X-ray Tube Notes PDF

X-ray Tubes II UCD School of Medicine Scoil an Leighis UCD 1 1 Conclusion (part i) The x-ray tube is a vacuum device for the production of x-ray radiation Cat...

X-ray Tubes II UCD School of Medicine Scoil an Leighis UCD 1 1 Conclusion (part i) The x-ray tube is a vacuum device for the production of x-ray radiation Cathode (-ve) to emit electrons into the vacuum Anode (+ve) target to attract the electrons Filament in the cathode emits electrons via thermionic emission Electron beam is accelerated toward target anode (+ve charge) by the applied high voltage causing radiation producing events. Electrons interacting with anode nuclei are the primary source generating general radiation or Brehmsstrahlung Electrons colliding with anode electrons are a secondary source producing characteristic radiation 1000°C) 6 6  Cathode Assembly Filament and cathode provide a carefully shaped electron beam that bombards a precise target area on the anode. Cathode assembly is connected to the -ve high voltage supply. The filament is typically constructed of Tungsten Work function: 4.5eV Some advantages of Tungsten are – High melting Point (3370°C) – Malleable (easily shaped into a wire coil) – Strong – Low vapour pressure – does not easily evaporate (longer filament life, better for tube) 7 7  Space Charge Effect Thermionic emission is the release of electrons by the transfer of thermal energy The rate of release is limited by the Space charge: -ve charged electron cloud emitted from tungsten filament. Cloud -ve charge prevents further electron emission until electrons have acquired sufficient Thermal Energy to overcome space charge force. Tendency of space charge to limit further electron emission is known as space charge effect. When filament is heated to its emission temperature, a state of equilibrium is reached in that the number of electrons returning is equal to number being emitted. 8 8  Space Charge Effect Filament current (A) -> filament temperature (T) -> thermionic emission rate When kVp = 0 an e- cloud (space charge cloud) forms around filament Space charge cloud shields the electric field for tube voltages of ≤ 40 kVp → only some e- are accelerated towards the anode: space charge limited v≥ 40 kVp the space charge cloud effect overcome by V applied and tube current (mA) limited only by the emission of e- from the filament: emission-limited operation Tube current about 5-10 times less than the filament current in the emission-limited range 9 9 Space Charge Effect From: Bushong, Radiological Science for Technologists. Ch. 6 10 10  Beam Control The shape of the electron beam must be well controlled so that the electrons bombard only the target area of the anode The filament is located within a -ve charged nickel cathode (high melting point; high thermionic work function so poor thermionic emitter). The Cathode is shaped to precisely control the beam geometry. This is known as a focussing cup 11 11 Beam Control - The negatively charged focussing - cup causes the electron stream to converge- onto the anode - target in an acceptable, focussed size and shape. - -ve charge - - - - - -ve charge 12 12 Beam Control - Dual Filament Broad focus beam – 1.0 - 2.0mm wide (large exposures) Fine focus beam – 0.4-1.0mm wide (small exposures – fine detail) Why not just use fine focus all the time? 13 13  Filament Power Supply Low voltage (8-12V) AC from a filament transformer. Current (3-5 A) Thermionic Emission from this heating current results in a tube current from the liberated electrons. Tube current, measured in miliamperes (1mA = 0.001A), refers to number of electrons flowing per sec from filament to target. Remember: filament current is not the same as tube current! A tube current of 100mA ~= 0.1C/s 6.25 x 1017 electrons across the tube in 1 sec 14 14  Quiz: Cathode Review? The electron source is typically a helical ___ wire filament surrounded by a focussing cup Filament circuit - approx ___ V and ___ A Electrical resistance heats the filament and releases electrons via ___ ___ (also lights up - incandescence) Filament current controls tube current, which is the rate of ___ flow from cathode to anode and is measured in ___ The focussing cup is typically made of ___ Bombardment of an unacceptably large area of the anode is prevented by the focusing cup, which surrounds the ___ Broad focus is typically >= ____ Fine focus is typically < ____ 15 Tungsten ~10 V and ~4A. Via thermionic emission. Rate of electron flow. Measured in mA. Nickel Filament Broad >= 1mm Fine < 1mm 15 Anode Rotating disk made of Tungsten due to its heat capacity (remember 99% heat - 1 % radiation) or Molybdenum (very high heat capacity and half the density) Face of anode is angled Target area: Tungsten-Rhenium about 1mm thick – Easier heat removal – Rhenium content reduces surface pitting (more elastic than tungsten) Face is attached to a Molybdenum stem that connects with a rotor, stator windings and induction motor 16 16 So why rotate the anode? The heat is concentrated on a small spot The anode cannot sustain a high current without damaging the target area Stationary Anode Rotating Anode 17 17 Rotor and Stator Rotating anode forms part of an induction motor. Rotor (copper) attached to anode by a molybdenum stem Molybdenum stem restricts heat since it is a poor heat conductor but has good heat capacity Rotor revolves around a central axle which forms the positive electrode (+75kV) Rotation speed – 3000 or 3600rpm at 50/60Hz The stationary Stator windings induce the rotating magnetic field to spin the rotating Rotor assembly. 