Diagnostic Radiology IV

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Diagnostic Radiology IV

• Factors affecting x-ray emission
• Power ratings and heat loading

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Factors affecting x-ray emission

• Output of an x-ray tube described by the terms
  quality, quantity and exposure
• Quality describes penetrability of an x-ray beam
• Quantity refers to the number of photons
  comprising the beam
• Exposure is nearly proportional to the energy
  fluence of the x-ray beam and therefore has
  quality and quantity associated characteristics

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Factors (cont.)

• X-ray production efficiency, exposure, quality
  and quantity are determined by
• X-ray tube target material
• Voltage
• Current
• Exposure time
• Beam filtration
• Generator waveform

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Target (anode) material

• Affects efficiency of bremsstrahlung radiation
  production
• Output exposure roughly proportional to atomic
  number
• Energies of characteristic x-rays depend on
  target material
• Target material affects quantity of
  bremsstrahlung radiation and the quality of
  characteristic radiation

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Tube voltage (kVp)

• Determines the maximum energy in the
  bremsstrahlung spectrum and affects the quality
  of the output spectrum
• Efficiency of x-ray production is directly
  related to tube voltage
• Exposure approximately proportional to the square
  of the kVp in the diagnostic range

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Tube voltage (cont.)

• Changes in kVp must be compensated by
  corresponding changes in mAs to maintain the same
  exposure
• Additional consideration of technique adjustment
  concerns the x-ray attenuation characteristics of
  the patient
• To achieve equal transmitted exposure through a
  typical patient, the mAs varies with the fifth
  power of the kVp ratio

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Tube current (mA)

• Tube current is equal to the number of electrons
  flowing from the cathode to the anode per unit
  time
• Exposure of the beam for a given kVp and
  filtration is proportional to the tube current

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Exposure time

• Exposure time is the duration of x-ray production
• Quantity of x-rays is directly proportional to
  the product of the tube current and exposure time
  (mAs)

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Beam filtration

• Beam filtration modifies the quantity and quality
  of the x-ray beam by selectively removing
  low-energy photons in the spectrum
• This reduces the photon number (quantity) and
  shifts the average energy to higher values,
  increasing the quality

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Generator waveform

• Generator waveform affects the quality of the
  emitted x-ray spectrum
• For the same kVp, a single-phase generator
  provides a lower average potential difference
  than a three-phase or high-frequency generator
• Both the quantity of x-rays produced and the
  quality of the x-ray spectrum are affected

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Summary

• X-ray quantity is approximately proportional to
• X-ray quality depends on kVp, generator waveform,
  and tube filtration
• Exposure depends on both quality and quantity
• Compensation for changes in kVp with radiographic
  techniques requires adjustments of mAs on the
  order of the fifth power of the kVp ratio

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Power rating

• Describes the energy per unit time that can be
  supplied (generator) or received (tube)
• Power rating in kilowatts (kW) is the average
  power delivered by the maximum tube current for
  100 kVp and 0.1-second exposure time

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Heat loading

• The heat unit (HU) is used to express the energy
  deposition on and dissipation from the anode of
  an x-ray tube

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Heat loading (cont.)

• HU underestimates energy deposition for
  three-phase, high-frequency or constant potential
  generators
• Multiplicative factor of 1.35 to 1.40 compensates
  for this difference
• For fluoroscopy

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Exposure rating charts

• Used to determine operational limits of the x-ray
  tube for single and multiple exposures and
  permissible heat load of the anode and the tube
  housing
• Specific to a particular x-ray tube and must not
  be used for other tubes

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Single-exposure rating chart

• Provides information on allowed combinations of
  kVp, mA, and exposure time for a particular x-ray
  tube, focal spot size, anode rotation speed, and
  generator type (no accumulated heat on the anode)
• Family of charts for specific focal spot size and
  anode rotation speed

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0.3 mm focal spot, 10 kW power, 3000 RPM rotor
speed
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0.3 mm focal spot, 10 kW power, 10,000 RPM rotor
speed
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1.2 mm focal spot, 120 kW power, 3000 RPM rotor
speed
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1.2 mm focal spot, 120 kW power, 10,000 RPM rotor
speed
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Use of rating charts

• Find the intersection of the requested kVp and
  exposure time
• Determine the corresponding mA (interpolating
  between adjacent curves, if necessary). This is
  the maximum mA allowed by the tube focal spot
• Compare the desired mA to the maximum mA allowed.
  If the desired mA is larger, the exposure is not
  allowed.
• For mA versus time plots with various kVp curves,
  the rules are the same but with an exchange of
  kVp and mA labels

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Anode heat input/cooling chart

• Shows the remaining heat load of the anode versus
  time as the anode cools
• Maximum anode heat load is the upper value on the
  y-axis of the chart
• After a series of exposures, total heat load
  accumulated on the anode is calculated as the sum
  of the HU incident per exposure
• If it is necessary to wait before reusing the
  tube in order to avoid anode damage, the cooling
  chart specifies how long to wait

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Anode heat chart (cont.)

• Same chart shows heat input curves that
  correspond to to continuous heat input resulting
  from fluoroscopy operation
• Curves initially rise very fast but reach a
  plateau
• Rate of heat energy input into anode equals rate
  of heat energy dissipation by radiative emission
• Useful for determining amount of accumulated heat
  on anode after a given amount of fluoroscopy time

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Housing cooling chart

• Heat generated in the anode eventually transfers
  to the tube housing
• Chart is used in much the same way as the anode
  cooling chart
• Housing heat load typically exceeds that of the
  anode

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