Saturday, July 25, 2015
A heterogeneous beam of X-rays is produced by an X-ray generator and is projected toward an object. According to the density and composition of the different areas of the object a proportion of X-rays are absorbed by the object. The X-rays that pass through are then captured behind the object by a detector (film sensitive to X-rays or a digital detector) which gives a 2D representation of all the structures superimposed on each other. In tomography, the X-ray source and detector move to blur out structures not in the focal plane. Computed tomography (CT scanning) is different to plain film tomography in that computer assisted reconstruction is used to generate a 3D representation of the scanned object/patient.
Medical and industrial radiography
Applications of radiography include medical radiography and industrial radiography. If the object being examined is living, whether human or animal, it is regarded as medical; all other radiography is regarded as industrial radiographic work.
Taking an X-ray image with early Crookes tube apparatus, late 1800s.
Radiography started in 1895 with the discovery of X-rays, also referred to as Rontgen rays after Wilhelm Conrad Rontgen who first described their properties in rigorous detail. These previously unknown rays (hence the X) were found to be a type of electromagnetic radiation. It wasn't long before X-rays were used in various applications, from helping to fit shoes to the medical uses of today. The first radiograph used to assist in surgery was taken a year after its invention in Birmingham by the British pioneer of medical X-Rays, Major John Hall-Edwards. X-rays were put to diagnostic use very early, before the dangers of ionizing radiation were discovered. Indeed, Marie Curie pushed for radiography to be used to treat wounded soldiers in World War I. Initially, many kinds of staff conducted radiography in hospitals, including physicists, photographers, doctors, nurses, and engineers. The medical specialty of radiology grew up over many years around the new technology. When new diagnostic tests were developed, it was natural for the radiographers to be trained in and to adopt this new technology. Radiographers now often do fluoroscopy, computed tomography, mammography,ultrasound, nuclear medicine and magnetic resonance imaging as well. Although a nonspecialist dictionary might define radiography quite narrowly as "taking X-ray images", this has long been only part of the work of "X-ray departments", radiographers, and radiologists. Initially, radiographs were known as roentgenograms.
Equipment
Sources
A number of sources of X-ray photons have been used; these include X-ray generators, betatrons, and linear accelerators (linacs). For gamma rays, radioactive sources such as 192Ir, 60Co or 137Cs are used.
Detectors
A range of detectors including photographic film, scintillator and semiconductor diode arrays have been used to collect images.
Theory of X-ray attenuation
X-ray photons used for medical purposes are formed by an event involving an electron, while gamma ray photons are formed from an interaction with the nucleus of an atom. In general, medical radiography is done using X-rays formed in an X-ray tube. Nuclear medicine typically involves gamma rays.
The types of electromagnetic radiation of most interest to radiography are X-ray and gamma radiation. This radiation is much more energetic than the more familiar types such as radio waves and visible light. It is this relatively high energy which makes gamma rays useful in radiography but potentially hazardous to living organisms.
