Download Chapter 6: The X

Chapter 6
The X-ray Tube
The electrical production of x-rays is only possible under very special conditions
including a source of electrons, an appropriate target material, a high voltage, and a
vacuum.
The x-ray tube is the device that permits these conditions to exist.
The tube consists of a cathode and an anode enclosed within an envelope and then
encased in a protective housing.
The cathode is the negative side of the x-ray tube. The function of the cathode is 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 filaments, focusing cup, and associated wiring.
The filament
The filament is a small coil of thin thoriated tungsten wire set in the cathode assembly
within the focusing cup.
Tungsten is the material of choice because of its high melting point (3,370*) and because
it is difficult to vaporize.
The high melting point permits the filament to operate at the high temperatures
requited of an x-ray tube.
Vaporization produces particles that deposit on other surfaces and reduce the vacuum
within the tube.
The length and width of the filament have a great effect on the ability of the particular xray tube to image details.
Most diagnostic x-ray tubes have dual filaments called a dual focus arrangement.
The function of the filament is to provide sufficient resistance to the flow of electrons so
that the heat produced will cause thermionic emission to occur.
Thermionic emission causes electrons to leave the surface of the filament wire and form a
thermionic cloud. Then 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.
Vaporized tungsten deposited on the inner surface of the glass envelope causes a
mirrored appearance and eventually high voltage arcing when sufficient current is
attracted to a deposit during an exposure. Arcing immediately destroys the tube.
Evaporization deposits on the glass envelope also cause increased filtration of the
primary beam and this decreased tube efficiency. (grounded metal envelopes reduce this)
Another major cause of tube failure is the breaking of the filament. Filaments become
increasingly thin as vaporization (10% subject to breakage) continues.
 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. The activation of the rotor switch causes the rotor to turn and 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 the vaporization of the filament. (Rotor switch = filament heat)
 Filament life is 6 – 9 hours (10,000 – 20,000 exposures)
All units have electronic interlocks that will not permit the exposure to occur until the
rotor has brought the anode up to the proper speed.
The focusing cup
The focusing cup is a shallow depression in the cathode assembly designed to house the
filament. It is made of nickel and its purpose is to marrow the thermionic cloud as it is
driven toward the anode.
The focusing cup is provided with a low negative potential, which focuses the electrons
toward one another in a convergence pattern.
A biased focusing cup is used to decrease the size of the focal spot. It maintains the cup
at a more negative voltage than the filament. This causes the exiting electron beam to be
focused into a narrower stream as it heads toward the anode.
As more and more electrons build up in the area of the filament, their negative charges
begin to oppose the emission of additional electrons, called the space charge effect, it
limits x-ray tubes to maximum mA ranges of 1,000-1,200.
The filament saturation current has been achieved when there are no further thermionic
electrons to be driven toward the anode. An increase in kVp will not increase the tube
mA- must increase the filament mA.
The addition of a more negative potential difference (approximately 2000 volts) at the
focusing cup causes the cup to attract the thermionic cloud.
The anode
The anode is the positive side of the x-ray tube and has three functions:
 it serves as a target surface for the high voltage electrons from the filament,
thereby becoming the source of the x-ray photons
 it conducts the high voltage from the cathode back into the x-ray generator
circuitry

and it serves as the primary thermal conductor.
The anode target surface is where the high-speed electrons from the filament are
suddenly stopped, resulting in the production of stray photons. The entire anode is a
complex device referred to as the anode assembly. This assembly consists of the anode,
stator and rotor and serves as the path for the high voltage flow during exposure.
Rotating anodes turn during the exposure, presenting a much larger target area.
The faster the anode rotates, the better the heat is dissipated.
Nearly all units designed for diagnostic radiography utilize rotating anodes because of
their greater efficiency.
Stationary anodes are composed of rhenium-alloyed tungsten (rhenium has greater
elasticity) imbedded in a 45* angled end of a copper rod.
The anode’s function as the source of x-ray photons as the primary thermal conducting
device is enhanced by the use of rhenium-alloyed tungsten as the target focal track
material.
Tungsten is the metal of choice for the source of x-ray photons for the following reasons:
 High atomic number: Tungsten’s atomic number (74) enhances the production of
diagnostic range photons.
 High melting point (1,000 – 2,000* C during normal use)
 Heat conducting ability
Mammography units use molybdenum as target material due to ability to emit a more
uniform range of lower energy photons.
The portion of the anode where the high voltage electron steam will impact is called the
target, focus, focal point, focal spot, or the focal track.
