Transcript X-ray tube 2

X-ray Tube -2
Line focus principle
Heal effect
Ratings
Tube failure
Line-Focus Principle
 The Effective Focal
Spot is the beam
projected onto the
patient.
 As the anode angle
decreases, the
effective focal spot
decreases.
 Diagnostic tube target
angles range from 5 to
15°.
Advantages & limitations of LFP
Advantages
 The advantage of Line focus is it
provides the sharpness of the small
focal spot with the heat capacity of
the large large focal spot.
 Smaller target angles will produce
smaller effective focal spots and
sharper images.
limitations
 Area covered by the beam reduces
with target angle
 To cover a 17” the angle must be 12°
 To cover 36” the angle must be 14°
 Anode heel effect (next slide)
Anode Heel Effect
 Due to the absorption of
x-rays by the anode heel
the radiation
intensity on the
cathode side of the xray tube is higher
than the anode side.
 This effect is
increased when the
target angle is
reduced
Influence of Anode heel effect
 The difference in the intensity can
vary by as much as 45%.
 If the center is 100% the anode side
of the beam can as low as 75% and
the cathode as much as 120%.
 The heel effect should be
considered when positioning areas
of the body with different thickness
or density.
 The cathode side should be over the
area of greatest density.
Anode Heel Effect on Focal spot size and
Resolution
 The effective focal spot
size and shape change
across the projected field
 The sharpness of the
image can be dependent
upon which area of the
beam coverage you are
looking at.
 Similar to the shape
distortion when the tube
is not centered.
Off Focal Radiation
 The electrons can rebound
and interact with other
areas of the anode other
than the focal area.
 These interations can
produce x-rays too.
 This is called Off-Focal
Radiation.
Control of Off Focal Radiation
 A diaphragm is placed
between the tube and the
collimator to reduce off
focus rays.
 But the off focus
radiation completely
 The percentage of off
focus radiation increases
with kV
X-ray Tube Rating Charts
 The that provide information
on the safe exposures and
safe operating conditions of
x-ray tubes are called tube
rating charts
 With careful use, the x-ray
tube can provide long
periods of service.
 Inconsiderate or careless
operation can lead to
shortened life or abrupt
failure.
 X-ray tubes are very
expensive. Cost varies from
$2,000 to $20,000.
Types of rating charts
 Radiographic rating chart
 Anode cooling chart
 Housing cooling chart
Radiographic rating chart
 This is the most important of the three charts
 It conveys which radiographic exposures are safe and which
are unsafe
 The chart shows a family of curves for different mA
 The two axes X & Y show scales of Time and kV respectively
 For a given mA, any combination of kVp and time that lies
below the curve is safe
 Any combination that lies above the curve of desired mA is
unsafe
 Modern x-ray systems have a microprocessor control that
does not allow unsafe exposure to be made.
Use of Series of radiographic rating
charts
 Important to use the correct rating chart e.g
 Rating charts for different filament sizes (focal spot sizes)
 For different anode rotation speeds
 For different anode angles
 For the type of high voltage rectification
 Radiographic rating charts can be used to check the proper
operation of microprocessor control protection circuit
Question
 Radiographic examination of the abdomen with a tube that
has a 0.6 mm focal spot and anode rotation of 10,000 rpm
requires technique factors of 95 kVp, 150 mAs.
 Calculate using the correct rating chart, the shortest possible
exposure time for this examination.
Anode cooling chart
 Anode cooling charts contain the information about the
thermal capacity of an anode and its heat dissipation
characteristics.
 It does not depend on the filament size and the speed of
rotation
 Usually the cooling is rapid at first and slows as the anode
cools
 In addition to knowing the maximum heat capacity the chart
is used to determine the length of time required for
complete cooling after any level of heat input.
Anode cooling chart
Anode thermal capacity, HU & Joule
 The maximum amount of heat that can be stored in the
anode without thermal damage
 In x-ray applications, It is measured in Heat Units (HU)
 1HU = 1kVp x 1 mA x 1s (for single phase)
 Actual heat energy is measured in Joules (J)
 1 J = 1 volt x 1 amp x 1s

= 1 kV x 1 mA x 1s (here the kV is the direct or the
RMS voltage)
 For single phase kVp = 1.4 kV (rms)
 For 3 phase or high frequency kVp = kV (rms)
 Therefore for three phase/high frequency,
HU=1.4 x kVp x mA x s
Questions
Radiographic examination of the lateral lumber spine with
a single-phase imaging system requires 98 kVp, 120 mAs.
How many heat units are generated by this exposure?
2. A fluoroscopic examination is performed with a singlephase imaging system at 76 kVp and 1.5 mA for 3.5
minutes. How many heat units are generate?
3. Six sequential skull films are exposed with a three-phase
generator operate at 82 kV, 120 mAs. What is the total heat
generate?
1.
Questions
A particular examination results in delivery of 50,000 HU
to the anoe in a matter of seconds. How long will it take
the anode to cool completely? (use the anode cooling chart
given in slide 16)
2. How much heat energy (in joules) is produce during a
single phase mammographic exposure of 25 kVp, 200
mAS?
1.
Housing Cooling Chart
 The cooling chart for the housing of the x-ray tube has a
similar shape as the anode cooling chart.
 The maximum heat capacity of the housing is in the range of
several million heat units.
 Complete cooling after maximum heat capacity requires
from 1 to 2 hours
X-ray tube failure
 All causes of tube failure relate to the thermal characteristics
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of the tube.
When the temperature of the anode during a single exposure
is excessive, localized melting and pitting occurs.
These surface irregularities lead to variable and reduced
radiation output.
If the melting is severe, the tungsten vaporizes and can plate
the port. This can cause added filtering or interference with
the flow of electrons.
If the temperature of the anode increases to rapidly, the
anode can crack and then become unstable in rotation.
 Maximum radiographic techniques must never be
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applied to a cold anode.,
During long exposures (1 to 3 seconds) the anode may actually
glow like a light bulb.
The heat may cause a failure of the bearing for the anode or a
crack in the glass envelope.
Because of the high heat of the filament, the inside of the glass
envelope. This will tungsten atoms are slowly vaporized and plate
eventually lead to arcing and tube failure.
Continuous high mA radiography will actually lead to the
filament breakage.
Anode faults
a. New
b. Surface
damage
due to
repeated
over load
c. Pitting due
to slow
rotation
C
d
d. Surface
damage
due to
Exceeding
of heat
capacity
Tube Warm-up Procedures
 By warming the anode through a series of exposures and increasing kVp
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settings, the anode will build up heat that is needed to avoid fracture of
the anode.
This process takes a little over one minute put will add to the life of the
tube.
Close shutters of collimator.
Make exposure of 12 mAs @ 70 kVp
Wait 15 seconds
Make exposure of 12 mAs @ 80 kVp
Wait 15 seconds
Make exposure of 12 mAs @ 90 kVp
Tube warm up is now complete.
END
V.G.Wimalasena
Principal
School of radiography