X-ray Tube -2

1
X-ray Tube -2

• Line focus principle
• Heal effect
• Ratings
• Tube failure

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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.

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Advantages limitations of LFP

• Advantages

• limitations

• 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.

• 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)

4
Anode Heel Effect

• Due to the absorption of x-rays by the anode heel
  the radiation intensity on the cathode side of
  the x-ray tube is higher than the anode side.
• This effect is increased when the target angle is
  reduced

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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.

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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.

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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.

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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

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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.

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Types of rating charts

• Radiographic rating chart
• Anode cooling chart
• Housing cooling chart

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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.

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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

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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.

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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.

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Anode cooling chart
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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,
• HU1.4 x kVp x mA x s

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Questions

1. 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
   single-phase 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?

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Questions

1. 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?

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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

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X-ray tube failure

• All causes of tube failure relate to the thermal
  characteristics 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.

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• Maximum radiographic techniques must never be
  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.

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Anode faults

1. New
2. Surface damage due to repeated over load
3. Pitting due to slow rotation
4. Surface damage due to Exceeding of heat capacity

d
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Tube Warm-up Procedures

• By warming the anode through a series of
  exposures and increasing kVp 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 _at_ 70 kVp
• Wait 15 seconds
• Make exposure of 12 mAs _at_ 80 kVp
• Wait 15 seconds
• Make exposure of 12 mAs _at_ 90 kVp
• Tube warm up is now complete.

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END

• V.G.Wimalasena
• Principal
• School of radiography