Chapter 9: X-ray Interaction with Matter

What two factors determine how x-rays interact with matter?

Wavelength and energy level

What are the three primary components of an atom that x-rays interact with?

Atoms, electrons, or nuclei

Which two interaction mechanisms are most important in diagnostic radiology?

Compton scattering and the photoelectric effect

What are the two alternative names for coherent scattering?

Classical Scattering and Thomson Scattering

At what energy range does coherent scattering typically occur?

Below approximately $10\,keV$

Describe the wavelength relationship in coherent scattering.

The scattered x-ray has the same wavelength as the incident x-ray ($\lambda = \lambda\prime$)

In coherent scattering, what change occurs to the x-ray's direction?

The x-ray changes direction, but there is no energy transfer or ionization

True or False: Coherent scattering results in ionization of the target atom.

False

In which direction is most coherent scattering directed?

Forward-directed

How does coherent scattering affect image quality at higher energies like $70\,kVp$?

It can introduce slight image noise and reduce image contrast

What is the primary contribution of coherent scattering to diagnostic imaging?

It contributes minimally but can cause general graying or noise

In what energy range does Compton scattering occur?

Throughout the diagnostic range

Which electron shell does an incident x-ray interact with in Compton scattering?

An outer-shell electron

What is the ejected electron in a Compton interaction called?

A Compton electron

How is the energy of a Compton electron calculated?

It is equal to its binding energy plus the kinetic energy with which it leaves the atom

What happens to the energy of the incident x-ray after a Compton interaction?

It continues in a different direction with reduced energy

How much energy does a scattered x-ray retain if it is deflected $180$ degrees?

Approximately two-thirds of its original energy

What is backscatter radiation?

X-rays scattered back towards the direction of the incident beam

Give an example of a radiographic artifact caused by backscatter radiation.

A cassette-hinge image on a radiograph

How does Compton scattering affect image contrast?

It reduces image contrast by scattering x-rays in various directions

What are the two sources of image noise mentioned in relation to Compton scattering?

Scattered x-rays and uniform optical density

Why is Compton scattering considered a radiation hazard in fluoroscopy?

Because scattered x-rays can reach radiographers

Compton scattering is most likely to occur with which types of electrons?

Outer-shell or loosely bound electrons

What happens to the probability of Compton scattering as x-ray energy increases?

Compton scattering decreases ($\approx 1/E$)

How does the atomic number of the absorber affect Compton scattering?

It has no effect on Compton scattering

How does the mass density of the absorber affect Compton scattering?

There is a proportional increase in Compton scattering as mass density increases

What occurs during the photoelectric effect?

An incident x-ray is completely absorbed by an inner-shell electron, which is then ejected

What is the name of the electron ejected during the photoelectric effect?

A photoelectron

How is the kinetic energy of a photoelectron calculated?

It is the incident x-ray energy minus the electron binding energy

What determines the kinetic energy of a photoelectron in low atomic number atoms?

The kinetic energy is nearly equal to the incident x-ray energy due to low binding energy

What is produced when a vacancy in the K-shell is filled following a photoelectric interaction?

Characteristic x-rays

Are characteristic x-rays considered secondary radiation?

Yes

What is the diagnostic value of characteristic x-rays produced in the patient?

They contribute no useful information to the diagnostic image

What is the threshold requirement for a photoelectric interaction to occur?

The incident x-ray energy must be equal to or greater than the electron binding energy

What is the mathematical relationship between the probability of the photoelectric effect and x-ray energy?

It is inversely proportional to the cube of the energy ($1/E^3$)

What is the mathematical relationship between the probability of the photoelectric effect and atomic number ($Z$)?

It is directly proportional to the cube of the atomic number ($Z^3$)

Is the photoelectric effect more likely to occur with high-$Z$ or low-$Z$ atoms?