The target is considered to be a point source of x-ray photons and it is from this point that
all tube to object and image receptor distances are measured.
The focal track is used to represent the circular path that will be impacted by the electron
beam.
The terms target, focus, focal point, and focal spot refer to the area of the focal track that
is impacted by the electron beam at one time.
The actual focal spot is used to describe the physical area of the focal track that is
impacted.
The effective focal spot is used to describe the area of the focal spot that is projected out
of the tube toward the object being radiographed.
The line focus principle is used to reduce the effective area of the focal spot. This
permits the best resolution of detail while permitting as large an actual area as possible to
increase thermal conductivity.
The effective focal spot is controlled by the size of the actual focal spot (which is
controlled by the length of the filament) and the anode target angle.
When the target angle is less than 45*, the effective focal spot is smaller than the actual
focal spot.
When the angle is decreased, smaller focal spots can be achieved.
The effective focal spots vertical dimension is the one that is stated as the focal spot size.
The anode heel effect is caused by the geometry of an angled anode target and results in
the radiation intensity being greater on the cathode side.
The induction motor electromagnets comprise the stator that turns the anode.
The anode is composed of a hollow copper cylinder or cuff that is attached to the anode
disk by a molybdenum shaft.
Common rotating rotors revolve at 3,200 – 3,600 rpm.
High-speed rotating anodes that operate at 10,000 – 12,000 rpm are also available to
assist in dissipating heat.
Any rough or extremely fast movement of the housing is not healthy for an x-ray tube
and should be avoided.
Ball bearings eventually become imperfectly round. This leads to a grinding noise and
wobbling throws the focal track off center and tube efficiency drops dramatically.
Superheated melted tungsten that drips onto the envelope will destroy the tube.
Metal envelopes are increasingly becoming more common. They prolong tube life
because they eliminate the problem of tungsten vaporization.
The envelope must be sealed tight to maintain a high vacuum. At the point where the
primary x-ray beam exits the envelope a window segment is constructed.
The removal of the air permits electrons to flow from cathode to anode without
encountering the gas atoms of air.
The housing controls leakage and scatter radiation, isolates the high voltages and
provides a means to cool the tube.
When x-ray photons are produced at the anode, they are emitted isotropically. The
primary beam consists of photons emitted through the window.
The protective housing is composed of cast steel and is capable of absorbing most of the
unwanted photons.
Any photons that escape from the housing except at the port are leakage radiation.
Leakage radiation must not exceed 100mR/hr at 1 meter. The housing also serves to
cushion the x-ray tube from rough handling by operators.
The oil insulates the high voltage components from the tube housing, which is handled by
the radiographer, and absorbs much of the heat that is produced by x-ray production.
Under no circumstances should anyone be in contact with a tube housing during exposure.
Off-focus, or extrafocal radiation is composed of photons that were not produced at the
focal spot and may contribute as much as 25 – 30% of the total primary beam.
Tube rating charts are the most valuable because they provide a guide regarding the
maximum technical factor combinations that can be used without overloading the tube.
Any combination of factors at or under the curve is safe.
Each filament of each tube has a unique radiographic tube-rating chart.
Anode cooling charts permit the calculation of the time necessary for the anode to cool
enough for additional exposures to be made.
A heat unit is calculated as kVp x mA x rectification constant.
To find the length of time for the anode to cool can be calculated by:
 Find the total heat units applied on the vertical scale.
 Read from the heat units over to the cooling curve and then down to read the
corresponding time.
 Calculate the time necessary for the anode to cool to any desired level and
subtract the corresponding time of the initial exposure.
The anode will usually reach its limits ling before the housing.
Warming up the anode according to the manufacturers recommendations prevents
thermal shock (cracking of the anode).
Holding the rotor switch unnecessarily should be avoided. The rotor switch increases the
filaments thermionic emission to exposure levels. Thermionic emission removes the
electrons from the filament; deposits vaporized electrons in tube surfaces and decrease
the vacuum.
The rotor causes stress to the rotor bearings, double press switches should be completely
depressed in one motion and dual switched should have the exposure switch depressed
first, followed by the rotor switch.
Lower mA stations should be used when possible because high mA increases filament
thermionic emission.
The lower speed rotor should be used when possible because the high-speed rotor
increases rotor-bearing wear.
Rotating the tube housing rapidly from one position to another should be avoided because
the gyroscopic effect may crack or otherwise damage the rotor.
A wobbling anode disk can cause tube failure.