OMFPR 1: Radiation Physics - X-Rays, Their Nature, Production, and Interactions

Radiation

Energy in transit

Ionization

Removal of electrons from an atom resulting in the formation of an "ion pair"

Z-number (atomic #)

Atomic # = # of protons

Electrons

Z-number is same as number of _______ in neutral atom.

A-number (atomic mass #)

# of P + N

Electron binding energy

Attractive force that keeps electrons bound to the nucleus in their orbitals

Increased

Increased Z # = _________ binding energy

Electromagnetic radiation

Movement of energy through space as a combination of electric and magnetic fields

Quantum theory

Energy transfer in the form of "bundles" (or packets) of energy called photons (or "quanta")

Light; energy

EM radiation occurs at the speed of _____ and with specific amount of ______.

No

Do x-rays have mass or electric charge?

Waves; frequency

X-rays travel in _____ with a specific _________.

Speed of light

Specific frequency of x-rays

True

T/F: X-rays travel in straight lines.

Affect photographic film

Able to produce biological and chemical changes

How are x-rays able to ionize?

0.1A to 0.5A

Range of wavelengths for x-rays

Bremsstrahlung

Characteristic radiation

Two mechanisms of x-ray production

Bremsstrahlung

Electron to nucleus interaction

Characteristic radiation

Electron to electron interaction

Bremsstrahlung mechanism

High velocity electrons are suddenly decelerated when they pass close to the nuclei of high Z # absorbing material

In the Bremsstrahlung mechanism, x-rays are produced when...

X-ray tube

Primary source of radiation is generated by what in the Bremsstrahlung mechanism?

X-ray photons with varying energy levels (within a specific range)

What is produced from the Bremsstrahlung mechanism?

Characteristic mechanism

High velocity electrons interact with an inner shell electron and knocks it out of its orbit

In the characteristic mechanism, x-rays are produced when...

Outer shell electron

In the characteristic mechanism, the vacancy is filled by an...

Released; x-ray photon

In the characteristic mechanism, the difference in the energy levels is _________ and an _____ _______.

Small

Characteristic mechanism contributes a small or large fraction of photons in the beam?

True

T/F: In characteristic mechanism, x-ray photons with specific energy level that is characteristic of the atom is produced.

Cathode

Electron source

Focusing cup

Concentration of electrons

Potential difference or tube voltage kVp

Mechanism to accelerate electrons

Anode

Suitable target to stop electrons

Current heats cathode

Emission of electrons (thermionic)

Formation of electron cloud

Describe thermionic emission

99; 1

The x-ray tube allows conversion of kinetic energy with __% as heat and _% x-radiation.

High atomic number

High melting point

Low vapor pressure

High thermal conductivity

Why tungsten target?

Tungsten; copper

Anode has ________ target and _______ stem.

Good thermal conductor

Why is copper stem used?

Tube voltage (kVp)

Exposure time (S)

Tube current (mA)

Filtration

Collimation

Distance of x-ray tube from patient/receptor

Factors controlling x-ray beam

Tube voltage (kVp)

To accelerate the electrons from cathode to anode

Exposure time

Tube current

Heating the cathode to produce electrons

Filtration

To remove lower energy x-ray photons

Collimation

Adjust the shape/size of x-ray beam

# photons increases

Max energy in beam increases

Mean energy increases

What is the difference between 80 kVP and 90 kVP?

Filtration

Removal of low energy photons from the x-ray beam while allowing passage of high energy photons

Glass, oil

Inherent filtration in x-ray tube

Aluminum disk

Added filtration in x-ray tube

Total filtration in x-ray tube

Aluminum + inherent filtration

1.5mm

Total filtration of 50-70 kVp: ___ mm Aluminum equivalent

2.5mm

Total filtration above 70 kVP: ___ mm Aluminum equivalent

Increase; decrease

When filtration is increased, there is _______ in exposure time and ________ in patient skin exposure.

Reduces; 80%

Although filtration requires an increase in exposure time up to 50% to compensate for loss of intensity, it _______ patient skin exposure by as much as __%.

HVL

Useful way to characterize the penetrating quality of an x-ray beam

Quality

Mean energy of an x-ray beam

HVL (half value layer)

Defined as the thickness of an absorber that reduces the quantity of photons by 1/2

Increases

As average energy of beam (kVp) increases, HVL ________.

