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Commonly used terms


As Low As Reasonably Achievable (ALARA). ALARA should be consistent with economic and social factors which need to be taken into account.

Background radiation

Everyone on earth is exposed to a small amount of natural background radiation, which comes from natural radioactive materials present in small amounts in the soil, the buildings we live in, the food we eat, in our own body, and from radiation from space (cosmic rays). The amount of background radiation varies from location to location but the global average is 2.4 mSv per year as per United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).

Computed Tomography (CT)

Cross sectional images are produced by X ray exposure from a rotating tube and detected, usually by multiple banks of detectors as the lying patient passes through a doughnut-shaped gantry on a moving couch. This type of scan is particularly useful for head, abdominal and chest imaging but due to higher doses carries a higher radiation risk than plain X ray examinations.

Deterministic effects

A deterministic effect is one where the severity depends upon the radiation dose, e.g. skin burns. There is a threshold for deterministic effect. Deterministic name comes from “determined” to occur once threshold is crossed.

Diagnostic Radiology

This specialty uses X rays, sound waves and radio-frequency waves to produce images for diagnosis and also to guide biopsies and treatment. The techniques include X ray examinations, mammography, fluoroscopy, ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and interventional procedures (image-guided biopsy or treatment). Positron emission tomography, usually linked to CT (PET-CT), utilizes both a radioactive source and X rays and may be carried out in either a department of diagnostic radiology or a department of nuclear medicine. Ultrasound and MRI do not use X rays and are not associated with any known cancer risk.


Dual-energy X-ray absorptiometry (DXA, or formerly DEXA) is a technique used to measure bone mineral density (BMD). The technique relies on transmission measurements made at two photon energies to allow calcium, and thereby bone mineral, be assessed. There has been significant growth in the application of DXA over the last decade. The preferred regions for BMD measurement are lumbar spine, proximal femur and whole body.

Fluoroscopy (screening)

Fluoroscopy is an imaging technique commonly used by physicians to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope. In its simplest form, a fluoroscope consists of an X ray source and fluorescent screen between which a patient is placed. However, modern fluoroscopes couple the screen to an X ray image intensifier and CCD video camera, or use a flat panel detector allowing the images to be recorded and played on a monitor.

Interventional Procedures

Interventionalist (radiologist, cardiologist, electrophysiologist and a number of other medical specialists) can carry out treatment and biopsies accurately guided by X ray screening (fluoroscopy or CT fluoroscopy). In many cases interventional procedures provide a substitute for more invasive surgical procedures. Increasingly cardiologists diagnose and treat more heart conditions using similar fluoroscopy-guided techniques. Interventional techniques are often associated with moderate to high radiation dose, depending on the complexity of the procedure.

Magnetic Resonance Imaging (MRI)

This sophisticated technique does not use X rays and produces cross-sectional images using radio-frequency waves. Unlike CT, MRI is not associated with any known cancer risk and is increasingly used, when available, for scanning the head, spine, joints and many other parts of the body. Developing countries currently have limited access to MRI.


Images of the breasts using X rays are used to diagnose cancer and other abnormalities. This technique is widely used to detect early breast cancer in national screening programmes.

Medical Oncologist

A doctor specialised in the treatment of cancer using chemotherapy. She/He plans and prescribes treatment to cure or reduce the symptoms of cancer.

Medical physicist

A healthcare professional who specialises in the application of physics in medicine and has the knowledge and responsibility for the radiation protection of patients, staff and the public. The qualified medical physicist oversees quality assurance of both diagnostic and therapy equipment and checks radiation doses in diagnostic, interventional procedures and in radiotherapy.

Nuclear Medicine

This specialty involves the use of an injected, swallowed or inhaled carrier labelled with a weakly radioactive tracer for the purpose of diagnosis or treatment. The tracer is concentrated within the target organ and provides a low dose of gamma rays for producing the diagnostic image or a targeted high dose for treatment when used for therapy.

Positron Emission Tomography with X ray Computed Tomography (PET-CT)

This hybrid technique combines the information from selective uptake of an injected positron emitter labelled tracer with the excellent positional information available from CT on a series of cross-sectional images. This is particularly useful for cancer imaging and detecting re-growth of tumours but may carry a high radiation dose. Many countries have very limited access to this sophisticated technique

Radiation Dose

The radiation dose is a measure of the energy deposited by radiation in a target such as tissue. This is important because it is this absorption of energy that can cause radiation effect in tissue and related to the associated risk for developing cancer due to the radiation exposure. Since a number of tissues are involved in X ray examinations and procedures, the effective dose is a quantity used to express the overall amount of radiation dose. It is calculated from the radiation dose for different tissues exposed and takes into account radiosensitivity of the tissues involved that absorb the dose. The common unit for effective dose is the millisievert (mSv). For example the dose from natural background radiation sources is on average 2.4 mSv per year (UNSCEAR) and the effective dose from a PA chest X ray is 0.02 mSv (typical average value)

Radiation effects

The radiation effects are classified into Stochastic effects and Deterministic effects. Smaller doses encountered in diagnostic procedures mostly stochastic effects are a matter of concern.

Radiation Oncologist

A doctor specialising in the treatment of cancer using radiotherapy. She/He prescribes and plans radiation and other treatment to cure or reduce the symptoms of cancer.

Radiologist (Diagnostic / Interventional Radiologist)

A doctor specialising in diagnostic/interventional radiology. They interpret and advise on radiographs and scans. With specialization, they can also perform specialised scans and image-guided biopsies and treatment procedures.

Radiographer (Radiological Technologist)

A healthcare professional trained to operate radiological equipment and perform imaging and therapy procedures. Diagnostic radiographers work in a diagnostic radiology and nuclear medicine department. Therapy radiographers work in a radiotherapy department.


This specialty uses X rays, gamma rays and other types of radiations to treat cancer and other diseases. The radiation energy deposited in the tissue is used to destroy the cells of the tumour. Treatment may be carried out with:

  • External beam therapy
  • Sealed source therapy (brachytherapy)

Stochastic effects

A stochastic effect is one where the probability of occurrence increases with radiation dose but the severity of the result is the same e.g. development of cancer. There is no threshold for stochastic effect. Stochastic stands for something that occurs by chance and is random in nature.


This technique does not use X rays but utilizes sound waves to produce cross-sectional images and to show flowing blood. It is not associated with any known cancer risk. It is useful for scanning the abdomen and pelvis, particularly in pregnancy. It is also used in some facilities to scan the breast, testes and soft tissues of the neck and limbs. Although fairly portable and widely available, the value of this technique is dependent on highly-trained professionals and is limited by the physique of some patients.

X ray

Correctly a type of high-energy and penetrating electromagnetic radiation but also commonly used to mean the examination or image produced by a diagnostic exposure with X rays.

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