18 18 Rotor and Stator 19 19  Focal Spot Focal spot is the area on the anode that is bombarded by electrons It can become overheated so it should be as wide as possible to dissipate the heat But only small focal spots produce good diagnostic images. So can we do anything else besides spinning the anode to increase the heat dissipation area? 20 20  Line Focus Principle If the face of anode is angled, the apparent size of focal spot perpendicular to the x-ray tube will be smaller than the actual focal spot. Therefore it is possible to have a large target area for electrons but still produce a narrow beam of x-rays. 21 21 Heel Effect 22 22 Heel Effect Electrons bombarding the anode produce X-rays in all directions. Intensity varies depending on the angle at which the X-Rays were emitted from the focal spot. Intensity of x-ray increases a little towards the cathode side of film (as anode face is angled). With the intensity otherwise trailing off towards the edges of the film. This effect can lead to lower image quality (uneven density on film / signal in detector) and so films/detectors are either placed far from tube or smaller collimated area used to reduce variation in x- ray intensity. 23 23 Heel effect and distance 24 24  Heel Effect – Application Can you think of any times where we could use the anode heel effect? 25 25 Heel effect and dose? 26 26  Off-Focus Radiation Radiation produced when electrons interact with parts of the anode other than the target – Usually electron backscatter – electrons “bouncing” off target Extends focal spot size Produces lower-energy x-rays – Can increase patient dose – Can reduce contrast – Can lead to “shadow” image of areas outside primary beam Can be reduced by: – Having collimators / lead diaphragm as close to anode as possible – Metal envelope instead of glass - as stray electrons may be attracted to earthed metal housing and are conducted away instead of hitting anode and producing off-focus radiation 27 27 Bushong “Radiologic Science for Technologists” 28 28 Quiz: Anode Review? The anode disk of a general X-ray tube is typically constructed of ___ ___ is typically mixed with Tungsten in the target area to reduce pitting The two methods of increasing the heat dissipation area of the anode surface without increasing the effective focal spot are anode ___ and ___ of the anode disk target Tilting of the anode disk surface makes use of the ___ ___ principle to increase the heat dissipation area The intensity of radiation is higher on the ___ of the film due to the heel effect The stationary winding of the anode is known as the ___ The spinning component of the anode motor is known as the ___ 29 29  The Housing The housing is the casing that holds all the tube parts. In addition it is lead lined to attenuate X-rays that are not directed at the tube window/port 30 30 The X-ray Tube Insert This is the X-ray producing vacuum device which is enclosed within the casing It can be constructed of glass or metal Conventional borosilicate glass envelope Xray tube Metal envelope CT tube 31 31  Tube Cooling Tubes are rated by the heat they can sustain Measured in Heat Units (HU) storage capacity PAUSE: 5-10 mins cooling time once heat storage capacity is reached Heat is transferred by radiation from the anode through the vacuum and into the surrounding oil Heat carried away from cooling oil by housing air fans or else by circulating the oil from the housing through an external cooler/chiller 32 32 The Tube Window/Port The housing has one opening to allow the X-ray to exit the tube in the imaging direction. In addition it is lead lined to attenuate X-rays that are not directed at the tube window/port 33 33  The Diaphragm The Diaphragm limits the extent of the emitted X-ray beam from the tube It can be adjusted to control the exposed area 34 34 The Light Beam Diaphragm This not only restricts the X-ray beam but also projects a light across the area that will be irradiated. Used for positioning and control of radiation coverage 35 35 Cones Cones act like the Diaphragm as an aperture controller but in a different geometric shape 36 36 Quiz: What are the 4 steps in the production of X-rays with an X-ray tube? 37 37 Quiz: What could be the function of this tube component? ? 38 38 Quiz: What % of applied energy ends up as radiation? What features of the design of a tube help with the heat problem? Insert (metal or glass) Housing (Oil filled and air or oil circulation used to cool) Focal spot size Anode angulation (actual vs projected focal spot) Anode rotation (focal track vs focal spot) Materials used (tungsten, rhenium, molybdenum, graphite) Tube expansion bellows & pressure switch 150 KV Tube cooling pauses between scans 39 > 99% !!! 39  In conclusion We have reviewed – The Cathode Assembly including filament, focussing cup, broad & fine focus and the Filament Power Supply connected to the –ve of the HV supply – The Anode Assembly including anode disk, rotor and stator connected to the +ve of the HV supply – The Housing including shield, insert, window/port, filter, diaphragm, Light Beam Diaphragm (LBD) and cones – The challenge of heat in X-ray tubes and the measures employed against this problem 40 40  References *Carroll QB (2018) Radiography in the digital age: physics, exposure, radiation biology. Third edition. Charles C. Thomas, Springfield, Illinois. Chapter 9 *Bushberg JT, Seibert JA, Leidholdt EM and Boone JM (2020). The Essential Physics of Medical Imaging. 4th edition. Lippincott-Williams and Wilkins, Philadelphia. Chapter 6 Bushong, SC (2020) Radiologic Science for Technologists. 12th edition (or as in library). Mosby Carter, PH (1994). Chesney’s Equipment for Student Radiographers. 4th edition. Chapters 2 and 3. Graham DT and Cloke P (2003). Principles of Radiological Physics. 4th edition. Churchill Livingstone Huda W and Slone R (2003). Principles of Radiologic Physics. 2nd edition. Lippincott-Williams and Wilkins, Philadelphia 41 41