Chapter 13 Key Terms

Question 1

Annihilation Reaction

Answer 1

Matter being converted back into energy as a result of a positron combining with a negative electron, which creates two photons moving in opposite directions.

Example: An annihilation reaction occurs in PET scans.

Question 2

Attenuation

Answer 2

The reduction in the number of x-ray photons in the beam, and subsequent loss of energy, as the beam passes through matter.

Example: Attenuation is an important factor in determining image quality.

Question 3

Backscatter Radiation

Answer 3

Photons that deflect back toward the source, traveling in the opposite direction of the incident photon.

Example: Backscatter radiation can lead to increased radiation exposure for healthcare workers.

Question 4

Characteristic Cascade

Answer 4

The reaction of electrons dropping into the holes created during a characteristic interaction until there is only a hole in the outer shell.

Example: Characteristic cascade plays a role in the formation of characteristic x-rays.

Question 5

Characteristic Photon

Answer 5

An x-ray photon created by the electron transfers from one shell to another.

Example: Characteristic photons have specific energies related to the atomic structure.

Question 6

Coherent Scatter

Answer 6

An interaction between x-rays and matter characterized by interaction between a very-low-energy x-ray photon and matter causing the electron to vibrate at the same frequency as the incident photon, which then produces a secondary photon with the same energy and wavelength as the incident photon but that travels in a different direction; also called classical scatter or unmodified scatter.

Example: Coherent scatter contributes to image noise in radiography.

Question 7

Compton effect

Answer 7

An interaction between x-rays and matter characterized by an incident x-ray photon interacting with a loosely bound outer-shell electron, resulting in removal of the electron from the shell, which then proceeds in a different direction as a scattered photon; also known as Compton scattering.

Example: The Compton effect is a major contributor to scattered radiation dose.

Question 8

Compton (or recoil) Electron

Answer 8

The dislodged electron resulting from Compton scattering; also called a recoil electron.

Example: Compton electrons can contribute to image degradation in computed tomography.

Question 9

Compton Scattered Photon

Answer 9

The photon that exits the atom in a different direction as a result of Compton scattering.

Example: Compton scattered photons can affect image contrast in radiography.

Question 10

Compton Scattering

Answer 10

An interaction between x-rays and matter characterized by an incident x-ray photon interacting with a loosely bound outer-shell electron, resulting in removal of the electron from the shell, which then proceeds in a different direction as a scattered photon; also known as Compton effect.

Example: Compton scattering is a form of inelastic scattering.

Question 11

Negatron

Answer 11

A negatively charged electron resulting from pair production.

Example: Negatrons are created in positron emission tomography.

Question 12

Pair Production

Answer 12

An interaction between x-rays and matter characterized by the conversion of the energy of an x-ray photon into matter in the form of two electrons.

Example: Pair production is an important process in radiation therapy.

Question 13

Photodisintegration

Answer 13

An interaction between x-rays and matter characterized by the interaction between a high-energy photon and the nucleus. The high-energy photon strikes the nucleus; the nucleus absorbs all the photon’s energy and then emits a nuclear fragment.

Example: Photodisintegration can lead to the creation of unstable isotopes.

Question 14

Photoelectric Absorption

Answer 14

An interaction between x-rays and matter characterized by an incident electron with slightly greater energy than the binding energy of the electrons in the inner shells, ejecting an electron from the inner shell while being absorbed in the reaction, resulting in an ionized atom.

Example: Photoelectric absorption is a major contributor to image contrast in radiography.

Question 15

Photoelectron

Answer 15

An ionized atom with a missing inner-shell electron resulting from ejection of the electron due to photoelectric absorption.

Example: Photoelectrons can contribute to image artifacts in diagnostic imaging.

Question 16

Positron

Answer 16

A positively charged electron resulting from pair production.

Example: Positrons are used in positron emission tomography imaging.

Question 17

Radiation Fog

Answer 17

The result of scattered photons striking the radiographic film and placing a density on the film that is unrelated to the patient’s anatomy.

Example: Radiation fog can obscure diagnostic information on radiographic images.

Question 18

Scattering

Answer 18

The interaction of x-ray photons and matter that causes a change in direction of the photons.

Example: Scattering is a common phenomenon in radiography.

Question 19

Secondary Radiation

Answer 19

A characteristic photon created by the electron transfers from one shell to another but occurs outside the x-ray target.

Example: Secondary radiation can contribute to radiation dose to healthcare workers.