X-Ray Tube (LAMU) PDF

X-RAY TUBE LAMU COMPONENTS  The X-ray tube contains two principal elements:  cathode: provides a source of electrons  anode: acts as the target for electrons and releases x-rays  Additional components include:  exapnsion bellows (provide space for oil to expand)  tube envelope (evacuated...

X-RAY TUBE LAMU COMPONENTS  The X-ray tube contains two principal elements:  cathode: provides a source of electrons  anode: acts as the target for electrons and releases x-rays  Additional components include:  exapnsion bellows (provide space for oil to expand)  tube envelope (evacuated)  tube housing  cooling dielectric oil  rotor  induction stator  tube window IDEAL DIAGRAM Cathode Assembly Consists of the filament, focusing cup, and associated wiring. Filament: A small coil of thin thoriated tungsten wire. The Filament is where the electrons for the production of X-Rays are emitted. Focusing Cup: A shallow depression in the cathode assembly designed to house the filament. IDEAL CATHODE Dual Focus Dual Focus: A two-filament arrangement within the x-ray tube. Some tubes have two filaments so that the tubes can have a greater variety of exposures. The diagram below is an example of a Dual Focus filament. DIFFERENT TYPES Focusing Cup  The focusing cup is a place where the electrons are accumulating so that there are enough electrons to produce X-Rays.  The diagram below shows the focusing cup and the filament.  The focusing cup is negatively charged so that when the electrons are emitted by the filament they will congregate by the filament and will surge across to the anode when the exposure begins. FOCUSING CUP Space Charge Effect: As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons. Saturation Current: As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode. Anode Assembly: Consists of the anode, stator, and rotor. The anode is positively charged so that the electrons from the filament (cathode) are attracted to it to produce x-rays. The rest of the anode assembly are there so that the anode can rotate and have a bigger target for the electrons. Stator: The stator is the stationary part of an electric motor or alternator. Depending on the configuration of the motor the stator may act as the field magnet, interacting with the armature to create motion. The stator may be either a permagnet or an electromagnet. Rotor: The rotor is the non-stationary part of a rotary electric motor or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. ANODE ASSEMBLY Purpose of Anode: 1. serves as a target surface for the high-voltage electrons 2. conducts the high-voltage from the cathode back into the x- ray generator circuit 3. serves as the primary thermal conductor. TYPES OF ANODE Stationary Anode: An anode assembly that is immobile. Rotating Anode: An anode assembly that turns during exposure. In the diagram below the left anode is a stationary anode because it does not move when an exposure occurs. The anode on the right is a rotating anode because the anode rotates during an exposure. WHY MATERIAL OF CHOICE  The target part of the anode is made of tungsten or molybdenum.  Tungsten is material of choice for target because: 1. High Atomic Number (74) - High Atomic Number means that when an electron falls back into the L shell after ionization there is enough energy for an x-ray to be emitted. 2. High Melting Point (3422 C, 6192 F) - The anode will not melt easily under repeated exposures. (You can still damage an anode if you operate the machine incorrectly.) 3. Heat-conducting ability - The ability to conduct heat helps cool the anode and prevent melting.  Molybdenum (42) is used for soft tissue imaging. When the anode gets old or from miss use the target can get pock marked Target Area Target, focus, focal point, focal spot mean the same thing. This is where the high- voltage electrons hit the anode. Actual focal spot: The physical area of the focal track that is impacted. Focal Track: The portion of the anode where the high-voltage electron stream will impact. When discussing a rotating anode this describes the circular path that will be impacted by the electron beam. Effective focal spot: The area of the focal spot that is projected out of the tube toward the object being radiographed. The angle of the focal spot is one way to control the size of the effective focal spot. The larger the angle the larger the effective focal spot Line-Focus Principle: Used to reduce the effective area of the focal spot. The effective focal- spot size is controlled by the size of the actual focal spot and the anode target angle. The effective focal spot's vertical dimension is the one that is stated as the focal-spot size. Anode Heel Effect: Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side. As the figure below indicates the intensity of the x-ray beam is greater towards the cathode (filament) end of the tube. More x-rays will be able to get out of the anode on the right side (the angle side) than on the left side because of all the material the x-rays must pass through to get out of the anode. The graphs show the percent of exposure on the vertical axis and the horizontal axis is the distance from the center of the exposure position towards the anode (left) and the cathode (right). As you can see the percent of exposure goes up the closer you get to the cathode side of The Envelope The envelope is the glass housing that protects the tube. It is also used to help protect from excessive exposure to x-rays. The envelope is the first part of the filtration system. Vacuum The removal of the air permits electrons to flow from cathode to anode without encountering the gas atoms of air.  Protective Housing The housing controls leakage and scatter radiation, isolates the high voltages, and provides a means to cool the tube. Leakage Radiation: Any photons that escape from the housing except at the port. Leakage radiation must not exceed 100 mR/hr at 1 meter.  Off-Focus Radiation: Photons that were not produced at the focal spot or extrafocal radiation. These can be produced when an electron from the filament interacts with the tungsten. Then the electron that was given off from the ionization does the same thing to another tungsten atom and creates another x- ray. Rating Charts and Cooling Curves Radiographic Tube Rating Chart: A guide regarding the most common technical factor combinations that can be used without overloading the tube. When you use the Radiographic Tube Rating Charts for the X-Ray machine you want to make sure that the kVP setting and the time of the exposure intersect below the mA reading. To solve this I drew a line along the 90 kVp setting on the left axis and a vertical line up from 0.3 seconds until they meet. In this example the lines meet above the 500 mA setting therfore, this is not a safe exposure for the tube. X-Ray Tube Heating There are three main types of heat generation in an x-ray tube. The first is by convection. Convection: The transfer of thermal energy by actual physical movement from one location to another of a substance in which thermal energy is stored. Also known as thermal convection. The air is what is moving the heat energy around in the tube. If the tube has oil in it, then the oil is the convection material.  The second is by conduction.  Conduction:The flow of thermal energy through a substance from a higher-to a lower-temperature region. This is from the stator axel to the anode.  The third is by radiation.  Radiation: The energy radiated by solids, liquids, and gases in the form of electromagnetic waves as a result of their temperature. Also known as thermal radiation. This occurs when the electrons hit the anode target and heat up the anode. The x-ray radiation is given off which heats up the tube. Anode Cooling Charts: Permits the calculation of the time necessary for the anode to cool enough for additional exposure to be taken.  Generator Type Constant Single-phase 1.00 Three-Phase, Six 1.35 Pulse Three-Phase, Twelve 1.41 Pulse High-Frequency 1.45 To use the Anode Cooling Chart: 1. Find the total heat units applied on the vertical scale. 2. Read from the heat units over to the cooling curve and then down to read the corresponding time. 3. Calculate the time necessary for the anode to cool to any desired level and subtract the corresponding time of the initial exposure Housing Cooling Chart: Permits the calculation of the time necessary for the housing to cool enough for additional exposures to be made Recommendations for Extending Tube Life 1. Warm up the anode following manufacturer's recommendations. 2. Do not hold the rotor switch unnecessarily. Double-press switchs should be completely depressed in one motion. Dual switches should have exposure switch depressed first, followed by the rotor switch. Use lower mA stations when possible. 4. Use lower-speed rotor when possible. 5. Do not make repeated exposures near tube loading limits. 6. Do not rotate the tube housing rapidly from one position to another. 7. Do not use a tube when you can hear loud rotor bearings. TUBE FAULTS Filament failure Anode rotor bearing detoriation Anode Damage Glass puncture Gassy Tube Vapourised Tungsten