The technology involved in the production of X-Rays has come a long way since the days of Wilhelm Roentgen1, with various techniques being used to increase both the precision and continuity with which machines can produce X-Rays for diagnostic purposes. This Entry considers the structure of the X-Ray tube, a device which produces X-Rays through the use of high voltage electricity, and then proceeds to look at the various developments used to improve the usefulness of the tubes.
X-Rays are produced by converting electrical energy into an electromagnetic wave. This is done by accelerating electrons from an electrically negative cathode towards a positive 'target' anode. When the electrons hit the target they are decelerated rapidly, causing them to lose energy which is converted into heat energy and X-Rays. The anode and cathode effectively form a circuit which is completed by the flow of electrons through the vacuum of the tube. The basic layout of an X-Ray tube therefore contains the following objects:
If electrons are fired through a space containing any air or gas, the electrons will interact with the molecules of the gas, colliding with them and producing lower energy secondary electrons. This is not desirable in an X-Ray tube as it would make the quantity and quality of X-Rays produced very difficult to control. For this reason, the anode and cathode are surrounded by an airtight glass enclosure, thus allowing a vacuum to be created inside the tube. Glass expands less when heated than most metals, and so special alloys which expand roughly the same amount as glass when heated are used to seal the gaps between metal and glass components. Meanwhile, the tube is shaped so that the anode and cathode are far enough apart to avoid electrical discharge between the two.
During operation both the glass and metal parts of the tube become very hot, so the glass casing is usually surrounded by oil which acts as a coolant and heat distributor. The apparatus containing the oil, tube and electrical connections is known as the tube housing, and has a small window at the bottom to allow the X-Rays to leave. The rest of the tube housing is lined with lead to prevent stray X-Rays from escaping, and the housing also functions to protect those present from electrocution by the tube's electrical circuits.
The negative cathode consists of a thin filament, usually made of tungsten, which is connected to two separate circuits. The first of these usually runs at about 10V and 5A and heats the filament ready for use, while the second provides the precise high voltage and low current which produces the stream of electrons which are accelerated towards the anode. The cathode may also be surrounded by a negatively charged focusing cup, which acts to focus the beam of electrons towards the anode. The process through which electrons escape the cathode will be discussed later.
The anode consists of a small piece of tungsten embedded in a large lump of copper, with the former being bombarded by electrons while the latter acts as a heat sink to prevent the tungsten from becoming damaged. While tungsten has a high atomic number, which makes it good for X-Ray production, and a high melting point (3,370°C), it has been found that an alloy of 90% tungsten and 10% rhenium is more resistant to damage from overheating and continuous electron bombardment. However, the copper heat sink has a lower melting point (1,070°C) than tungsten, and so the tungsten target must be a little bigger than necessary so that it loses some of the heat before passing the rest on to the copper. The tungsten face may be grooved in places to allow easier expansion upon heating, and the back of the anode can be coated with carbon or a similar black compound to help aid dissipation of heat.
The tungsten edge of the anode is angled slightly so as to direct X-Rays down towards an exit window in the tube housing. Due to the high level of heat produced while in use, most modern anodes now consist of a large copper wheel with a continuous tungsten target forming the outermost section, with the anode being rotated rapidly during exposure so as to spread the heat produced across a large surface area. The process though which X-Rays are produced at the tungsten target will be discussed in a separate Entry.
Most X-Ray tubes also incorporate a beam restrictors, which are discussed below, and filters, which are a complicated matter and deserve their own Entry. The same goes for X-Ray grids, screens and film, all of which are separated from the x-ray tube by the patient and thus do not fall under the topic of X-Ray tubes.
Thermionic emission is the name given to the process through which the electrons escape the negative cathode on their way towards the anode. When the heating circuit is activated, the current flowing through the tungsten filament heats the wire to about 2,200°C, giving some of the electrons in the metal enough energy to move a little way from the surface of the metal. This leads to a cloud of electrons hovering around the filament - this is known as the Edison effect, and leads to a negative 'space charge' existing around the filament. Once a certain number of electrons have begun to hover around the wire, the space charge becomes so negative that it prevents any more electrons from joining the cloud, with excess electrons becoming repelled back into the wire, which is effectively more positive than the cloud. This is known as the space charge effect, and it limits the number of electrons that can escape from the filament at a given temperature.
However, once the high voltage circuit involving the anode and cathode is activated, things change. The tungsten filament now becomes extremely negative, forcing the electrons in the space charge cloud to escape into the vacuum of the tube, where they are attracted towards the positive anode. As the electrons generally spread out as they escape from the cloud, a negative circular focusing cup surrounds the cathode filament, repelling those electrons which stray from the correct path back onto the right track. In 'grid-controlled' tubes, the focusing cup can also be made negative enough to prevent any electrons from escaping the filament, allowing the current to be turned on and off rapidly. The electrons flowing from the cathode to the anode complete the circuit, with the number of electrons flowing from cathode to anode being determined by the ampage of the circuit, while their energy is determined by the voltage difference between the cathode and anode.
