Wilhelm Conrad Roentgen used a Crookes-Hittorf tube to produce the first x-rays in 1895. Early x-ray tubes had no shielding and emitted radiation in all directions, posing major hazards. Modern tubes are designed with shielding and safety features to overcome these problems. Key components of x-ray tubes include the cathode, which emits electrons via thermionic emission, the anode target, which converts the electrons' kinetic energy to x-rays, and various designs like rotating anodes to dissipate heat and improve performance.
2. The X-Ray Tube
Development
Wilhelm Conrad
Roentgen used a
Crookes-Hittorf tube
to make the first x-ray
image.
There was no
shielding so x-rays
were emitted in all
directions.
4. The X-Ray Tube Development
Two major hazards plagued
early radiography.
Excessive radiation exposure
Electric Shock
Moderns tubes are designed
to overcome these problems
The modern tube is based on the Coolidge
tube
14. X-Ray Tube
Converts Energy
FROM
electrical energy
To
Heat
> 99% of incident energy
Bad! Ultimately destroys tubes
X-Rays
< 1% of incident energy
*
15. Inside the Glass Insert
Filament
Similar to light bulb
Glows when heated
Target
Large (usually) tungsten block
target filament
16. Requirements to Produce X-Rays
Filament Voltage
High Voltage
+
filamentanode
filament
voltage
source
high
voltage
source
18. Space Charge
Electrons leave filament
filament becomes positive
Negative electrons stay close
Electron cloud surrounds filament
Cloud repels new electrons from filament
Limits electron flow from cathode to anode
+ -
-
-
*
19. X-Ray Tube Principle
Positive (high) voltage applied
to anode relative to filament
electrons accelerate toward anode target
Gain kinetic energy
electrons strike target
electrons’ kinetic energy converted to
heat
x-rays
+
*
20. Cathode
Fillament +
Focussing cup +
Connecting wires
Tungsten filament- 0.2mm
diameter wire is coiled to
spring of 0.2cm diameter and
1 cm length
21. Cathode (filament)
Coil of tungsten wire
Tungsten advantages
high melting point 3370’C
little tendency to vaporize
long life expectancy
Tungsten disadvantages
not as efficient at emitting
electrons as some other
materials
22. Cathode (filament)
1-3% Thorium added for better
emission
Cathode is source of electrons
filament heated by electric current
~ 10 volts
~ 3-5 amps
filament current is not tube current
23.
24. Filament (cont.)
Large Filament normally left on at
low “standby” current
boosted before exposure (prep or first trigger)
With time tungsten from hot filament
vaporizes on glass insert- sunburn
thins the filament
filters the x-ray beam
increases possibility
of arcing
electrons attracted to
glass instead of target
+
25.
26. Focusing Cup
Negatively charged
Focuses electron
stream to target
overcomes tendency of
electrons to spread
because of mutual
repulsion
+
Focusing
Cup
27.
28.
29. Focal Spot
portion of anode struck by electron
stream
Focal spot sizes affects and limits
resolution
+
30. Focal Spots
Most tubes have 2 filaments & thus 2
focal spots
only one used at a time
small focus
improved resolution
large focus
improved heat ratings
Electron beam strikes larger portion of
target
31. Focal Spot Size & Resolution
The larger the focal
spot the more it will
blur a tiny place on
the patient.
32. Focal Spot Size & Heat
The larger the area the
electron beam hits, the
more intense the beam
can be without melting the
target
33. Tube Current (mA)
Rate of electron flow from filament to target
Electrons / second
Measured in milliamperesmilliamperes (mA)
Limited by
filament emission (temperature / filament current)
space charge (see next slide)
+
36. Target Angle
Small
optimizes heat ratings
limits field coverage
+
Large Target Angle
(Small Actual Focal Spot)
+
Small Target Angle
(Large Actual Focal Spot)
• Large
– poorer heat ratings
– better field coverage
40. The Target
Tungsten-Rhenium is used as the target for
the electron beam.
