X-ray Spectrum Quantity

X-ray Spectrum Quantity

• X-ray spectrum quantity refers to the number of photons within the primary X-ray beam, measured in milliampere-seconds (mAs).
• The intensity of the beam is defined as the rate of flow of energy per unit area perpendicular to the beam, also known as energy fluence rate, with units of W.mm-2 or J·mm2·s–1.
• The intensity of the beam decreases by the inverse square law.
• Factors influencing X-ray beam quantity:
  • Tube voltage (kV): beam quantity is approximately proportional to the square of the tube potential.
  • Generator type/voltage waveform: reducing ripple increases beam quantity.
  • Beam filtration: increasing filtration reduces beam quantity.
  • Current (mA): beam quantity is directly proportional to current.
  • Exposure time (seconds): beam quantity is directly proportional to exposure time.
  • Anode material: beam quantity is directly proportional to the atomic number (Z) of the anode material.

X-ray Beam Quality

• The quality of an X-ray beam refers to its penetrating ability.
• The penetrating ability of an X-ray beam depends on the energy of the photons.
• The effective energy of the X-ray spectrum is the energy that corresponds to the penetration of a photon energy between the minimum and maximum energies of the spectrum.
• Adding filtration increases the penetration (HVL) of an X-ray beam by removing low-energy photons.
• HVL values are used to judge the adequacy of filtration.
• Regulations specify minimum acceptable HVL values for different KV values.

Penetration with Scatter

• In situations where Compton interactions are significant, some of the radiation removed from the primary beam by Compton interactions is scattered in the forward direction, creating the appearance of increased penetration.
• The effective penetration (Pe) is given by: Pe = P x S, where S is the scatter factor.
• Factors contributing to the amount of radiation scattered in the forward direction:
  • X-ray beam area, or field size.
  • Body section thickness.
  • KV.

Penetration Values

• HVL values provide useful information about the penetration of a specific radiation in a specific material.
• When an HVL value is known, the penetration through other thicknesses can be easily determined.
• HVL values for different materials are used in diagnostic imaging.

Photon Range

• The range of individual photons is the distance traveled before they are absorbed or scattered.
• The range of photons is exponential, with each thickness of material attenuating the same fraction of photons entering it.
• The average range of a group of photons is inversely related to the attenuation rate.
• The average range is equal to the reciprocal of the attenuation coefficient (µ).

Half Value Layer (HVL)

• HVL is the most frequently used quantity for describing the penetrating ability of specific radiations and the penetration through specific objects.
• HVL is the thickness of material penetrated by one half of the radiation, expressed in units of distance (mm or cm).
• HVL is related to, but not the same as, average photon range.
• HVL is inversely proportional to the attenuation coefficient.