Actual focal spot = The area of the tungsten target upon which the stream of electrons produced at the cathode impinge at high speed.

Angiocardiography = Radiography of the heart chambers and major vessels using water soluble iodine containing contrast to opacify the cardiovascular system.

Anode = The tube head target; the positively charged electrode upon which the electron stream from the cathode collide.

Arthrography = Contrast radiography of a joint space used to demonstrate capsule distension or rupture and articular cartilage problems.

Atom = The smallest possible particle of an element retaining all properties of the element in question; it consists of protons, neutrons and electrons.

Atomic number = The number denoting the unique quantity of protons possessed by an element.

Bronchography = Contrast radiography of the bronchial tree; used to diagnose conditions such as bronchial foreign bodies, tumours or lung lobe torsion.

Cathode = The tube head filament; the negatively charged electrode where the electron stream is generated by thermionic emission.

Clearing time = The time taken for removal of unexposed halide during fixing.

Collimation = Determination of the limits of an area of interest to be radiographed.

Compression band = A tight support used to reduce large areas such as the chest and abdomen during radiography.

Contrast = The differences between shades visible on a radiograph.

Contrast media = Agents used in order to opacify a structure or organ enabling better visualisation of the structure itself of mucosal surface.

Contrast radiography = Radiographic studies utilising contrast media allowing improved assessment of the shape, size and position of a structure or organ.

Controlled area = An area within a room designated specifically for radiography as demarcated by the radiation protection adviser.

Crossed grid = A type of stationary grid containing a series of lead strips alternating with strips of a radiolucent material; these strips run both vertically and horizontally making the crossed grid very efficient, but expensive.

Definition = The sharpness or clarity of the structures of interest visible on a radiograph.

Density = The degree of blackness visible on a radiograph.

Developer = An alkaline solution containing phenidone-hydroquinone responsible for the first stage of processing a radiograph. During development, the exposed silver bromide crystals of the film emulsion are converted into grains of black metallic silver while the bromide ions are released into solution.

Dichroic fog = Yellow staining marring a developed radiograph; the cause is insufficient rinsing or exhausted fixer.

Double contrast radiography = Radiography of a hollow organ using both a positive contrast agent (usually water soluble iodine) and a negative contrast agent (room air or oxygen). The inner lining of the organ is coated with the positive agent providing better visualisation of the mucosae, while the gas within the organ prevents the obscuring of small defects.

Effective focal spot = The small area from which the x-ray beam appears to originate due to the angulation (20 degrees to the vertical) of the target; this should ideally be a point source in order to produce fine radiographic definition.

Electron = A negatively charged particle revolving around the nucleus of an atom; electrons orbit in planes or "shells".

Element = A simple chemical substance that cannot be decomposed by usual chemical means; an element may be a constituent of a compound, which is a combination of 2 or more elements.

Filament = The tungsten coil of the tube head cathode (negatively charged electrode); it is from the filament that electrons are produced by thermionic emission.

Film focal distance = The distance between the focal spot and the radiographic film; usually 75cm-100cm.

Film-screen combination = The selection of a compatible film type and intensifying screen of a certain speed and definition.

Fixer = An acidic solution containing thiosulphate responsible for the removal of unexposed silver halide crystals leaving a permanent image that may be viewed in normal light. Fixer also hardens the film emulsion making it more resistant to scratches and abrasions.

Flat film = A low contrast radiograph showing mainly shades of grey; the cause is usually under-development.

Focal spot size = The size of the area from which the primary beam emerges. Fine focus will produce a smaller focal spot size and thus better definition than coarse (or broad) focus. Coarse focus is suitable for dense areas where higher exposures are required.

Focussed grid = A stationary grid consisting of central vertical strips of alternating lead and radiolucent material and gradually sloping strips on either side. The focussed grid is superior to parallel grids since the divergence of the primary beam is taken into account, thus avoiding "grid cut off".

Fogging = Term describing undesirable darkening of a radiograph. Causes may be environmental (safe-light fog), technical (over-exposure or scatter) or developmental (over-development). See also dichroic fog.

Gastrogram = A contrast radiograph of the stomach.

Grid = A special piece of radiographic equipment used to prevent scatter consisting of alternating strips of lead and radiolucent material. The grid is placed between the patient and the x-ray cassette where obliquely moving scatter is absorbed by the lead strips, whilst undeflected photons pass through the radiolucent interfaces.

Grid factor = The amount by which mAs must be increased by in order to compensate for the absorption of some of the primary beam when a grid is used. The grid factor is specific to each grid, but generally is around 2.5-3.

Grid ratio = The ratio of the height of the lead strips to the width of the radiolucent interspaces. Grids with a large ratio are more efficient at absorbing scatter, usual ratios are between 5:1-10:1.

Intensifying screen = A screen within the x-ray cassette comprising phosphorescent crystals that serve to intensify the effect of the x-rays upon the radiographic film.

Ionising radiation = The emanation of energy in the form of electromagnetic waves of x-rays.

