The X-ray Tube

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The X-ray Tube
Tube Housing
• Made of cast steel & is usually lead-lined
– Provides for absorption of most off-focus
radiation
• Purposes:
– Controls leakage & off-focus radiation
(discussed later)
– Isolates high
voltages
– Helps to cool
the tube
Glass Envelope
• Surrounds entire cathode & anode
assemblies except for the stator
– Made of several layers of Pyrex w/ varying
densities
– Glass is fitted to the metal
of the anode & cathode
ends
– Must be airtight to
maintain a good
vacuum
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Glass Envelope
• A target window is constructed in the glass
envelope to allow less scatter & attenuation
of the photons
– In most tubes - simply a thinner “cut” of glass
– In mammography - a special
metallic beryllium window
prevents attenuation of lower
energy photons
Cathode
• The cathode is the negative end of the x-ray
tube.
– Made up of the filament(s) and a focusing cup.
Filament
• Most x-ray tubes have a dual filament
cathode assembly - also known as dual focus
– The two filaments sit parallel to each other in
the focusing cup & share a common ground
wire.
– Most filament coils
are 7-15mm long ,
1-2mm wide,
0.1-0.2mm thick
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Filament
• Filaments must be able to:
– Boil off electrons (thermionic emission)
– Withstand great amounts of heat
• Filament materials
– Tungsten - most widely used material
• High boiling point (3,370° C)
• It is difficult to vaporize
– Rhenium (3,170° C)
– Molybdenum (2,620° C)
Filament
• Vaporization occurs over time
– When the particles vaporize (turn into a gaseous
form), they solidify on the glass of the x-ray
tube, called sun-burning or sun-tanning of tube.
• Reduce the x-ray output of the tube
• destroy the vacuum integrity of the tube, leads to
arcing and ultimately tube failure
• Thorium (a radioactive metallic element) is
added to the filament material to make the
tube last longer.
Focusing Cup
• The focusing cup helps control electron cloud
– The electrons repel each other & want to spread
out. The focusing cup forces the electrons to
form a small stream as they move toward the
target material
– Made of nickel
– Has a low
negative charge
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Grid-Controlled Focusing Cups
Some x-ray procedures require exposures be
taken at quick intervals.
• Grid-controlled focusing cups have a
variable charge applied to the focusing cup
that acts as an exposure switch
– When the tube is activated, the charge increases
& decreases rapidly
– Short bursts of electrons
flowing to the target.
Grid-Controlled Focusing Cups
• May be found in:
– portable capacitor discharge units
– digital subtraction angiography
– digital radiography
– Cineradiography
Anode
The Anode is the part of the x-ray tube
where accelerated electrons move to after
kV is applied to the tube.
• Two types:
– Stationary anode (old type) - just a tungsten
button imbedded in copper bar.
– Rotating anode consists of a molybdenum
disk(target) rotated by an induction motor.
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Rotating Anode Assembly
This is a diagram of a
rotating anode without &
with the tube.
Rotating Anode Stator and Rotor
Consists of two main parts:
• Stator
– Rests just outside of the glass tube
– Made up of a series of
electromagnets equally spaced
around the neck of the tube
• Designed to energize opposing pairs,
in sequence, so that they induce the
rotation of the rotor.
• Rotor
– Located within the glass tube
– Made up of copper bars & soft iron
around a molybdenum shaft
***Mutual Induction***
Rotating Anode Stator and Rotor
• When the rotor is rotating at the desired level,
the x-ray exposure may be completed.
• Most revolve at 3400 revolutions per minute
(rpm) minimum.
• By rotating the anode
we spread the
generated heat over a
larger surface area
allowing greater
technique loads.
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Anode Target Characteristics
• Anode target - the point on the anode where the
electrons strike
• Tungsten – rhenium alloy is the most common
material and is plated onto the surface of the
molybdenum disk
• Tungsten has:
– High atomic number (74)
– High thermal conductivity level
– High melting point
• Rhenium added to increase thermal capacity and
tensile strength
The Line-Focus Principle
• Actual focal spot - the area of the target material
being bombarded by electrons from the filament.
• Effective focal spot - the imaginary geometric line
that can be drawn based on the actual focal spot
size vs. the angle of the anode.
• Best described by the angle of the anode
– the smaller the angle of the anode, the smaller the
effective focal spot size (any angle <450 results in the
effective FS being smaller than the actual FS)
– 120 target angle most common because it is the
minimum that will cover a 14x17 at 40”
The Line-Focus Principle cont.
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The Anode Heel Effect
• Caused by the angle of the anode vs. the intensity
of the electrons striking it.
• X-rays exiting the target on the anode side have to
traverse the “heel” of the anode
– Photons directed toward the cathode end do not have
to travel through as much of the anode because of the
angle of the target so more make it out
– Those directed toward the anode end must travel
through more material so more are absorbed
– Results in the beam being of lower intensity on the
anode side.
The Anode Heel Effect
• As much as 20% more photons at the
cathode end of the tube & as little as 25%
fewer photons at the anode end of the tube.
• Most noticeable with:
– Small focal spot
– Short S.I.D.
– Large field
Production of Off-Focus Radiation
• Radiation produced from x-ray photons or
electrons that have reflected off of the anode
• These x-rays or electrons can strike a number of
things in the tube and produce scatter photons:
– Side of the focusing cup
– Tungsten particles from sun-burn
• Because they are not produced in the focal track
they are “off-focus” and while most are absorbed
by the housing, some make it out of the tube and
degrade the radiographic image.
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Extending Tube Life
• Practical methods
• Tube rating charts
– Determines if a technique is safe
– Used to test overload protection circuits
• Calculating heat units and using cooling
charts.
Practical Methods
The life of the tube is under your control!
• Proper warming extends tube life
• Avoid repeated exposures close to tube load
limit
• Do not hold the rotor switch unnecessarily
Listen to your equipment!
Tube rating charts
• Rules for use
– Select the correct chart
– Plot the point using technical factors
– ANYTHING ON OR ABOVE THE GIVEN
mA LINE IS UNSAFE
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Tube Rating Charts
Calculating Heat Units (hu)
kV x mA x time (s) x C x # of exposures
r
The heat unit rectification constants (C ) are:
r
– 1 φ 2 pulse (full wave)
= 1.00
– 3 φ 6 pulse
= 1.35
– 3 φ 12 pulse
= 1.41
– High frequency
= 1.45
An anode cooling curve based on
the tube’s rating chart must be
used when calculating multiple
exposures.
Calculating Heat Units (hu)
If 10 exposures of 80 kVp, 200 mA & 0.43 s.
is made on a high frequency unit, how many
heat units (hu) are produced?
kV x mA x time (s) x C x # of exposures
r
80kVp x 200mA x 0.43 sec x 1.45 x 10 =
99,760 hu
If the anode is at its maximum how long
must we wait before making the exposures?
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Anode Cooling Chart
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