X-Ray Beam Notes

X-Ray Tube Construction and Function

  • X-ray Tube Housing:
    • Made of metal or glass envelope.
    • Contains a negatively charged electrode (cathode) and a positively charged electrode (anode).

Cathode (Negative Electrode)

  • Contains filament and focusing cup.
  • Filament:
    • Source of electrons.
    • Filament current is typically 3-5A and 10V.
    • Thermionic Emission: Coiled tungsten wire.
    • Dual focus (large and small) for varying spatial resolution needs.
    • Creates thermionic emission, leading to the space charge effect.
  • Focusing Cup:
    • Negatively charged.
    • Focuses electrons due to the space charge effect.

Anode (Positive Electrode)

  • Contains molybdenum, copper, tungsten, and graphite for thermal and electrical conductivity.
  • Focal Track:
    • Usually Tungsten (90%) and Rhenium (10%).
    • Tungsten:
      • High melting point (34003400 degrees Celsius).
      • Efficient x-ray production (high Z).
    • Rotating or stationary.
  • Rotating Anode:
    • Rotates at 3k-10k rpm.
    • Requires a stator (electric motor that turns the rotor) and rotor (attached to the anode).
    • Copper ball bearings allow smooth rotation.
  • Target:
    • Decelerates and stops electrons.
    • Typically angled 5-20 degrees.
    • Energy converted to heat (99%) and x-rays.
    • Bremsstrahlung and characteristic interactions occur here.

Heat Dissipation

  • Most anode interactions produce heat which is transferred to the envelope, then to insulating oil surrounding the tube.
  • The tube may also have a fan.
  • Rotating anodes dissipate heat better than stationary anodes.
  • Stationary anodes are used in areas requiring small techniques.

Tube Housing

  • Made of glass or metal evacuated envelopes.
  • Metal is more common; glass may have tungsten evaporation and sedimentation.
  • Metal also better reduces off-focus radiation from leaving the tube.
  • Window is the only opening for the primary beam.
  • Functions:
    • Insulation from electrical shock.
    • Heat dissipation through oil insulation.
    • Lead-lined to reduce leakage radiation (leakage must be <100mR/hr at 1m from the source).
    • Provides electrical current through high voltage cables entering the top.

Atomic Structure

  • Protons, Neutrons, Electrons.
  • Atomic number: number of protons.
  • Atomic Mass: protons + neutrons.
  • Energy levels: k, l, m, n (1,2,3,4).
  • 2,8,18,322, 8, 18, 32 electrons respectively.
  • 2n22n^2: formula to determine the number of electrons per shell.
  • Energy Sublevels equal to the number of levels.
  • Sublevels have orbitals that hold the electrons (s,p,d,f,g).

X-Ray Exposure

  • Two switches on the console, sometimes combined as deadman switches.
  • Cathode Side:
    1. High negative charge strongly repels electrons.
    2. Electrons stream toward the anode (tube current).
  • Anode Side:
    1. High positive charge strongly attracts electrons in the tube current.
    2. Electrons strike the anode.
    3. X-rays and heat are produced.

X-Ray Spectrum

  • Polyenergetic beam, mainly produced by Bremsstrahlung interactions.

Potential Difference

  • The less the voltage ripple to the anode, the greater the efficiency.
  • Ripple describes the variability in voltage waveform.
  • Controlled by the generator type:
    • Single phase half rectified.
    • Single phase full rectified.
    • 3 phase 6 pulse.
    • 3 phase 12 pulse.
    • High-frequency generator.

X-Ray Beam: Quantity and Quality

  • Controlled by the technologist's settings on the control panel.
  • Quantity: Number of x-ray photons in the primary beam.
  • Quality: Penetrating power of the x-ray beam.
  • Prime Exposure Factors:
    • Kilovoltage (kVp):
      • Potential difference.
      • Determines the speed at which electrons move from cathode to anode.
      • Higher kVp helps overcome space charge effect.
      • Increases repulsion from cathode and attraction toward anode.
      • Determines quality (penetrability) and has a slight effect on quantity due to increased efficiency.
    • Milliamperage (mA):
      • Measures tube current.
      • Number of electrons per unit of time traveling from cathode to anode.
      • Changing mA stations changes tube current.
      • Determines QUANTITY.
      • mA increase results in a proportional photon production increase.
    • Exposure Time:
      • Length of time tube current is allowed to flow and produce x-rays.
      • Directly related to quantity.
    • mAs: mA × s=mAsmA \,\times\, s = mAs.
      • Determines QUANTITY.
      • Convert seconds to milliseconds.

