Flashcards in Ch. 6 The xray tube Deck (126):
There are 2 primary parts to the tube. They are:
Cathode and anode
What are the 3 main methods of support for the xray tube?
Ceiling, floor to ceiling, and C-arm
What is the most frequently used support for the tube?
Describe the floor to ceiling support.
a single column with rollers at each end, one attached to a ceiling mounted rail and the other attached to a floor mounted rail.
What are other variations of the c-arm called?
the L-arm and U-arm
The emissions of xrays are considered..
isotropic, or with equal intensity in all directions
Only xrays emitted through the ___ are useful
What are xrays that escape through the protective housing called?
How much is radiation reduced when the protective housing is properly designed?
to less than 1 mGy/hr at 1 m at maximum conditions
Protective housing guards against....
excessive radiation exposure and electric shock
How does the protective housing protect against electric shock?
it incorporates specially designed high voltage receptacles
How does the protective housing help the tube itself?
it protects the tube from damage caused by rough handling
What role does oil play in the protective housing?
it serves as both an insulator against electric shock and as a thermal cushion to dissipate heat.
How can the oil get cooled if it is too hot?
by a cooling fan that cools it
What part of the tube is the oil located?
the protective housing
An xray tube is...
an electronic vacuum tube with components contained within a glass or metal enclosure.
What are the 2 electrodes that are contained in the tube?
the cathode and anode
What kind of glass is the tube made of?
Why is the tube made of pyrex glass?
to withstand the large amount of heat produced
The enclosure maintains a ____ inside the tube.
What is the role of the vacuum within the tube?
It allows for more efficient xray production and a longer tube life
What would happen if even a little gas was in the tube instead of it being in a vacuum?
the electron flow from cathode to anode is reduced and fewer xrays are produced and more heat is generated
What is the modern xray tube called?
a Cooldige tube (it is a vacuum tube)
Why is a glass enclosure less desirable than a metal enclosure?
because as glass ages some tungsten vaporizes and coats the inside of the glass enclosure. This alters the electrical properties of the tube, allowing tube current to stray and interact with the glass enclosure, resulting in arcing and tube failure
Why is a metal enclosure better?
they maintain a constant electric potential between the electrons of the tube current and the enclosure. They have a longer life and are less likely to fail. Virtually all high capacity tubes now use metal enclosures.
What does a dual filament provide?
the 2 filaments supply separate electron beams to produce 2 focal spots.
What are the 2 primary parts of the cathode?
a filament and a focusing cup
What is the filament similar to?
the coil of wire in kitchen toaster
What happens when the filament is heated?
it emits electrons when the current is high enough. The outer shell electrons of the filament atoms are boiled off and ejected from the filament.
What is thermionic emission?
When the electrons are boiled off the filament in the cathode
What are filaments usually made of?
What is the most common cause of tube failure?
tungsten vaporization with deposition on the inside of the glass enclosure
When tungsten vaporizes in a metal enclosure, what tends to happen?
it deposits on the internal components and upsets the electric characteristics of the tube and can cause arcing and lead to tube failure.
Where is the filament located in cathode?
in the focusing cup
What is the job of the focusing cup?
It is negatively charged and electrostatically confines the electron beam to a small area of the anode
What does the effectiveness of the focusing cup depend on?
its size and shape, its charge, the filament size and shape, and the position of the filament in the focusing cup
Most rotating anode xray tubes have 2.....
filaments mounted in the cathode assemble side by side creating large and small focal spot sizes.
How do filaments that are in a biangle tube have to be aligned?
end to end with the small focus filament above the large filament
grid controlled tubes are designed to be turned...
off and on rapidly
Where are grid controlled tubes used?
in portable capacitor discharge imaging systems and in digital subtraction angiography, digital radiography, and cineradiography
What does the word grid refer to when talking about grid controlled tubes?
an element in the tube that acts as a switch
In a grid controlled tube, what is the grid, which is also the switch?
the focusing cup
How is the xray tube current adjusted?
by controlling the filament current
The relationship between the fillament current and xray tube current depends on...
the tube voltage
What is the cloud of electrons that are around the filament called?
In a space charge, why is it difficult for electrons to be emitted by the filament?
because of electrostatic repulsion
What is it called when electrostatic repulsion makes it difficult for electrons to be emitted by the filament?
space charge effect
Why is producing xray tubes with currents that exceed 1000 mA difficult?
Because it is difficult to produce an adequate space charge compensating device
Thermionic emission at low kVp and high mA can be...
space charge limited
What is the saturation current?
When an increase in kVp does not result in a higher mA because the maximum is reached and there are no more available electrons to be used.
Why is saturation current not reached at a lower kVp?
because of space charge limitation
When is the small focal spot used?
when better spatial resolution is required
When is the large focal spot used?
when large body parts are imaged and when other techniques that produce high heat are required
How is the focal spot generally selected?
with the mA station selector on the console
At what mA is the small focal spot used?
at 300 or less a small is used. a large can also be used but a small is not used above 300 mA.
Why is a large focal spot used at 400 mA or higher?
Because the heat capacity of the anode could be exceeded if the small focal spot were used.
How big is a small focal spot?
0.1 to 1 mm
How big is a large focal spot?
0.3 to 2 mm
the anode is the _____ side of the tube
There are 2 types of anodes. They are:
stationary and rotating
Where are stationary anodes used?
in dental xray systems, some portables, and other special purpose units where high tube current and power are not required
The anode is the positive side of the tub that ....
conducts electricity and radiates heat and contains the target
The anode has 3 functions:
1. electrical conductor
2. provides mechanical support for target
3. a good thermal dissipater
How is the anode a good electrical conductor?
