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Where do the x-rays come from
from the anode rotating disk after it receives emitted electrons from the cathode side
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What is Kv
kilovoltage is the strength or power of the electrons or photon beam
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what is ma
ma is milliamperage which is the number of electrons or photons emitted across the anode and cathode space or produced
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the higher the kv the stronger the ____?
x-ray
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What allows x-rays to be made under very special conditions including a source of electrons, an appropriate target material, a high voltage and a vacuum?
the x-ray tube
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List the structures located on the cathode side of the x-ray tube?
- focusing cup
- filaments
- filament wires (associated wires)
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List the structures located on the anode side?
- anode rotating desk made of tungsten
- focal spots on rotating desk
- molybdenum neck shaft
- rotor that houses the shaft
- ball bearings located around the shaft inside the rotor housing
- stator located externally of the x-ray tube
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What is the anode side of the xray tube?
cathode side?
- positive side
- negative side
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Where is the area that the electrons hit after thermionic emission?
focal point on the anode rotating disk
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What is the cathode and anode housed in?
glass envelope
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what is the glass envelope housed in?
a steel protective covering, it is the part that the radiographer actually moves and touches
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What part of the x-ray is always located outside of the x-ray tube
the stator
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In what shape does the xray beam comes from after it hits a focal point
divergence shape of photons that target the material
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Where is the source of electrons?
on the cathode electrode of the xray tube
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What 3 parts consist of the cathode end?
- focusing cup
- filament tungsten wiring
- associated wiring entering in for the electrons
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Where are the filaments located?
in the focusing cup
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What element is the filament made out of in the focusing cup?
tungsten
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Why is tungsten used? (2 reasons)
- high melting point
- it is difficult to vaporize
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What is melting point of tungsten?
3370 degrees C
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What is the main function with filament size
provides clarity of the x ray image
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what size is the best filament to use
small filament
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When do we not use a small filament
when not imaging bigger and thicker anatomy like the femur abdomen pelvis
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what happens when you increase mas?
the higher the mas the more electrons get applied to filament, thus more electrons hit the focal point which results in more xray beams
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What is another word for Mas?
current
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what is the space charge
it is when electrons are liberated or hang around the surface of the filament (wires) and form an electric cloud called space charge
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what is thermonic emission?
It is the process by which electrons are boiled off the filaments or when they bounce off enter the anode side
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At what temperature will the tungsten filament no exhibit significant thermonic emission?
at 2200 degrees Celsius if it is below it will not be emitted it has to be hot enough
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What is the main function of the filament in general?
to provide enough resistance to the flow of electrons so that heat is produced and cause thermionic emission (which is when the electrons bounce off the filament and hit a focal spot)
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What is the first step in an xray exposure?
rotor switch
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What happens when the xray tube is turned on?
a mild current is sent to the filament, which allows it to be preheated and ready for exposure at all times
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What can be done in prevention of degradation of the xray tube?
pressing the rotor switch and the exposure buttn at the same time
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What is the approximate life of an xray tube and how much is needed to replace them if damaged?
- 6-9 hrs
- costs 10k to replace
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What happens if the rotor switch is activated prior to exposure?
degradation of the tube happens earlier which can be prevented
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What divergence has better quality in regards to detail?
the smaller divergence beam because the amt of photons that are emitted outside of the central ray come at a smaller angle than if it were to come down on a wider angle creating a blurry image on the outside of the area of interest
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Why is the middle ray or beam the beast out of all the photons?
b/c it is the straightest
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What causes a bigger divergence beam?
larger focal spot
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What is the focusing cup
it is a shallow depression on the cathode side or assembly designated to house the filaments
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What element is the focusing cup made out of?
nickel
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What is the purpose of the nickel made focusing cup?
its purpose is to narrow the amount of thermionic emission to the anode
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What happens when two atoms are of the same charge ?
