Imaging with equipment that combines positron emission tomography and computed tomography (PET/CT) provides the special benefits of both in one procedure, that is, a highly sensitive imaging technique used in oncology, cardiology, neurology and in infectious and inflammatory diseases. The information from the PET scan and from the CT scan are very different but complementary to each other. The PET scan shows areas with increased metabolic activity, while the CT scan shows detailed anatomical locations. A combination of these two images together enables a doctor to tell whether a region with high metabolic activity is significant, and if so, to state definitively where that location is. Often the PET/CT is repeated to monitor the effect of treatment of a particular disease. Most commonly PET utilizes 18F-FDG as a radiotracer, the short half life of which (110 min) reduces radiation exposure compared with other commonly used radionuclides such as 99mTechnetium (6 hours) and 201Thallium (72 hours).
The radiation exposure from 18F results in internal exposure to the patient and low level external exposure to other people in their vicinity. The radiation (X rays) from the CT scanner only radiates the patient and only during the CT scan. Whenever a repeat PET/CT scan is necessary, it should be performed with low dose CT.
A PET/CT test has two components: a PET scan and a CT, which are done together. The radiation exposure from CT has a very wide range depending on the type of the test, the area of the body scanned and the purpose of the test. In its simplest form, a CT scan is used only for the localization of abnormalities seen on a PET scan (non-diagnostic scan). The radiation dose from such a scan can be low (e.g. an effective dose of about 7 mSv for a whole body study). However, the effective dose from a high resolution diagnostic scan can be quite high (up to 30 mSv for a whole body CT scan). The effective dose from a PET scan is modest and depends on the activity of the injected FDG (18F-Fluoro deoxyglucose) and is typically 8 mSv for adults using 400 MBq and is the same whether a part of the body or the whole body is imaged. Major reductions in radiation doses from PET/CT scans can be achieved by modifying the acquisition parameters for CT. Conventional radiographic examinations such as chest, abdominal and bone X rays also give a radiation dose but only a fraction of that resulting from a CT examination. Examinations such as ultrasonography and magnetic resonance (MR) imaging do not involve exposure to ionizing radiation. Further information on dose reduction in CT-examinations can be found »
Yes. Radiation effects are known to be cumulative in nature. However, the repair mechanisms in the body are quite active and spacing the procedures with suitable time intervals helps reduce radiation effects, as is done in radiotherapy. At the moment there is no formal mechanism to record and track cumulative radiation exposure to a patient, as is done for staff. Patients are advised in their own interest to let all physicians know about their previous radiological examinations.
No. Although a small amount of radioactive glucose is injected for the PET examination, this will not affect the diabetes. It is important, however, that the PET imaging team are aware that the patient has diabetes, to ensure that the best results can be achieved from the scan and unnecessary radiation dose is avoided.
The PET scan will not affect the performance of these investigations. However, if other imaging tests are also planned, it is important to avoid any unnecessary duplication of the tests by ensuring that all physicians involved in the patient’s care are aware of the imaging tests being performed on the patient.
Yes. Children are more sensitive to radiation than adults. All radiation carries a theoretical risk of inducing cancer. The effective dose to the patient from a typical PET/CT examination may be in the range of 5-18 mSv. If the procedure is justified and is expected to give significant benefit to the patient, the benefit/risk ratio is high. Thus it is important to justify each examination. Once the procedure is justified, it is necessary to perform the procedure with due optimization so as to reduce the radiation dose to the patient without compromising diagnostic information.
It is advisable not to bring children along to the PET/CT centre. Following injection of radioactive material and before the scan starts, it is important that the patient is relaxed, so that the staff can get the best scan possible. The radiation exposure to accompanying child from the patient, although small, is better avoided as far as possible.
It is not desirable. Although the radiation dose from the person undergoing a scan is fairly low, it is desirable to keep the radiation exposure to the foetus as low as reasonably achievable. Should a pregnant woman’s presence be necessary to comfort a small child, specific advice to keep their distance from the child and from other patients who have undergone PET scans or other diagnostic and therapeutic radionuclide procedures should be provided. In such a case, the contact time should be as short as possible.
Some of the administered 18F-FDG might be excreted in small amounts in breast milk. Normally, the scan should be delayed until breast feeding has stopped. But if the scan is needed urgently, then it is advisable to collect milk before the scan, so that this can be used to provide a feed after the scan. Furthermore, milk should be collected and discarded for 2 hours after the scan. Normal breast feeding can resume after that.
There is no significant risk involved in such an exposure. For details »
There is no danger to other travellers on public transportation following a PET scan with due attention to pregnant women and children as in Qn. 7. However, in some countries, radiation detectors in public areas and specific locations such as airports can be inadvertently activated by even small amounts of radiation. Radiation detectors are now more sensitive than ever before. Nonetheless, the isotopes used for PET imaging decay so rapidly that after only 24 hours there is no danger of activating a radiation detector. However, it may be a good idea for the patient to obtain a document from the PET Centre stating that they have undergone a PET/CT scan, in case they are questioned.
No. Although the scan involves injection of a radioactive substance, which will lead to radiation exposure of persons in his/her vicinity, the amount of radiation coming from the patient following the scan is low. The patient can carry out all routine activities without any risk to others, with the consideration of limiting contact with pregnant women and children as in Qn. 7.
18F-FDG is by far the most commonly used radiotracer. Other tracers with shorter half lives (e.g. 82Rubidium, 13N-ammonia, 11C and 15O water) decay much faster than FDG, and therefore require no additional restrictions. Radiation dose rates from non-fluorine tracers with longer half lives have not been established in clinical use and are used very rarely in research studies. Individualized instructions would be required if these tracers are used.
No, there is no significant risk to the staff taking care of these patients. However, radiation from patients undergoing other diagnostic and therapeutic radionuclide procedures such as bone scans or radioiodine therapy may pose a risk of radiation exposure to medical staff and does require attention. Patients undergoing PET/CT scan would add to this radiation exposure. Following simple guidelines for reducing contact time and increasing distance would suffice to minimize the radiation exposure to staff. For patients with urinary catheters or incontinence, standard precautions for dealing with biohazardous material would be sufficient to prevent undue radiation exposure and contamination.
No, there is no need for restrictive advice for non-radiation workers who have only occasional contact with patients who have undergone a PET/CT examination. The usual principles of reducing contact time and keeping distance apply.
The basic training requirements and guidelines set by each country for each category of staff (nuclear medicine physicians, radiologists, medical physicists and technologists or radiographers as appropriate) should be followed for the PET/CT. Its inter-disciplinary nature will, in many instances, be best met through a collaboration and consensus among professional bodies on training requirements, and judicious use of continuing education programmes. If PET/CT is located in a nuclear medicine facility the physicians may need to gain the knowledge and skills required to interpret CT, and the nuclear medicine technologists may need to be able to perform CT examinations. On the other hand, if it is in a radiology department, the radiologists and radiological technologists may need to acquire knowledge and skills in nuclear medicine. In either case, the physicians, radiologists and technologists involved must be well educated and trained in PET/CT imaging procedures and radiation protection principles.