Milliamperage and Time

  • Milliamperage×time=mAsMilliamperage \times time = mAs
  • 200 mA×0.25 s=50 mAs200 \,mA \times 0.25 \,s = 50 \,mAs
  • 400 mA×0.25 s=100 mAs400 \,mA \times 0.25 \,s = 100 \,mAs
  • 200 mA×0.50 s=100 mAs200 \,mA \times 0.50 \,s = 100 \,mAs
  • Changing the mA or time will change the quantity of x-rays produced.

Quality Control

  • Most tests are done with a dosimeter.
  • Exposure timer accuracy: Spinning top test device.
    • Variability allowed: +/−5%+/-5\% for times >10ms, +/−10%+/- 10\% for times <10ms.
  • Radiation output measurements: Reproducibility.
    • Checks consistency of radiation output with repeated exposure factors.
    • Must be +/−5%+/- 5\%.
  • mAs reciprocity:
    • Verifies consistent exposure when mA and time are adjusted keeping mAs the same.
    • Must be +/−10%+/- 10\%.
  • mA and time linearity:
    • Verifies proportional changes in mA or time have the expected effect on density/intensity.
    • Must be +/−10%+/- 10\%.
  • Half Value Layer (HVL):
    • Different kVp’s have different penetrability; therefore, different HVL.
    • By determining the HVL, you can see the ACTUAL beam kVp.
    • Penetrability usually changes due to a build-up of tungsten in the envelope, hardening the beam.

Line Focus Principle

  • Relationship between actual and effective focal spot.
  • Actual focal spot: area on anode target exposed to tube current electrons.
  • Effective focal spot: focal spot size as measured directly under the anode target.
  • The smaller the anode angle, the smaller the effective focal spot size.
  • Large focal spot dissipates heat better.
  • Small focal spot provides improved spatial resolution.

Anode Heel Effect

  • X-rays are more intense on the cathode side of the x-ray tube.
  • The intensity of the x-rays decreases toward the anode.
  • Some photons traveling toward the anode are absorbed by the anode itself.
  • Intensity may differ by up to 45%.
  • Place the thicker anatomic area under the cathode end for more even exposure to the image receptor.

Beam Filtration

  • Aluminum filtration is added to the x-ray beam to absorb low-energy photons.
  • Total Filtration:
    • Inherent: Permanent part of the path of photons (envelope, oil, window).
    • Added: Usually aluminum.
  • Reduces patient exposure.
  • Government requires machines operating above 70kVp must have 2.5mm aluminum or equivalent.
  • Increasing filtration increases quality but decreases quantity.

Compensating Filtration

  • Added to the primary beam to alter its intensity.
  • Types: Wedge filter, Trough filter.
  • Used to image non-uniform anatomic areas.
  • The thicker part of the filter is lined up with the thinner part, allowing for more even exposure to the image receptor.

Heat Units

  • Energy is neither created nor destroyed.
  • During x-ray production, most of the electron's kinetic energy is converted to heat and can damage the x-ray tube.
  • Heat units (HU) = mA×time×kVp×generator factormA \times time \times kVp \times generator\, factor.

Avoiding Tube Damage

  • Warm-up the tube according to the manufacturer’s specifications, especially if it has not been energized for 2 hours or more.
  • Avoid excessive heat unit generation by not repeatedly using exposure techniques near an x-ray tube’s limit.
  • Do not hold down the rotor button without making an exposure.
  • Use lower tube currents with longer exposure times when possible to minimize wear on the filament.
  • Do not move the tube while it is energized to prevent damage to the anode and anode stem due to torque.
  • If the rotor makes noticeable noise, stop using the tube until it has been inspected by qualified service personnel.

Electron Target Interactions

  • Interactions of electrons with the anode target result in kinetic energy being converted to other forms of energy.
  • KE=12mv2KE = {1 \over 2} mv^2.
  • If kVp is increased, both quality and quantity are increased.

Electron Target Interactions

  • The kVp selected determines the maximum energy the electrons will have.
    • Ex. 70 kVp = maximum electron energy 70 keV.
  • Electrons traveling from the cathode to the anode are known as the tube current.
  • The kinetic energy of the tube current is transferred to the anode when the electrons clash with the target.
  • As these interactions occur, the electrons slow until they are nearly still.
  • Electrons interact with orbital electrons (characteristic) or the nuclear field of the target atom (Bremsstrahlung).
  • These interactions cause the conversion of KE to electromagnetic energy (x-rays and thermal).

Characteristic Radiation Production

  • Produced by an interaction between an electron from the x-ray current with a K-shell electron in the target.
    1. Incoming electron collides with K shell electron.
    2. If the incoming electron has greater energy than the K shell binding energy, the K shell electron may be ejected.
    3. If an electron is ejected, a void is left in the K shell.
    4. An outer shell electron (from any shell) fills the void, leaving a void in its respective shell.
    5. Each transition of an outer shell electron to a lower shell electron results in the release of an x-ray photon.
    6. The energy of the photon released is equal to the difference between the binding energy of the vacated electron position and that of the electron coming to fill the position.
    7. K shell x-rays are produced from this type of interaction.
  • L, M, N, etc. x-rays from outer shell characteristic interactions occur but are too low in energy to be useful in diagnostics.

