The X-ray tube Flashcards
What’s in the internal?
Cathode and Anode
What is external?
Support structure
Protective housing
Glass or metal enclosure
What are the conditions that make a xray production possible?
(4)
-source of electrons
-vacuum
-high voltage
-apporiat target material
what side is the cathode?
negative side
What are the cathode’s functions?
(3)
-produce a thermionic cloud
-Conduct high voltage between the cathode and anode
-Focus the electron stream towards the anode
What does the cathode assembly consist of?
Filament
Focusing cup
Associated wiring
What is filament?
Small coil of thoriated tungsten wire
-0.1 to 2.0 mm thick
-Could also be made of rhenium and Molybdenum
Most xrays tubes have..
dual focus arrangement (2 filaments)
What is the function of the filament?
(Sufficient resistance?)
-Provides sufficient resistance to the flow of electrons so that the heat produced will cause an thermionic cloud
What is the space charge effect?
As more electrons build up in the filament there negative charges begin to oppose the emission of more electrons
What is the focusing cup made of?
nickel
What does the focusing up do?
narrows the thermionic cloud
Its a shallow depression in the cathode assembly designed to house the filament?
Focusing cup
What does the grid biased tubes do?
(grid-pulsed or grid-controlled)
Focusing cup pulses between negative and positive charge to remove electrons from use (rapid)
What causes tube failure?
High voltage arcing which is short circuit in the tube (glass cracking)
Types of tube failure
Breaking of the filament
Rough handling the equipment (jarring)
Holding the rotor switch to long prior to exposure
Routinely delaying exposure can shorten tube life by
50-60%
What side is the anode?
Positive side
Anode functions?
-Target surface for electrons
-Conduct high voltage from the cathode back into the xray generator
-Thermal conductor
What is the anode assembly?
Anode (TARGET)
Stator
Rotor
What are the two types of anode
Stationary and Rotating
What does the rotating anode do?
Turn during the exposure
What is the common rotating anodes revolve at?
3,200-3,600 rpm
What is the high speed anodes revolve at?
10,000-12,000 rpm
What is the focal track made of?
tungsten rhenium alloy
Why is tungsten a metal of choice for anode ?
High atomic number #74
High melting point
Heat conducting ability
What is pitting?
Normal use of rotating the anode and will eventually vaporize the target material
-reduces the tube efficiency
What is cracking?
its a failure to warm up the tube range of operation before use
What is called when the anode relief itself from stress?
Stress relief
What can the focal track also be called?
-target
-focus
-focal point
-focal spot
What is actual focal spot?
The physical area of the focal track that is impacted
What is effective focal spot?
area of the focal spot that is projected out of the tube toward the object being radiographed
What does the line-focus principal do?
its used to reduce the effective area of the focal spot
Effective focal spot is controlled by?
-size of the actual focal spot
-size of filament
-anode target angle
What is the limits of the xray tube mA ranges?
1,000-2,000 mA
To cover a 14x17 IR at 40” you need a minimum target angle of…
12 degrees
The use of the line-focus principle causes a problem that is known as…
anode heel effect
Why is the anode heel effect a problem for the line-focus principle?
Bc of its geometry of an angled anode target which the radiation is greater on the cathode side
Anode heel effect is more noticeable at…
-short SID
-Large image receptor
-small focal spot
The anode side is
(less thick or thicker)
less thick
The cathode side is
(less thick or thicker)
thicker
What is the stator?
Uses magnets to turn on the rotor
What does the stator turn and where is it located?
turns the anode and its located outside the vacuum of the envelope
If the stator fails the rotor…
Won’t turn the anode and result in melting the spot on target
Where is the rotor located?
its located inside the stator and inside the envelope
How does the rotor look like ?
Hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft
What is inside the rotor?
Contains sliver plated steel ball bearings around the shaft and is anchored to the envelope
What is rotor failures?
high- speed harmonic damage which is caused by the tone produced by their rotating
(5,000 and 7,000 rpm)
If a high speed anode tube housing is quickly rotated from one position to another, what is it called?
gyroscopic effect
What does the gyroscopic effect produced?
the centrifugal force of the rotation
Another rotor failure is…
bad bearings because of long use at high temperature
What is in the envelope ?
the entire cathode and anode assembly EXECPT the STATOR enclosed within a glass
Why is metal envelopes a increasingly more common choice than glass?
Prolong tube life because it eliminates problem of tungsten vaporization
What is the vacuum’s function?
Removable of air which permits electrons to flow from cathode to anode
What does the housing control?
leakage
scatter radiation
isolates the high voltage
Cools the tube
When xray photons are produced at the anode they are emitted
isotopically (in all directions)
The primary beam consists of photons emitted
throughout the window
Leakage radiation must not exceed…
100mR/hr at 1 meter
What is dielectric oil used for?
-its used to fill space between the envelope and the tube housing
-insulated high voltage components from the tube housing
What is off-focus or extrafocal radiation?
its photons that were not produced at the focal spot
What causes ghosting structures?
scattered electrons or photons when they have sufficient energy remaining and strike another object in the tube
When talking about focal track what does it represent?
Represents the circular path that will be impacted by electron stream
Xrays are produced and most are emitted at angles between…
45 and 90 degrees
