RD1044 The X-ray Tube Design PDF

RD1044 The X-ray Tube Design Cletus Amedu [email protected]  What am I learning today?  Design features of the stationary anode X-ray tube  Functions of glass envelope, cathode, filament, anode, target, tube housing  Describe how heat is transferred from the X-ray tube  Electrical sa...

RD1044 The X-ray Tube Design Cletus Amedu [email protected]  What am I learning today?  Design features of the stationary anode X-ray tube  Functions of glass envelope, cathode, filament, anode, target, tube housing  Describe how heat is transferred from the X-ray tube  Electrical safety & radiation protection  The function of a rotating anode  Overview of the operation of the X-ray tube/circuit  Potential faults in an X-ray tube  Cathode - Anode + Tube Tube Tube Housing  Tube housing (shield)  The X-ray tube is usually encased in a protective housing to shield users from radiation exposure.  Typically made of earthed aluminium or steel and coated with 3mm of lead- lining (except at the window) to minimize radiation leakage, hence safety for Radiographer and patients.  Acts as a physical barrier which protects the radiographer and patients from electric power in the tube  Also protects the tube from damage during mild to moderate collision  Thermal dissipation  Tube housing (shield)  Provides support for x-ray tube & HT cables  The housing is filled with pure oil to act as an insulator and coolant.  A metal bellows or neoprene diaphragm at one end of the shield allows for expansion of the oil when it is heated. Carter 1994  A microswitch (pressure sensitive switch which disconnects the tube’s kV) may prevent more exposures if oil is too hot.  There is a tube port (radiolucent window) which allows the useful beam to leave the tube via the light beam diaphragm Two types of x-ray tube Stationary anode Rotating anode Stationary anode X- ray tube Dendy and Heaton 1999  Stationary anode X-ray tube  It has limited use e.g. dental units.  While unit is on filament is at ‘idling’ temperature.  ‘Prep’ heats filament to required temperature (2400° C)  Exposure button fully pressed, kV applied across tube, electrons accelerated towards target. Rita Roque, 2018  Glass envelope  Made of borosilicate glass  Strong enough to support a vacuum (If atoms were present the electrons would collide with the gas atoms and slow down (lose KE).  Holds electrodes in their precise positions  Joined at cathode and anode end by re-entrant seals which are slightly different from glass (This allow thermal expansion to prevent cracking)  The glass must be a good electrical insulator to prevent current flowing through it when a potential difference is applied between the cathode and anode  Rounded to prevent build up of high amounts of static charge  Radiolucent to transmit the X-ray beam (allows radiation to pass through it)  Cathode – negative charge  Electrode operates at a high negative potential  Consists of the filament, focussing cup, connecting wires and cathode support  The focussing cup is made of nickel or stainless steel  Has a high melting point and is a poor thermionic emitter  The area of the focal spot is reduced by the negative bias on the focussing cup  Negative bias is also referred to as "negative potential" or "negative voltage" on the focusing cup.  It is used to control and focus the electron beam emitted by the cathode toward the anode target  Filament – part of the cathode  Made of tungsten  Low work function (4.5 eV)  Emits electrons easily to form a cloud of negatively charged electrons called a space charge  High melting point (3370ºC)  Tungsten has low vapour pressure (pressure at which a substance no longer evaporates) and so does not easily evaporate. This prevents the wire becoming thin and prevents the formation of tungsten on the inner wall of the glass envelope.  Tungsten is strong and can be drawn into a thin spiral of wire to increase surface area  May be two filaments side by side, which is known as dual focus  A low voltage is applied across the filament, from the filament transformer within the x-ray generator.  Anode – positive charged target  Copper cylinder supports the rectangular target (Tungsten)  Acts as an electrical conductor (positive)  Conducts heat from the target  Copper also has a low atomic number (19), making it a poor choice as a source of X-rays  The remote end of the cylinder lies outside tube insert, surrounded by oil  Oil acts as an electrical insulator  Oil carries heat away to tube housing by convection  Anode – positive charged target  When the target gets hot during an exposure a temperature gradient is established. Heat always travels from hot to cold.  The second cooling pathway is where heat leaves the cylinder by the process of radiation, across a vacuum, to and through the glass envelope. The process of heat radiation occurs most efficiently when the source temperature is high. It thus mainly concerns the target itself and its immediate surroundings.  