What are the 3 possible outcomes for x-ray photons passing through tissue?
What is the definition of attenuation?
The removal of X-ray photons from the x-ray beam as a result of absorption and scatter
What is Beer's Law formula?
I = I0e-μx, where mu is the linear attenuation coefficient with units m-1
Why is soft tissue not distinguishable on xray?
the mu values for blood and muscle are too simlar
What is the mass attenuation coefficient and what is its formula with units?
- Probability of an x-ray photon interacting with matter. Depends on no of atoms encountered.
What are the factors that cause attenuation?
- 1.Pair production
- 2.Photoelectric absorption
- 1. Coherent scattering
- 2. Incoherent scattering (compton)
Explain the process of pair production. Is it relevant in diagnostic imaging?
1. Photon converted to electron positron pair
2. Positron combines with and electron producing 2 photons at 180° to each other.
Occurs at high energies (>1.02 MeV).
- Not relevant in diagnostic imaging.
- Present in radiotherapy
Explain photoelectric absorption. Is it relevant in clinical imaging?
1. An incident photon impacts an inner shell orbital electron with all of its energy E=hf is transferred to the electron
2. A photoelectron is ejected.
it is relevant
- 3. Electrons from upper orbitals fill the empty shell
- 4. emission of characteristic xrays occurs
What is photoelectric absorption proportional to and what causes it to increase/decrease
- Decreases with energy proportional to 1/E3 Sharp discontinuities just above ionisation energies
- Increases as xray photon energy reaches ionisation level of K-shell electrons and then decreases
- Increases with atomic no and is proportional to Z3
- Increases with and prop to density (ρ)
Draw the graph for mass attenuation coefficient vs xray energy and what it shows about the primary elements of soft tissue.
The primary elements of soft tissue have low absorption edges (<<10keV)
Show how barium can be used for image contrast
Z of barium = 56.
Z of soft tissue = 7.4
So photoelectric probability of barium is (56/74)3 times greater in soft tissue.
Explain the 3 step process of coherent scatter (Rayleight/thompson).
Is it relevant in diagnostic radiology?
1. Photons interact with atom
2. No energy loss to atom
3. Scattered photon is usually emitted from the atom in a forward direction.
Accounts for only 5% of all photon interactions at diagnostic imaging energies.
- Of minimal concern in diagnostic radiology
Explain compton scattering (incoherent) in a 3 step process.
Is it relevant clinically?
- 1. Incident photon impacts on an outer shell valence electron
- 2. Electron is ejected from the atom
- 3. photon is scattered
What is kinetic energy of compton electron?
The energy of the incident photon - scattered photon
Explain how the features of compton scattering change as you change the incident photon energies
Scattered photons may really move in any direction/angle (even back the way it came)
Higher photon energy means photons scatter in forward direction
The less the incident photon scatters, the more enrgy is retained by scattered xray
At energies of 100keV (diagnostic range) scattering is more isotropic
What is probability of compton scattering proportional to?
- proportional to electron density
- Proportional to physical density (ρ)
- Is nearly independent of energy for diagnostic enrgy range
- Decreases in a 1/E fashion(approximately) for higher energies. No need to worry about this for diagnostic ranges.
Which two interactions are relevant to xray interaction?
- Photoelectric absorption
- Compton scattering
What interactions does Atomic no affect
- Photoelectric absorption
- Not compton scattering
What contributes to energetic electrons (xray hazard)?
Photoelectric absorption more prominent at low energies.
Compton scattering compromises the great majority of interactions near diagnostic imaging range
Which interactions does an energetic electron affect (xray hazard)?
- Both compton and photoelectric absorption contribute
- You get more energy deposited in tissue with photoelectric absorption
What interactions does physical density affect?
both photoelectric absorption AND compton scattering.
How are x-rays hazardous?
Energetic electrons cause ionisation and break chemical bonds of tissues
What is the difference between stochastic and deterministic effects of xrays?
of effect (not severity) increases with dose. Eg cancer,genetic effects
(also called tissue reactions)
- Require higher doses
- Below a threshold, nothing occurs
- Eg cataracts, erythema, fibrosis
How goes the hierarchy of dose terminology?
- Kerma [Gy]
- Absorbed dose, D [Gy]
- Equivalent dose[Sv] (organ dose is in [Gy]
- Effective dose[Sv]
Whats the difference between somatic and genetic effects?
Somatic is to cells other than sex organs
Genetic effects can be passed to offspring?
What is KERMA? What are its units?
Kinetic Energy Released In Media
from x-ray beam to charged particles per unit mass of a medium [J/kg] . Units called Gray (Gy)
- At high energys (>1MeV), some of the enrgy goes to bremstrahlung
- Energy transferred to electrons will be deposited away from that initial location cos of increased electron range.
What is equivalent dose? What are its units?
Different types of radiation have different effects on tissues, which this takes into account
for diagnostic x-rays wr
So equivalent and absorbed dose would have the same value but different name units.
What is absorbed dose? what is its units. Whats the difference between it and KERMA?
Radiation energy deposited per unit mass of a medium
Unit: Gray (Gy) J/kg
What is effective dose, what is its formula
Different organs and tissues have different radiosensitivity
How can one reduce x-ray hazard?
from source due to inverse square law with distance I∝ 1/d2
Allow more time for same exposure
- Has a high attenuation coefficient
- High density
Fill in the gaps a-d for the mass attenuation coefficients of soft tissue
- a: Rayleigh scattering (coherent)
- b: Photoelectric absorption
- c: Compton
- d: Pair production