However, there are exceptions to this rule. Below a peak kilovoltage2 of 40 (or 40kVp for short), not all the electrons in the space charge cloud are able to escape as soon as they are emitted. This leads to the existence of a residual space charge, which remains around the filament even though many of the electrons are able to escape directly. This negatively charged barrier reduces the flow of electrons from cathode to anode, effectively reducing the current between the two. Above 40kVp (the saturation voltage), the residual space charge is very small, and the small changes in current caused by increasing the kVp are then adjusted for automatically by reducing the temperature of the filament very slightly. Different X-Ray tubes have different saturations voltages and require different amount of adjustment to assure a constant current with changing peak kilovoltages.
The Line Focus Principle
As mentioned above, the tungsten face of the anode is angled so as to direct the X-Rays it produces down towards the exit window in the tube housing. The focus spot is the area on the anode which is bombarded by electrons and produces X-Rays as a result, but becomes overheated in the process and should ideally be as wide as possible to spread out the heat. However, the focal spot should preferably be as small as possible, as only small focal spots produce good diagnostic images. This is where the line focus principle becomes important. Basically, if the face of the anode is angled at less than 45°, the apparent size of the focal spot perpendicular to the X-Ray tube will be smaller than the actual focal spot. Think of a right-angled triangle with sides of five, 12 and 13 units, with the angled side (13) representing the face of the anode. In this example the stream of incoming electrons would be 12 units wide, but the beam of X-Rays produced would only be five units wide. Though this is a simplification of the situation, this example shows that it is possible to have a large target area for the electrons, but still produce a narrow beam of X-Rays.
The Heel Effect
Although there is a high intensity of X-Rays directly perpendicular to the X-Ray tube, the electrons bombarding the anode do in fact produce X-Rays in all directions. This becomes important when using the X-Rays produced to make an X-Ray film, as the intensity varies depending on the angle at which the X-Rays were emitted from the focal spot. When looking at the plane in which an X-Ray film would be placed, the intensity of the X-Rays increases a little towards the cathode side of the film3, with the intensity otherwise trailing off towards the edges of the film. This effect leads to lower image quality, and so films are either placed far from the X-Ray tube or smaller films are used, thus reducing the variation in X-Ray intensity across the film.
Beam restrictors are lead obstacles placed near to the anode of X-Ray tubes and are used to control the width and breadth of the x-ray beam allowed to pass through the patient and onto the x-ray film. Beam restrictors are important as they keep patient exposure to a minimum and also help reduce the number of scattered x-rays reaching the film4, but there is another important reason for restricting the x-ray beam.
As the X-Rays are produced by a focal spot on an anode, they do not all originate from exactly the same point. While the centre of the X-Ray film is exposed by X-Rays produced by the entire surface area of the focal spot, film at the edges of the X-Ray field will only be able to 'see' part of the focal spot due to the angles involved. This area at the edge of the X-Ray field is known as the 'penumbra'5, and leads to reduced exposure of the film at the edges. Since the image quality in the penumbra is reduced, it is best to decrease the size of the penumbra as much as possible.
Basic Beam Restrictors
The more basic types of restrictors include aperture diaphragms, which are essentially leads sheets with holes in the middle through which allow X-Rays to pass through. These restrict the beam and therefore reduce patient exposure, but do not affect the size of the penumbra produced. There are also various other restrictors based around the shapes of cones and cylinders, although these effectively do the same job as an aperture diaphragm. Most basic restrictors also come with the disadvantage that they can only produce one size of X-Ray beam, and therefore must be swapped over if the X-Ray tube is to be used for a different application.
The best form of beam restrictor is the collimator, which consists of two sets of four sliding shutters which move independently to restrict the beam, with each shutter being a lead plate which cuts off one side of the beam to help give it a rectangular shape. The upper set of shutters sits close to the focus spot and determines the width and breadth of the beam, while the second set cuts off as much penumbra as possible by shielding the parts of the film that would otherwise only be able to see part of the focus spot.
Determining the X-Ray Field Size
As the beam size produced by a set of collimators can vary greatly, it is important to have some indication as to the size and location of the X-Ray field with respect to the patient. A mirror is often placed between the two sets of plates, allowing a light to be shone through the lower plates and onto the patient, thus indicating the width and breadth of the X-Ray field currently dictated by the collimators. The mirror must be placed at 45° to the X-Ray beam and at its centre, while the light bulb must be the same distance from the mirror as the X-Ray tube's focus spot is, lest the light pass through the lower shutters at a different angle to the X-Ray beam. The alignment of the light beam and X-Ray beam has to be checked regularly by placing lead clips at the edges of the rectangular light beam and then seeing if the X-Ray beam produces the same size rectangle with the clips at the edges of the exposed region on the film.
Positive Beam Limiting Devices
Most X-Ray machines are now equipped with automatic collimators in which the shutters are driven by small motors. These machines use 'positive beam limiting' techniques, whereby the machine automatically detects the size of the X-Ray film loaded and adjusts the collimators so that the X-Ray beam matches the size of the film, thus preventing unnecessary patient exposure.