Rhenium is used to increase the surface
properties to minimise the pitting and cracking
of the target
41. The Rotating Anode
The rotating
anode allows the
electron beam to
interact with a
much larger
apparent target
area.
The heat is not
confined to a
small area.
44. Rotating Anode
Advantages
Better heat ratings
Disadvantages
More complex ($)
Rotor drive circuitry
motor windings in housing
bearings in insert
45. Rotating Anode
Larger diameter
Better heat ratings
heavier
requires more support
costly
Materials
usually tungsten
high melting point
good x-ray production
molybdenum (and now Rhodium) for mammography
(sometimes)
low energy characteristic radiation
47. The anode stem
The anode stem is made from
molybdenum(2600’)
It is made appropriately thin as to
minimize the heat conduction towards
the rotor
48. Lubricatio
n
Not oil
Not graphite
Silver is the best available in high vaccum,
bearing wear is negligible
50. Breaking The Rotating Anode
When the anode is spinning at the correct
speed, the exposure can be made.
After the exposure is completed, it slows
by reversing the motor.
This is necessary to avoid excessive wear
and tear of the bearings
52. Beam Intensity
Product of
# photons in beam
energy per photon
Units
Roentgens (R) per unit time
Measure of ionization rate of air
Depends on
kVp
mA
target material
filtration
53. Grid-controlled tubes
Grid used to switch tube on/off
grid is third electrode
relatively small voltage
controls current flow
from cathode to anode
Negative grid voltage repels electrons from filament
Grid much closer to filament than target
Applications
speedy switching
required
+
grid
54.
55. Tube Housing
Shields against leakage
radiation
lead lined
leakage limit
100 mR / hour when tube
operated at maximum continuous
current for its maximum rated
kilovoltage
*
56. Tube Housing (cont.)
Shields against high
voltage
Housing filled with oil
Oil
Vacuum
Insert
59. Dissipation of heat from the target
Even with the anode rotating, some melting
occurs.
The heat must be rapidly dissipated from the
target.
The anode dissipates heat by radiating
towards the glass envelop
Step up transformer to produce 50,000 to 150,000 V
Vaccum avoid ionisation…… See the change
Connecting wires of alloys with same coefficient of expansion as pyrex glass
2mm lead thickness
.
We need mechanisms to dissipate heat n heat resistant materials
.
High voltage applied
Overcome by raising kVp, emission limited, saturation voltage
.
Add something for better emission…. Something more unstable…
Add something for better emission…. Something more unstable…
.
2 filaments in focusing cup
standby” current 5mA…. Automatic filament boosting circuit for exposure……arcing puncture.. so Metal enclosures used now
Negative charge repel and spread
.
Surrounds filament… nickel
Negative charge…. Modify e- trajectories…. 1st equal, 2nd more
.
Target dosent get damaged… next page importance
Look sharpness of vertebrae… L2,3 clear border since x rays focussed here
.
Current flow anode to cathode
Target is angulated
.
.
.
More vertical
Tungsten 2-3mm thick n &gt;1cm dimensions… fixed to copper(conductor)… repeated bombarding caused pitting…
There was need to increase focal spot size… to distribute heat…. Without blurring
Bevelled edge, 3000rpm, exposed every 1/60th second, total area = ht of e- beam* circumference
Rotor on principle of electro-magnetic induction
.
.
Heat dissipation was still a problem
Tungsten rhenium target embedded, back coated carbon black, slits n grooves for expansion
Molybdenum poor conductor high melting 2600’…avoid expansion and jamming rotor. Length minimum…… for heavy anode to decrease inertia
Ant anode pneumonic
.
Speedy switching
.
Protect unnecessary exposure n fogging of film…. High voltage cables
housing also contains oil seal without air that provides more electrical insulation and a thermal Convection to metal shield, compact tubes, microswitch..Never hold the tube during an exposure.Never use the cables or terminals as handles. Some housing have a fan for cooling.
Many e- hit other areas on the anode n thus different areas on the patient
.
.
.
Oil convection and expand …. So expansion bellows….. Heavy large anode best… but support insufficient… so add more support
2 ball bearings,, larger better anode…. Metal protective casing