Physics - Chapter 7

X-ray interactions with matter

What are 2 other names for Classical Interactions?

Coherent Scattering

Thomson Scattering

The incident x-ray photon interacts with an orbital electron of a tissue atom and changes direction

Classical / Coherent / Thomson Scatter

Which interaction has the lowest energy ~10 keV

Classical / Coherent / Thomson

True or False: Classical interactions involve ionization of the atom

False - they do not, but they do contribute to patient dose (slightly)

Which interaction involves moderate energy photos ~ 20-40 keV?

Compton Scattering

Which interaction involves an incident x-ray photon entering a tissue atom, interacting with an orbital electron (usually a middle or outer shell electron) and removes it from its shell, losing about 1/3 of its energy in the process and is deflected in a new direction?

Compton Scatter

Name the 3 things Compton Scattering does when it interacts with matter?

1) it ionizes the atom, making it unstable; creates a characteristic cascade (outer shell electrons filling inner shell vacancies and emitting x-ray photons) called secondary photons that only contribute to patient dose.

2) The ejected electron called Compton Electron (or secondary electron) leaves the atom with enough energy to go through interactions of its own in adjacent atoms (depends on the type of energy it has and the type of atom it interacts with).

3) The incident electron is now a Compton scattered photon ejected in a new direction that has enough energy to go through other interactions in the tissues or exit the patient and interact with the IR creating “image fog.”

What is a problem with Compton scatter interacting with the IR?

It doesn’t follow its original path through the body and strikes the IR in the wrong area thus contributing no useful information to the image.

Which type of interaction is responsible for most of the scatter that fogs the image?

Compton Scattering

True or False: Compton Scattering only occurs in soft tissue?

False - Soft tissue and bone

Which type of scattering is the major source of occupational exposure?

Compton Scattering

A Compton scatter photon still retains about how much of its original energy?

2/3

What energy range do photoelectric interactions contain?

20-120 kVp

The incident x-ray photon interacts with the inner-shell electron of a tissue atom and removes it from orbit and in the process is totally absorbed.

Photoelectric Interactions

The energy transfer between the incident photon and inner-shell electrons is equal to

the incident photon energy minus the binding energy of the orbital electron

True or False: The energy of the photoelectron is nearly equal to that of the incident x-ray photon because the binding energy of the soft tissue atom is very low

True

True or False: In bone, the energy of the photoelectron is more because the orbital electron-binding energy of bone atoms is greater and the incident xray photon has to expend more energy to remove it.

False: The energy of the photoelectron is less

True or False: Photo electric events result in the total absorption of the incident photon

True

What 3 things does the probability of photoelectric interactions depend on?

The energy of the incident photons

The atomic number of the tissue atoms it interacts with

the incident x-ray photon energy, which must be greater than or equal to the inner-shell binding energy of the tissue atoms involved

The probability of photoelectric interactions is directly proportional to

the 3rd power of the atomic number of the absorber

Inversely proportional to the 3rd power of the x-ray energy

True or False: The higher the atomic number of the tissue atom, the greater the number of photoelectric events

True

Occurs only with very high-energy photons of 1.02 MeV or greater; interacts with the nucleus of a tissue atom

Pair Production

What two particles make up the pair production

Positron - positively charged electron

Electron / negatron

Which type of interaction causes an annihilation event?

Pair Production

Photons with extremely high energies of more than 10 MeV may strike the nucleus of the atom and make it unstable; does not occur in radiography

Photodisintegration

The difference between the x-ray photons that are absorbed photoelectrically and those that penetrate the body

Differential absorption

True or False: Different body structures absorb x-ray photons to different extents

True

Refers to those x-ray photons that pass through the body and reach the IR; creates the dark shades of the image

Transmission

Refers to those photons that are attenuated (absorbed) by the body and do not reach the IR; results in the lighter shades of the image

Absorption

What does absorption depend on?

The density of body tissues through which the x-ray photons are passing

Body structures that readily absorb x-rays are called

Radiopaque

Less dense structures have a much lower probability of absorption and are said to be

Radiolucent