Diagnosis and Therapy

The x-ray tube was perhaps the first of many complex electrical machines that have transformed medical care.

The use of x-rays revolutionized diagnostic medicine.

To view x-ray images of other parts of humans, click here

X-rays can also be used for soft tissue, by using special techniques.

X-rays have paved the way for other imaging techniques.

Mammography can detect growths as small as a pea, which might go undetected by feeling the breast.

The calcium atoms in bones absorb x-rays, giving rise to clear shadows on x-ray film.

X-rays have two important uses in medicine
1. diagnosis
2. radiation therapy.
Both x-ray diagnosis and therapy benefit the patient but, as with the use of any form of radiation, the benefit must outweigh the risk for it to be used.

1. Diagnosis

X-ray photographs, called radiographs, are used extensively in medicine as diagnostic tools. The value of radiographs for diagnostic purposes is a consequence of the penetrating properties of x-rays. Diagnosis of a suspected bone fracture is possibly the first encounter with x-rays for most people and this is routine work in the accident and emergency departments of hospitals.

Radiographs (x-rays photographs) are commonly viewed as shown on the left. Recently, digital images have become more common as shown on the right.

Demo of diagnostic x-rays

X-ray images (radiographs) are made as follows. Some photographic film is placed behind the leg, for example. The x-rays are directed at the leg and pass through it to the photographic film. X-rays pass readily through the soft tissue but are absorbed to a greater or lesser extent by any dense matter like bone that they encounter. Thus, less x-rays will strike the photographic film in parts of the image that correspond to denser parts of the leg, and these denser parts will stand out as lighter areas in the image when the film is developed. X-ray images are therefore good for revealing the condition of bones without the need for surgery.

Chest x-rays are also used for the diagnosis of tuberculosis and cancer. Pictures of the lungs were easy to interpret because the air spaces are more transparent to x-rays than the lung tissues. Various other cavities in the body can be filled artificially with contrasting media, either more transparent or more opaque to x-rays than the surrounding tissue, so that a particular organ is brought more sharply into view. Barium sulphate, which is highly opaque to x-rays, is used for the x-ray examination of the gastrointestinal tract. Such dyes can have serious side effects.
A recent x-ray device, used without dyes, offers clear views of any part of the anatomy, including soft organ tissues. This device called the CAT scanner is described below.
Women have mammograms, in which their breasts are x-rayed to search for early signs of breast cancer. Soft x-rays and highly sensitive x-ray film are used in mammography.

Photographic film is sensitive to x-rays but much more so to light. To rely only on x-rays would require high exposure times. Owing to the ionising ability of x-rays this is not desirable. Exposure times must always be kept to a minimum (e.g. for a skull x-ray, about 0.40 s). Therefore an image-intensifying cassette is used. A fluorescent material such as zinc sulphide is used to coat the intensifying screens; this absorbs the x-rays and re-emits the energy as light. It is the resultant light that affects the photographic film. The small atoms found in tissue (carbon, hydrogen, oxygen and nitrogen) rarely absorb medical x-rays, while the larger atoms found in bone (calcium) absorb x-rays frequently. That is why bones cast clear shadows onto x-ray film. Tissue shadows are also visible but they are much less obvious.

X-rays photons interact with tissue and bone in four major processes
· elastic scattering
· photoelectric effect
· Compton effect
· Electron-positron pair production.
Those who chose to study the “Particle Physics” option on the Leaving Certificate Physics course may wish to explore how these four processes are involved in medical x-rays, by pursuing the link Physics of the human body (Pdf pages 44-47).

Dentists use x-rays to locate problems of tooth decay and abscesses.

2. Radiation Therapy

In radiotherapy, x-rays are used to treat certain diseases, notably cancer, by exposing tumours to x-radiation. In a process known as radiotherapy, x-rays are directed at a tumour to destroy it. The x-rays are produced by a linear accelerator generating energies up to 6 MeV (compared with energies of 120 keV for diagnostic purposes). The x-ray machine is rotated in an arc around the patient. This ensures that the tumour receives a maximum dose but healthy tissue surrounding it does not receive too much.

CAT Scan

A recent development in the application of x-rays is the computerized axial tomography (CAT or CT) scanner. This offers a way of obtaining a sectional view of the body without shadows from other organs being imaged.

It works by having an x-ray source moving in an arc around the patient and producing short bursts of x-rays.

A large number of x-ray detectors are arranged, also in an arc, on the opposite side of the patient from the source. The detectors consist of sodium iodide crystals to detect the x-rays and photodiodes to record the scintillations from the crystals. A computer is used to collect information from the photodiodes and to reconstruct a slice of the body on a TV screen in a few seconds. The method is particularly good in examining soft tissue. Using a similar dosage of radiation as that of the conventional x-ray machine, an entire "slice" of the body is made visible with about 100 times more clarity. CAT scanners can detect tumours, blood clots, etc., which a conventional x-ray machine would be unable to detect.

The CAT scanner was invented by Godfrey Hounsfield who was awarded the Nobel prize for medicine in 1979.