Introduction to PET/CT Imaging

Peeyush Bhargava, MD, FACNM
Affiliation: www.nuclearmd.com
Last Modified: May 21, 2013

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What is a PET scan?

  • PET stands for Positron Emission Tomography. It is an imaging exam through which your doctor looks at the metabolic activity of a radioactive compound inside your body.
  • The most common radioactive compound used for PET imaging is called FDG (18-Fluro-deoxy-Glucose), which is an analogue of glucose, and its metabolism in the body can be seen on PET images.
  • Since cancer cells are metabolically more active, they use more glucose (and FDG) and "light up" on the scan as abnormal activity.
  • Unlike the MRI and CT, which provide pictures of the physical structures of the body, a PET scan provides metabolic imaging that show how the body's organ systems are functioning. The PET is able to show whether tissue is normal or abnormal based on cellular metabolism.
  • PET has been investigated for more than 40 years, and much of the early work was performed at the University of Pennsylvania, by pioneers like Dr. Abass Alavi.

What is a PET/CT scan?

  • PET/CT is new and highly advanced imaging technique, which combines PET (positron emission tomography) and CT (computed tomography) in one machine.
  • The scanner merges PET and CT images together to provide a unique combination of both functional and structural information.
  • It helps physicians to diagnose cancer and provide additional information that can help determine which therapies are appropriate for the patient. It also helps physicians monitor a patient's response to treatment or identify metastasis that can affect treatment.

How does PET work?

  • A PET scan may also be referred to as an "FDG-PET scan." FDG is a radioactive analog of glucose (sugar). For a PET scan, a small amount of radioactivity is attached to sugar (glucose) molecules and given to the patient through an IV.
  • Upon entering the body, the FDG is processed by organs and tissues as part of their normal function and is detected by the PET scanner. A computer then creates a picture of where the FDG is taken up (called "activity") using colors to highlight the different levels of activity.
  • PET has the ability to demonstrate abnormal metabolic activity in organs sometimes even before there is a structural abnormality present (that could be seen on CT or MRI).

How can the PET detect cancer cells?

  • Tumor cells have a high rate of metabolism and because of this, they take up more of the radioactive glucose (FDG).
  • This makes the cancer cells appear more "active" than other areas on the scan pictures. The rate of uptake of FDG by the tumor cells is proportional to their metabolic activity.
  • In addition to the color picture, the amount of uptake on PET is often given a number called "SUV" (standard uptake value); most active cancers will have an SUV greater than 2.5.
  • Cancer cells are not always the only "PET avid cells" (or cells that take up the FDG) in the body; it is important to realize that a PET scan is not able to distinguish metabolic activity due to tumor from activity due to non cancerous processes such as inflammation or infection.
  • Therefore, it is important to discuss with your healthcare provider the clinical significance of your PET scan results.

PET/CT and Cancer

  • PET/CT imaging with FDG has become an indispensable part of cancer care. A PET scan's ability to detect the metabolic activity levels of an abnormal area seen on CT scan, allows the reader to distinguish between a benign and cancerous tumor- the cancerous one will appear more active.
  • In some cases, after treatment for cancer, there may be dead cancer cells left behind, which would still be seen on MRI or CT scan, but the PET can show that they are not active.
  • PET can be used to determine how a tumor is responding to treatment or to detect metastases. Chemotherapy causes changes in cellular activity and these changes can be detected with a PET scan well before they would be seen on CT or MRI alone.

How to prepare for your PET Scan

  • Confirm your appointment 24 hours prior to the test. Because of the high demand for a PET/CT scan, and the short half-life of the tracer, your slot may be given to another patient.
  • Bring a list of the medications that you are taking, brief medical history, and treatments that you have had (chemotherapy, radiation therapy, or surgery). If you take any medications for diabetes, you may be instructed not to take these as they could interfere with the FDG.
  • Patients must fast overnight prior to a PET scan. Afternoon patients may eat a light meal six hours prior to exam. They can and should drink plenty water while they are fasting.
  • You should plan to be at the facility for 2-3 hours on exam day. You should avoid vigorous physical exercise the day before, on the day of the scan, and until the scan is completed.
  • If you had other imaging studies performed such as CT scans, MRI, ultrasound etc, it is crucial that the films and reports are made available to the radiologist who is interpreting the PET/CT scan.

The PET Imaging Procedure

  • After you register, you will be taken to the PET prep room. An IV will be put in and you will be given the radioisotope (FDG).
  • There is a period of approximately 60-90 minutes wait time during which the FDG circulates through the body.
  • When it is time for the PET scan to begin, you will lie on a table that slides into the scanner, a donut-shaped machine that resembles a CT scanner. The table will slide through the PET scanner.
  • For most patients, the scan takes about 30-60 minutes, depending on whether the whole body, or a particular part of the body is scanned. You will need to lie still during that period.
  • For most cancers, the body is scanned from just below the brain to the mid-thigh. In certain cancers such as melanoma, a full body scan, from the top to the head to the toes, is used.
  • You may go home when the study is finished and resume your regular diet and medication. You are encouraged to drink water to help clear the radioactivity from your body.
  • If you have received sedation during the course of the exam, you will need someone to drive you home.

Will my health insurance cover PET?

  • Most insurance companies reimburse for PET procedures, including Medicare. Contact your insurer directly to learn about payment reimbursement.
  • The staff at your doctor's office or the imaging center can also provide more information and help with preauthorization for insurance reimbursement.

How much radiation does a PET scan expose the patient to?

  • Because the radioisotope used in a PET scan is short-lived, the amount of radiation exposure the patient receives is small and is the same as from a CT scan of the chest.
  • While this dose is very low, women who are pregnant or breastfeeding should discuss the risks of radiation exposure with their doctors.

Do people experience any reactions as a result of FDG injection or the PET scan?

  • Patients typically do not experience any reactions as a result of the PET scan, because the body processes the radioactive compound naturally. Therefore, no side effects are expected.

What are the other clinical applications of PET Imaging?

  • Traditionally, PET has been used in oncology, but the use of PET is not limited to oncology. FDG is the most common tracer used in Oncology followed by F-18 Sodium Fluoride for imaging bone metastasis, and novel Ga-68 labeled peptides for imaging of neuroendocrine tumors.
  • PET has also been used in the fields of neurology and cardiology. For example, PET can measure blood flow and glucose metabolism in the muscles of the heart. Areas of low blood flow but normal metabolism in the heart muscle indicate heart muscle that may be repaired by restoring blood flow, while areas of matched low blood flow and metabolism may already be irreversibly damaged and so require other treatments. In addition blood flow in the heart may be measured at rest and after exercise to detect partly obstructed arteries in the heart responsible for angina.
  • In the field of neurology, PET has been used to study seizure disorders and Alzheimer's disease. PET imaging can localize the source of seizure focus in patients with medically uncontrolled seizures, for potential surgical management. Novel agents are being developed to evaluate patients who present with cognitive decline and may have Alzheimer's dementia.

About the author: Dr. Bhargava is a Board Certified Nuclear Medicine Physician based in Houston, TX. He has 10 yrs of experience in Nuclear Medicine and has worked as an Attending Physician at St. Luke's Roosevelt Hospital in Manhattan, Chief of Nuclear Medicine at the Michael E Debakey VA Medical Center in Houston, and Medical Director of Nuclear Medicine at the Excel Diagnostics and Nuclear Oncology Center, also in Houston. He has published more than 30 articles in peer reviewed Nuclear Medicine journals and is a sought after speaker for American Board of Radiology review courses. He developed and maintains www.nuclearmd.com.

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