Line Focus Principle
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The X-Ray is produced when electron is accelerated or deccelarated. (That's why young kids are suggested to keep their distance from old TVs). Tungsten is a good source to produce this kind of ray. 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 loose energy or may not. If it does, and does not, elastic and inelastic scattering will be produced. The radiation is named white radiation.
Electrodes are inside vacuum for independent controlling of the speed of the accelerated electrons.
The structure of generators is that, electrodes which are responsible for generating and directing the electrons are sealed in a vacuum, for controlling the speed of the electrons crushing the tungsten. Cathode is positive and is composed of two parts. The first one is filament and the second one is tungsten.
Filament is a meander coil tungsten with the indicated size. When current passes through the wire, heat is produced and gets absorbed by the electrons. They escape from the metal and they are named space charge. Because of high melting point, tungsten is very useful for this job.
Line Focus Principle
Most of energy carried by the electrons are converted to heat. So that, a large focus point is needed because of huge amount of heat which is produced. Better images need a small focal point. The needed focal size is related to the length of actual focal size. f = Fsin (angle) where angle is the anode angle. Larger anode angle means a larger focal point.
In general, X-Ray's beam intensity is not uniform. When it focuses to target, the conical shape appears. 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 and this effect is called as heel effect. The heat problem produced by anode is solved by rotation of anode up to 10,000 rpm, it 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.