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Physics module 7
| Question | Answer |
|---|---|
| X-ray tube rating chart | A chart that shows how the peak kilovolt level, x-ray exposure, and current measured in milliamps interact to produce tube heat. The chart also shows the maximum heat limit for a single exposure with the tube. |
| Anode cooling chart | A chart used to prevent damage caused by not letting an anode cool sufficiently before the tube is used again in another x-ray exposure. The anode cooling chart shows how long it takes the tube to cool from its maximum level of heat. |
| Housing cooling chart | A chart that shows how much time is needed for the housing to cool down after the maximum heat capacity has been reached |
| Anode | The positively charged disc at one end of a vacuum tube; the disc in an x-ray tube, positively charged relative to the cathode; electrons from the cathode strike the tungsten target of the anode and produce x-rays |
| Cathode | The negatively charged element in a vacuum tube or x-ray tube |
| Induction motor | An AC motor consisting of a rotor and stator. The electromagnets in the stator have a changing magnetic field that causes the rotor within to turn. |
| Protective housing | A structure that surrounds the x-ray tube and protects both the tube and the people nearby. The housing is lead-lined to absorb x-rays. |
| Why is the glass envelope generally made out of Pyrex glass? | To withstand the high temperatures associated with x-ray production |
| What is the purpose of the glass envelope enclosure? | To establish and maintain a vacuum |
| Leakage radiation | X-rays not in the controlled beam that escape through the protective housing surrounding the x-ray tube |
| What is the purpose of the lead lining in the housing? | To reduce leakage radiation to less than 100mR/hr |
| One other purpose of the housing is to serve as a thermal cushion to help protect the x-ray tube from heat damage | |
| Another purpose of the housing is to serve as a mechanical support for the x-ray tube and protects it from damage | |
| Arcing in the x-ray tube may be a result of which of the following? 1)slow anode rotation 2)Technique too high 3)overheating 4)loss of vacuum? | Loss of vacuum |
| How does surrounding the x-ray tube with oil cool it? | The oil draws the heat away from the tube |
| The protective housing serves as an electrical insulator? T or F | True |
| How is the target window made for general x-ray tubes? | Glass ground thinner in that spot |
| The stators of the anode induction motor are inside the glass envelope? T or F | False |
| Stationary anode | An anode that is fixed in place so that an electron beam always hits the same place. X-ray equipment with stationary anode tubes is used in circumstances where only low exposure factors are needed and less heat is generated in the anode. |
| Rotating anode | An anode that, like a stationary anode (see below), is struck by an electron beam, emitting x-rays. A rotating anode turns in many revolutions per minute (rpm) to avoid the problems of heat build-up in a single spot on its surface. |
| What is the target material for the rotating anode? | Tungsten |
| What may also be added to the tungsten to increase thermal capacity and tensile strength? | Rhenium |
| Why is Molybdenum used as the disc base and the core in a rotating anode? | Dissipates heat well and it is a light, but strong, alloy that makes it easier to rotate the anode. |
| How does the anode rotate? | Rotation is achieved with an induction motor and electromagnetic mutual induction |
| Where is the rotor located in a rotating anode? | Inside the glass envelope |
| Where is the stator located in a rotating anode? | Outside the glass envelope |
| Where is the anode disk mounted on the rotating anode? | Molybdenum shaft |
| The copper on the end of the shaft has what embedded in it? | Iron bars |
| What kind of material are the iron bars? | Ferromagnetic |
| What kind of material is the glass envelope? | Nonmagnetic (so it will not interfere with the process) |
| Line focus principle | The fact that different target angles create different size focal spots in the x-ray beam |
| What is the net result of the anode heel effect? | To create a variation in intensity of the x-ray beam |
| Which metal is used in the design of the anode because it is a good conductor of both heat and electricity? | copper |
| Iron bars are incorporated into which of the following? 1)Rotor 2)Anode disk 3)Glass envelope 4)Housing | Rotor |
