Week 4: Production, Filtration and Prime Factors Flashcards
(5 cards)
Bremsstrahlung Interactions
Occurs when a negative electron is attracted to the positive tungsten nucleus in the atom. The incident electron does not have enough energy to penetrate the nucleus so it slows down as it passes by and the direction is changes.The energy from the braking/slowing is emitted as an x-ray. The energy of a brems photon is exactly the difference between the incident energy of the election and its exit energy. Majority of diagnostic beam is Brems interactions.
High Energy Brems
Some electrons pass very near the nucleus or interact with it directly, these very strong interactions causes the electron to either interact with the nucleus directly and lose all its energy or brake nearly completely losing most of its energy. These interactions give off an x-ray with an every nearly equal or equal to the total energy of the incident electron. Not very common.
Characteristic Interactions
They only produce x-rays of very specific energies that are characteristic of the target material. Incident electron actually collides with an electron bound around the tungsten atom. The electron receives enough energy to escape its binding around the tungsten nucleus and its ejected. This creates a hole that will immediately be filled by an electron bound in a shell further away from the nucleus. A that electron drops closer to the nucleus of the tungsten an x-ray is emitted that matches the difference between the binding energy of the near and far shell. They will always have an energy equal tot he difference between the binding energy of the outer and inner shells between which they dropped. Only seen above 70kv because that is the energy needed to remove the K shell electron from the tungsten atom.
Filtration
Selectively absorbs low energy photons (don’t contribute to image but increase patient dose). hardens the beam, increases the average energy but the maximum energy is unchanged. Increase in exposure factors required due to the overall exposure decrease but the patients dose overall decreases despite the increase in technical factors.
Half Value Layer Test
To confirm that most of the low energy photons have been removed from the primary beam. The HVL is the amount of absorbing material that is required to reduce the intensity of the beam by half. The goal is to halve the beam intensity but increase its average energy.
