Part of the electro-magnetic spectrum; first discovered in 1895 by Wilhelm Röntgen, their wavelengths are between 109 and 1013 metres; in behaviour and energy they are identical to the gamma rays emitted by radioactive isotopes. Diagnostic X-rays are generated in an evacuated tube containing an anode and cathode. Electrons striking the anode cause emission of X-rays of varying energy, the energy being largely dependent on the potential difference (kilovoltage) between anode and cathode. The altered tissue penetration at different kilovoltages is used in radiographing different regions, for example in breast radiography (25–40 kV) or chest radiography (120–150 kV). Most diagnostic examinations use kilovoltages between 60 and 120. The energy of X-rays enables them to pass through body tissues unless they make contact with the constituent atoms. Tissue attenuation varies with atomic structure, so that air-containing organs such as the lung offer little attenuation, while material such as bone, with abundant calcium, will absorb the majority of incident X-rays. This results in an emerging X-ray pattern which corresponds to the structures in the region examined. Their use for diagnostic imaging (radiology) and for cancer therapy (see RADIOTHERAPY) has been an integral part of medicine for over one hundred years. Many other forms of diagnostic imaging have been developed in recent decades, sometimes also loosely called ‘radiology’. Similarly the use of chemotherapeutic agents in cancer has led to the term oncology, which may be applied to the treatment of cancer by both drugs and X-rays.
The recording of the resulting images is achieved in several ways, mostly depending on the use of materials which fluoresce in response to X-rays.
Many body organs are not shown by simple X-ray studies. This led to the development of contrast materials which make particular organs or structures wholly or partly opaque to X-rays. Thus, barium-sulphate preparations are largely used for examining the gastrointestinal tract: for example, barium swallow, barium meal, barium follow-through and barium enema. Water-soluble iodine-containing contrast agents that ionise in solution have been developed for a range of other studies.
More recently a series of improved contrast molecules, chiefly non-ionising, has been developed, with fewer side-effects. They can, for example, safely be introduced into the spinal theca for myeloradiculography – contrast X-rays of the spinal cord. Using these agents, it is possible to show many organs and structures mostly by direct introduction, for example via a catheter (see CATHETERS). In urography, however, contrast medium injected intravenously is excreted by the kidneys which are outlined, together with ureters and bladder. A number of other more specialised contrast agents exist: for example, for cholecystography – radiological assessment of the gall-bladder. The use of contrast and the attendant techniques has greatly widened the range of radiology.
The relative insensitivity of fluorescent materials when used for observation of moving organs – for example, the oesophagus – has been overcome by the use of image intensification. A faint fluorographic image produced by X-rays leads to electron emission from a photo-cathode. By applying a high potential difference, the electrons are accelerated across an evacuated tube and are focused on to a small fluorescent screen, giving a bright image. This is viewed by a TV camera and the image shown on a monitor and sometimes recorded on videotape or cine.
X-ray images are two-dimensional representations of three-dimensional objects. Tomography (Greek tomos – a slice) began with X-ray imaging produced by the linked movement of the X-ray tube and the cassette pivoting about a selected plane in the body: over- and underlying structures are blurred out, giving a more detailed image of a particular plane.
In 1975 Godfrey Hounsfield introduced COMPUTED TOMOGRAPHY (CT) (CT). This involves (i) movement of an X-ray tube around the patient, with a narrow fan beam of X-rays; (ii) the corresponding use of sensitive detectors on the opposite side of the patient; (iii) computer analysis of the detector readings at each point on the rotation, with calculation of relative tissue attenuation at each point in the cross-sectional plane. This invention has enormously increased the ability to discriminate tissue composition, even without the use of contrast.
The tomographic effect – imaging of a particular plane – is achieved in many of the newer forms of imaging: ULTRASOUND, magnetic resonance imaging (see MRI) and some forms of nuclear medicine, in particular positron emission tomography (PET SCANNING). An alternative term for the production of images of a given plane is cross-sectional imaging.
While the production of X-ray and other images has been largely the responsibility of radiographers, the interpretation has been principally carried out by specialist doctors called radiologists. In addition they, and interested clinicians, have developed a number of procedures, such as arteriography (see ANGIOGRAPHY), which involve manipulative access for imaging – for example, selective coronary or renal arteriography.
The use of X-rays, ultrasound or computerised tomography to control the direction and position of needles has made possible guided biopsies (see BIOPSY) – for example, of pancreatic, pulmonary or bony lesions – and therapeutic procedures such as drainage of obstructed kidneys (percutaneous nephrostomy), or of abscesses. From these has grown a whole series of therapeutic procedures such as ANGIOPLASTY, STENT insertion and renal-stone track formation. This field of interventional radiology has close affinities with MINIMALLY INVASIVE SURGERY (MIS).
The two chief sources of the ionising radiations used in radiotherapy are the gamma rays of RADIUM and the penetrating X-rays generated by apparatus working at various voltages. For superficial lesions, energies of around 40 kilovolts are used; but for deep-seated conditions, such as cancer of the internal organs, much higher voltages are required. X-ray machines are now in use which work at two million volts. Even higher voltages are now available through the development of the linear accelerator, which makes use of the frequency magnetron which is the basis of radar. The linear accelerator receives its name from the fact that it accelerates a beam of electrons down a straight tube, 3 metres in length, and in this process a voltage of eight million is attained. The use of these very high voltages has led to the development of a highly specialised technique which has been devised for the treatment of cancer and like diseases.
Protective measures are routinely taken to ensure that the patient's normal tissue is not damaged during radiotherapy. The operators also have to take special precautions, including limits on the time they can work with the equipment in any one period.
The greatest value of radiotherapy is in the treatment of malignant disease. In many patients it can be used for the treatment of malignant growths which are not accessible to surgery, whilst in others it is used in conjunction with surgery and chemotherapy.