It receives electrons from the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator
How much of the thermionic emissions is converted into heat?
What are the most common anode materials?
copper, molybdenum, and graphite
What is the target?
The area of the anode struck by the electrons from the cathode
In stationary anode tubes, what is the target made of?
a tungsten alloy embedded in the copper anode
In a rotating anode tube, what is target?
the entire rotating disc is the target.
What can help the tungsten have added mechanical strength to withstand the stresses of high speed rotation and effects of repetitive thermal expansion and contraction?
Alloying the tungsten
What is layered under the tungsten to make the anode lighter and easier to rotate?
molybdenum or graphite
How much more area does a rotating anode have than a stationary one?
500 times more area
What is possible with the rotating anode?
higher tube currents and shorter exposure times
What can be improved by increasing the speed of the anode?
most rotating anodes rotate at ____rpm; anodes of high capacity xray tubes rotate at ____ rpm.
Where is the stem of the anode?
it is the shaft between the anode and the rotor
What is the stem made of?
molybdenum due to poor heat conduction
What can cause the anode to pit or crack?
If the rotor mechanism fails it can over heat and cause pitting or cracking, ultimately tube failure.
How does the motor of the anode rotate without being hooked up to cables and gears?
an electromagnetic induction motor is used to turn the anode
An induction motor has 2 basic parts. They are:
1. a part outside the glass or metal enclosure called the stator
2. a part inside the glass or metal enclosure called the rotor
What is the stator?
a series of electromagnets equally spaced around the neck of the tube
What is the rotor?
a shaft made of bars of copper and soft iron fabricated into one mass
How is a rotating anode powered?
by an electromagnetic induction motor.
How does the induction motor work?
Through electromagnetic induction, kinda like a transformer. Current in each stator winding induces a magnetic field that surrounds the rotor. the windings are energized sequentially so that induced magnetic field rotates on the axis of the stator. The magnetic field interacts with the ferromagnetic rotor causing it to rotate synchronously with the activated stator windings.
After the exposure button is pushed, why is there are delay before the exposure is made?
the delay allows the rotor to accelerate to its designated rpm while the filament is heated.
When is the kVp applied to the tube?
When the filament is heated
When the filament is heated, the filament current is...
increased to provide the correct xray tube current.
Why does a high speed rotor slow down so quickly (within 1 minute)?
because the induction motor is put into reverse
What is the focal spot?
the area of the target where xrays are emitted
What is the line focus principle?
when the target is angled to make the effective area of the target much smaller than the actual area of electron interaction
What is the effective focal spot or target area?
the area projected onto the patient and the IR
When the target angle is smaller, then effective focal spot is...
What are the diagnostic xray tube target angle ranges?
What is the advantage of the line focus principle?
it simultaneously improves spatial resolution and heat capacity
The line focus principle results in an effective focal spot size that is...
much less than the actual focal spot size.
The usual shape of a focal spot is..
the double banana
What are the differences in xray intensity across the focal spot controlled by?
the design of the filament and focusing cup and by the voltage on the focusing cup.
when are round focal spots important?
for high resolution magnification radiography and mammo.
What is the heel effect?
When the intensity of the xrays that are emitted through the heel of the target is reduced because they have a longer path through the target and therefore increased absorption. (anode side)
What effect does the anode angle have on the heel effect?
the smaller the anode angle, the larger the heel effect
How much can the radiation intensity vary across the useful beam?
as much as 45%
When is the heel effect important?
When imaging anatomical structures that differ greatly in thickness or density.
For a more uniform exposure, what end of the tube should be positioned toward the thicker part of the anatomy?
the cathode side
Which side of the tube is the effective focal spot size smaller?
the anode side and larger on the cathode side
What is off focus radiation?
When some electrons bounce off the focal spot and then land on other areas of the target, causing xrays to be produced from outside of the focal spot
Why don't we like off focus radiation?
it causes increase in skin dose and reduce image contrast
How can off focus radiation be reduced/
by designing a fixed diaphragm in the tube housing near the window of the xray tube. Or a metal enclosure will extract and conduct the reflected electrons away.
Does the use of a grid help with off focus radiation?
How can heat be dissipated from the tube?
Radiation is the...
transfer of heat by the emission of infrared radiation
the transfer of energy from one area of an object to another
the transfer of heat by the movement of a heated substance from one place to another
What does excessive heat lead to?
reduced xray tube life.
Which 3 modes of heat transfer happen in the xray tube?
What kind of radiation does the anode always emit?
What can happen if the temperature of the anode is excessive during a single exposure?
localized surface melting and pitting of the anode can occur
What can happen if surface melting is severe?
the tungsten can be vaporized and can plate the inside of the glass enclosure causing filtering of the xray beam and interfere with electron flow from the cathode to the anode.
maximum radiographic techniques should...
never be applied to a cold anode.
Keeping the anode hot for too long can cause...
heating of the rotor bearings causing more friction and imbalance of the rotor anode assembly
What is the cause of abrupt tube failure?
electron arcing from the filament to the enclosure because of vaporized tungsten
What is an open filament?
When the filament get so thin that it breaks
What xray tube rating charts are most important to techs?
radiographic rating chart, the anode cooling chart, and the housing cooling chart
out of the 3 charts, which one is the most important?
the radiographic rating chart
What does the radiographic rating chart tell us?
for a given mA, any combination of kVp and time that lies below the mA curve is safe. Any combination that lies above the curve representing the desired mA is unsafe.
What is contained in an anode cooling chart?
the thermal capacity of an anode and its heat dissipation characteristics and is used to determine the length of time required to complete cooling after any level of heat input.