Reviewed by Dr Melissa Hickson, specialist registrar in radiology
|The X-rays are converted into light and the more energy that has reached the recording system, the darker that region of the film will be.|
X-rays were first discovered in 1895 when, during experiments with electric currents passed through a vacuum tube, Wilhelm Conrad Röntgen noted that a nearby fluorescent screen glowed when the current was being passed.
When the current was switched off the screen stopped glowing. He attributed this effect to previously unknown rays which, X being the symbol for an unknown quantity, he called X-rays.
We now know that these rays are, like light and radio waves, a form of electromagnetic radiation.
X-rays have high energy and short wavelength and are able to pass through tissue.
On their passage through the body, the denser tissues, such as the bones, will block more of the rays than will the less dense tissues, such as the lung.
A special type of photographic film is used to record X-ray pictures. The X-rays are converted into light and the more energy that has reached the recording system, the darker that region of the film will be.
This is why the bones on an X-ray image appear whiter (less energy passes through) than the lungs (more energy passes through).
In the early days of X-rays, images, such as that of Mrs Röntgen's hand, were produced differently: the bones appeared dark rather than white.
Within two months of their discovery, X-rays were being used both in Europe and North America, not just to take pictures of the internal organs of living people but also to treat a wide variety of diseases.
The energy that does not pass through the body is deposited within it and it is this energy that causes the biological effects of radiation.
The machines used to take X-ray pictures produce X-rays with energies of around 120,000 electron volts.
The X-rays used for cancer treatment are much more powerful, with energies of between 2 million and 20 million electron volts.
Staff involved in X-ray tests and treatment
A radiographer is a trained professional concerned with the operation of X-ray equipment and the care of patients during radiological examinations or X-ray treatment.
Radiographers who operate equipment used for taking X-ray pictures are known as diagnostic radiographers: those who operate equipment used for treatment are known as therapy radiographers.
A radiologist is a qualified doctor who is concerned with interpreting X-ray pictures and performing certain types of investigation and treatment that rely on imaging techniques.
A radiotherapist (sometimes known as a clinical oncologist) is a doctor who specialises in the treatment of disease using X-rays and/or cell-killing medicines.
In order to practise in the UK as a radiologist or radiotherapist, it's compulsory that a doctor becomes a fellow of the Royal College of Radiologists (RCR), and this can only happen once they have completed their specialist training and have passed the RCR membership exams.
The RCR is responsible for overseeing the training of radiologists and radiotherapists, setting examinations and maintaining professional standards.
A simple X-ray image can be extremely informative. For example it can show whether or not a bone is broken or whether or not there is a shadow on the lung.
Special X-ray techniques can also be used to investigate other problems with individual structures within the body.
By injecting special dye into arteries and/or veins the blood vessels can be made visible and the soft tissues can be differentiated from each other more easily.
By swallowing special dye the gullet and stomach can be examined. Similar dye can be introduced via an enema to examine the rest of the small bowel, colon and back passage.
CT scanning is a further development of the use of X-rays, and it's fast becoming the most frequently used investigating imaging technique in hospitals.
It consists of a sophisticated scanner connected to a computer, which produces and detects multiple x-rays at the same time.
The resulting pictures are far more detailed, and therefore allow more accurate examination of the bones and soft tissue in cross-sectional views.
The technology behind CT scanning is rapidly evolving, with faster scanners being developed which give increasingly accurate images.
What happens during an X-ray test?
The X-rays are produced by an electrical machine and the patient stands between the machine and a special screen used for obtaining the image.
Patients are asked to remove any metal objects, such as watches and jewellery, that might appear on the picture and cause confusion.
Patients may also be asked about whether they have any metal implants, such as prosthetic hips and knees, as these could cause some distortion of the final x-ray image.
Female patients of a reproductive age will also be asked about their pregnancy status prior to undergoing the procedure ie. whether they are pregnant or whether there is a possibility they could be. This is important as it allows the amount of harmful radiation to the foetus to be minimalised.
The patient is asked to keep as still as possible for the few seconds it takes for each image to be obtained.
For a CT picture to be obtained, patients will lie within the CT scanner, which is similar to a bed surrounded by a doughnut shaped machine.
The procedure is entirely painless and because the amount of radiation used is very small, there are generally no side-effects.
The pictures are checked for technical quality by the radiographer and then sent off to the radiologist for reporting.
For more complex examinations this process may take a few hours so the official result of the test is not usually available immediately.
Is X-ray radiation dangerous?
It is one of the ironies of radiological practice that X-rays can both cause cancer and be used to treat it.
Nowadays, with the use of very small doses of radiation to produce high quality X-ray images, the risk of cancer after properly supervised X-ray examinations is extremely small; so small as to be of little consequence to any individual.
Certain X-ray techniques do result in a higher exposure to radiation compared to other techniques, and therefore are associated with a higher risk.
However the benefits of being able to make an accurate diagnosis thanks to X-ray imaging, which in turn determines the best possible clinical management for the patient, far outweigh the risks.
Therefore in almost all patient cases, justification can be given for carrying out the examination.
Radiologists are trained to make the decision about whether a procedure is justified or not, therefore minimising dose as much as possible and only exposing patients to radiation when it is required to do so.
Because staff in the X-ray department work with X-rays all the time they would, if they stayed beside every patient, over the course of time, be exposed to quite a high dose of radiation. This is why they go behind a screen when the X-ray beam is switched on.
The cumulative effect would be significant for them in a way that it is not significant for an individual patient.
Radiation can cause damage to a foetus, which is why, as far as possible, the use of X-rays during pregnancy is kept to the absolute minimum.
Any woman who suspects that she is pregnant, and who has been referred for an X-ray examination, should make sure that the radiographers and doctors caring for her know about her condition.
Based on a text by Erik Fangel Poulsen, specialist
Last updated 25.07.2012