Line Focus Principle
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X-rays beams are produced when an electron is accelerated or decelerated. (This is why young children are warned to keep their distance from old televisions). Tungsten is a good source for producing this kind of radiation. The high-energy electrons can interact with the nuclei of the tungsten and produce white radiation or general radiation.
When an electron is attracted toward the nucleus due to deviation from its original path, it may (or may not) lose energy. If it does (or does not), elastic (inelastic) scattering will be produced. This radiation is referred to as white radiation.
Electrodes are located inside a vacuum for the purpose of independently controlling the speed of the accelerated electrons.
The structure of generators is such that the electrodes that are responsible for generating and directing the electrons are sealed in a vacuum for the purpose of controlling the speed of the electrons striking the tungsten. The cathode is positive and composed of two parts, a filament and tungsten.
The filament is a meander coil tungsten of a specified size. When current passes through the wire, heat is produced and gets absorbed by the electrons. The electrons escape from the metal and are considered space charge. Because of its high melting point, tungsten is very useful for this job.
Line Focus Principle
Most of the energy carried by the electrons is converted to heat. As a result of the great amount of heat produced, a large focus point is needed. Better images need a smaller focal point. The focal size required, f, is related to the length of the actual focal size. f = F*sin (angle), where the angle is the anode angle. A larger anode angle means a larger focal point.
In general, an X-ray's beam intensity is not uniform. When it focuses to a target, a conical shape appears. The intensity of the positive anode is lower than the intensity of the negative cathode. Because intensities are different, a visible difference in the picture is produced; this effect is called the heel effect. The heat problem produced by the anode is solved by rotation of the anode up to 10,000 rpm. This rotation also increases the total area.
- Fung KK, Gilboy WB (2000). "Anode heel effect" on patient dose in lumbar spine radiography". Br J Radiol 73 (869): 531–536.