MRI - Magnetic Resonance Imaging - CT - Computed Tomography - Xray - Ultrasound - Digital X-rays

What are X-rays?

The German scientist Wilhelm Roentgen discovered X-rays in 1895.Sources of Radation

The use of radiation in medicine is of great value but it contributes by far the greatest part of our man-made radiation dose. Although this is on average much less than the dose we get from nature, some people get quite high doses. Most of the medical dose is due to X-ray examinations.

How X-rays are used

A high voltage accelerates a beam of electrons towards a target in an evacuated tube. When the electrons collide with the target X-rays are produced.

X-rays can be used to produce medical images because some parts of the body, such as bones, are not as transparent to X-rays as the surrounding soft tissues.

About X-rays

Radiation Protection

There should be a clinical need for all X-rays examinations and they must be carried out according to accepted practice with doses as low as reasonably achievable. It is assumed that any radiation dose, however small, carries a degree of risk, but the risks from a necessary examination carried out carefully should be small compared with the benefits received.

Radiographs of the limbs and joints, the chest, and the teeth are the commonest uses of X-rays, fortunately these radiographs requires a relatively small amount of radiation.

The radiation exposure from X-ray examinations can be compared with naturally occuring background radiation. For example, the radiation exposure from a chest X-ray is similar to 10 days natural background radiation. Some other examples are:


Radiation Dose

Comparable Natural Background Radiation


Computed Tomography (CT)-Abdomen 10 mSv 3 years
Computed Tomography (CT)-Colonography 5 mSv 20 months


Radiography-Spine 1.5 mSv 6 months
Radiography-Extremity 0.001 mSv Less than 1 day

Central Nervous system:

Computed Tomography (CT)-Head 2 mSv 8 months
Computed Tomography (CT)-Spine 10 mSv 3 years


Computed Tomography (CT)-Chest 8 mSv 3 years
Radiography-Chest 0.1 mSv 10 days

Face and neck:

Computed Tomography (CT)-Sinuses 0.6 mSv 2 months


Coronary Artery CT 6 mSv 2 years

Women's Imaging:

Bone Densitometry (DEXA) 0.01 mSv 1 day
Mammography 0.7 mSv 3 months

Doses vary greatly from patient to patient and from clinic to clinic for the same x-ray examination. NRL's staff aim is to keep radiation doses down while maintaining the quality of the image.

NRL's Medical Physicist provides expertise on x-rays, equipment, and radiation protection. Much of the physicist's work relates to NRL's quality assurance programme, a major goal of which is to get the best image at the minimum dose.

The National Radiation Laboratory carries out studies of radiation doses received by patients and issues advice on radiation protection.