Interaction of X-Radiation with Matter - Matter Interaction

What is Beam Quality?

1. Penetrating power

2. Beam energy

3. Controlled by kVp

What is Beam Quantity?

1. Tube current

2. Intensity

3. Beam photon number

4. Controlled by mAs

What is Absorption? (general definition)

Transfer of EM energy from photon to biologic tissue

What is Attenuation (overall definition)?

Reduction of photon number in beam as partial energy is transferred to biologic tissue

What is Transmission?

No EM transfer from photon to body

What is Absorbed Dose (D)?

Amount of EM energy absorbed per unit of body mass

What is interaction in the tube?

Electron-electron interaction

What is interaction in the body?

Photon-electron interaction

What 2 types of photon-electron interaction is in the body?

Either attenuated or transmitted

What is Attenuation?

Primary beam intensity (number) reduced by (1) absorption or (2) scatter

What is Direct Transmission?

Photon is neither absorbed or scattered (attenuated), reaching the IR unchanged

What is Indirect Transmission?

Photon reaches IR with a change in direction an energy (attenuated - absorbed or scattered)

What is Absorption?

Total loss of energy

What is Scatter?

Partial loss of energy & change of photon direction

How much of its original energy does scatter retain?

Retains at least 2/3 of original energy

What does small-angle scatter cause?

Fog

What does side/backscatter cause?

Occupational dose

What is the best way to reduce both fog and occupational dose?

Distance

What are the 5 x-ray interactions with matter?

1. Classical (coherent) scatter

2. Compton scatter

3. Photoelectric absorption

4. Pair production

5. Photodisintegration

What are the 2 x-ray interactions with matter that are important in the diagnostic range?

1. Compton scattering

2. Photoelectric absorption

What is are the names for Coherent Scatter?

Classical, Thompson, unmodified scatter

Coherent scatter has ___ loss of energy as photon scatter

No

Coherent scatter is less than ___ keV

10 keV

Coherent scatter has a __ change in direction (

Slight

Coherent scatter has ___ ionization

No (no free electrons produced)

Coherent scatter interacts with the ___ atom

Whole

What type of interaction is Compton Scatter?

Outer shell photon-electron interaction

Compton scatter ___ the atom

Ionizes (produces free electron & scattered x-ray photon)

What can Compton Scatter cause?

Additional biologic interactions (fog patient) or reach IR (fog image)

X-ray photon changed both energy & direction = ___

Nondiagnostic

What does Compton Scatter do to image quality?

Degrades

Fog = ___ contrast resolution

Decreased

Compton scatter ___ patient dose

Increases (nondiagnostic exposure)

What type of scatter is the primary source of occupational exposure?

Compton

Why is Compton Scatter potentially harmful to both the image and the operator?

Reduces contrast and increases dose

What is the best way to control scatter?

Collimation

What is Photoelectric absorption?

Inner shell photon-electron interaction that creates a free electron (ionization)

Photoelectron = patient ___

Dose

Photoelectric absorption absorbs the ___ x-ray photon

Entire

When the x-ray photon is completely absorbed in photoelectric absorption there is a ___ in the inner shell

Vacancy

What is the Electron Cascade?

Vacancy in the inner shell is filled by an outer-shell electron, producing an energy release of x-ray (Characteristic) due to binding energy change for equilibrium

What type of radiation is Characteristic x-ray?

Fluorescent radiation

In body, characteristic x-rays are ___ energy = __ patient dose

Very low energy = low patient dose

What 2 things Photoelectric Absorption responsible for?

Patient dose & Image Contrast

What does the tube have to produce x-rays?

Electron cascade

What photoelectric absorption ejection occurs in the tube?

Filament electron ejects an inner shell target electron

In the tube, once the ejection occurs, what occurs?

Cascade energy change releases high energy x-ray photons for the primary beam

What type of interaction occurs in the tube?

Electron to electron interaction

What photoelectric absorption ejection occurs in the body?

Primary x-ray ejects an inner shell tissue electron

What happens in the body once the ejection occurs?

