Biomedical imaging Xray 2

• Unaffected
• Absorbed,
• Scattered

The removal of X-ray photons from the x-ray beam as a result of absorption and scatter

I = I₀e^-μx, where mu is the linear attenuation coefficient with units m^-1

the mu values for blood and muscle are too simlar

• Probability of an x-ray photon interacting with matter. Depends on no of atoms encountered. 
• 

Absorption

• 1.Pair production
• 2.Photoelectric absorption

Scattering:

• 1. Coherent scattering
• 2. Incoherent scattering (compton)

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

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.

• 3. Electrons from upper orbitals fill the empty shell
• 4. emission of characteristic xrays occurs

it is relevant

• 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)

The primary elements of soft tissue have low absorption edges (<<10keV)

Z of barium = 56.

Z of soft tissue = 7.4

So photoelectric probability of barium is (56/74)³ times greater in soft tissue.

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
• 

• 1. Incident photon impacts on an outer shell valence electron
• 2. Electron is ejected from the atom
• 3. photon is scattered

Its relevant

The energy of the incident photon - scattered photon

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

• 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.

• Photoelectric absorption
• Compton scattering

• Photoelectric absorption
• Not compton scattering

Photoelectric absorption more prominent at low energies.

Compton scattering compromises the great majority of interactions near diagnostic imaging range

both?

• Both compton and photoelectric absorption contribute
• You get more energy deposited in tissue with photoelectric absorption

both photoelectric absorption AND compton scattering.

Energetic electrons cause ionisation and break chemical bonds of tissues

Stochastic effect:

Probability of effect (not severity) increases with dose. Eg cancer,genetic effects

Deterministic effect (also called tissue reactions)

• Require higher doses
• Below a threshold, nothing occurs
• Eg cataracts, erythema, fibrosis

• Kerma [Gy]
• Absorbed dose, D [Gy]
• Equivalent dose[Sv] (organ dose is in [Gy]
• Effective dose[Sv]

Somatic is to cells other than sex organs

Genetic effects can be passed to offspring?

Kinetic Energy Released In Media

Energy transferred 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.

Different types of radiation have different effects on tissues, which this takes into account



for diagnostic x-rays w_r = 1

So equivalent and absorbed dose would have the same value but different name units.

Radiation energy deposited per unit mass of a medium

Unit: Gray (Gy) J/kg

Different organs and tissues have different radiosensitivity

Increase distance from source due to inverse square law with distance I∝ 1/d²

Lead shielding

• Has a high attenuation coefficient
• High density

Allow more time for same exposure

• a: Rayleigh scattering (coherent)
• b: Photoelectric absorption
• c: Compton
• d: Pair production