Interactions of Photons in Medical Imaging

The video explains two major ways x-rays interact with tissue: the photoelectric effect and Compton scattering. The photoelectric effect involves a photon being absorbed by an electron, leading to characteristic radiation. Compton scattering involves partial energy transfer, affecting image quality and increasing radiation dose. The likelihood of photoelectric absorption increases with atomic number and decreases with photon energy. Materials like lead are used for shielding due to high atomic numbers. The k-edge concept is crucial for contrast agents like iodine and gadolinium, enhancing x-ray absorption. In nuclear medicine, Compton scattering dominates in low atomic number materials, while the photoelectric effect is prevalent in high atomic number materials.

What happens to a photon during the photoelectric effect?

• It passes through the tissue without interaction.

• It is absorbed by an outer shell electron.

• It is absorbed by an inner shell electron.

• It is scattered with reduced energy.

What is characteristic radiation?

• Radiation emitted when an electron moves to a lower-energy shell.

• Radiation emitted when an electron is ejected from an atom.

• Radiation emitted when a photon is scattered.

• Radiation emitted when a photon passes through tissue.

In Compton scattering, what happens to the photon?

• It transfers part of its energy and continues traveling.

• It is completely absorbed.

• It is absorbed by the nucleus.

• It transfers all its energy to an electron.

Why does Compton scattering decrease image quality?

• It increases the patient's dose.

• It creates scattered photons that interfere with the image.

• It only occurs in high atomic number materials.

• It absorbs all the x-rays.

What increases the likelihood of photoelectric absorption?

• Increasing photon energy

• Decreasing photon energy

• Decreasing atomic number

• Increasing atomic number

Why is lead used for shielding against x-rays?

• It is transparent to x-rays.

• It is lightweight and easy to handle.

• It has a low atomic number.

• It has a high atomic number, increasing photoelectric absorption.

What is the k-edge?

• The energy level where Compton scattering is most likely.

• The energy needed to eject an outer shell electron.

• The energy needed to eject a K-shell electron.

• The energy level where all photons are absorbed.

Why are iodine and gadolinium used as contrast agents?

• They have k-edges that match the x-ray spectrum.

• They are inexpensive and easy to obtain.

• They are transparent to x-rays.

• They have low atomic numbers.

In nuclear medicine, which interaction is dominant in low atomic number materials?

• Characteristic radiation

• Photoelectric effect

• Compton scattering

• Pair production

Which interaction is dominant in materials with high atomic numbers?

• Compton scattering

• Elastic scattering

• Pair production

• Photoelectric effect