So, what charge is the focusing point provided with?
they repel one another and spread out or bloom
low negative charge potential b/c of its shape it focuses electrons toward one another
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what is the biased focusing cup
and what does it cause
it is the cup that maintains at a more negative voltage than the filament
causes the the electron stream to become narrower as it goes towards the anode
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where is biased focusing cup used often?
in mammography
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what is called when it is possible to decrease size of the focal spot
based focusing cup
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What is the space charge effect?
happens when more and more electrons build up in the area of the filament and their negative charges begin to oppose the emission of additional electrons
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What is the phenomenon that limits the tubes maximum mA range from 1000-1200?
space charge effect
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What is the maximum mA range of the space charge effect?
1000-1200
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what is the other phenomenon that deals with the efficiency of the xray tube?
saturation current
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When is the filament saturation achieved?
when there are no further thermionic electrons
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What happpens to thermionic emission as the kVp increases?
as kVp increases more electrons r a greater percentage of electrons are driven to the anode
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What is positively charged electrode?
the anode
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What is the purpose of the anode assembly?
serves to decelerate the electrons as well as store and dissipate (disperse)
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What area or part of the xray tube serves as target area for the high voltage electrons from the filament?
anode side
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What is the target surface?
it is where the surface where the electrons are suddenly stopped, resulting in the production of x ray photons
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what are the three main portions of the anode assembly?
- anode roatating disk
- rotor
- stator
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What are the two types of anodes?
rotating and stationary anodes
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What are all x-ray tubes in radiology departments? (certain type)
rotating
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When does the rotating anode actually start turning and why?
during exposure to make a much larger target area
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The faster the the anode rotates the _____ the ______ dissipation ability
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What is the outer material of the focal track or the target area made out of?
Inner?
- tungsten
- molybdenum 5-13 cm in diameter
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What is the rpm range of heat dissipation on the anode rotating disk?
3400-10000 rpm
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what is the outer covering of the focal track composed of?
rhenium alloyed tunsgten
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whats another purpose of anode assembly?
it serves as a pathway for the high voltage flow during exposure
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what is the rotor attached to?
the molybdenum shaft which is part of the anode rotating disk
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Why is tungsten the primary metal of choice? (3 reasons)
- high melting point
- heat conducting ability (difficult to vaporize)
- high atomic number
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what is the main function of the glass envelope?
The main function of the glass envelope surrounding the x-ray tube is to create a vacuum environment for more efficient x-ray production
If Air/Gas present. ē's from electron stream interact with air/gas molecules. result in harder to control x-ray production
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How does tube failure happen?
Loss of the tube's vacuum reduces x-ray production, produces more heat & leads to eventual tube failure
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The glass used in x-ray tubes must be...?
- Pyrex or heat resistant variety
- Heat is the main product of ē interactions at the target
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How does tube failure happen?
Loss of the tube's vacuum reduces x-ray production, produces more heat & leads to eventual tube failure
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To keep the rotor bearings from burning out, what materials is placed between the disc and rotor
molybdenum or graphite or a combination of such materials
Molybdenum has a high melting point like tungsten but is a relatively poor conductor of heat (more of an insulator than tungsten)
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In a x-ray tube, vaporized tungsten can come from the _______ or _______.
filament or anode target surface
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what is the purpose of collimation?
to limit the amount of xray exposure towards the patient and that radiation only goes to area of clinical interest
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what part of the xray tube serves as a source of the electrons
the cathode
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the _____ and _____ have a great effect on the ability of a particular xray tube to image fine details
length and width
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what happens every time the rotor switch is pressed or depressed?
it loses life in the filament
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what happens when high voltage is released at exposure
the entire thermionic cloud is avalable to be driven toward the anode target where the xray photons will be produced
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What happens to the electrons as kVp increases?
a greater percentage of thermionic electrons are driven towards the anode
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when is filament saturation achieved?
when there is no further thermionic electrons to be driven towards the anode
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What will happen if you try to increase kVp at the level of filament saturation?
- it will not increase the mA
- the ray circuitry automatically adjusts this when factors are changed by the operator
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what motor turns the rotor?
stator
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