Radiation Risk of Medical Imaging for Adults and Children
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PET stands for “positron emission tomography”. It is a nuclear medicine imaging test in which an injected radiolabeled molecule enables functional imaging, typically of a metabolic pathway or cell surface receptor. To date, this has mainly been applied for cancer imaging, but it is also useful for imaging inflammation, infection and the function of a range of other organs, including the brain and heart.
The radioactive substance most commonly used in PET scanning is a radioactive sugar analogue called fluorodeoxyglucose (FDG). After intravenous injection, FDG accumulates in areas of glycolytic metabolism where it emits energy in the form of gamma rays. These are detected by the PET scanner and further converted into three dimensional images. This is ideal for cancer imaging, as tumours primarily use glycolytic metabolism for growth.
There are several different positron emitters now used for clinical PET imaging including fluorine-18, gallium-68 and iodine-124. These can be labelled to a variety of different molecules for imaging different pathways. This includes prostate-specific membrane antigen (PSMA) PET for imaging prostate cancer, and DOTATATE PET for imaging neuroendocrine tumours.
PET scanners are combined with computed tomography (CT) scanners, called PET-CT scanners. The PET-CT combination allows any abnormality on the PET scan to be precisely anatomically located within the body.
There are a wide range of indications for PET including:
More detailed indications for PET imaging can be found in:
As with any imaging test, a referral for a PET-CT scan implies that the result will have an impact in the diagnosis, management and prognosis of a patient’s condition. Patients should have had routine diagnostic imaging to identify underlying pathology before functional molecular imaging with PET is carried out.
There are no absolute contraindications for a PET scan.
This study may not be suitable for pregnant women. The benefit versus risk should be discussed with the nuclear medicine specialist. Women who are breast-feeding may need to make special preparations (see InsideRadiology: Nuclear Medicine) after the test to stop breast-feeding for a short time and to avoid close contact with young children due to the small amount of radioactivity released for a while after the test. Patients should discuss this with their referring doctor or the nuclear medicine practice where they will have the test for details.
If the patient is unable or not willing to remain still for a prolonged period of time, you may need to consider another procedure.
If the patient is claustrophobic, you may need to consider another procedure, although this can usually be managed with conscious sedation.
For FDG PET-CT, it is important that the patient does not have markedly elevated blood sugar levels. The patient’s blood sugar level will be checked before the PET scan and the scan may need to be rebooked if it is markedly elevated. Ideally, levels should be between 4–10 mmol/L. If the level is above 12.5 mmol/L, the scan would usually be rescheduled.
Recent chemotherapy or radiotherapy can make interpretation difficult. Depending on the clinical question, the scan may be rebooked for a later time.
The PET or PET-CT scan results in a small amount of radiation exposure, typically less than a diagnostic CT scan. Allergic reactions to the radioactive tracer generally do not occur, as the actual substance is administered in micro- or nanogram quantities.
Conventional imaging, such as CT scanning, X-rays or ultrasound, will provide anatomical information about the presence or absence of disease, but not necessarily cellular disease activity. Delayed comparison imaging may be helpful in determining anatomical changes that reflect changes in tumour growth. Sometimes image-guided or surgical biopsy may be required to confirm or exclude disease activity in areas of concern seen on PET imaging.
Page last modified on 5/3/2018.
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