Calculating Energy of Characteristic Photons

  • K shell binding energy−binding energy of electron filling the vacancy=photon energyK \, shell \, binding \, energy - binding \, energy \, of \, electron \, filling \, the \, vacancy = photon \, energy
  • Tungsten (W) is element 74.
  • Shell binding energies (keV) for Tungsten:
    • K-shell: 69
    • L-shell: 12
    • M-shell: 3
    • N-shell: 0.6
    • O-shell: 0.1

Bremsstrahlung Radiation

  • A moving electron from the tube current interacts with the electrical field of the target atom nucleus.
  • This attraction slows the electron’s motion and results in a change of direction and the transfer of KE to electromagnetic energy (x-ray photon).
  • Reaction may cause electron to lose all, none, or some of its energy, resulting in photons of unpredictable energies.
  • The closer to the nucleus the incident, and the greater the KE energy loss, the greater the energy of the produced x-ray.
  • Most x-rays are Bremsstrahlung!
  • The electron beam of the tube current must have at least 70 keV energy to eject a K shell interaction creating a useful characteristic x-ray.
  • Bremsstrahlung radiation can be produced at any energy; at 60 kVp, the beam will be 100% bremsstrahlung; at 100 kVp, the beam will be 15% characteristic.

Anode Heat

  • Electron stream interactions with outer shell electrons of the target that do not have enough energy to dislodge the electron excites it.
  • The excited electron raises to a higher energy level, then quickly drops back down, releasing infrared radiation.
  • Most anode heat is caused by these interactions. 99% of KE from the cathode is converted to heat; 1% creates the x-rays.
  • Heat production is directly proportional to current and kVp.
  • The efficiency of x-ray production is independent of current but directly proportional to kVp.
  • At 60 kVp 0.5% electrons converted to x rays
  • At 100 kVp 1% is converted to x rays

X-Ray Emission Spectrum

  • A spectrum is a range of energies.
  • A discrete spectrum contains only specific values.
  • A continuous spectrum contains all possible values.
  • The x-ray emission spectrum demonstrates the relative number of x-rays of each possible energy.
  • Some technologies are available to absorb energies in a beam and create an emission spectrum.
  • The spectrum demonstrates the quality of the image.
  • Changes in mA, kVp, and filtration will alter the emission spectrum.

Characteristic X-Ray Spectrum

  • A spectrum demonstrating discrete energies characteristic of the differences between electron binding energies of a particular element.
  • A characteristic x-ray spectrum can have 1 of 15 different energies.
  • Vertical lines represent different energies of photons created by different shells electrons dropping into the K shell.

X-Ray Emission Spectrum

  • The range of energies in an x-ray beam.
  • The shape of the spectrum is always similar.
  • The position along the energy axis changes according to the quality of the beam.
  • The size and shape of the spectrum can be affected by:
    • Changing current: change the amplitude.
    • Change of voltage: change amplitude.
    • Added filtration: change amplitude.
    • Target material: change amplitude and characteristic line position.
    • Change voltage waveform: change amplitude.

Effect of Added Filtration on the X-Ray Emission Spectrum

  • Adding filtration reduces beam intensity and increases the beam’s average energy.
  • Filtration absorbs more low-energy x-rays.
  • This reduces the bremsstrahlung x-ray emission spectrum more on the left than the right.
  • May be called “hardening” the beam.
  • The characteristic spectrum is not affected.

Effect of Changing the Target Material on the Emission Spectrum

  • The target material affects the quality and quantity of x-rays.
  • Target material with higher atomic numbers increases the efficiency of bremsstrahlung radiation production.
  • Moves the characteristic spectrum to the right due to a higher binding energy in the K shell electrons.

Photon Emission Spectrum and The 15% Rule

  • The effects of changing the emission spectrum describe the reasoning for technique calculations.
  • A 15% increase in kVp results in an optical density change equal to doubling the mAs (to double the output intensity a 40% increase would be necessary)
  • 15% doubles OD because it increases the number of photons AND increases penetration allowing for more to reach the IR

Summary

  • Thermionic emission from the cathode occurs during rotoring.
  • The anode has a positive charge, attracting the electrons from the cathode when kVp is applied during exposure.
  • The tube is responsible for dissipating heat, filtering, and containing radiation.
  • Most photons are produced in Bremsstrahlung interactions (100% under 70kVp).
  • Manipulation of mA, kVp, and time adjust beam quantity and quality.
  • Tube care and the understanding of heat units are essential to appropriate x-ray operation.