Reduces size and modifies shape of beam

Reduces volume of tissue irradiated

Improves image quality

Collimator is a metallic barrier with an aperture that:

Inversely proportional ( I = 1/D^2)

For a given beam, the intensity is ________ ___________ to the square of the distance from the source.

Changing the distance between the x-ray tube and patient has marked effect on beam intensity, so position the cone of x-ray as close to the patient (not touching) as possible

Clinical significance of inverse square law

4m

2m

1m

0.25 Gy at _m

1 Gy at _m

4Gy at _m

1. No interaction (9%)

2. Coherent scattering (7%)

3. Photoelectric effect (27%)

4. Compton scattering (57%)

4 types of interactions possible

Coherent

Only 7% of total interactions are ________ scattering.

Photoelectric effect

27% of all interactions in a dental x-ray beam

Higher

Z (atomic #) of bone is ______ than Z of soft tissue.

Z bone > Z soft tissue

Differential photoelectric absorption within different types of tissues makes production of a radiographic image possible!

What is the clinical significance of photoelectric effect?

Compton scattering

57% of all interactions in a dental x-ray beam

Outer orbital

In Compton scattering, an incident photon interacts with an ____ ______ electron.

Fogging (darkening) of the film/receptor while carrying no useful information due to altered path

In Compton scattering, the scattered photon travels in all directions with a lower energy, causing...

Bones = radiopaque

Sinuses = radiolucent

Are bones radiopaque or radiolucent?

Differential

Making a radiographic image requires ___________ absorption.

Equivalent dose

A measure of comparison of biological effectiveness of different types of radiation

Sievert (Sv)

SI unit of equivalent dose

Effective dose

Used to estimate risk in humans by comparing different exposure types (panoramic vs. chest radiograph)

Volume/radiosensitivity of tissue irradiated

Biologic effectiveness of radiation

Effective dose takes into account...

Radiation biology

The study of effects of ionizing radiation on living systems

X ray, y ray

Uncharged, non-particulate (EM)

n

Uncharged, particulate

Beta particles

Alpha particle

Proton

Heavy ions

Charged, particulate

Direct effect

Ionization of biologic macromolecules directly by a photon of ionizing radiation

Direct

1/3 of all radiation-induced biologic effects caused by x-rays is ______ effect.

Indirect

Majority of all radiation-induced biologic effects caused by x-rays is _______ effect.

Photon absorbed by water (in living tissues), causing ionization of some water molecules to form free radicals (radiolysis of water)

Free radicals interact with macromolecules to produce biologic changes

2 steps of the indirect effect

Indirect

2/3 of all radiation-induced biologic damage caused by x-ray exposure is by ________ effects.

DNA

Primary target for cell damage from ionizing radiation

Cell death

Genetic mutations

Carcinogenesis

What can a DNA double-strand break cause?

Tissue reactions (or deterministic effects)

Radiation injury to organisms resulting from killing of large number of cells (e.g. erythema, cataract formation)

Stochastic effects

Sublethal damage by radiation to individual cells leading to carcinogenesis or heritable mutation

Deterministic

Examples: mucositis, cataract formation post radiation therapy

Stochastic

Examples: radiation-induced cancer, genetic effects

Deterministic

Caused by killing of a large number of cells

Stochastic

Caused by sublethal damage to DNA

Deterministic

Is there a safe threshold dose for deterministic or stochastic effects?

Deterministic

Severity of clinical effects and dose are proportional to dose

Stochastic

Severity of clinical effects and dose are independent of dose

Deterministic

Probability of having effect and dose = effect definitely seen ONLY above threshold dose

Stochastic

Probability of having effect and dose = greater the dose, greater chance for having effect

True

T/F: Even the smallest doses of radiation as those associated with dental x-rays have the potential for cancer formation.

Linear

Radiation carcinogenesis has a ______ non-threshold response.

Radiosensitivity of cell/tissue

Stage in cell cycle

Reproductive capability

Age

O2 and temperature (higher = more damage)

Volume of tissue

Host factors influencing radiation damage

Type of radiation / LET

Dose

Dose rate

Radiation factors influencing radiation damage