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Diagnostic Radiology IV Factors affecting x-ray emission Power ratings and heat loading 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

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Diagnostic radiology iv l.jpg

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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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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