Tube Heat Ratings
Only one percent of the energy lost by decelerating electrons is actually converted into X-Rays, with the remainder being converted to heat. This represents a serious problem, as overheating would melt the anode and destroy the tube. For this reason, each tube has a rating chart which indicates the maximum current that can be used for a particular kVp and exposure duration6. If these limits are followed, the tube's tungsten target should never exceed 3,000°C, giving a reasonable safety margin between maximum temperature and melting point (3,370°C).
As 99% of the energy from the electron stream is converted into heat energy, the heat produced during an exposure can be calculated by multiplying the peak kilovoltage, the current and the exposure time, thus giving the amount of heat energy produced in joules. The amount of heat produced through repeated exposures in a certain short period of time can therefore be calculated, and should not exceed the amount of heat that could safely be produced from one exposure lasting for the same short period. This is due to the fact that it takes time for the heat to travel through the tungsten, into the copper part of the anode, then into the oil contained within the tube housing, and finally into the surrounding environment. Charts also exist detailing the ability of anodes to store and dissipate heat energy over periods of several minutes, thus indicating the maximum heating that the tube can withstand during repeated use.
Tubes are also given a kilowatt rating, which can be found by multiplying the peak kilovoltage with the maximum current that can safely be maintained for 0.1 seconds at that particular kVp. For instance, a tube which can maintain a current of 500mA for 0.1 seconds at 70kVp would have a kilowatt rating of 35kW7.
Although glass would seem a sensible material for the casing surrounding the vacuum, metal can also be used provided that ceramic insulators are used to separate the casing from the high voltage components. The advantages of a metal/ceramic tube are threefold:
Reduction of off-focus radiation - during an exposure, some electrons may be scattered backwards from the anode, colliding with the tungsten target again. Some of these electrons end up colliding with the target outside of the focal point, thus creating off-focus X-Rays. Thus problem is reduced in metal tubes, as the grounded metal casing attracts the scattered electrons away from the anode.
Longer tube life expectancy - during the lifetime of a glass tube, small bits of the tungsten anode are vaporised and become deposited upon the glass, giving it a 'sunburnt' appearance. Eventually, enough tungsten is deposited that current can flow across the inside of the glass, causing arcing between the filament and the glass. This is not a problem in metal tubes, as the metal enclosure is grounded to earth.
Higher Heat Capacity - as the tube can be more easily shaped, the tube enclosure can be made to support a much larger rotating anode with supports at both ends of the tube. This larger anode can store and dissipate more heat, and so the tube has a higher heat loading capability.
Although this Entry will not cover the complex electronic circuits used to supply power to and control X-Ray tubes, switching plays an important role in controlling the amount of radiation a patient is exposed to. The high voltage, low current circuit used to create a negative voltage on the filament and a positive voltage on the tungsten target is known as the secondary circuit. This high-voltage circuit is powered by a step-up transformer which draws its power from a low voltage, high current circuit known as the primary circuit. Controlling the tube by switching on and off the primary circuit is known as primary switching, while control techniques involving the secondary circuit are known as secondary switching.
Primary switching is most commonly used where exposure times of around one millisecond or longer are required, although the thyristors used to control the circuit cannot cope with switching on and off continuously to make repeated exposures of this length. Meanwhile, secondary switching is used in applications which require rapid repeated exposure as short as half a millisecond, with this generally being achieved through control of the focusing cup voltage in grid-controlled circuits8.
Switching generally occurs automatically, and is controlled by an exposure timer which deactivated the tube once the required X-Ray exposure has been produced. Although some exposure timers are simple electronic timers set to shut off the tube after a certain time has elapsed, modern timing circuits actually measure the amount the X-Ray film has been exposed and shuts off the tube once the required exposure has been achieved. This is done using one of the following detectors, which are placed between the patient and the X-Ray film9. In each case, once sufficient input has been detected, the detector will send a signal to shut off the X-Ray tube, and the detector will then reset itself.
Photomultiplier Detectors - a thin plate of lucite coated with light-emitting phosphor is placed in front of the X-Ray film. The phosphor produces light when irradiated with X-Rays, and this light is transmitted through the lucite to a photomultiplier, which detects the level of light output electronically. It is important to note that the phosphor is more sensitive to lower kV X-Rays, and so the circuit must be adjusted according to the peak kilovoltage being used for the exposure.
Ionisation Chambers - an ionisation chamber consists of two sheets of aluminium or lead foil, which are charged with a potential difference before each use and have air between them. The interaction between the X-Rays and the air leads to a proportionate number of air molecules gaining a charge, causing them to be drawn towards the plates where they dissipate their charge. This leads to a reduction in the difference in voltage between the two plates, thus allowing the number of X-Rays passing through the apparatus to be measured.
Solid State Timers - solid state or 'semiconductor' technology can also be used to detect X-Rays, with these detectors having the advantages of being small, consistent, rapid and practically invisible to the X-Ray beam.
All exposure timers come with a failsafe back-up circuit which automatically terminates the exposure after the patient has been exposed to a certain level of radiation, thus preventing equipment failure from jeopardising the health of patients.