What is combined PET/CT?


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Combined PET/CT uses two imaging methods, CT and positron emission tomography (PET), in one procedure. CT is done first to create anatomic pictures of the organs and structures in the body, and then PET is done to create colored pictures that show chemical or other functional changes in tissues.

Different types of positron-emitting (radioactive) substances can be used in PET. Depending on the substance used, different kinds of chemical or functional changes can be imaged. The most common type of PET procedure uses an imaging agent called FDG (a radioactive form of the sugar glucose), which shows the metabolic activity of tissues. Because cancerous tumors are usually more metabolically active than normal tissues, they appear different from other tissues on a PET scan. Other PET imaging agents can provide information about the level of oxygen in a particular tissue, the formation of new blood vessels, the presence of bone growth, or whether tumor cells are actively dividing and growing.

Combining CT and PET may provide a more complete picture of a tumor’s location and growth or spread than either test alone. The combined procedure may improve the ability to diagnose cancer, to determine how far a tumor has spread, to plan treatment, and to monitor response to treatment. Combined PET/CT may also reduce the number of additional imaging tests and other procedures a patient needs.

Is the radiation from CT harmful?

Some people may be concerned about the amount of radiation they receive during CT. CT imaging involves the use of x-rays, which are a form of ionizing radiation. Exposure to ionizing radiation is known to increase the risk of cancer. Standard x-ray procedures, such as routine chest x-rays and mammography, use relatively low levels of ionizing radiation. The radiation exposure from CT is higher than that from standard x-ray procedures, but the increase in cancer risk from one CT scan is still small. Not having the procedure can be much more risky than having it, especially if CT is being used to diagnose cancer or another serious condition in someone who has signs or symptoms of disease.

It is commonly thought that the extra risk of any one person developing a fatal cancer from a typical CT procedure is about 1 in 2,000 (2). In contrast, the lifetime risk of dying from cancer in the U.S. population is about 1 in 5 (3).

It is also important to note that everyone is exposed to some background level of naturally occurring ionizing radiation every day. The average person in the United States receives an estimated effective dose of about 3 millisieverts (mSv) per year from naturally occurring radioactive materials, such as radon and radiation from outer space (1). By comparison, the radiation exposure from one low-dose CT scan of the chest (1.5 mSv) is comparable to 6 months of natural background radiation, and a regular-dose CT scan of the chest (7 mSv) is comparable to 2 years of natural background radiation (1).

The widespread use of CT and other procedures that use ionizing radiation to create images of the body has raised concerns that even small increases in cancer risk could lead to large numbers of future cancers (4, 5). People who have CT procedures as children may be at higher risk because children are more sensitive to radiation and have a longer life expectancy than adults. Women are at a somewhat higher risk than men of developing cancer after receiving the same radiation exposures at the same ages (6).

People considering CT should talk with their doctors about whether the procedure is necessary for them and about its risks and benefits. Some organizations recommend that people keep a record of the imaging examinations they have received in case their doctors don’t have access to all of their health records. A sample form, called My Medical Imaging History, was developed by the Radiological Society of North America, the American College of Radiology, and the U.S. Food and Drug Administration. It includes questions to ask the doctor before undergoing any x-ray exam or treatment procedure.

What are the risks of CT scans for children? 

Radiation exposure from CT scans affects adults and children differently. Children are considerably more sensitive to radiation than adults because of their growing bodies and the rapid pace at which the cells in their bodies divide. In addition, children have a longer life expectancy than adults, providing a larger window of opportunity for radiation-related cancers to develop (7).

Individuals who have had multiple CT scans before the age of 15 were found to have an increased risk of developing leukemia, brain tumors (8), and other cancers (9) in the decade following their first scan. However, the lifetime risk of cancer from a single CT scan was small—about one case of cancer for every 10,000 scans performed on children. 

In talking with health care providers, three key questions that the parents can ask are: why is the test needed? Will the results change the treatment decisions? Is there an alternative test that doesn’t involve radiation? If the test is clinically justified, then the parents can be reassured that the benefits will outweigh the small long-term risks.

What is being done to reduce the level of radiation exposure from CT?

In response to concerns about the increased risk of cancer associated with CT and other imaging proceduresthat use ionizing radiation, several organizations and government agencies have developed guidelines and recommendations regarding the appropriate use of these procedures.

• In 2010, the U.S. Food and Drug Administration (FDA) launched the Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging. This initiative focuses on the safe use of medical imaging devices, informed decision-making about when to use specific imaging procedures, and increasing patients’ awareness of their radiation exposure. Key components of the initiative include avoiding repeat procedures, keeping doses as low as possible while maximizing image quality, and using imaging only when appropriate. The FDA also produced Reducing Radiation from Medical X-rays, a guide for consumers that includes information about the risks of medical x-rays, steps consumers can take to reduce radiation risks, and a table that shows the radiation dose of some common medical x-ray exams.

• The NIH Clinical Center requires that radiation dose exposures from CT and other imaging procedures be included in the electronic medical records of patients treated at the center (10). In addition, all imaging equipment purchased by NIH must provide data on exposure in a form that can be tracked and reported electronically. This patient protection policy is being adopted by other hospitals and imaging facilities.

• NCI’s website includes a guide for health care providers, Radiation Risks and Pediatric Computed Tomography (CT): A Guide for Health Care Providers. The guide addresses the value of CT as a diagnostic tool in children, unique considerations for radiation exposure in children, risks to children from radiation exposure, and measures to minimize exposure.

• The American College of Radiology (ACR) has developed the ACR Appropriateness Criteria®, evidence-based guidelines to help providers make appropriate imaging and treatment decisions for a number of clinical conditions. These guidelines and supporting documents are available on ACR’s website

• ACR has also established the Dose Index Registry, which collects anonymized information related to dose indices for all CT exams at participating facilities. Data from the registry can be used to compare radiology facilities and to establish national benchmarks.

• CT scanner manufacturers are developing newer cameras and detector systems that can provide higher quality images at much lower radiation doses.