Ch 10 xray interaction with matter

principles of med imaging

What are the 5 x-ray interactions with matter?

1. Coherent Scattering

2. Compton Effect

3. Photoelectric Effect

4. Pair Production

5. Photodisintegration

What are the only two interactions that are important in making an x-ray image?

Compton and Photoelectric effect

Coherent Scattering interactions occur with matter at energies below?

Approximately 10 keV

What is another term for Coherent Scattering?

Classical Scattering or Thompson Scattering

What are the two types of Coherent Scattering?

1. Thompson scattering: involves a single electron in the interaction

2. Rayleigh scattering: involves all of the electrons of the atom in the interaction

-Both have same basic results

What happens in a Coherent Scattering Interaction?

-Incident x-rays interact with a target atom & causes it to become excited (vibrate)

-Releases excess energy as scattered x-ray

-WAVELENGTH & ENERGY of the scattered x-ray is equal to the incident x-ray

-Direction of scattered x-ray is different

Does energy transfer or ionization occur in Coherent Scattering interactions?

No

Most Coherent scattered x-rays are scattered in the ____ direction

Forward

Coherent Scattering has little importance to diagnostic radiology.

True

At what range can Coherent Scattering occur?

-70 kVp

-Few % of x-rays undergo coherent scattering

-Contributes to image noise

What is image noise?

Graying of an image that reduces contrast. This results in deterioration of the radiographic image.

What is Compton Scattering?

-X-rays interact with outer-shell electrons

-Ionizes the atom

-Scatters the x-ray

-Reduces its energy

The ejected electron is also called?

-Compton electron

-Secondary electron

In what direction does Compton Scattering occur?

In a different direction with less energy

What is Compton-Scattered X-ray Energy equal to?

The different between the energy of the incident x-ray & and energy of the ejected electron

What is the energy of the ejected electron equal to?

Its binding energy plus the KE with which it leaves the atom

What is the Compton Effect equation?

Ei = Es (Eb + Eke)

Ei - energy of the incident x-ray

Es - energy of the scattered x-ray

Eb - electron binding energy

Eke - electron's kinetic energy

During Compton scattering, the incident photon energy is divided between the?

Ejected (Compton) electron & the scattered photon

During Compton scattering, scattered photon retains?

Most of its original energy

Both the scattered x-ray and the Compton electron have sufficient ____ to undergo additional ____ interactions before they lose all their energy

-Energy

-Ionizing

Compton scattering photons can be deflected in what direction?

Any direction

If Compton scattered photons are deflected at 0 degrees?

No energy is transferred

If Compton scattered photons are deflected at 180 degrees?

More energy is transferred to the Compton electron and Scattered photon retains approximately 2/3 of its original energy

What is backscatter radiation?

-X-rays that are scattered back in the direction of the incident x-ray beam (source)

-X-radiation backscatters from the wall or exam table

-Responsible for the cassette-hinge image

The probability of Compton effect

Decreases as x-ray energy increases

The Probability of Compton effect is _ proportional the x-ray energy (1/E) & independent of atomic number

Inversely

Compton Scattering can create a serious radiation exposure hazard in?

Radiography and particularly fluoroscopy

What does Compton Scattering reduce?

Image contrast

Scattered x-rays provide what information on radiograph?

-No useful information on radiograph

-Produce a uniform optical density on the film radiograph (image fog)

-Uniform intensity on the digital radiograph resulting in reduced image contrast

What is the photoelectric effect?

-X-rays in the diagnostic range interact with inner shell electrons ejecting s k-shell photoelectron

-X-ray is completely absorbed, resulting in vacancy in the k-shell… corrected when an outer-shell electron (usually L) drops into the vacancy

-Incident photon disappears

-Characteristic x-rays are produced after a PE interaction and do not contribute to diagnostic value

What is a photoelectron?

-The electron ejected from its inner-shell orbit during the process of photoelectric absorption

-The KE is equal to the difference between the energy of the incident x-ray & binding energy of the electron (inner shell)

PE effect can't occur unless

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

If the incident x-ray does have sufficient energy

The probability that it will undergo a PE effect decreases with the third power of the photon energy (I/E^3)

The probability of the PE effect is _ proportional to the third power of the x-ray energy (1/E)^3

Inversely

Pair Production _ _ occur during x-ray imaging.

Does not

What is Pair Production very important in?