Cascade energy change releases low energy x-ray photon that only contributes to patient dose

What type of interaction occurs in the body?

X-ray to electron interaction

The tube has ___ characteristic electron cascade

High

The body has __ characteristic electron cascade

Low

What does photoelectric absorption depend on?

Incident photon energy & atomic number of the tissue

Photoelectric absorption increases as incident photon energy ___

Decreases

Lower energy x-ray photons will be absorbed more __

Easily

Photoelectric absorption ___ at atomic number of tissue increases

Increases

X-ray photons will be absorbed more easily in ____ than in ____ or ___

Compact bone; Soft tissue; Air

What is the Z# of bone?

13.8

What is the Z# of soft tisseu

7.4

What is the Z# of air?

7.6

The thicker the ___, the ___ absorption

Density; more

Compression paddles = ___ absorption

Less

What can abosrption differ due to?

1. Different body structures (differences in chest densities)

2. Pathology (density loss due to decrease in bone calcium)

What are the body tissues from least to most dense?

1. Gas

2. Fat

3. Cartilage

4. Hollow organs (empty)

5. Muscles

6. Solid organs

7. Hollow organs (filled with contrast media)

8. Bone

9. Tooth enamel

Higher tissue density = ___ photoelectric absorption = ___ exposure to IR

More; Decreased

Tissue density = tissue ___

Compactness

Additive pathology = ___ absorption

Increased

What are examples of additive pathologies?

1. Pleural effusion

2. Ascites

3. Tumor

Destructive pathology = ___ absorption

Decreased

What are examples of destructive pathologies?

1. Emphysema

2. Osteoporosis

Contrast media = ___ k-shell absorption due to high Z# ___

More; Differences

Contrast media = ___ photoelectric absorption

Increased

What is contrast media used for?

Demonstrate similar structure

Contrast media has a ___ atomic number than surrounding tissue

Higher

What is the Z# of Barium?

56

What is the Z# of Iodine?

53

What is absorbed by contrast media?

X-rays

Contrast has ___ outlines compared to surrounding structures

Sharp

Why are there different degrees of absorption in the body?

Varying atomic numbers of tissues

What is differential absorption varying in different tissues responsible for?

Image contrast

High kVp = ___ absorption & ___ transmission through tissue

Less; More

Increased kVp = ___ in all interactions, but ___ impact on photoelectric

Decrease; more

What is the Pprobaboility of Compton effect?

Inversely proportional to kVp or x-ray energy (1/E^1)

What is the Probability of the Photoelectric Effect?

Inversely proportional to the 3rd power of kVp or x-ray energy (1/E^3)

Low energies = ___ photoelectric

More

High energies = ___ Compton

More

What are Compton interaction independent of?

Z#

Increased kVp results what probability of all interactions?

1. Fewer Compton interactions

2. Even fewer photoelectric interactions

3. More transmission through tissue

4. Less differential absorption

Increased Z# results what probability of all interactions?

1. No change in Compton interactions (independent)

2. Many more photoelectric interactions

3. Less x-ray transmission through tissue

4. More differential absorption photoelectrically

Increased Tissue Mass Density results what probability of all interactions?

1. Increase in Compton interactions (more electrons available)

2. Increase in photoelectric interactions (more electrons available)

3. Reduction in x-ray transmission through tissue

4. Less differential absorption

What are Photoelectric Interactions dependent on?

Both Tissue Z# and kVp

Compton interactions are ___ of Z#, but depdent on ___

Indepdent; kVp

What counts as a higher energy incident x-ray?

Above 1.022 MeV

What do high energy incident x-rays interact with?

Nucleus (attractive force)

In Pair Production, when the x-ray disappears, what is created?

2 electron equivalence (positron & electron)

What happens to the electron?

Continues to ionize & eventually will fill an empty shell

What will happen to the Positron?

Unites with a free electron & converts to energy via annihilation radiation

What type of interaction is Annihilation Radiation?

Matter-antimatter

Pair production does ___ occur in diagnostic radiology

Not