Frequently asked questions



1. How do PET/CT radiation doses compare with doses from other examinations?

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 »

2. Do repeat radiological procedures on the same patient increase the radiation risk?

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.

3. Does diabetes have any additional radiation risk issues in PET scanning?

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.

4. Can a patient have a CT scan, diagnostic X ray examination or MRI scan on the same day as the PET/CT scan?

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.

5. Do children undergoing a PET/CT scan have higher radiation risks?

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.

6. Can a child accompany a patient to the PET/CT centre?

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.

7. Can a pregnant woman accompany a friend, partner or child who is having a scan?

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.

8. Can a patient breastfeed after a scan?

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.

9. What if an ancillary staff member is in the early stage of pregnancy and is exposed to a patient who has undergone PET/CT?

There is no significant risk involved in such an exposure.

10. After a PET scan, how long does a patient need to wait before using public transportation without setting off radiation detectors?

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.

11. Does a patient need to restrict his or her activities after a PET/CT scan?

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.

12. Are any additional restrictions required for a patient’s behaviour for positron emitting tracers other than 18F-FDG?

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.

13. Are members of the nursing and ancillary staff at risk when taking care of patients after PET/CT examination?

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.

14. Are there any radiation risks to other non-radiation workers, e.g. anaesthetists, para medical staffs, nurses?

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.

15. What training requirements should be met by staff involved in PET/CT?

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.