Kilovoltage (kV) = The potential difference applied between the filament and target of the x-ray tube head; the penetrating power of the primary beam.

Light beam diaphragm = Adjustable device used to control the size of the emergent x-ray beam; device for the purpose of collimation.

Line voltage compensator = Device used to control fluctuations in the mains electricity.

Milliamperage (mA) = The quantity of x-rays in the emergent beam; allows determination of the tube current and the intensity of the primary beam.

Milliampere seconds (mAs) = Combination of x-ray quantity and exposure time.

Monochromatic film = Film sensitive only to blue light emitting screens.

Movement blur = Undesirable effect found on exposed films due to instability of equipment such as the tube head or cassette or patient activity.

Myelography = Radiographic examination of the spinal cord following the introduction of a radio-opaque contrast medium by means of cisternal or lumbar puncture into the sub-arachnoid space.

Negative contrast agent = Radio-opaque medium allowing better visualisation of the structures of interest on a radiograph; negative agents are gases (usually air or oxygen) and appear darker than the surrounding tissues.

Neutron = Uncharged charged particles found within the nucleus of an atom.

Non-screen film = Radiographic film that is not used with intensifying screens; it is contained within a plastic envelope rather than a cassette. Very fine detail is produced although longer exposure times are necessary. Non-screen film is used almost exclusively for intra-oral radiography.

Object film distance = The distance between the area of interest to be radiographed and the film; they should be as close to one another as possible to avoid magnification and distortion of the image.

Orthochromatic film = Radiographic film sensitive to both blue and green light emitting screens.

Parallel grid = A type of stationary grid comprising alternating vertical strips of lead and radiolucent material. The perimeter of the exposed radiograph may be of poor quality due to the absorption of x-ray photons from the edge of the primary beam.

Penumbra effect = Undesirable effect causing blurring of the image due to the use of a large focal spot size.

Photon = Energy packet released when rapidly moving electrons are slowed down or halted.

Pneumocystogram = A contrast radiograph of the bladder utilising gas as a negative contrast agent to enable better visualisation.

Portal venography = Contrast radiography of the vasculature of the liver involving the introduction of a contrast agent via the hepatic portal vein.

Positive contrast agent = An element used in radiography to outline endothelial or mucosal surfaces thus allowing improved visualisation of the position, size and shape of the structure or tissues of interest. Positive agents appear whiter than normal tissues due to their high atomic number.

Potter Bucky diaphragm = An expensive, but highly effective moving grid incorporated in the radiography table-top. Grid lines are not seen on the exposed film unlike those found following the use of stationary grids.

Primary beam = The useful beam of polychromatic x-ray photons used to create an image on radiographic film.

Proton = Positively charged particle contained within the nucleus of an atom.

Pseudo-focussed grid = A type of stationary grid comprising vertical slats that become progressively shorter towards the edges, thus reducing the amount by which the perimeter of the primary beam is absorbed.

Radiation protection advisor (RPA) = Experienced external adviser responsible for the demarcation of the controlled area and the drawing up of local rules and written systems of work.

Radiation protection supervisor (RPS) = A senior veterinary surgeon responsible for ensuring that all radiographic procedures are undertaken safely and in accordance with the local rules and written systems of work.

Radiography = The production of diagnostic film records of internal body structures by the exposure of film specially sensitised to x-rays.

Radiology = The science of radiation and its use in the diagnosis and treatment of disease.

Reduction = The chemical process by which a radiographic image is produced during development; the silver bromide ions of the film emulsion are converted into crystals of metallic silver whilst the bromide ions are lost in solution.

Retrograde urethrogram = Contrast radiograph of the male urethra using water soluble iodine containing contrast medium in order to provide improved visualisation; indicated in cases such as haematuria, dysuria and urinary incontinence.

Retrograde vaginourethrogram = Contrast radiograph of the female urethra using water soluble iodine containing contrast medium introduced via a Foley catheter.

Scattered radiation = Secondary radiation produced by lower energy x-ray photons that emerge from the patient in random directions; representative of a serious hazard if steps are not taken to prevent risk to personnel.

Soot and whitewash = Term describing an exposed film showing very high contrast and few shades of grey; the cause is the use of a too low a kilovoltage.

Stop bath = The intermediate wash bath in manual development; usually water, but may be a weak acid which is better able to halt the development and more compatible with the fixer which is also acidic.

Tanning = Term describing the hardening of the developed radiograph during fixing.

Target = The positively charged electrode of the x-ray tube head (anode) upon which the electrons generated at the filament impinge. The target may be stationary and mounted on a copper stem to transport heat to the oil bath, or rotating and mounted on molybdenum rod to prevent heat being conducted to motor.

Thermionic emission = The process by which an electron cloud is released following heating of the filament, bypassing an electrical current through it.

X-ray tube head = The part of the x-ray machine where x-rays are generated; it contains the filament and target which are essential components necessary for the formation of ionising radiations.