The X-Ray Tube
DMI 50B
Kyle Thornton
Evolution Of The X-Ray Tube
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Roentgen experimented with a Crookes tube
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In 1913, W.D. Coolidge redesigned the tube, calling it a
hot cathode tube
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Evacuated tube containing two electrodes
Could not control the number of electrons moving between the
electrons (tube current)
Replaced the negative electrode with a small spiral-wound
tungsten wire
Current was applied to this wire creating heat to free electrons
Hot cathode tubes became the standard for x-ray tubes
Cold Cathode X-Ray Tube
Early 20th century
Cold Cathode Tube Fluorescence
Coolidge Tube w/Stationary Anode
Helpful website
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http://www.ndted.org/EducationResources/HighSchool/Radio
graphy/xraygenerators.htm
http://www.youtube.com/watch?v=Bc0eOjWkxp
U
Common Components Of The XRay Tube
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Glass envelope
Cathode
Anode
Glass Envelope
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A vacuum is created prior to
sealing the tube
The glass envelope:
– Provides support for the
electrodes
– Provides electrical insulation
– Assists in the removal of heat
– Is made of very thick glass to
withstand heat
– Is thinner at the exit window
where the x-rays emerge
from the tube
Cathode
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Negative electrode of the
x-ray tube
Consists of a filament
and a focusing cup
The filament provides a
source of electrons
Electrons are freed when
the filament is heated
The filament is a long
thin tungsten wire
shaped into a spiral coil
Cathode
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About 10 volts and 3 – 5 amperes are applied to the
filament to heat it
Tungsten is used because of its high melting point 3370° C
It is also malleable
Most modern tubes contain two filaments
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The longer filament is used when large numbers of electrons
are needed
The shorter filament is used when lower tube currents and
maximum detail are needed
The Focusing Cup
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The filaments are mounted within a focusing cup
Generally made of nickel, stainless steel, or
molybdenum
A negative charge is placed on the focusing cup
Focuses the electrons on a smaller spot of the anode
This improves detail on the film
Benefit of the Focusing Cup
X-Ray Tube Focusing Cup
http://www.youtube.com/watch?v=LyWuvSZRSLc
&feature=related
The Anode
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The anode is the positive
electrode
It is struck by the electron
stream
The area struck is the anode
target
Usually made of tungsten
A great deal of heat is
produced at the anode
– Depends on the voltage,
the current, and the length
of time the anode is struck
by electrons
Stationary Anodes
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Used when lower heat quantities are produced
Consists of a tungsten target and copper block and
stem
Copper conducts the heat away from the tungsten
target
The heat can damage the anode target, causing pitting
–
Results in x-ray beam of reduced intensity due to scattering
and absorption in the uneven surface
Rotating Anodes
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The first was made by Philips Medical Systems
in Holland in 1929
Consists of a tungsten alloy disk on a
molybdenum base
Rotating anodes range in size from about 5 cm
to 12.5 cm
Disk sizes determine the thermal load
Anodes have an angle of about 7° to 20°
Rotating Anodes
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Rotating anodes assure that the same area of
the target being struck over and over is rare
Heat energy is distributed more evenly over the
anode face
More rapid exposures are possible
The rotating anode is driven by an induction
motor
Purposes Of The Anode
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Serves as a positive electrode
Provides structural support for the target plate
Provides a means for of dissipating heat to the
target
The Benefits Of Using Tungsten As
A Target Material
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Malleability
High melting point 3370° C
High Z number
Resists vaporization at
high temperatures
Ability to conduct heat
away from area of heat
production
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Its density
Ability to absorb heat
without raising the
temperature of the
conductor
Its availability makes it
cost-effective
The Induction Motor
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Works on the principle of Lenz’ law
Using opposing magnetic fields, the copper bar is
made to rotate
Rotation of the anode is accomplished more efficiently
this way
The anode is surrounded by electromagnets that are
switched on and off in rapid sequence
Anodes utilize 60 Hz AC
Anodes usually rotate at 3600 rpm
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Newer anodes may rotate at 10,000 rpm
Induction Motor Diagram
Target Plate Angulation
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The area of the target struck by the electron
stream is the focal spot
Image sharpness is improved when the focal
spot is small
By angling the target, a small area can be
struck, but at the same time provides a large
space for heat dissipation
Line-Focus Principle
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By angling the target, the
effective area is much
smaller than the actual
area of electron
interaction
As the target angle
decreases, so does the
effective focal spot
Dual Focus Anodes
Heel Effect
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A consequence of the
line focus principle
The beam intensity on
the cathode side is more
intense than on the
anode side
Patients must be
positioned accordingly
for some exams
Heel Effect
Off-Focus Radiation
Off-Focus Radiation
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Radiation produced when
electrons bounce off the
target area of the anode and
strike other areas and
produce x-rays
About 15% of the electrons do
this
This reduces image contrast
Can be controlled by applying
an additional diaphragm close
to the focal spot
Tube Exit Window
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The useful beam is emitted from the tube exit
window
This section of glass is generally much thinner
than the rest of the tube
The Tube Housing
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Supports and houses the x-ray tube
Provides insulation
Prevents shock
Is lined with a lead tube shield to prevent
leakage radiation
Oil surrounds the tube within the tube shielding
The Tube Housing
Causes Of X-Ray Tube Failure
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A single excessive exposure
Long exposure times
Filament vaporization
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The most common
Safe Operation With Tube Rating
Charts
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Radiographic rating chart
Anode cooling chart
Housing cooling chart
Tube Rating Charts
Here’s Some Questions
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Which of the following exposures are unsafe?
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95 kVp, 150 mA, 1 sec. 3400 rpm, .6mm focal spot
80kVp, 400 mA, .5 sec. 3400 rpm, 1mm focal spot
125 kVp, 500 mA, .1 sec 10,000 rpm 1 mm
75 kVp, 700 mA, .3 sec, 10,000 rpm 1 mm focal
spot
88 kVp, 400 mA, .1 sec., 10,000 rpm .6mm focal
spot
Heat Units
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The product of mA, T,
and kVp
HU = mA X T X kVp
Used to determine
thermal capacity of an
anode or tube housing
The tube housing
cooling chart is similar to
that of the anode cooling
chart
Anode Cooling Chart
Here’s Some More Questions
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If 50,000 HU are delivered to the anode, how
long will it take to cool completely?
How many heat units are produced if 6 films
are exposed using a three phase, six pulse
generator at 82 kVp and 120 mAs