologist and your healthcare team can interpret them.

PET Scan

A PET scan uses imaging to see how the body's organs work at a cellular level.


PET scans have many uses in diagnosing certain conditions, including:

  • Diagnosis or monitoring of cancer
  • Identification of certain infections
  • Evaluation of blood flow in the heart and viability of heart tissue
  • Evaluation of sarcoidosis
  • Diagnosis of certain brain disorders such as dementia and seizure disorders

PET scans are often combined with a CT scan (known as a PET-CT) or magnetic resonance imaging (PET-MRI) for more detailed images of the body. Using specific radioactive tracers, PET allows imaging of different conditions.

How It Works

PET scanning involves the use of a radioactive tracer that is tagged with a specific molecule or protein. The tagged portion allows it to travel to the area of interest in the body. For example, fluorodeoxyglucose (FDG) is a tracer that resembles the simple sugar molecule glucose.

Cancer cells grow rapidly and take up a lot of glucose. FDG is taken up more by cancer cells than healthy cells and can identify tumors and metastases in the body because those areas show up as bright on the PET scan.

Preparing for a PET Scan and What to Expect

The day before your PET scan, you may be asked to refrain from exercising and to eat a dinner low in carbohydrates, followed by a six-hour fast. Check with your healthcare provider whether you can drink any water. Typically water is allowed at this time, but do not drink any coffee, tea, or other beverages.

When you arrive for your scan, an IV will be placed, and the tracer will be administered through the IV. You will be asked to wait for an hour or so to give enough time for the tracer to be absorbed in your body.

Then you will have the imaging portion of the scan. You will be asked to lie on a table where a donut-shaped camera will detect the tracer and create images of your body. This portion takes about half an hour.

Comparing PET Scans to CT Scans

While PET scans and CT scans are both types of imaging tests that use radiation, they are looking at different things and use different types of energy to create images.

PET scanning examines the function of organs and tissues, and uses a tagged radiotracer. The radioactivity from the tracer is detected by the PET scanner to create and image. CT scanning, on the other hand, use x-rays to create detailed images and provide information about the structure of organs and tissues in the body. CT scans are much faster to perform than PET scans and require less preparation.

CT and PET are often combined to provide even more detail than either test alone is able to provide.

Imaging Tests for Diagnosing Cancer

Several different types of imaging are used to diagnose cancer. Depending on the type of cancer, PET scans or other nuclear imaging, CT scans, MRI, ultrasound, or a combination may be used. Each of these tests has different benefits, including regarding resolution of tissues, differentiating tumors from nearby structures, and the ability to provide information on the function of organs.

Your oncologist will be able to discuss which type of imaging test is the most appropriate for you or your family member's specific type of tumor. A radiologist who specializes in reading medical imaging tests will help interpret the tests.

Does a PET Scan Show All Cancers?

PET scans detect cancer in many different organs, but they have some limitations. Some early stages of cancer and certain types of cancer, such as stomach cancers, do not show up well on a PET scan. Other types of tests are used to diagnose gastric cancer.


A PET-CT scan is a combination imaging test that involves the PET scan in addition to a CT scan. Performing both tests together allows for more detailed images and the localization of potential cancers or pathologies (deviations from what's normal) in the body. The PET portion uses nuclear tracers that go to a certain part of the body and light up. In contrast, the CT scan provides high-resolution images to help localize the abnormality.


CT scans and PET scans are both essential tools in the diagnosis and monitoring of many health conditions, particularly cancer. They use different types of radiation, with PET using nuclear radiation and CT scans using X-rays.

Depending on the specific tracers used, PET scans look at function and can show as a lit-up area of the scan when abnormalities are present. CT scans show detailed images of the body's structure. PET-CT scan is a combination of these two imaging tests that allows for even better identification of abnormalities.

Frequently Asked Questions

  • What does PET scan show that CT does not?

    PET scans can show metabolic activity within the body. Nuclear tracers are directed to particular areas of interest where they appear as a lit-up area on the scan. CT scans, on the other hand, show details about the anatomic structure of the body and its organs and tissues.

  • Does a PET scan show more cancer than a CT scan?

    PET scans can often localize cancer sooner than it could be seen on a CT scan because they can show abnormal uptake of tracers like F18-fluorodeoxyglucose (FDG). However, some types of cancer aren't seen well on a PET scan and other types of imaging, including CT may better detect them. An oncologist will help select the most appropriate type of imaging depending on the location or type of cancer that is being evaluated.

  • Which imaging tests show more details?

    Each type of medical imaging test has pros and cons. Computed tomography (CT) and magnetic resonance imaging (MRI) scans can show a high level of detail and resolution. Depending on the part of the body, they often provide higher level of detail than other tests like simple X-rays and ultrasound.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Angela Ryan Lee, MD
Angela Ryan Lee, MD, is board-certified in cardiovascular diseases and internal medicine. She is a fellow of the American College of Cardiology and holds board certifications from the American Society of Nuclear Cardiology and the National Board of Echocardiography. She completed undergraduate studies at the University of Virginia with a B.S. in Biology, medical school at Jefferson Medical College, and internal medicine residency and cardiovascular diseases fellowship at the George Washington University Hospital. Her professional interests include preventive cardiology, medical journalism, and health policy.