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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
X ray tube consists of what?
cathode and anode within an envelope and then encased in protective housing
the negative side of the x-ray tube
cathode
produces a thermionic cloud
cathode
conducts the high voltage to the gap between the anode and cathode
cathode
focuses the electron stream as it heads for the anode
cathode
Cathode assembly consists of
filament or filaments
focusing cup
associated wiring
a small coil of thin thoriated tungsten wire
filament
How big is the wire?
0.1-0.2 mm thick
How big is the coil?
1-2 mm wide by 7-15 mm long
What is the material of choice because of it's high melting point?
tungsten
What is the melting point of tungsten
3370 C
What else are desirable materials other than tungsten?
rhenium and molybdenum
A two filament arrangement within the x-ray tube
dual focus
When the high voltage is released at exposure what happens?
the entire cloud is available to be driven toward the anode target where x rays photons be produced
A shallow depression in the cathode assembly designed to house the filament
focusing cup
What is the focusing cup made off?
nickel and its purpose is to narrow the thermionic cloud as it is driven toward the anode
As more and more electrons build up around the filament, their negative charges begin to oppose the emission of additional electrons
space charge effect
as kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode
saturation current
What are the anodes functions?
1) serves as a target surface for the high-voltage electrons from the filament
2) conducts the high voltage from the cathode back into the x-ray generator circuitry
3) serves as primary thermal conductor
1,2, and 3
What does Anode assembly consist of?
1) anode
2) stator
3) rotar
1, 2 and 3
What are the two types of anodes?
stationary and rotating
anode assembly that is immobile
stationary
an anode assembly that turns during exposure
rotating
The faster the anode rotates, the ___________ the heat dissipation
better
Stationary anodes are composed of
rhenium-alloyed tungsten imbedded in a 45o angled end of a copper rod.
Primary reasons for choosing tungsten for the x-ray source:
1) High atomic number
2) High melting point
3) Heat conducting ability.
1,2, and 3
What reduces the efficiency of the tube?
Pitting of the anode target focal track
How do you avoid pitting?
use an anode warm-up procedure whenever the machine is activated after a long period of time
What is the target?
the point source of x-ray photons, and it is from here that all tube-to-object and image receptor distances are measured
Actual focal spot
the physical area of the focal track that is impacted.
Effective focal spot
the area of the focal spot that is projected out of the tube toward the object being radiographed
Line-focus principle
used to reduce the effective area of the focal spot permitting the best resolution of detail while permitting as large an actual area as possible
The effective focal-spot size is controlled by
the size of the actual focal spot
Anode heel effect
due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side
Stator
the only part of the cathode or anode assemblies that is located outside the vacuum of the envelope
The anode heel effect
the reason why each radiographic table has a standard natural "head" end
induction-motor electromagnets comprise what?
comprise the stator that turns the anode
Common rotating anodes revolve at
3,200-3,600 revolutions per minute (rpm)
High speed rotating anodes operate at
10,000-12,000 rpm
A common cause of tube failure is
bad bearings caused by long use at high temperatures
envelope is made of
Pyrex glass and is about 10" long, 6" in diameter at the center, and 2" in diameter at the ends
The window
The point where the primary x-ray beam exits the envelope
The primary function of the envelope is
to maintain the vacuum between the cathode and the anode
Leakage radiation
any photons that escape from the housing except at the port
Leakage radiation must not exceed
100 mR/hr at 1 meter.
Tube rating charts
provide a guide regarding the most common technical factor combinations that can be used without overloading the tube
Anode cooling chart
permits the calculation of the time necessary for the anode to cool enough for additional exposures to be taken
Housing cooling chart
permits the calculation of the time necessary for the housing to cool enough for additional exposures to be made
Charts to help avoid thermal damage to the tube:
1) Radiographic tube rating charts
2) Anode cooling charts
3) Housing cooling charts
1, 2 and 3
When about 10% of the diameter of the filament has vaporized, what happens?
a filament becomes subject to breaking
The portion of the anode where the high-voltage electron stream will impact is called by different
names:
1) the target
2) the focus
3) the focal point
3) the focal spot
4) the focal track
1, 2, 3, and 4
When the target angle is less than 45 degrees, the effective focal spot is
smaller than the actual focal spot.
The most common diagnostic radiography target angle is
12 degrees
Tubes are available with angles ranging from
7 to 17 degrees
Diagnostic tubes are available with focal spots of
0.1mm to 3.0mm.
Stator
the only part of the cathode or anode assemblies that is located outside the vacuum of the envelope
Where is the rotor located?
inside the stator and inside the envelope
What is the rotor composed of?
a hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft
Metal envelopes are becoming more common because
they eliminate the problem of tungsten vaporization
The removal of the air 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
The housing does what?
1) controls leakage and scatter radiation
2) isolates the high voltages
3) provides a means to cool the tube
1,2, 3
What does the dielectric property of the oil do?
insulates the high-voltage components from the tube housing that you, the radiographer, touch
What will the tube housing absorb?
most of the off-focus radiation but enough can exit causing "ghosting" of structures adjacent to the edge of the primary beam
The ball bearings use
silver plating as a high-temperature lubricant because liquids would produce gas at high temperatures which would reduce the vacuum in the tube.
Some electrons produced in the thermionic cloud will
vaporize and deposit on the inside of the tube
When the electrons vaporize and deposit inside the tube, the deposits give the glass envelope what kind of look?
a mirrored appearance and can cause high-voltage arcing which can destroy the tube.
The deposits of vaporized tungsten can cause
increased filtration of the primary beam
What does the thermionic cloud deposits do to the inside of the tube?
reduces vacuum and makes the tube gassy
With a rotating anode target area, the term focal track is used to
represent the circular path that will be impacted by the electron beam
Rotating anode disks
range from 5-13 cm in diameter and are composed of molybdenum
The heated cathode was created by
William Coolidge for GE in 1915.
In the early days of radiography, what kind of cathode was used?
a cold cathode with an unheated filament