Target – part of the anode Tungsten is used as target material:  High melting point (3370°C)  Low vapour pressure (5000 kPa) at high temperatures  High atomic number (Z=74)  Tungsten is an efficient converter of the bombarding electrons’ kinetic energy into x-rays (bremsstrahlung and characteristic)  Tungsten can be made into a smooth block (2-3 cm) with an expansion rate similar to copper, keeping it in place when its temperature rises  Good thermal conductivity – heat transfer to copper  Target – part of the anode Tungsten’s Atomic Number:  High atomic number (Z=74)  The atomic number of an element is a fundamental property that defines the element and its place on the periodic table.  It is represented by the symbol "Z."  The atomic number is the number of protons (positive charge) in the nucleus of an atom of that element.  Each element on the periodic table has a unique atomic number, which determines its chemical identity.  The atomic number is crucial because it dictates the number of electrons in a neutral atom of that element  Therefore, tungsten is an efficient converter of the bombarding electrons’ kinetic energy into x-rays  Focussing cup – making sure our electrons end up in the right place a) No focusing cup on the cathode b) Concave focusing cup directs the electrons towards central axis - smaller area (W) on the anode (uses a slight negative bias)  Electron beam focussing  Electrons experience two forces  One towards the anode (the set kV)  One towards the central axis of the beam (negative bias on cup).  The force towards the central axis of the beam is greater than the force of electrostatic repulsion between the electrons and so the plume of electrons is focussed on to a small area on the anode.  Focal spot size  Unlike visible light, x-rays cannot be focussed and are therefore not produced from a point source  The effect of the source’s finite size is that every X-ray shadow is surrounded by a zone of partial shadow, termed the penumbra or geometric unsharpness  To restrict geometric unsharpness an x-ray tube is designed to have a small radiation source area.  This is achieved by ‘electron beam focussing’ and ‘angulation of the target’.  Angulation of target  The target is set to allow free access for approaching electrons and a wide exit path for emerging x-rays.  The target bombarded by the electrons is a rectangular area (‘actual’ or ‘true’ focal area.  Viewed from the perspective of the central ray it is foreshortened to a square (‘apparent’ or ’effective’ or ‘nominal’ focal area or just x-ray tube’s focal spot)  Electrical safety a) Structure of HT cable b) X-ray tube shield is made electrically safe by connecting the copper wire braiding of HT cables to both the shield and HT transformer casing (securely earthed) Graham 1996  Electrical safety  There are three basic principles: 1. Insulation of live components – This exists between the live components and the housing in the form of oil in its housing. The resistance of an insulator diminishes as the its temperature rises and so the role of the oil in heat dissipation is also important from the point of view of electrical safety 2. Earthing of component housings – The tube housing is connected to earth via the outer braiding of high-tension cables and so the casing will always remain at earth potential. If a live wire within the casing becomes disconnected and touches the shield, then the current will flow to earth and the casing will provide minimal electrical hazard to someone touching it at the time. Layers of rubber and other insulators are used to provide resistance between the innermost conducting core and the outside to prevent current flow across the cable. High tension cables operate up to potentials of at least 150 kV. 3. Restricted access to live components – The live components are secured inside the tube shield and the ends of the high-tension cable connectors are securely fixed. This means there is no access to the live components.  Circuits may be isolated from the mains by switches, fuses and breakers.  Radiation protection  The lead lining of the tube housing limits the radiation leakage from the tube and provides protection for staff and patients.  X-rays are emitted in all directions from the focus  Useful beam is allowed to leave through the tube port.  The radiation leakage rate must not exceed an air kerma of 1.00 mGy per hour at a distance of one metre from the focus. If there is a break in the lead lining then this will increase so QA checks are required.  Radiation protection 1. Inherent filtration refers to the removal of low-energy photons by the glass envelope and the oil as the beam passes to the tube port. 2. Total filtration refers to inherent filtration plus added filtration (aluminium). 3. The use of a light beam diaphragm to reduce the field size will help to reduce the volume of tissue irradiated which in- turn will reduce scatter and therefore patient dose Radiation protection Light beam diaphragm http://www.medimaging.co.uk/product_display.php?id=25  Problem of heat 99% heat Only 1% X-rays produced Temperature rise must be reduced to save life of tube by: 1. Minimising the quantity of heat produced – i.e. mA, kVp, and time. One may need to use faster film/screen combinations. 