The time that the radiation source is actually producing radiation.

A stream of electromagnetic or particulate radiation that could be collimated and is generally unidirectional or divergent usually from a small source and restricted to a small-solid angle.

See also Ion Beam, Broad Beam, Useful Beam and Beam On Time.

Conventional (also called analog, plain-film or projectional) radiography is a fundamental diagnostic imaging tool in the detection and diagnosis of diseases. X-rays reveal differences in tissue structures using attenuation or absorption of x-ray photons by materials with high density (like calcium-rich bones).
Basically, a projection or conventional radiograph shows differences between bones, air and sometimes fat, which makes it particularly useful to asses bone conditions and chest pathologies. Low natural contrast between adjacent structures of similar radiographic density requires the use of contrast media to enhance the contrast.
In conventional radiography, the patient is placed between an x-ray tube and a film or detector, sensitive for x-rays. The choice of film and intensifying screen (which indirectly exposes the film) influence the contrast resolution and spatial resolution. Chemicals are needed to process the film and are often the source of errors and retakes. The result is a fixed image that is difficult to manipulate after radiation exposure. The images may be also visualized on fluoroscopic screens, movies or computer monitors.
X-rays emerge as a diverging conical beam from the focal spot of the x-ray tube. For this reason, the radiographic projection produces a variable degree of distortion. This effect decreases with increased source to object distance relative to the object to film distance, and by using a collimator, which let through parallel x-rays only.
Conventional radiography has the disadvantage of a lower contrast resolution. Compared with computed tomography (CT) and magnetic resonance imaging (MRI), it has the advantage of a higher spatial resolution, is inexpensive, easy to use, and widely available. Conventional radiography can give high quality results if the technique selected is proper and adequate. X-ray systems and radioactive isotopes such as Iridium-192 and Cobalt-60 for generating penetrating radiation, are also used in non-destructive testing.

See also Computed Radiography and Digital Radiography.

Fluoroscopy is used to study moving body structures in real time. A fluoroscope is used to produce a continuous (advanced fluoroscopy machines provide pulsed techniques to lower the amount of radiation) x-ray beam, passing through the body part being examined and transmitted to a monitor so that dynamic images of deep tissue structures can be visualized. Fluoroscopy is primarily used for gastrointestinal exams, genitourinary studies, cardiovascular imaging and for invasive procedures performed by interventional radiologists and angiographers under fluoroscopic guidance. Fluoroscopy can also produce a static record of an image formed on the output phosphor of an image intensifier. The image intensifier is an x-ray image receptor that increases the brightness of a fluoroscopic image by electronic amplification and image minification. Modern fluoroscopy systems combine less radiation with better image quality due to digital image processing and flat-panel technology.
Roentgen's discovery of x-rays related directly to fluoroscopy, because fluorescence on the material in the room draws his attention to the x-ray's properties. In 1896, Thomas A. Edison created the first fluoroscope, consisting of a zinc-cadmium sulfide screen that was placed above the patient's body in the x-ray beam and provides a faint fluorescent image. In first-generation units, the exam room required complete darkness. The users wear red goggles for up to 30 minutes prior to the examination, to adapt the eyes to darkness. After this, the radiologist stared directly at a yellow-green fluorescent image through a sheet of lead to prevent the x-ray beam from striking the eyes.

The focal spot is the point where the electron beam impinges on the tube anode and from which x-rays are emitted. The size of the focal spot is determined by the size of the filament and cathode, which is determined by the manufacturer.
Most x-ray tubes have more than one focal spot size. The size of the focal spot has influence on spatial resolution. The smaller the focal spot, the better the limiting spatial resolution of the x-ray system, especially in magnification mammography. The use of a small focal spot concentrates heat onto a smaller portion of the anode therefore, more heat is generated and a longer cooling time is necessary.