chapter 8 lecture

Objectives

• Understand electron target interactions.

• Identify the source and characteristics of electrons.

• Discuss heat, characteristic radiation, and Bremsstrahlung radiation at the target.

• Analyze the X-ray emission spectrum and influencing factors.

Electron Sources

• Electrons originate from the cathode:

  • Boiled off the filament and shot to the target.

• Target Material: Tungsten is primarily used due to its high melting point and atomic properties.

Electron Target Interactions

• Types of Interactions:

  • Heat Production: Majority of energy (99%) from kinetic energy converts to heat at the target.

  • Characteristic Radiation: Points of X-ray production due to electrons interacting with inner shell electrons.

  • Bremsstrahlung Radiation: Produced when electrons slow down near the nucleus of tungsten atoms.

Emission Spectrum

• X-ray Emission Spectrum:

  • Discusses the quality and quantity of X-rays produced based on electron interactions.

  • Factors Affecting Spectrum:

    • kVp (kilovolt peak): Determines the energy and quality.

    • mA (milliamperage): Adjusts the quantity of electrons.

    • Filtration: Cleans up low-energy X-rays that contribute less.

Filtration

• Definition: Attenuates lower energy X-rays; increases average energy of the X-ray beam.

• Notable Function: Reduces patient dose by eliminating ineffective low-energy photons.

Technical Factors Affecting Spectrum

• kVp increases:

  • Stretches the curve, increasing both amplitude and position.

• mA increases:

  • Changes the amplitude (height) of the curve without altering the kVp endpoint.

  • Doubling mA results in doubling the X-ray production.

Characteristic Radiation

• Production Process:

  • An incoming projectile electron knocks out an inner shell electron and causes outer shell electrons to fill this void, emitting an X-ray during the transition.

  • Atomic Binding Energy: The energy difference between the shells generates quality X-rays, mainly through the K shell (K characteristic).

• X-ray Energy Levels:

  • K shell of tungsten has a binding energy of 69 keV.

Bremsstrahlung Radiation

• Definition: Produced when incoming electrons get close to the nucleus, slowing down and changing direction, which leads to energy loss depicted as X-rays.

• Key Characteristics:

  • Bremsstrahlung radiation involves a wider range of energies and is influenced by kVp settings.

X-ray Production Efficiency

• Efficiency Increases: With increasing kVp but always remember 99% of interactions turn to heat instead of X-rays.

• Target Material: Affects characteristic radiation efficiency and intensity.

  • Tungsten vs. Gold: Gold provides high efficiency but has a lower melting point, making tungsten more suitable.

15% Rule

• Adjustment Protocol: An increase of kVp by 15% is similar to doubling the mA concerning X-ray output, leading to less patient exposure.

Summary of Findings

• Interactions:

  • Excitation leads to heat.

  • Characteristic radiation occurs when an outer shell fills an inner shell void.

  • Bremsstrahlung occurs when the projectile electron is influenced by the nucleus.

• Determining Factors in X-ray production and spectrum: Influence comes from adjustments in mA and kVp settings, filtration, and the choice of target material.

  • All interactions relate back to their impact on energy, image quality, and patient dose.