| Why is it desirable to have a large actual focal spot? | Better heat dissipation |
| Why are stationary anodes impractical for general x-ray exams? | Cannot use large techniques |
| Focusing cup | A shell-like device behind and to the sides of the filament that prevents spreading of the electrons as they move from the filament toward the anode |
| Filaments | A small coil of wire through which current is passed. In an x-ray tube or a vacuum-tube rectifier, the filament is used to produce a stream of electrons. |
| What is a filament? | A coil of wire usually 7-15 mm long and 1-2 mm wide |
| What is the filament made of? | Filaments are usually made of tungsten with 1-2% thorium (a radioactive metallic element) added to increase thermionic emission glossary and tube life |
| Thermionic emission | A process that occurs when a strong current meets the resistance of a filament and causes it to become hot and emit electrons. |
| How are the filament selected for heating? | The filament is tied in with the mA station, so which ever mA you choose. The lower the mA stations the smaller filament out of the two will be chosen and the higher mA stations will be tied with the larger filament. |
| What size is the small filament? | 0.3mm to 1mm wide |
| What size is the large filament? | 1mm to 2.5mm wide |
| What is the focus cup made of? | Nickel |
| What does the anode initially provide? | The target and the large potential difference, which, in turn, provided kinetic energy to the filament electrons. |
| What does the cathode initially provide? | The source of electrons. These electrons are the vehicles for the kinetic energy that is carried to the target with tremendous velocity |
| Generally, an x-ray tube will have how many filaments? | 2 |
| What determines which filament is selected? | mA station selected |
| The boiling off of electrons is called what? | Thermionic emission |
| The filaments are wired to which circuit section? | Filament circuit |
| Most of the energy used to produce x-rays results in which of the following? 1)X-rays 2)Protons 3)Electrons 4)Heat | Heat |
| What would happen to the filament electrons without the focusing cup? | Repel each other and scatter |
| The filament electrons will physically penetrate the target. T or F? | True |
| Where does the voltage become kilovoltage in the x-ray circuit? | Step-up transformer |
| The two major components of the cathode are within the glass envelope. T or F? | True |
| What is the purpose of the target window of the glass envelope? | Minimize interference with x-rays leaving the tube |
| Suntanning of the tube is a result of? | Vaporized tungsten |
| The protective housing serves as an electrical insulator. T or F? | True |
| The housing serves as a thermal cushion by? | Cooling the tube |
| There is a risk of burns from touching an x-ray tube housing. T or F? | True |
| One of the roles of the anode is to? | Give filament electrons kinetic energy |
| What metal do we use as the target because of its atomic complexity? | Tungsten |
| What is the primary limitation of the stationary anode? | Heat capacity |
| By rotating an anode disk, we are able to spread heat over a larger surface area. T or F? | True |
| By what method is the rotor of the x-ray tube turned? | Mutual induction |
| Which target angle will produce the smallest effective focal spot? 1)17 2)12 3)10 4)7 | 7 |
| Why is the beam less intense on the anode side? | Some photons have to penetrate the heel of the target |
| What is added to the filament to increase thermionic emission? | Thorium |
| Why do we limit the smaller filaments to the lower mA stations? | Less heat produced with smaller stations and techniques |
| Small changes in filament current will cause large changes in tube current. T or F? | True |
| How does the focusing cup "focus" electrons on the anode? | By electrostatic repulsion |
| Rectifiers serve to do what? | Route electricity through tube the same way |
| The force of attraction between the anode and filament electrons is so strong that the electrons reach 1/2 the speed of light. T or F? | True |
| How many heat units are produced using a high-frequency machine and the following factors for one exposure? 72kVp, 200mA, 0.5sec | 10,150 |
| Generator type, single-phase | 1.0 |
| Generator type, three-phase, 6-pulse | 1.35 |
| Generator type, three-phase, 12-pulse | 1.41 |
| Generator type, high frequency | 1.45 |
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volcomgrl12