2. Minimising the rate at which heat is produced – This is determined by the manufacturer as the tube rating. Sometimes one may need a short exposure time to reduce movement unsharpness and therefore the need to increase the rate of x-ray production, which will increase heat production. 3. Increasing the area of bombardment of electrons - The angle of the anode controls the ratio between the true and effective focal areas. The steeper the angle the larger the heated area and the smaller the x-ray source. However, there is a limit to how small this angle can, be known as the anode heel effect. 4. Providing efficient cooling pathways – as described earlier Heat loss from a stationary anode tube  A large amount of heat is generated during an exposure and so efficient heat loss is necessary to protect the tube against thermal damage and to allow short exposures of high intensity. The sequence of events is as follows: 1. An exposure is made with a considerable amount of heat energy being deposited at the focal spot of the target. This must be removed to prevent thermal damage. 2. Some heat is lost from the focal spot by radiation through the vacuum to be absorbed by glass envelope and oil. 3. Most heat is lost by conduction through the tungsten target into copper block and along anode stem. 4. The end of the stem is in contact with oil and this sets up convection currents which warm the oil. The oil expands and eventually expansion bellows trip a microswitch to prevent further exposures. Heat loss from a stationary anode tube  5. Heat then passes from the oil to the metal housing by conduction.  6. The warm casing sets up convection currents in the air in the room.  Heat loss mainly by conduction (C) but convection (CV) and radiation (R) also play a part in the dissipation of heat from the target to the atmosphere of the room. H=housing; L=lead lining; M=microswitch; B=expansion bellows; E=envelope; V=vacuum in envelope; K=cathode; F=filament; T=tungsten target; A=copper anode. Heat transfer Graham 1996  Rotating anode  Area bombarded by electrons is a broad, ring-shaped focal track  Rotation carries the recently heated area out of the way of the incoming electrons  This can achieve shorter exposure times and increased tube rating Rotating anode X-ray tube  Anode heel effect 1. The x-rays produced are slightly below the surface of the target material. 2. More x-rays are absorbed along path E and therefore a lower intensity. 3. Anode heel effect means that x-rays at the anode end have a lower intensity than the central axis while the intensity at the cathode end will be greater than at the central axis. 4. This increases as the target angle is reduced and increases with tube age (target becomes pitted).  Operation of x-ray tube  Operation of x-ray tube  X-rays are produced when electrons are accelerated from cathode to anode.  The number of electrons is controlled by the mA selector and  Kinetic energy of the electrons (photon energy) is controlled by the kV selector.  Tube current (mA) and filament current 1. When selecting mA, the radiographer is selecting the required filament heating current to produce the required mA. 2. NB: the filament heating voltage is about 10 V and current is about 10 A.  Tube voltage (kVp) 1. KVp controls the peak potential difference across the tube. 2. If increased, then the greater the force of attraction between the anode and cathode. The electrons will strike the target with greater kinetic energy and produce higher energy photons.  X-ray tube faults  X-ray tube faults  With age the tube deteriorates and faults can occur in any part:  Glass envelope and enclosed vacuum 1. Deposition of tungsten resulting in reduced insulating properties of the glass, making puncture more likely. This results in loss of vacuum with drawing in of oil and immediate uncontrollable tube current through ionisation. 2. Glass may be broken by careless handling 3. Stress factures due to strain of anode assembly weight and rapid rotation of anode  X-ray tube faults  Anode, target and rotor 1. Heat can lead to crazing (development of cracks or fine fractures)of the target surface, causing unsharpness and reduction in X-ray output. 2. Anode can melt or split leading to gassy tube resulting in uncontrollable tube current (too high mA from ionisation of gas).  Filament  Break in filament due to thinning through vaporisation which depends on temperature. If broken then no X-rays produced.  Oil seal and shield  Radiation leakage  Stator windings  Break in windings or supply cable results in no anode rotation or intermittent rotation which are harmful to target. Safety circuit should prevent exposure if this fault occurs. References  Ball J. & Moore A.D. (2008) "Essential Physics for Radiographers" 4th edition. Wiley Blackwell.  Graham D., Cloke P., Vosper M. (2007) Principles of Radiological Physics. 5th Ed. Churchill Livingstone. ISBN: 0443070733  Graham D., Cloke P., Vosper M. (2012) Principles and Applications of Radiological Physics. 6th Ed. Churchill Livingstone. ISBN: 0702052159 School of Health & Psychological Sciences City, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 5060 E: [email protected] www.city.ac.uk/department