Chapter 5 Flashcards
The x-ray tube consists of a _______ and an ______ enclosed within an _______, and then encased in a _______.
cathode; anode; envelope; protective housing
An X-ray tubes life can be significantly extended by ________.
Proper care and handling by professional radiographers
Is the cathode the positive or negative side of the x-ray tube?
Negative
What is the function of the cathode?
To produce a thermionic cloud, conduct the high voltage to the gap between cathode and anode, and focus the electron stream as it heads for the anode.
The cathode assembly consists of?
The filament or filaments, focusing cup, and associated wiring.
What is a filament?
A small coil of thin tho rusted tungsten wire.
How thick is the filament wire?
0.1-0.2 mm
What are the dimensions of the filament?
1-2mm wide by 7-15mm long.
_________ is the material of choice because of its high melting point.
Tungsten
What is the melting point of tungsten?
3,370*C
What are two desirable materials to use as a filament other than tungsten?
Rhenium & molybdenum
Why is tungsten not easily vaporized a good material to used in a cathode?
Vaporization produces particles that deposit on other surfaces and reduce the vacuum witching the tube.
What is another name for x-ray tubes that have dual filaments?
Dual-focus arrangement
What is the function of the filament?
To provide sufficient resistance to the flow of electrons so that the heat produces will cause thermionic emission to occur.
What temperature does tungsten need to reach to exhibit significant thermionic emission?
Above 2,200*C
Older tubes are much more sensitive to ______ and a _____ that is jarred can break prematurely.
Rough handling; thin filament
What is an average diagnostic x-ray tube filament life?
6-9 hours (10,000-20,000 exposures)
What is the primary cause of premature turn failure?
The radiographer’s habit of holding the rotor switch prior to making exposures. Every second the rotor switch is depressed, life is removed from the filament.
Routinely delayed exposures while the filament is enduring maximum current can shorten tube life by ___-___% (To 5,000-12,000 exposures)
50;60
Single-phase generators are usually capable of initiating exposure within ____ milliseconds.
10 (0.01 second)
High-frequency generators are usually capable of initiating exposure within ___ millisecond.
1 (0.001 second)
Who developed the cathode using a hot filament?
William D. Coolidge
When was the Coolidge tube developed?
1915
What other development did Coolidge contribute to radiography other than the hot filament?
The focusing cup, the imbedded anode target, and various anode cooking devices.
Why is the focusing cup?
A shallow depression in the cathode assembly designed to house the filament.
What is the focusing cup made of?
Nickel
What is the focusing cup’s purpose?
To narrow the thermionic cloud as it is driven toward the anode.
Why do electrons have a tendency to diverge rather than travel in straight lines?
Because they possess a negative charge
What is space charge effect?
As more electrons build up in the area of the filament, their negative charge begin to oppose emission of additional electrons. It also limits x-ray tubes to maximum mA range of 1,000-1,200.
What is saturation current?
Affects the efficiency of the x-ray tube. As kVp increases, a grater percentage of the thermionically emitted electrons are driven toward the anode. The filament amperage curve flattens out when the kVp is driving the entire thermionic cloud toward the anode. Saturation current is achieved when there are no further thermionic electrons to be driven toward the anode.
The anode is the positive or negative side of the x-ray tube?
Positive
Why are the anode side of the x-ray tubes 3 functions?
-Serves as a target surface for the high voltage electrons from the filament(becoming the source of x-ray photons)
-it conducts the high voltage form the cathode back into the x-ray generator circuitry
-it serves as the primary thermal conductor
The ________________ is where the high-speed electrons from the filament are suddenly stopped, resulting in the production of x-ray photons.
Anode target surface
The anode assembly consists of?
The anode, static and rotor and serves as the path for the high-voltage flow during exposure.
What are the two types of anodes?
Stationary anodes and rotating anodes
When we’re rotating anodes developed?
1936
How long is a given area of the target bombarded in modern rotating anodes?
7-50 microseconds
The _______ the anode rotates, the better the heat dissipation.
Faster
Stationary anodes comprise _______________ imbedded in a ___* angled end of a copper rod.
Rhenium-alloyed tungsten; 45
Rotating anode disks range from ____ to ____ cm in diameter and comprise ____________.
5; 13; molybdenum
Tungsten is the metal of choice for the source of x-ray photons for what three reasons?
- High atomic number(74)
- High melting point
- Heat-conducting ability
During normal use, the focal track reaches a temperature between _________C and _______C
1,000; 2,000
What does using an anode warm-up procedure designed to do?
To bring the anode heat form room temperature to near the range of operation.
A tape measure attached to the side or bottom of a tube collimating may begin at 12 cm because?
The target is considered the point source of x-ray photons and it is from this point all tube-to-object and image-receptor distance are measured.
Line-focus principle is used to
Reduce the effective area of the focal spot. This permits the vets resolution of detail while permitting as large an actual area as possible
As the actual focal-spot size increases, the effective focal-spot size ____________.
Also Increases
When the target angle is less than ____*, the effective focal spot is smaller than the actual focal spot.
45
What is the most common diagnostic radiography target angle is?
12*
When the target angle is decreased, ________ focal spots can be achieved.
Smaller
Why is a disadvantage of extremely small target angles?
The geometry of the angle can limit the size of the primary beam field at short source-to-image receptor distances.
To cover a 14”x17” field at 40”, a minimum of ____* target angle is required.
12
Target angles of ______ than 14* have been shown to produce some degree of ______ as they age
Less; cutoff
Why are x-ray focal spots rectangular?
Because the line-focus principle applies only in the direction is the angle.
X-ray tube targets must be ________ to maintain the small effective focal-spot size in the horizontal direction, usually not over 2 mm.
Narrow
The line-focus principle permits the actual focal spot’s vertical dimension to as much as 6 mm to gain __________________________________.
Maximum thermal conductivity
The effective focal spot’s vertical dimension is the only one stated as the ______________.
Focal-spot size
Who establishes standards for focal-spot size?
The National Electrical Manufacturers Association (NEMA)
The ________ is the only part of the cathode or anode assemblies that is located _________ the vacuum of the envelope.
Outside
Where is the rotor located?
Inside the stator and inside the envelope
What is the rotor comprised of?
A hollow cylinder or cuff that is attached ti the anode disk by a molybdenum shaft.
Common rotating anodes revolve at _____-______ revolutions per minute.
3,200-3,600
A common cause of then failure is _______________ caused by long use at high temperatures.
Bad bearings
What is the primary function of the envelope?
to maintain the vacuum between the cathode and anode
What does the removal of air permit from the vacuum pump do?
permits electrons to flow from cathode to anode without encountering the gas atoms of air and greatly increases the efficiency of the tube’s operation
Modern x-ray tubes must be mounted inside what?
a protectives housing
What does the housing control?
leakage and scatter radiation, isolates the high voltages, and provides a means to cool the tube
When x-ray photons are produced in the anode, how are they emitted?
isotropically (in all directions)
What is leakage radiation?
any photons that escape from the housing except from the port
What must leakage radiation not exceed?
100 mR/hr at 1 meter
Off-radiation may contribute as much as __-__ percent of the total beam.
25-30
What are the three types of rating charts available to help radiographers avoid thermal damage to x-ray tubes?
radiographic tube rating charts, anode cooling charts, and housing cooling charts
What do anode cooling charts permit?
the calculation of the time necessary for the anode to cool enough for additional exposures to be made
What is the formula for calculating heat units?
kVp x mA x time x rectification constant
