Chapter 8 The X-ray tube
Since the discovery of X-rays in 1895 by Roentgen and the heated cathode X-ray tube by Coolidge, X-ray tubes have developed into complex pieces of electromechanical engineering. They comprise around 350 parts, taking 150 assembly operations. The cost (at date of publication) can be as much as £20 000.
Figure 8.1 Rotating anode X-ray tube. A Anode disk, B Ball bearings, C Collimator lamp, D Collimator diaphragm, E Glass envelope, F Focal track, HT High-tension cable socket, K Cathode assembly, L Lead lining, M Mirror, Ms Microswitch, N Expansion diaphragm, O Oil, P Tube port, R Rotor assembly, S Anode stem, T Rotor support, V Plastic window, W Stator windings.
The tube housing protects the delicate insert from damage during use (Fig. 8.2). It is made of steel or aluminium with an external protective coat of paint to allow easy cleaning and an internal lining of lead to reduce radiation leakage to below the required maximum. The cover has special mounting rings, trunions for attachment to the tube suspension equipment, and sealed terminals and sockets for the high-tension cables and other associated control equipment connections. On the external surface the tube is marked to indicate the position of the focus point of the anode and a plate indicates the electrical characteristics and date of manufacture.
The cover is lined with lead to reduce radiation leakage, except for the X-ray port, which is made of plastic or beryllium. Beryllium is used as it has low X-ray absorption due to its proton number of 4.
Legislation varies in different countries; however, a typical figure for the maximum radiation leakage from an X-ray tube housing at 1 meter distance from the tube, with collimators closed, is 1 mGy per hour when the tube is operating at its maximum factors.
The tube housing is earthed to provide shock proofing and contains mineral oil surrounding the insert to electrically insulate it and aid cooling. Expansion bellows within the tube housing allows expansion of the oil when the X-ray tube heats up during use.
In order for the X-ray tube to operate, the anode and cathode need to be contained in a vacuum;this vacuum is contained within the tube envelope. The envelope needs to be strong enough to support the anode and cathode assemblies, provide electrical insulation between the two and maintain the vacuum. The tube vacuum envelope is generally made of glass although some high-power tube envelopes are made of metal or ceramic.
Glass tube envelopes are made of borosilicate glass, which provides the required strength, low coefficient of thermal expansion and electrical insulation. Metal and combined metal with ceramic insulation are alternative methods of construction with the advantages of greater strength and mechanical stability compared to previous tubes made of glass; heat dissipation is also improved.
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.
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.