PET imaging in nuclear medicine

During Pair Production, the incident x-ray:

Comes close enough to the nucleus of an atom to be influenced by the strong nuclear field

During Pair Production, the x-ray disappears &:

2 electron appear. 1 positively charged (positron) & 1 negatively charged e-

During Pair Production, the incident photon must have at least _ of energy BC 2 electrons are formed

1.02 MeV

During Pair Production, the electron:

Loses its energy through excitation & ionization & eventually fills a vacancy

During Pair Production, the positron:

-Unites with a free electron

-Gives rise to 2 photons moving in opposite directions & each possessing energies of 0.51 MeV

-Mass of both particles is converted to energy in a process called annihilation radiation

Photodisintegration _ occur in diagnostic radiology

Does not

During Photodisintegration, x-rays with energy above ___ are totally absorbed directly by the nucleus

Approximately 10 MeV

During Photodisintegration, the nucleus is then raised to an excited state & instantly emits:

A nucleon or other nuclear fragment

Why does differential absorption occur?

BC of Compton scattering, Photoelectric effect, & x-rays transmitted through the patient

Compton-scattered x-rays contribute _ useful information to an image.

No

What happens when Compton-scattered x-rays interact with the IR?

Assumes that the x-ray came straight from the source

Photoelectric absorption of x-rays:

-Provides diagnostic information to IR

-Do not reach the IR (absorbed)

-Representative of anatomical structures with high x-ray absorption characteristics: Radiopaque structures

-Produce the light areas in an image: Correspond to bone

What is differential absorption?

An x-ray image resulting from the difference between x-rays absorbed photoelectrically in the patient. X-rays that are transmitted to the IR.

What are the three types of x-rays that are important for image formation?

-Compton scatter (no useful information)

-Photoelectric absorption (produces the light areas on the image)

-Transmitted x-rays (produce the gray/dark areas on the image)

During differential absorption, ____ of incident x-rays reach the IR

Approximately 1%

During differential absorption, how many x-ray reach the IR and interact to form the final image?

Less than half

A radiographic image results from _ of x-rays emitted from the tube

Approximately 0.5%

Differential absorption __ as the kVp is reduced. Resulting in increased patient dose.

Increases

Compton effect is ____ of the atomic number of tissue

Independent

Probability of Compton scattering for bone atoms & soft tissue atoms is approximately:

Equal & decreases with increasing x-ray energy

With energy, the decrease in Compton scattering is not as rapid as the decrease in PE effect

Increasing

Probability of Compton effect is _ proportionally to x-ray energy (1/E)

Inversely

Probability of the PE effect is _ proportional to the third power of the x-ray energy (1/E)^3

Inversely

At __ energies, most x-ray interactions are photoelectric

Low

At __ energies, most are Compton scattering

High

As x-ray energy (kVp) is , the chance of any interaction decreases

Increased

To image small differences in soft tissue, one must use:

Low kVp to get maximum differential absorption

High kVp technique is used for?

-Barium studies and chest radiography in which intrinsic contrast is used

-Results in lower patient dose

-Amount of scatter radiation from surrounding soft tissue contributes little to the image

What do you use when scatter radiation is great?

Grids

Below 20 keV (soft tissue), which interaction is predominant?

Photoelectric effect

Above 20 keV (soft tissue), which interaction is predominant?

Compton effect

What is mass density?

Quantity of matter per unit volume. Related to the mass of each atom & tells how tightly the atoms of a substance are packed.

What is mass density measured in?

Kilograms per cubic meter (kg/m^3) or grams/cubic centimeter (g/cm^3)

What is the mass density of ice?

917 kg/m^3

What is the mass density of water?

1000 kg/m^3

The interaction of x-rays with tissue is ___ to the mass density of the tissue regardless of the type of interaction.

Proportional

When mass density is doubled, the chance for x-ray interaction is ____

Doubled

Low kVp technique (below 80 kVp) produces

Excellent high-contrast radiographs of organs of GI tract

Higher kVp techniques (above 90 kVp) can be used in examinations to

Outline organs under investigation & to penetrate the contrast medium so the lumen of organs can be visualized

Air is a _ contrast agent

Negative

Barium is a __ contrast agent

Positive

Ba and iodine are called contrast agents

Positive

Relative ___ of interaction is dependent on the atomic number, mass density, & x-ray energy

Frequency

What is attenuation?

The total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue

_ is the product of absorption & scattering

Attenuation

What does it mean when x-rays are attenuated exponentially?

-They do not have a fixed range in tissue

-They are reduced in number by given percentage for each incremental thickness of tissue they go through