The X-ray tube

Chapter 8 The X-ray tube





KEY POINTS
















FILAMENT ASSEMBLY


The filament is the source of electrons used in the production of X-rays. Electron production occurs when the filament is heated to around 2000 °C, this is achieved by passing a current through the filament. The temperature of the filament determines the number of electrons produced and is controlled by the milliamperes (mA) selected by the operator. The filament assembly is constructed as an electromagnetic lens so that it focusses the accelerated electrons to a small area of the anode – the focal spot.


There are usually two filaments: a small one with low output for better geometric resolution and a larger filament for higher output capacity, with wire diameters of 0.22 mm and 0.3 mm diameter, respectively. The filament is constructed as a spiral, with dimensions calculated to maximise the even density of the electrons produced. An alternative electron source is the flat emitter filament instead of a helix. This is used for some modern mammography tubes and allows a better X-ray intensity distribution than with the helix, thus improving image quality.


The filament is generally made of tungsten as it is:






and has:




Tungsten’s low coefficient of linear thermal expansion ensures the dimensions change little when it is heated, and the low vapour pressure ensures little tungsten is vaporised. This is important because, when deposited on the inside of the glass tube, tungsten reduces output and increases the possibility of arcing, causing severe damage to the tube. The addition of between 1% and 2% thorium to the tungsten improves thermionic emission.


Feb 20, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on The X-ray tube
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