Lecture 20 Photon Interactions with matter pt 2

What is Compton scatter?

Scattered radiation that has sufficient energy to exit the patient and degrade the image, posing an exposure hazard to radiology personnel. (Compton is bad, bad, bad!)

What happens during a Compton interaction?

Incident photon interacts with an outer shell electron and ejects it; the photon loses energy equal to the binding energy of the electron.

What are the results of a Compton interaction?

1. Recoil (Compton) electron may cause further interactions. 2. Scattered photon may cause further Compton interactions, decreases contrast resolution, and adds to occupational dose. 3. Atom is ionized.

How does photon energy affect Compton interactions?

As photon energy increases, the chance of any interaction decreases, including Compton, but higher energy Compton photons have higher energy and scatter forward, reaching the image receptor.

What is Pair Production?

An interaction that occurs with high-energy photons above 1 MeV.

Describe the process of Pair Production.

1. Incident photon interacts with the electromagnetic field around the target nucleus and is absorbed. 2. Nucleus temporarily jumps to a higher energy state and releases a positron and an electron. 3. The positron interacts with an electron in an annihilation event. 4. Two photons are released, each with half the energy of the original photon, traveling 180° from each other.

What is a positron?

A particle the same size as an electron but with a positive charge.

At what photon energy does photodisintegration occur?

At photon energies greater than 10 MeV.

Describe the process of photodisintegration.

1. Incident photon is absorbed by the nucleus. 2. The nucleus jumps to a higher energy state. 3. The nucleus releases a nuclear fragment.

What is the result of photodisintegration?

It has no influence in radiography but changes the atomic number of the target atom.

What are the characteristics of the photoelectric effect?

Inner shell electron interaction; contributes to image contrast; photon energies just above the binding energy of inner shell electrons are most desirable; higher atomic number of material increases probability of photoelectric interaction.

What are the characteristics of Compton interactions?

Outer shell electron interaction; contributes to occupational dose; results in poor image quality and decreased contrast resolution; higher kVp causes more scatter radiation.

Why do higher kVp levels cause more scatter radiation?

Because as kVp increases, photon energy increases, producing more forward-scattered photons that reach the image receptor.

How does kVp affect photoelectric and Compton interactions?

As kVp increases, the probability of both interactions decreases and more photons transmit through the patient.

What is the relationship between influencing factors and interactions?

Photoelectric effect: kVp (inverse above K-shell binding energy), Atomic number (directly), Tissue density (directly), Tissue thickness (directly). Compton effect: kVp (directly), Atomic number (no effect), Tissue density (directly), Tissue thickness (directly).