Unit 9: X-ray interaction with matter

Photons tend to interact with things that have a similar

wavelength

Low - energy photons tend to interact with

whole atoms

Moderate - energy photons tend to interact with 

electrons

High-energy photons tend to interact with 

nuclei

What are the 5 types of x-ray interactions with matter?

1. coherent scattering

2. compton scattering

3. photoelectric effect

4. pair production

5. photodisintegration

Of the 5 types of interactions, which are most important to us as x-ray techs?

compton scattering and photoelectric effect

Coherent scattering AKA

classical scattering or Thompson scattering

Coherent scattering has ____ energy x-rays

low (below 10 keV)

Coherent scattering involves interaction with

the whole atom

lower energy = _____ wavelength

longer

Because coherent scattering involves x-rays below 10 keV, those x-rays are not in the

diagnostic range

What removes a lot of the photons involved in coherent scattering?

filtration

In coherent scattering, incident x-rays

excite the target atom

After exciting the target atom, (coherent scattering)

the target atom releases a scattered x-ray with a wavelength equal to incident x-ray

The trajectory of the scattered x-ray is (coherent scattering)

different from the incident x-ray

Coherent scattering involves a change in _____ of x-ray without a change in ______

direction; energy

Most coherently scattered x-rays are scattered in the ______ direction

forward

No _______ occurs in coherent scattering

ionization

Coherent scattering is _____________ to diagnostic radiology

of little importance

coherent scattering can contribute

slightly to image noise

image noise can

reduce image contrast

Compton scattering involves x-ray energies

throughout the diagnostic range

The incident x-ray (compton scattering)

ejects outer-shell electron

Because the incident x-ray ejects an outer-shell electron, compton scattering involves

ionization

What is the name of the outer-shell electron that is ejected during compton scattering?

compton electron

The scattered x-ray in compton scattering continues

in a different direction with less energy

The energy of the ejected electron (compton scattering)=

binding energy + kinetic energy the electron leaves with

Energy of compton scattered x-ray =

energy of incident x-ray - energy of ejected electron

What is the formula for scatter photon (compton scattering)

Es = Ei - (Eb + Eke)

A 50 keV x-ray ionizes an atom of barium by ejecting an O-shell electron with 20keV of kinetic energy. What is the energy of the scattered x-ray? (the binding energy of barium O-shell electrons = 0.04keV)

29.96keV

In compton scattering, the scattered x-ray usually 

retains most of the energy

The scattered x-ray and compton electron may have

further ionizing events before they lose all energy

The scattered x-ray is eventually (compton scattering)

absorbed photoelectrically

The compton electron eventually loses

all KE and falls into a vacant electron shell

The scattered x-ray in compton scattering can be deflected

in any direction

The degree of deflection dictates the (compton scattering)

energy that is transferred

0 degree deflection (compton scattering) =

no energy is transferred

180 degree deflection (compton scattering) =

x-ray retains at least approx. 2/3 of the original energy

Backscatter radiation =

x-rays scattered back in the direction of the incident x-ray (doesn’t have to be exactly 180, just back in the general direction)

The probability of compton scattering decreases as

x-ray energy increases 

probability of compton scattering and x-ray energy are ____ proportional

inversely

As mass density of absorbing material increases, there is also a proportional increase in

compton scattering

Compton scattering is independent of 

Z#

compton scattering provides no

useful information to the radiograph

compton scattering produces

uniform optical density and uniform intensity (blanket noise) over the image that reduces contrast

What is the number one source of occupation exposure?

scatter radiation from the pt. (pt. is primary source)

Increasing kVp will _____ scatter (compton)

decrease

Probability of compton scatter is different between bone and soft tissue due to 

mass/density difference (not Z#)

in 1921 Albert Einstein

won the nobel prize in physics

Photoelectric effect involves x-ray energies

throughout the diagnostic range

Photoelectric effect involves _______-shell electron interaction

inner

During photoelectric effect interactions, the atom is

ionized

During photoelectric effect interactions, the incident x-ray is

totally absorbed

The incident x-ray ejects _______ and is ______ (photoelectric effect)

inner-shell electron; completely absorbed

The incident x-ray in photoelectric effect AKA

photoelectron

The photoelectron has kinetic energy equal to

energy of incident x-ray - binding energy of the elctron

What is the equation for photoelectron kinetic energy?