High-$Z$ atoms

What is the atomic number of Hydrogen?

$$1$$

What is the K-shell binding energy of Hydrogen?

$$0.02\,keV$$

What is the atomic number of Carbon?

$$6$$

What is the K-shell binding energy of Carbon?

$$0.3\,keV$$

What is the atomic number of Nitrogen?

$$7$$

What is the K-shell binding energy of Nitrogen?

$$0.4\,keV$$

What is the atomic number of Oxygen?

$$8$$

What is the K-shell binding energy of Oxygen?

$$0.5\,keV$$

What is the atomic number of Aluminum?

$$13$$

What is the K-shell binding energy of Aluminum?

$$1.6\,keV$$

What is the atomic number of Calcium?

$$20$$

What is the K-shell binding energy of Calcium?

$$4.1\,keV$$

What is the atomic number of Molybdenum?

$$42$$

What is the K-shell binding energy of Molybdenum?

$$19\,keV$$

What is the atomic number of Rhodium?

$$45$$

What is the K-shell binding energy of Rhodium?

$$23\,keV$$

What is the atomic number of Iodine?

$$53$$

What is the K-shell binding energy of Iodine?

$$33\,keV$$

What is the atomic number of Barium?

$$56$$

What is the K-shell binding energy of Barium?

$$37\,keV$$

What is the atomic number of Tungsten?

$$74$$

What is the K-shell binding energy of Tungsten?

$$69\,keV$$

What is the atomic number of Lead?

$$82$$

What is the K-shell binding energy of Lead?

$$88\,keV$$

What is the effective atomic number of Fat?

$$6.3$$

What is the effective atomic number of Soft tissue?

$$7.4$$

What is the effective atomic number of Lung?

$$7.4$$

What is the effective atomic number of Bone?

$$13.8$$

What is the effective atomic number of Air?

$$7.6$$

What is the effective atomic number of Iodine?

$$53$$

What is the effective atomic number of Barium?

$$56$$

What is the effective atomic number of Concrete?

$$17$$

What is the effective atomic number of Molybdenum?

$$42$$

What is the effective atomic number of Tungsten?

$$74$$

What is the effective atomic number of Lead?

$$82$$

Photoelectric effect occurs most likely with which electrons?

Inner-shell or tightly bound electrons

At what energy level is the photoelectric effect most likely?

When x-ray energy is just higher than the electron binding energy

As x-ray energy increases, what happens to the penetration through tissue?

It increases

What happens to the photoelectric effect relative to Compton scattering as x-ray energy increases?

There is less photoelectric effect relative to Compton scattering

How does mass density affect photoelectric absorption?

There is a proportional increase in photoelectric absorption as mass density increases

What is the minimum energy required for pair production?

$$1.02\,MeV$$

Explain why $1.02\,MeV$ is required for pair production.

To create an electron-positron pair, where each particle has an energy equivalence of $0.51\,MeV$

In pair production, the incident x-ray interacts with what part of the atom?

The strong nuclear force field

What two particles are created during pair production?

An electron and a positron

What happens to any incident energy exceeding $1.02\,MeV$ in pair production?

It is distributed equally as kinetic energy between the electron and the positron

What defines a positron?

A positively charged particle

What eventually happens to the electron created in pair production?

It loses energy through excitation and ionization, eventually filling an orbital shell vacancy

Define annihilation radiation.

The process where a positron combines with a free electron, converting their masses into two gamma photons

Where does pair production primarily occur?

In the vicinity of the nucleus of high-$Z$ atoms

Is pair production significant in standard x-ray imaging?

No

In what imaging modality is pair production crucial?

Positron emission tomography (PET)

What energy level is required for photodisintegration?

Approximately $10\,MeV$ or higher

What happens to the nucleus during photodisintegration?

It absorbs the x-ray photon directly and enters an excited state

What does the excited nucleus emit during photodisintegration?

A nucleon or other nuclear fragment (such as protons or neutrons)

Why is photodisintegration not relevant in diagnostic imaging?

The energy threshold is too high

In what fields is photodisintegration significant?

High-energy astrophysics and nuclear research

What interaction involves the excitation of electrons in the whole atom at low energies?

Classical (Coherent) Scattering

Which interaction results in a positron and electron?

Pair production

What interaction results in nuclear fragments?

Photodisintegration

Define differential absorption.

The variation in how different tissues absorb x-rays, producing contrast and detail

What is the main role of the photoelectric effect in differential absorption?

It provides diagnostic information by being absorbed in dense tissues like bone

Define radiopaque.

Structures that absorb x-rays and appear light on a radiograph

Define radiolucent.

Structures that allow x-rays to pass through and appear dark on a radiograph