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Physics 2 Unit IV
Xray tube
| Question | Answer |
|---|---|
| What are the general functions of the tube housing? | *contains the tube insert *protects tube from shock *protects operator & pt. from electric shock *cools the anode; cools heat off tube *rotates the anode *protects the operator & patient from leakage radiation |
| What is the metal tube housing made out of? | steel |
| What does the oil inside the housing do? | fills space between shield and tube insert; acts as an insulator & heat transfer media |
| What side is the expansion diaphragm on? | cathode side |
| What is the port on the tube housing? | opening in tube which the xrays pass thru. provides attachment for the collimator covered by filters. |
| What are the cable receptacles? | made of insulating plastic; receive cables from high voltage transformer; lubricated with petroleum jelly to prevent spark |
| what is the stator? | stationary part of induction motor; rotates between 3,000-3,600rpm; 3 phase anode rotate 10,000rpm |
| What type of glass is the tube insert made out of? | pyrex glass; can withstand high temps; maintains a high vacuum |
| What is the cathode made out of? | nickel |
| What is the function of the focus cup? | surrounds the filament; when exposure is made the same negative potential is applied to prevent focal spot blooming. |
| Why is thoriated tungsten used in the filament? | it adds e- & raises melting point |
| What is the area on the anode target that is struck by the e- stream? | focal track |
| What are the 2 parts of the target? | *body *facing |
| What is the facing part of the target (anode)? | part where e- hit; has a high melting point & high atomic number. There is a small amount |
| T/F: higher atomic number means higher melting point. | false |
| T/F: the anode stream inhibits transfer of heat from target to rotor. | true |
| Describe the ball bearings of the rotor. | made of tungsten & cobalt; coated with silver lubricant |
| Which tube did not have a high vacuum? | crooke's tube |
| Why didn't the crooke's tube not last long? | had gas inside the tube that ionized and did not have a vacuum. once the gas ionizes it produces + and - particles. the + became attracted to the cathode that made even more powerful photons that hit the glass wall in the same spot to make xrays. |
| What was the air-cooled tube? | had a vacuum and hot cathode tube. |
| What are the special types of tubes? | *magnification tubes *therapy tubes *Mammography tubes *grid-operated tubes |
| How do grid-operated tubes work? | grid placed between cathode and anode = no e- flow; the grid has a negative charge and repels e- and prevents exposures. These are used in portables & cine radiography |
| How are therapy tubes cooled? | circulating water is used to cool the tube |
| What kind of photons are produced by mammography tubes? | low energy |
| T/F: Therapy tubes operate at high kV and low mA and long exposure times. | true |
| T/F: Mammography tubes have less filtration than diagnostic xray tubes. | true; because lower kV (20-40) is used with mammography and only imaging soft tissue. Requires .5mm Al/eq |
| What factors affect tube performance? | *line focus principle *target angle *actual focal spot *effective focal spot |
| What shrinks the dimensions of the filament? | target angle |
| What focal spot is the actual dimensions of the filament? | actual focal spot |
| What is the size of the focal spot after the line-focus principle? | effective focal spot |
| Tube 1: T.A.= 17 degrees; A.F.S = 3.45mm; E.F.S=1.0mm Tube 2: T.A.= 10 degrees; A.F.S = 3.45mm; E.F.S= 0.6mm Which tube can make more exposures? Which tube has better detail? | Both tubes can load the same amount of heat because they have the same actual focal spot size. Tube 2 has better detail because the effective focal spot is smaller than tube 1. |
| What is the relationship between the target angle on the anode and the effective focal spot? | Bigger the target anode angle = bigger effective focal spot size; a smaller target anode angle gives a smaller effective focal spot size. |
| Tube 1: T.A.=17 degrees; E.F.S=1mm; A.F.S.=3.45mm Tube 2: T.A.=10 degrees; E.F.S=1mm; A.F.S.=5.75mm Which tube is used for better detail? Which tube can load more exposures? | Both tubes have the same effective focal spot so the detail is the same. Since tube 2 has a bigger actual focal spot size it can load more exposures. |
| What is the amount of e- passing from the cathode to anode; measured in mA? | tube current |
| What is the amount of current applied to filament; needs to be heated up to boil off e-? | filament current |
| What current is measured in amperes (A)? | filament current |
| What happens to the filament when the temperature is increased? | more e- are boiled off which increases the tube current. |
| What is the process called when e- are boiled off the filament? | thermionic emission |
| T/F: a change in filament current = change in tube current. | true; tube current is dependent on filament current. |
| What is a filament circuit? | controls the temp of filament to control the amount of mA created. |
| What material is the filament made out of? | tungsten |
| What are 3 devices that can vary filament current? | *Rheostat *Choke coil *mA selector |
| Which device that varies filament current is a variable resistor? | rheostat |
| T/F: When using a mA selector you are selecting mA. | false; you are selecting filament heat in order to boil off more e-. |
| What is it called when the exposure is made and all emitted e- are attracted to the anode? | saturation |
| T/F: when the tube is operating at lower kV levels, not all the emitted e- are attracted to the anode. | true |
| What is the space charge effect? | the mass of e- has a strong negative effect that individual e- tend to be pushed backwards towards the filament. |
| What does it mean when the tube is operating at below saturation? | not all e- are boiled off are attracted to the target |
| T/F: tube current is limited by the number of e- that are collected by the anode, not by the number emitted by the filament. | true |
| What factors affect tube performance? | *anode heel effect *anode angle & area of IR coverage |
| What is the anode heel effect? | intensity of the xray beam decreases closer to the anode due to absorption of the beam by the target |
| Smaller the target angle, __________ xray beam. | smaller |
| What is the disadvantage for smaller target angles when using larger IRs? | smaller the target angle = smaller the xray beam which cannot cover as much IR surface area. Smaller anode angles yield smaller effective focal spot sizes which are used for detail work. |
| What is the equation for single phase for HU? | HU= kVp X mA X s |
| What is the equation solving for HU when using a 3 phase or high frequency machine? | HU= 1.4 X kVp X mA X s |
| T/F: the housing of the tube can tolerate less heat than the anode. | false; the tube housing can tolerate a lot more heat than the anode. |
| What factors can affect tube rating? | *cooling rate of the anode *thermal capacity of housing *cooling rate of housing |
| What are some faults that can occur to the glass envelope? | mirror surfacing & stress fractures |
| What is mirror surfacing? | vaporized tungsten condenses on inside of flass over the window; acts an additional filter this decreases the tubes ability to radiate heat. |
| how can the glass envelope crack? | from the weight of the anode or rough handling of the tube. |
| What is "crazy paving" of the anode? | roughing of the target surface caused by overloading the tube or repetitively making rapid exposures |
| What does "crazy paving" do to sharpness and beam intensity? | decreases both |
| How might an anode crack? | making heavy/rapid exposures on a cold anode (imbalance of the tube) |
Created by:
mokapis
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