Lesson 4, 8-9, & 10-11 Test Flashcards
4 conditions necessary to produce x-rays
Source of free electrons (thermionic emission, cathode filaments)
Acceleration of electrons (kilovoltage)
Focusing of electrons (focusing cup)
Deceleration of electrons (anode/target)
The ejection of electrons from the surface of the filament wires due to increased heat, causing an electron cloud; “boiling off” of electrons
While current passes through the filament wire, heat (therm) causes the separation if electrons (ionic) which will be released from the wire (emission)
Thermionic emission
Space charge cloud
Negative side of the x-ray tube
Cathode
What is the function of the cathode?
To produce a thermionic cloud, conduct the high voltage to the cap between the cathode and anode, and focus the electron stream as it heads for the anode
3 things the cathode assembly consists of
Filament(s)
Focusing cup
Associated wiring
4 things the x-ray tube consists of
Cathode
Anode
Enclosed within an envelope
Encased in a protective housing
The entire cathode and anode assembly is housed within this glass or metal structure
Envelope
The entire cathode and anode assembly is housed within this metal structure
Protective housing
Small coil of wire set in the cathode assembly within the focusing cup; electron production happens here
Filament
What is the filament made of?
Thoriated tungsten; rhenium and molybdenum are also desirable materials
2 reasons tungsten is the material of choice for the filament
High melting point- permits the filament to operate at the high temperatures required of an x-ray tube
Difficult to vaporize- vaporization produces particles that deposit on other surfaces and reduce the vacuum within the tube
Positive side of the x-ray tube; serves as a target surface for high-voltage electrons from the filament, conducts the high voltage from the cathode back into the x-ray generator circuitry and serves as the primary thermal conductor
Anode
A shallow depression in the cathode assembly designed to house the filament; surrounds the two filament wires of the cathode
Holds the released electrons around the filament wires in a space charge cloud until the exposure switch is engaged; when the kilovoltage is applied, during the exposure, the electrons are focused into a narrow beam that is directed toward the anode because they repel each other
Because electrons all possess negative charges, their tendency is to diverge rather than travel in straight lines; this is provided with a low negative potential which focuses the electrons toward one another in a convergence pattern
Focusing cup
The parts within the cathode assembly that connect the elements of the assembly together
Wiring
A two-filament arrangement within the x-ray tube
Dual-focus
What is the function of the filament?
To provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur
This process causes electrons to leave the surface of the filament wire and form a thermionic cloud
When the high voltage is released at exposure, the entire cloud is available to be driven toward the anode target where x-ray photons will be produced
Why are there two filament wires in the cathode?
They are different sizes because there are two different focal spots that handle different mA
2 focal spots and their associated mA
Large = high mA (greater than 100), more electrons flowing; large filament diameter Small = 100 mA or below
What percentage of x-ray exposure is heat versus x-rays?
Heat = 99% X-rays = 1%
Forces the electrons across the tube giving them kinetic energy
Kilovoltage (kVp)
The higher the kVp the _______ energy the accelerating electrons will have
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