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Mr Anthony Wallace
Dr Timothy Cain
Date last modified: December 24, 2013
X-ray procedures provide a measurable additional radiation dose to our annual background radiation exposure. The contribution of medical X-rays to the average population radiation dose has increased over the past few decades, as more sophisticated imaging tests have become available; this is especially true of CT scans.
Radiology is the medical specialty that uses X-rays, ultrasound and magnetic resonance imaging (MRI) to produce images or pictures that help to diagnose disease or injury, or guide procedures that treat the disease or condition. Ultrasound and MRI do not use or produce ionising radiation.
Nuclear medicine is the medical specialty that uses radiopharmaceuticals (radioactive medicines) to show how an organ is working. Certain radiopharmaceuticals can also be used to treat some medical conditions, such as some cancers.
These medical specialties are often carried out in the same radiology facilities and are jointly called ‘medical imaging’.
There are generally four types of technology used to carry out different types of medical imaging procedures:
1. Imaging using an X-ray tube
X-rays are a type of ionising radiation produced by an X-ray tube, which is found in equipment used for the following types of imaging procedures:
The radiation exposure from having an X-ray, fluoroscopy, mammography or CT examination only occurs while the machine is on and activated by the operator. Any operator of X-ray machines has to be trained in the use of these machines and holds a government licence.
2. Magnetic resonance imaging (MRI)
MRI uses strong magnetic fields and radio waves to make images. It does not use ionising radiation (see Magnetic Resonance Imaging (MRI)).
Ultrasound uses high frequency sound waves that the human ear cannot detect to make images. It does not use ionising radiation (see Ultrasound).
4. Nuclear Medicine
Nuclear medicine uses small amounts of a radioactive material (radiopharmaceutical), which is normally injected into a patient’s vein, but can be given through the mouth or inhaled. The patient becomes slightly radioactive for a short time, and images are made from the radiation given off from the patient. Positron emission tomography (PET) is a special kind of nuclear medicine study that uses short-lived radiopharmaceuticals that provide specialised information, particularly for investigation of cancers and epilepsy (see Nuclear Medicine and PET scan).
The radiation exposure a patient receives will depend on the type of examination and the purpose of the imaging study. Generally, plain X-rays, mammography and fluoroscopy give a lower radiation dose than CT, but complex procedures using fluoroscopy can result in doses similar to extensive CT examinations. Ultrasound and MRI do not use ionising radiation. Because children are more sensitive to the effects of ionising radiation it is important, where possible, to use tests that do not require ionising radiation (i.e. ultrasound and MRI) providing they will give the answers required by the referring doctor.
These risks are difficult to accurately measure, but it has been shown that the risk of developing cancer is slightly increased if you have been exposed to additional ionising radiation above background levels. The risks are not the same for all people; females are slightly more sensitive to the effects of ionising radiation compared with males. Children are also more sensitive, as the cells that make up their growing tissues and organs are dividing more rapidly. Children also live longer, so the effects of radiation have more time to become visible.
Some people have genetic differences that predispose them to the effects of ionising radiation. There are other risks from high exposures to ionising radiation, but these are not expected at the dose levels used in diagnostic imaging.
The increased risk is small, and usually less than the risk from not identifying or treating a disease or condition properly. It is important to make sure that every test has a definite benefit to balance the small radiation risk of the test. If you are referred for a CT scan or other test involving ionising radiation, it is important that you discuss the relative risks and benefits with your referring doctor so that you understand how you will benefit from having the study.
All operators of an X-ray machine (including CT) have been trained to use only enough X-rays to provide quality pictures for the specialist. The dose of ionising radiation is therefore kept to a minimum.
The purpose of diagnostic radiology is to provide the radiologist or nuclear medicine specialist (specialist doctors) with images of sufficiently high quality, so that they can report the results of the test to your doctor to assist in understanding and explaining your medical problem or symptom, and confirm either the presence or absence of disease or injury.
X-ray imaging procedures, in the main, offer the advantage of being:
It is important that any request for an imaging test is provided by your doctor, in consultation with you. It is your own doctor who will be able to make an assessment of whether the benefits of the X-ray procedure outweigh any possible risks. The radiologist or nuclear medicine specialist supervising the procedure will also assess if it is the most appropriate test, taking into account the information your doctor has written on the request form together with your medical history. If there are concerns, then the radiologist/nuclear medicine specialist might want to speak to your doctor before the test is carried out.
Your decision should be made in close consultation with your referring doctor. Ask your doctor about the procedure and how it will help to provide information about your symptoms or the presence of disease or injury. Ask your doctor about the risks of the procedure and what the risks would be of not having the procedure; that is, if your doctor needs the information in order to identify and plan the most appropriate treatment.
Although there is a small risk of harm from ionising radiation, there could be a greater risk of not having the information; for example, failure to detect potentially serious disease that could be easily treated at an early stage, but is harder to treat or is incurable if detected later.
Discuss any concerns with your doctor, and access reputable websites to find out information. You might also be able to obtain information from the hospital or private practice where your doctor has referred you for the procedure.
It might also be as beneficial to you to confirm the absence of disease or injury as it is to confirm its presence.
X-rays, CT scans, nuclear medicine studies, MRI and ultrasound each have a greater or lesser ability to scan and provide an image of specific parts of the body and/or to identify the presence or absence of certain conditions or disease. MRI and ultrasound studies are usually used in preference to other imaging tests when it is possible to do so. Your referring doctor will consider which imaging procedure is most appropriate depending on the type of information required and your medical history. Your doctor can also discuss the most appropriate choice of test with the radiologist or nuclear medicine specialist.
The amount of ionising radiation that a child is exposed to in X-rays is usually altered to take into account their body size; smaller bodies require less X-rays to make the images, as there is less tissue to ‘stop’ the X-rays. It is important that this also happens when a child has a CT scan. Older CT scanners were not designed to alter the radiation dose for small patients, but newer CT scanners have much better detectors (that measure the X-rays passing through the patient), and can significantly reduce the radiation dose to minimise the absorbed dose of ionising radiation. It is important that a modern CT scanner is used for children, and that the operator adjusts the imaging parameters to reduce the radiation dose to an acceptable level.