Chapter 13 X-ray tubes

The X-Ray Tube
Consists of:
1) Glass Envelope
2) Cathode
3) Anode
4) Surrounded by oil to cool tube
(Contained in protective housing)
Consists of Three Sub-Assemblies:
1) Filament
2) Support wires
3) Focusing cup
• Tungsten wire
• 3-5 A, 10 – 12 V applied
• Produces space charge through thermionic
• Double focus tubes have 2 filaments to
produce large and small focal spots
• Filament problems include:
1) filament evaporation over time
- Progressively less current is needed
to achieve desired mA as this occurs
- may lead to tungsten on envelope &
“arching of the tube”
Double focus filaments
2) filament breakage
inside focusing cup
Focusing Cup
• Negatively charged
• Used to focus electrons
Filaments inside the
focusing cup
Three Support Wires
• Delivers voltage and amperage to filaments
• Wiring – Lines 1 and 3 – large filament lights
Lines 2 and 3 – small filament lights
Line 3 common to both filaments – produces
high negative potential
May be:
1) Stationary – Used in dentistry, embedded in Cu
2) Rotating
Anodes are composed of:
1) tungsten – Primary metal because it has:
- has high melting point
- high atomic no. (74) enhances diagnostic
- high ability to dissipate heat
2) rhenium – Increases elasticity of focal track
3) graphite and molybdenum may also be added
to increase heat dissipation
Stationary Anode
Rotating Anode
Anode Focal Spots
Actual Vs Effective (Apparent)
Actual Focal Spot – The area on the anode hit
by the bombarding electron stream
- Approximately 2 X 6
Effective (Apparent) Focal Spot – The focal
spot that is projected towards the image receptor.
- Smaller effective focal spots yield
improved spatial resolution
- Approximate range from .1 mm - .6 mm
Line Focus Principle – The effective focal
spot is smaller then the actual focal spot.
- Due to angle of anode
- Always true with anodes less than 45°
Key Facts to Remember
• The size of the effective focal spot decreases
as the angle of the anode decreases (becomes
Advantages/Disadvantages of Steep Anodes
Advantages –
1) Better spatial resolution
2) Increased short exposure rating
- Disadvantages
1) Cutoff at short SID’s
Key Facts to Remember
• Steeper (decreased angled) anodes accentuate
the anode heel effect.
- Anode Heel Effect – The x-ray intensity
is greater at the cathode versus the
anode end of the tube.
- In addition this effect is more pronounced
at short SID’s
Rotating Anodes
Benefit – Heavier exposures at shorter exposure
• Tungsten – Primary ingredient
• Coated with Rhenium – Increases elasticity of
focal track, decreases roughness
• Molybdenum/graphite added to dissipate heat
• Cu added in rotor to dissipate heat
• 3-5 inches in diameter
• Angle range 10° - 17° (12° most common)
• 3,600 – 10,000 rpm
• Turned with induction motor operating on the
principle of electromagnetic induction
• Effective focal spot sizes of .6 mm (large),
.1 mm - .3 mm (small)
Anode Warm Up Procedure - Done to decrease
possible damage to anode from heat.
Typical Warm-Up Procedure
80 kVp X 100 mA X 1 sec
80 kVp X 100 mA X 2 sec
80 kVp X 100 mA X 3 Sec.
Space Charge Compensation
Space Charge Effect – The tendency to resist
further emission of electrons during thermionic
- Occurs as a result of repelling effects of
space charge around filament
- Space charge has definite size that depends
on: 1) Size of filament wire 2) Applied mA
Space Charge Limited Region – When an increase
in mA automatically occurs with an increased
Saturation Current – That point where an increase
in kVp will not automatically increase mA
Space Charge Compensator – A device in the
filament circuit that automatically lowers filament
current as kVp is increased
- allows independent control of kVp & mA
Factors Affecting Tube Life
Over heating of the filament & anode
Leads to:
• Vaporized tungsten deposited on the glass
envelope - May cause arcing (sparkover)
in the tube which is most common cause of
tube failure
• Increased filtration
• Overheating the filament may also cause
thinning and eventual burnout (breaking) of
the filament
• Overheating in filament is minimized by
filament booster Circuit
- Limits filament current until exposure
is made
• Overheating the anode may lead to a cracked
or pitted anode, damage to rotor bearings
Anode Factors Affecting Tube Life
Life of the anode and the ability to use high mAs
and kVp is related to the ability of anode to
accumulate, store and discharge heat.
Extension of tube life involves paying attention to:
1) Tube rating chart - A chart indicating the
maximum safe exposure time for any selected
kVp/mA combination for a single exposure.
- Assumes use of a cold anode
2) Anode Thermal Capacity – Amount of heat that
can be safely accumulated by an anode.
3) Anode Cooling Curve – A chart that indicates
the rate of heat dissipation from the anode
over a specified time period.
Tube Rating Chart
Ignoring the tube rating chart may lead to:
• Melted or cracked anode
• Vaporized tungsten on glass envelope
• Pitting of the focal track/damage to rotor bearings
Reading the tube rating chart:
•Exposures above and to the right of the slanted
line are not ok.
•Exposures below and to the left of the line are ok.
Tube rating charts are effected by:
• Type of rectification
• Size of tube focus
• Tube design
• Cold Vs hot tube
• Type of power supply
• Application
• Anode rotational speed
Anode Thermal Capacity
Thermal capacity is measured in heat units (HU).
H.U. –
single phase = kVp X mA X time
3 Phase, 6 pulse = kVp X mA X time X 1.35
3 phase, 6 pulse = kVp X mA X time X 1.41
high frequency = kVp X mA X time x 1.45
An increase in kVp and decrease in mAs (all
other factors constant) will decrease heat units.
Typical thermal capacity (general diagnostic tube)
70,000 – 400,000 H.U. (angiography, CT tubes
have much higher capacities)
Increasing mass, surface area or rotational speeds
of the anode will increase thermal capacity &
short exposure ratings.
Pitting – Occurs due to repeated minor overloading
Cracking – Occurs due to overloading a cold tube
Tube Housing Factors Effecting
Thermal Dissipation
Thermal dissipation – Cooling that results from
the transfer of heat to:
1) Oil (surrounds glass envelope)
- some tubes equipped with forced oil
2) Tube Housing
3) Housing Fan – dissipates heat from tube
Practical Steps in Extending Tube Life
• Decrease boost and hold time
- Extends filament & bearings life
- Decreases tungsten deposits on glass
• Never exceed instantaneous tube ratings
- Avoids cracked or pitted anode
• Use the correct tube rating and cooling charts
• Use an anode warm-up procedure
- Avoids cracked anode
• Follow exposure rating charts and anode
cooling charts to extend filament and
anode life span