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Physics 2 Unit IV

Xray tube

QuestionAnswer
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