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
More
2 reasons why you have to wait when you push the x-ray button
The anode needs to rotate
Filament wires need to be hot enough to boil off electrons
2 causes of tube failure by the cathode
Vaporization of tungsten (from both filament and anode)
Breaking of the filament itself
What is the focusing cup made of?
Nickel
As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons, limiting x-ray tubes to maximum mA ranges of 1,000-2,000
Space charge effect
As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode
Saturation current
What is the average diagnostic x-ra tube filament life?
6-9 hours (10,000-20,000 exposures)
When the x-ray machine is first turned on, a mild current is sent to the filament. The filament remains in this preheated mode until immediately prior to an exposure. When the switch labeled “rotor” is activated prior to an exposure, not only dose the rotor begin to turn by a higher current is sent to the filament to bring the thermionic cloud to the proper size for the mA selected. This increase in filament heating is what causes most of this of the filament. Tungsten (from both filament and anode) is also gradually deposited on the inner surface of a glass envelope
Vaporization
Positive side of the x-ray tube
Surface is where the high-speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons
To convert the electrons’ kinetic energy into x-ray energy the accelerating electrons must be slowed down or stopped (deceleration)
Anode
3 functions of the anode
Serves as a target surface for the high-voltage electrons from the filament, thereby becoming the source of the x-ray photons
Conducts the high voltage from the cathode back into the x-ray generator circuitry
Serves as the primary thermal conductor
3 things the anode assembly consists of
Anode
Stator
Rotor
Induction-motor electromagnets that turn the anode
Stator
A hollow copper cylinder or cuff that is attached to the anode by a molybdenum shaft ad is affected by the electromagnetic field of the stator, causing it to turn
Located inside the stator and inside the envelope
The inside contains silver-plated steel ball bearings around a shaft that is anchored to the envelope; use silver plating as a high-temperature lubricant between the cuff and the anode shaft
Rotor
An anode assembly that turns during exposure
Rotating anode
What are rotating anode discs made of?
Molybdenum
What is the focal track material?
Rhenium-alloyed tungsten
3 reasons tungsten is the metal of choice for the source of x-ray photons
High atomic number
High melting point
Heat-conducting ability
Normal use of a rotating anode will eventually vaporize sufficient target focal tract material to roughen the target area; reduces the efficiency of the tube
Pitting
The portion of the anode where the high-voltage electron stream will impact (4 names); the precise point at which the x-ray photons are created
Area of the focal track that is impacted by the electron beam at one time
Rectangular area; have to focus on getting a good resolution because x-rays come from an area instead of a spot and cause the edges of the image to blur
Target
Focus
Focal point
Focal spot
The portion of the anode where the high-voltage electron stream will impact; the circular path that will be impacted by the electron beam
Focal spot/target/focus/focal point makes up this so the electrons aren’t hitting the same spot on the anode
Focal track
2 focal spots on the anode
Actual
Effective
The physical area of the focal track that is impacted, in the tube
Actual focal spot
The area of the focal spot that is projected out of the tube toward the object being radiographed, what the patient sees
Effective focal spot
2 things size of the effective focal spot is determined by
Anode target angle
Actual focal spot size
What is the actual focal spot size controlled by?
Length of the filament
By angling the target, one makes the effective (focal spot) area of the target much smaller than the actual (focal spot) target area
Design incorporated into x-ray tube targets to allow a large area for heating while a small focal spot is maintained
Line-focus principle
What is the most common diagnostic radiography target angle?
12°
Why can the target angle be no smaller than 12°?
To cover a 14” x 17” field at 40”
Caused by the line-focus principle
Because of the geometry of an angled anode target, the radiation intensity is greater on the cathode side
Photons that are emitted toward the anode end are more likely to be absorbed by the target material itself than those that are emitted toward the cathode end
This can cause as many as 20% more photons at the cathode end of the tube and 25% fewer photons at the anode end; a total variation of approximately 45% exists parallel to the anode-cathode axis
The 45% variation is significant enough to cause a visible difference in exposure during radiographic examinations when large film sizes are used at short distances
Anode heel effect
Because the cathode end of the x-ray tube has a more intense beam, it should be positioned toward the ________ part of the body
Denser/thicker
Where is the stator located?
Outside the vacuum of the envelope; the electromagnetic effect that causes the rotor to turn can function through the envelope, permitting electrical isolation of the stator coils from the high voltage of the exposure
At what speed do high-speed rotating anodes operate?
10,000-12,000 revolutions per minute (rpm)
A structure where the primary x-ray beams exit the envelope, which allows less absorption or scatter of photons
Window
3 things the protective housing does
Controls leakage and scatter radiation
Isolates the high voltages
Provides a means to cool the tube
