Chapter 10 Bushong

5 Interactions with Matter

5 Interactions with Matter

1. classical/coherent/thompson

2. compton scatter

3. photoelectric

4. pair production

5. photodisintegration

Two Interactions Associated with Radiology

1. compton

2. photoelectric

Classical Scattering

1. incoming x-ray interacts with target atom itself

2. becomes excited

3. produces scattered x rays

4. no energy is transmitted or loss

5. no ionization occurs

kVp Range -

• 10 keV or lower

• at 70% kVp, 5% undergo classical scattering

Contribution to image-

• image fog/noise

Interaction-

• between low energy x rays and atoms

• x ray loses no energy, but changes direction slightly

• wavelength of incident x ray = wavelength of scattered x ray

Compton Scattering

1. incoming x ray interacts with loose outer shell electron

2. ejects it

3. becomes a compton electron

4. ionizes the atom

kVp Range-

• moderate diagnostic range

• 40-120 keV

Interaction

• occurs between moderate level energy x rays and outer shell electrons

• results in ionization of the target atom

• changes x ray direction

• reduces x ray energy

• wavelength of scattered x ray is greater than incident x ray

Extra info

• associated with backscatter radiation- x rays that have interacted with an object and are deflected back

• probability is determined by energy of xrays, not atomic number

• higher the energy, less chance of compton

• probability decreases with increasing x ray energy

• source of most occupational radiation (common in fluoroscopy)

• contributes to image fog/noise

Photoelectric Absorption

1. incoming x ray interacts with inner k-shell electron

2. incident photon is totally absorbed and disappears

3. knocks out k-shell electron

4. becomes photoelectron

5. ionization occurs

Energy Range

• 30-120 keV

Interaction

• occurs when an incident x ray is totally absorbed during ionization of an inner shell electron

• the incident photon disappears and the k-shell electron that is now called a photoelectron, is ejected from the atom

Extra Info

• sometimes produces characteristic x rays (secondary radiation)

• probability

  • you need high kV and something with a high atomic # in order for photoelectric to occur

• depends on atomic number of material

• important to radiology: this is where we get our diagnostic info that we see on the radiograph

• has a greater chance occurring in bone due to higher atomic number than soft tissue

Pair Production

1. incoming high energy x ray interacts with nucleus of atom

2. incident x ray disappears

3. results in formation of two electrons

Energy Requirement-

1.02 meV

Interaction

• occurs in x rays that have 1.02 meV or higher energies

• the x ray interacts with the nuclear force field

• 2 electrons that have opposite charges are created

Extra Info

• occurs in PET scans

Photodisintegration

1. high energy x rays are totally absorbed by the nucleus

2. become excited

3. instantly emits a nucleon or nuclear fragment

Energy Requirement

• 10 meV

Interaction

• occurs between high energy x rays and the nucleus

• the x ray is absorbed by the nucleus and a nuclear fragment is emitted

Extra Info

• occurs in nuclear reactors, never diagnostic imaging

Differential Absorption

different degrees of absorption in different tissues results in image contrast and formation of the x ray image

• increases as the kVp is reduced

  • downside of this is that reducing the kVp to increase differential absorption, increases patient dose

Differential absorption occurs because of?

1. compton scattering

2. photoelectric scattering

3. x rays transmitted throught the patient

Differential absorption depends on what 3 factors?

1. atomic number of tissue

2. mass density of the tissue

3. energy of kVp

What does differential absorption look like for the three factors?

• Compton scatter

provide no useful information, only noise.

• Photoelectric

provide diagnostic information to the image (show radiopaque anatomic structures w/ high absorption characteristics.

• Transmitted x-rays

produce the dark areas on images

(show radiolucent anatomical structures)

Attenuation

defined- reduction in the # of x-rays left in an x-ray beam after it goes through tissue

• Attenuation is the product of absorption and scattering

Which two compounds help demonstrate internal organs in x rays? What type of compounds are they?

1. barium (atomic #56)

2. iodine (atomic #53)

• positive compounds

Air

is also a contrast medium that is a negative contrast and is used along with barium (double-contrast examination)