The X-ray tube

Published on 01/04/2015 by admin

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Last modified 22/04/2025

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Chapter 8 The X-ray tube

KEY POINTS

TUBE HOUSING

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.

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.

THE FOCUSSING CUP

The filament is set centrally in a slot machined into a metal focussing cup: the cathode cup.The shape of the cup, along with the electrostatic forces, prevents the electron beam fanning out, concentrating it on the focal spot of the anode (Fig. 8.3).

This design is called an electronic lens system with a resulting focal spot width that depends upon:

Cathode cups are typically manufactured from molybdenum, nickel or an iron alloy to ensure dimensional stability during use when the assembly becomes hot.

A separate filament transformer in the high-tension tank separates the primary low voltage side from the secondary high tension (HT) side of the transformer. The HT is connected to a common lead for both filaments and the focussing cup. Both filaments and focussing cup make-up the cathode and have negative polarity.

When the X-ray set is turned on the filament is supplied with a lower than operating current to heat the filament and prepare it for the higher current needed during exposure. This produces a cloud of electrons around the filament. The electrostatic field limits the number of electrons produced at the filament, thereby limiting the maximum tube current possible; this is called the ‘space charge effect’. As the tube voltage increases, the tube current increases up to a point when all the electrons in the space charge have been used up and the tube is then said to have reached saturation current.

THE ANODE

Original X-ray tubes were designed with a tungsten anode set in a block of copper. The tungsten produces the X-rays and the copper carries the heat away from the tungsten. This design limits the X-ray output as the rise in temperature would eventually lead to melting of the tungsten. Stationary anode designs are still used in low-output applications, such as dental radiography, as they are simpler to construct, robust and cheaper.

In 1929 Bouwers at Phillips produced the first commercial rotating anode tube, known as the Rotalix. Here the anode was a rotating disc where the area bombarded by the filament electrons – the focal spot – became a focal track with a much larger surface area and volume and with a correspondingly larger heat capacity.

The anode is the positive terminal of the X-ray tube; it serves to conduct the tube current, provide support of the target and provides a means ofdissipating the heat away from the target. X-rays are produced by the rapid deceleration of fast moving electrons and tungsten is used as the material of choice for the combination of its properties.

These stationary anodes are found in basic X-ray tubes with low power requirements such as those in dental equipment, mobile C-arm units for fluoroscopy and low-load radiography. The low power requirements of these applications results in much lower heat generation and these requirements can be met using a stationary anode tube.

THE STATOR ASSEMBLY

The anode disc needs to rotate at high speed and this is achieved by attaching the anode via a stem to a large copper rotor, which forms the armature of a motor. The target disc, or rotor, is mounted on a shaft, the stem extending from a rotor body which can spin on internal bearings on the rotor shaft. This rotor shaft extends through the end of the insert to the outside of the insert vacuum for connection to the anode wire, and also is the mounting point for the insert inside the housing.

The rotor consists of a copper cylinder and rests in ball bearings for smooth movement. The bearings cannot be lubricated with ordinary grease because it would affect the vacuum and the high-tension characteristics of the tube. Soft metals such as lead and silver are applied to separate the ball bearings and the running surfaces, in order to prevent the possibility of ‘jamming’ in the vacuum. This form of lubrication limits the lifetime of the bearings in the X-ray tube to about 1000 hours.

The exposure switch controls the rotation. The anode only rotates when radiation is required and is braked immediately afterwards. The high inertia of the heavy metal disc leads to some delay in the rotor reaching operational speed. The delay is up to 2.5 seconds, depending on the type of starting device and the anode. An interlock ensures exposure can only take place after the anode has reached its final speed.

The heat of the anode should be prevented from reaching the stator arrangement. The stem is designed to limit the transfer of heat to the rotor assembly; molybdenum is used as it has lower thermal conductivity than tungsten and is made as thin as possible to reduce heat conduction towards the bearings (Fig. 8.8).

ANODE COOLING

The amount of heat stored in the anode at the time of exposure is measured in ‘heat units’ (HU) and this is calculated as a product of exposure (in kV) multiplied by the tube current and time (mA s).

As an example, a typical chest exposure of 90 kV at 2 mA s produces 180 HU, which requires only a short cooling time; however, fluoroscopy at 90 kV for 3 min at 2 mA (360 mA s) produces 32 400 HU and requires a cooling time of over 5 min. Cooling time is exponential with time and depends upon the temperature of the surrounding materials and requires a correction factor for different voltage waveforms. Tube housings also have cooling charts and these are used in a similar manner to the anode cooling charts.