Eke= Ei - Eb

A photoelectric effect interaction will emit a

characteristic x-ray

the characteristic x-ray emitted by photoelectric effect is ____ radiation, has no )_____, and does not ________

secondary radiation; diagnostic value; penetrate to the image receptor

Photoelectric interaction cannot occur unless the incident x-ray has

equal or greater energy than the electron binding energy

A 75keV x-ray has a photoelectric interaction with a barium atom. What is the kinetic energy of the photoelectron and the energy of the characteristic x-ray, if an L-K transition occurs?

(k-shell binding energy = 37keV) (L-shell binding energy= 5.989keV)

Eke=75keV - 37keV= 38keV

Ex= 37keV- 5.989keV = 31.011keV

The probability of the photoelectric effect is inversely proportional

to the 3rd power of the x-ray energy (1/E)³

The probability of the photoelectric effect is proportional to the

3rd power of the atomic number of absorbing material (Z)³

As mass density of absorbing material increases, there is also a proportional increase in

photoelectric absorption

Pair production

does not occur during x-ray imaging

During pair production, a high-energy incident x-ray 

avoids interaction with electrons and gets close enough to the nucleus to be influenced by the strong nuclear field

After the incident x-ray is influenced by the strong nuclear field it (pair production)

disappears

Pair production interaction produces

one electron and one positron (positive electron)

Positron AKA

antimatter

After the electron and positron are produced, the positron will

meet up with a free electron and they will annihilate each other and turn to pure energy

What is the energy equivalence of an electron?

0.51meV

Because 2 electrons are produced, the incident x-ray photon must have an energy of at least (pair production)

1.02meV

The 1.02meV of the incident x-ray is (pair production)

equally distributed between the two electrons as kinetic energy

X-ray with less than 1.02meV energy cannot

undergo pair production

The electron produced by pair production will eventually

lose energy through various interactions and fill a vacancy in an atomic orbital shell

Pari production is very important in what modality?

PET (positron emission tomography) - picks up on positron interactions

Photodisintegration occurs in diagnostic imaging (true/false)?

false

During photodisintegration, x-rays with energy 

above 10meV interact directly with the nucleus and are absorbed

Because the x-rays are absorbed in the nucleus, (photodisintegration)

the nucleus is raised to an excited stated

Because the nucleus is raised to an excited state, it then (photodisintegration)

emits a nucleon or nuclear fragment

The difference between x-rays absorbed photoelectrically and those transmitted to the image receptor

differential absorption

Differential absorption increases as kVp is

reduced

compton-scattered x-rays contribute ____ useful information to the image

no

x-rays absorbed through the photoelectric effect provide 

diagnostic information because they do not reach the image receptor

The photoelectric effect accounts for the _____ of the radiograph

white areas/structures with high x-ray absorption characteristics(radiopaque)

X-rays that penetrate the body provide diagnostic information because they are

transmitted to the image receptor

X-rays that penetrate the body account for the ____ areas of the radiogrpah

black/dark/structures with low x-ray absorption characteristics/radiolucent

An x-ray image results from

the difference between those x-rays absorbed photoelectrically in the pt. and those transmitted to the IR

Approx. __% of the x-rays incident on a pt reach the IR

1

Fewer than ____ of the x-rays that reach the IR interact to form the image

half

The radiographic image results from about ____% of the x-rays emitted from the tube

0.5

an x-ray photon will have a higher probability of being absorbed photoelectrically in

tissues with a higher Z#

Compton scatter probability is independent of

Z#

Probability of compton scatter decreases with 

increasing x-ray energy

At low energies, most x-ray interactions with tissue are

photoelectric

At high energies, most x-ray interactions with tissue are

compton scattering

As x-ray energy is increased, the chance of any interaction at all

decreases

mass density =

kg/m³ (matter per unit volume)

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

doubled

Why does doubling mass/density double the chance for x-ray interaction?

because there are twice as many electrons available for interaction