LINE FOCUS PRINCIPLE

LINE FOCUS PRINCIPLE

The region present at the tungsten anode in the x-ray tube where the electron stream (stream) emitted from the cathode is called the focal spot. 99% of these incident electrons are converted into heat, only less than 1% is converted into the required x-ray. This emitted energy fails uniformly in the focal spot area.
Therefore, the larger the focal spot area, the higher the heat generated by the collision of the electron anode, the less hot the target (anode) will be, so it is less likely to melt, the smaller the focal spot, the smaller the energy of the focal spot. Causes it to melt. Therefore, to increase the heat loading capacity of the anode, it is necessary that the focal spot be taken as large as possible.

If the focal spot is enlarged, the radio graphic details are affected. The smaller the focal spot, the better the radio graphic detail on X-ray film. Therefore, to increase the lifespan of the x-ray tube it is necessary that the focal spots are enlarged and to get an X-ray of good radio graphic details, it is necessary that the focal spots should be taken as small as possible.  In 1918, the Line Focus Principle was presented, which solved all these problems.

The size and shape of the focal spot depends on the size and shape of the electron beam emitted from the anode, and the shape and size of the electron beam depends on the dimensions of the tungsten filament, the formation of the focusing cup, and the position in the focusing cup of the filament.The target surface on which the electron beam collides and tilts slightly from the vertical plane of the electron beam, this inclination from the vertical plane of the electron beam of the anode is called anode angle. The anode angle is different for an x-ray tube of different design, and in a diagnostic radiology x-ray tube it is usually about 6⁰ to 20⁰.