ch 9

• Quantity/Intensity

  • Refers to the number of X-ray photons in the beam.

  • Also known as beam intensity.

  • Measured in coulombs per kilogram or gray (for air kerma).

  • Directly proportional to the mass of X-ray photons produced.

• Average Energy:

  • Not measured by quantity; relates to quality.

Measurement Definitions

• Air Kerma (K):

  • K: Kinetic energy

  • R: Released

  • M: Matter

  • Measures the number of ion pairs produced in air from X-ray photons.

• Ionization in Air:

  • More X-ray photons = more ionization.

Effects on Quantity

1. Milliampere (mA):

   • Tube current affecting the flow of electrons.

   • Increase mA -> More electrons -> Increased X-ray quantity (intensity).

2. Exposure Time:

   • Longer exposure leads to more X-ray photons produced.

   • Increase exposure time -> Increased intensity.

3. Kilovoltage Peak (kVp):

   • Voltage difference between cathode and anode in the X-ray tube.

   • Higher kVp increases both quantity and quality of X-ray photons.

   • Low kVp results in low quantity (low intensity).

   • Relationship: doubling kVp increases intensity by a factor of about four (not directly proportional).

4. Distance:

   • Inversely related to beam quantity: Increase distance -> Decrease intensity (in accordance with the inverse square law).

   • Closer distance = high intensity.

5. Filtration:

   • Removes low-energy photons (reducing quantity).

   • Filters improve image quality but decrease the number of X-rays used.

   • Increases average energy of the beam but decreases overall quantity.

Summary of Relationships

• Directly Proportional Relationships:

  • Increasing mA, exposure time, or kVp = Increased X-ray intensity.

• Inverse Relationships:

  • Increasing distance or filtration = Decreased X-ray intensity.

Half Value Layer (HVL)

• Refers to the thickness of a material required to reduce the intensity of the X-ray beam by half.

• Important for understanding how much of an absorber is needed to affect the beam intensity.

Filtration Definitions

• Inherent Filtration:

  • Built into the X-ray machine’s design (e.g., glass or metal housing).

• Added Filtration:

  • Additional filters that can be placed in the path of the X-ray beam to improve quality.

• Compensating Filters:

  • Used when parts of the body vary significantly in thickness; ensures uniformity in exposure.