Rotating anode and line focus principle

2.

The principles of the rotating anode (C: 14-17; H: 24)

·         The ability of the x-ray tube to achieve high x-ray outputs is limited by the heat generated at the anode

·         The purpose of the rotating anode is to spread the heat produced during exposure over a large area of the anode.

·         The rotating anode consists of a large disc of tungsten or an alloy with a beveled edge (6^o – 20^o) which rotates at a very fast speed (e.g. 3600 rpm)

·         Because of the rotation, the electrons accelerated from the cathode will bombard a constantly changing area of the target.

·         For 3600 rpm any one area is only exposed to the beam every 1/60 seconds, and the remainder of the time heat generated during the exposure is dissipated.

·         The faster the anode rotates the greater is its ability to withstand heat.

·         The diameter of the disc determines the total length of the target track and affects the maximum permissible loading of the anode.

·         The stem of the rotating tube is made from Molybdenum (a poor heat conductor) to prevent the anode assembly and bearings from overheating and binding/ sticking.

·         The inertia of the heavier anode causes a delay between the application of force to the anode assembly and the time at which maximum angular velocity is reached – therefore a circuit is incorporated to prevent the exposure until the rotor has reached its full speed.

·         Also the stem is made shorter to decrease the inertia as much as possible.

·         A rotating tube greatly increases the effective target area used during an exposure and therefore raises the heat capacity

The line focus principle (C: 13; BB: 108-109; H: 25)

·         Beveling of the anode edge (6^o– 20^o) takes advantage of the line focus principle

·         Most of the energy of the electrons in the tube current is converted to heat

·         A large focal spot allows the accumulation of larger amount of heat before damage (i.e. a larger focal spot will allow greater heat loading)

·         But for good radiographic detail a small focal spot is required

·         The size and shape of the focal spot is determined by the size and shape of the electron beam which is determined by:

o   The dimensions of the filament

o   Construction of the focusing cup

o   Position of the filament in the cup

·         But when the target is beveled, the focal spot, when viewed from the direction in which the x-rays emerge, is foreshortened and appears small

·         The EFFECTIVE/ APPARENT focal spot is smaller than the ACTUAL focal spot on the target – the size being related to the sine of the angle of the target (Effective focal length = Actual focal length x sinQ)

·         As the angle of the anode is made smaller the apparent focal spot becomes smaller

·         But for practical purposes there is limit to which the anode angle can be decreased as dictated by the heel effect (the point of anode cut-off)

·         There are three trade-offs to consider for the choice of anode angle:

o   Heat/ power loading

o   Effective focal spot size (small = good resolution)

o   Field coverage (small angle = small field coverage)

·         The line focus principle is used to permit larger heat loading while minimizing the size of the focal spot  by orientating the anode at a small angle to the direction of the x-ray beam irradiating the patient