Term
| Describe the space-charge effect |
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Definition
| As electrons build, their charges begin to oppose the emission of additional electrons, limiting the mA range to 1000-1,200 |
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Term
| What is the atomic # of tungsten? |
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Definition
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Term
| What is the only part of the anode or cathode assembly that is located outside the vacuum of the glass envelope? |
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Definition
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Term
| According to the NCRP, what is the maximum allowable radiation leakage from the tube? |
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Definition
Must not exceed 100 mR/hr
(at 1 meter from the tube) |
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Term
| What is the primary purpose of the tube glass housing? |
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Definition
The envelope is the glass housing that protects the tube.
(Also, to provide a vacuum, permitting electrons to flow from cathode to anode without interacting with atoms in air.) |
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Term
| Describe the anode-heel effect |
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Definition
| Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side. |
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Term
| Describe the Actual Focal Spot (AFS) |
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Definition
| The physical area on the focal track that is impacted. |
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Term
| Describe the Effective Focal Spot (EFS) |
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Definition
| Area that is projected out of the tube |
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Term
| How does the Line Focus Principle relate to EFS? |
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Definition
Used to reduce the EFS thus increasing resolution.
** permits the best resolution of detail while permitting as large as actual area as possible. |
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Term
| Describe the focusing cup and what it does. |
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Definition
Shallow depression in the cathode assembly that houses the filaments - made of nickel.
Operates to narrow or focus the electron cloud during thermionic emission to be directed toward anode
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Term
| What is responsible for the noise during rotor acceleration? |
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Definition
| Ball bearings inside the cuff of the rotor |
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Term
| Describe what it means when the x-ray tube is gassy. |
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Definition
| During thermionic emission, if vaporization occurs, you end up with a gassy tube which decreases the vacuum inside the glass envelope. |
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Term
| What material commonly makes up the target for mammographic units? |
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Definition
Molybdenum (atomic # 42)
** because of it's ability to give a more uniform range
of lower energy photons. |
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Term
What material is used for the anode target area?
Why is it used? |
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Definition
Tungsten
- Higher x-ray production efficiency
- Thermal conductivity
- High melting point
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Term
| What happens if the stator malfunctions? |
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Definition
The rotor will not turn and a spot will melt on the anode.
(picture p. 119) |
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Term
| What happens if the rotor malfunctions? |
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Definition
| A malfunctioning bearing can prevent the rotor from turning and produce melting in one spot on the anode. |
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Term
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Definition
| A coil of thin tungsten wire set within the focusing cup |
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Term
| Why is the filament made of tungsten? |
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Definition
- High atomic # of 74 - has higher x-ray production efficiency
- Thermal conductivity
- High melting point
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Term
| Is the Cathode side of the tube negative or positive? |
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Definition
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Term
| Is the anode side of an x-ray tube negative or positive? |
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Definition
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Term
Why is a rotating anode more desirable
than a stationary anode? |
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Definition
There is better heat dissipation from the rotating anodes, which permits bombardment of electrons from the filament to a given area in a short amount of time.
Stationary anodes are low-power and inefficient for heat dissipation. |
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Term
| What is the Heat Units (HU) formula? |
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Definition
| kVp x mA x time x rectification constant = HU |
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Term
What are the 4 main components of the
x-ray tube assembly? |
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Definition
- Cathode (the source of electrons)
- Anode (target)
- Glass envelope (vacuum)
- Protective housing (made of very heavy lead)
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Term
| What 3 things make the Cathode assembly? |
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Definition
- Filament(s)
- Focusing cup (located in the filament)
- Associated wiring
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Term
| What 3 things make up the anode assembly? |
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Definition
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Term
| What are the 3 functions of the Anode assembly? |
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Definition
- Serve as target surface for electrons from the filament, eventually becoming the source of x-ray photons
- Conducts high-voltage from the cathode back into the generator circuitry
- Serves as the primary thermal conductor
** x-ray photons are made at the anode ** |
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Term
| List other names for the 'Target Area' |
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Definition
Target
Focus
Focal point
Focal spot
Focal track |
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Term
| Where do you measure the "source" when measuring for SID? |
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Definition
Target Area
(portion of the anode where focused electron stream impacts) |
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Term
| Describe Thermionic Emission |
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Definition
| The negative electrode (also called the cathode) is a wire filament that emits electrons when heated. This process is called thermionic emission and the wire filament is called an 'electron gun'. |
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Term
| What are 'Tube Rating Charts'? |
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Definition
| They provide a guide to the maximum technical factors that can be set without overheating the tube. |
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Term
| What is an 'Anode Cooling Chart'? |
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Definition
| Permits calculation of time necessary for anode to cool enough for additional exposures to be made. |
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Term
| Describe 'Off focus radiation' |
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Definition
Composed of photons that were not
produced at the focal spot.
These photons are produced away from the focal spot and have sufficient
energy that when they strike another object in the tube, they
produce photons and can produce an image. |
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