PET-CT (positron emission tomography computed tomography)

Massachusetts General Hospital Imaging provides PET-CT (positron emission tomography – computed tomography) services in a comfortable, caring environment using the latest technology, and every scan is read by a radiologist with specialty training.
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About This Procedure


Before your appointment for a CT, please print and complete the Patient Procedure Screening Form.

Massachusetts General Hospital Imaging provides PET-CT services on the Mass General Imaging - Boston, Mass General Imaging - Chelsea and Mass General/North Shore Center for Outpatient Care in Danvers. No matter which facility you come to, our staff places priority on making your journey through the imaging process comfortable, safe, and successful. All images are read by a radiologist with specialty expertise in the area of the body being studied.

PET-CT overview

  • PET (positron emission tomography) looks at how the cells in your body process a radioactive tracer material that you drink, inhale, or have injected.
  • CT (computed tomography) uses X-ray technology to produce detailed images.
  • By combining two scans, a PET-CT shows both your anatomy and how your cells are behaving.
  • Do not eat or drink anything except water for at least 6 hours before the exam.
  • Many PET-CT exams also involve contrast—an injection that makes the images more vivid and informative.
  • If you are over 60 or have kidney disease, diabetes, lupus, or multiple myeloma, a blood test before your exam is necessary to make sure the contrast is safe for you.
  • The exam may take two hours or more. Actual scanning time is 30 to 45 minutes.
  • The trained professional performing your exam, called a technologist, will be nearby and able to talk to you throughout the scan.
  • Every exam is interpreted by a radiologist with specialty expertise in the specific area of the body under study. A specialist is capable of seeing and understanding subtle things due to advanced training and singular focus.
  • We use the latest technology, and the capabilities of our state-of-the-art scanners play a key role in tailoring each exam to your specific needs and reducing radiation exposure.

PET-CT in depth

What is a PET-CT exam?


A PET-CT exam combines two types of scans to help pinpoint abnormal activity in the body.

A PET (positron emission tomography) scan creates an image of your body's metabolic activity and shows the rate at which your body's cells break down and use sugar (glucose). This is done by injecting a small amount of radioactive material (FDG) into your blood stream and waiting for it to disperse to the area of focus. The PET scan is then performed to detect the radioisotope and creates an image on the computer screen.

A CT (computed tomography) scan is a noninvasive medical test that uses special X-ray equipment to produce multiple images or pictures of the inside of the body and a computer to join them together in cross-sectional views of the area being studied.


A PET-CT combines the functional information from a PET scan with the anatomical information from a CT scan. When a CT scan is superimposed over a PET scan, doctors can pinpoint the exact location of abnormal activity. They can also see the level and extent of that activity. Even when an abnormal growth is not yet visible on a CT scan, the PET scan may show the abnormal activity.

Common uses

PET-CT scans are commonly used to find changes in the body during the early stages of disease and for staging and restaging of cancers.


  • CT: CT examinations improve health care and are an essential part of diagnosis and treatment planning. However, there are some risks associated with the level of radiation exposure during a CT and therefore the medical benefit of conducting the exam should always outweigh any risks involved. No direct data have shown that CT examinations are associated with an increased risk of cancer; extrapolations from studies of radiation exposure suggest there is a very small incremental risk.

    At Mass General Imaging, we pay special attention to minimizing radiation exposure—without giving up image quality. Radiation reduction has long been a priority for our entire staff, including radiologists, the technologists who administer most exams, researchers, and equipment engineers. We use many strategies to reduce radiation exposure, from employing the latest technology to customizing exams for each patient.

    Learn more about how Mass General Imaging reduces radiation dose.
  • PET: The dose of radiotracer administered is small, resulting in minimal radiation exposure. Nuclear medicine has been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure. Allergic reactions to radiopharmaceuticals may occur but are extremely rare.

    Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. Some of the pharmaceuticals that are used for the study can pass into the mother's milk and subsequently the child will consume them. To avoid this possibility, it is important that a nursing mother inform her physician and the nuclear medicine technologist about this before the examination begins. Usually, you will be asked to discontinue breast-feeding for a short while, pump your breasts in the interim and discard the milk. Breast-feeding can often resume shortly afterwards.

What should I expect BEFORE my PET-CT exam?

  • Medications: If you need to take your medication in the morning prior to the scan, take it with water. Most claustrophobic patients are able to tolerate a PET-CT or PET scan. Talk to your doctor if you think you need some additional anti-anxiety medication for the scan. We cannot prescribe or supply medication.
  • Food and drink: Do not eat or drink anything for at least 6 hours before the exam, except water. If your doctor has told you to take your regular medicine, take it with plenty of water. If you are diabetic, do not drink or eat anything for at least 4 hours prior to your scan. Take your diabetic medication as usual. If your doctor has told you to take your regular medicine, take it with plenty of water. Avoid: candies, gum or beverages other than water.
  • Exercise: Please do not exercise for at least 24 hours before the exam.
  • When to arrive: Please check in 30 minutes before your appointment time.
  • What to wear: Wear loose-fitting, comfortable clothes with no metal (zippers, under wire bras, etc) for the test. Leave your watch, jewelry, and other valuables at home.
  • Intravenous preparation: Many patients receive a contrast agent intravenously (IV) during their PET-CT test. If your doctor or the radiologist has determined that this procedure will enhance your PET-CT scan results, the technologist will place an IV in your arm or hand prior to going into the test. If you've ever had an allergic reaction to contrast, tell your doctor and the technologist. The doctor may prescribe special medicine for you to take before the exam and also while you are here for the exam. You should bring the last two doses of medicine with you.

What will I experience DURING my PET-CT exam?

  • Scanning: You will be required to lie flat with your arms raised above your head. If you think you will be unable to keep your arms above your head for approximately 35 minutes, please notify the technologist and they may accommodate you.
  • Length of exam: You should plan to be here for approximately 2-3 hours. The actual scanning and preparation time varies with the type of scan you are having.
  • Contrast medium: A contrast medium, or contrast agent, highlights your organs and blood vessels and helps the radiologist see them better. The contrast agents in use today carry a low risk of allergic reaction and cause little discomfort for most people. If you are over 60 or have kidney disease, diabetes, lupus, or multiple myeloma, you’ll need a blood test beforehand to make sure the contrast will be safe for you.

    The high speed of our state-of-the art scanners means we are able to produce high-quality images using less contrast than in the past; contrast dilutes fairly quickly into your bloodstream, but our fast scanners take their pictures before the dilution occurs.

What should I expect AFTER my PET-CT exam?

  • Instructions: You can drive and resume normal activities immediately after leaving, unless you have taken medication to relax you. It is important that you drink as much water or fluids as possible for the rest of the day and empty your bladder as often as possible. This will result in a more rapid clearance of radioactivity and contrast from your body.
  • Exam results: All PET-CT exams are read by a Mass General radiologist specialty trained in PET-CT imaging and dedicated to the specific area of interest for your study.

    Rapid results are essential not only for your peace of mind, but also for your physician to begin planning your treatment immediately, if necessary. After the scan has been read, the results are sent to your physician, who will discuss them with you.


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Request an Appointment

Call to schedule an appointment 617-724-9729

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