[Cat 1] Describe relevant luminescence processes in the context of photostimuable phosphors.
[Cat 1] Describe the interaction processes of photoelectric effect & Compton scattering.
[Cat 1] Discuss the impact of field size, kVp and patient thickness on scatter production.
[Cat 2] Distinguish between atomic ionisation and excitation.
[Cat 2] Describe the coherent scattering interaction process.
[Cat 2] Demonstrate knowledge of the process described by attenuation.
[Cat 2] Describe the attenuation of monoenergetic and polychromatic radiation in terms of linear and mass attenuation coefficients and HVLs.
[Cat 2] Demonstrate knowledge of the factors that impact on attenuation.
[Cat 3] Describe very simply how luminescent screens work
Introduction
There are 5 principal interactions of x-rays with matter:
- Compton effect *
- Photoelectric effect *
- Coherent (Rayleigh) scattering – a lower-energy (<10 keV) x-ray photon excites an atom causing the atom to release an x-ray photon of equal wavelength but different direction to it’s incidence path. Has little effect on image quality.
- Pair production – a high-energy (must be >1.02 MeV) photon interacts with the nucleus of an atom and is converted into an electron (511 keV) and a positron (511 keV).†
- Photodisintegration – a high-energy (~ 15 MeV) photon is absorbed by a nucleus, resulting in immediate disintegration of the nucleus.†
* The photoelectric effect and Compton effect are the two most pertinent photon-matter interactions in clinical radiology and are therefore discussed in further detail below.
† Mostly encountered in megavoltage radiotherapy.
Photoelectric effect
Concept
The photoelectric effect (PE) or photoelectric absorption is a form of photon interaction with tightly bound inner-shell electrons. It occurs when a photon is totally absorbed by an inner-shell electron and the kinetic energy of the photon exceeds the electron-binding energy, resulting in the ejection of the inner-shell electron as a photoelectron. The resultant energy of the photoelectron becomes the difference between the incident photon energy and electron binding energy.
The photoelectron then ionises other atoms in the tissue locally via ionisation and excitation, contributing to the patient dose.
An Auger electron is an outer-shell electron with binding energy less than the energy difference of the electron transition.
The inner-shell vacancy is then occupied by an outer-shell electron seeking a lower energy state. This release a photon of discrete energy level equivalent to the difference in orbital shells and is termed characteristic x-ray.
The electrons rearrange, producing an electron cascade
The characteristic x-ray produced via photoelectric absorption in tissue is of lower energy than the characteristic x-rays produced in an x-ray tube. This is because the atomic number and inner-shell binding energy of soft tissue is far lower than that of the target anode (usually tungsten).
Probability
- For the PE effect to occur, the incident x-ray must have energy equal to or greater than the binding energy of the inner-shell electron.
- The absorption of photons increases markedly as the x-ray photon energy is increased from below to above the binding energy of the K-shell electrons (K-edge).
- The probability of PE absorption then decreases rapidly as the photon energy (E) further increases above the K-edge and is proportional to:
PE ∝ Z3/E3
Where Z = atomic number, E = photo energy.
- The PE effect predominates at energies just above the K-edge of the absorber.
- The probability of PE absorption increases significantly with atomic number and is proportional to Z3.
- The more tightly bound an electron is, the greater is the probability of the PE effect, if E is greater than the binding energy.
- Photoelectric absorption is thus highest for K-shell electrons, which are most tightly bound in an atom, followed by the L-shell, and so on.
- The PE effect is important if the atomic number (Z) is high and the photon energy is just above the K-edge.
- Photoelectric absorption is inversely proportional to photon energy and therefore attenuates the lower energy photons.
Incident photon must have nergy > binding energ
AND PE absorbtion most liely to occur when photn and energy and binding energy are almost the same
The tighter an electorn is bound, the more likely it is to be inovlved in photoelectric reaction
The photoelectric effect yields 3 end products
- Characteristic radiation
- Ejective electron
- Atom deficient of one electron
How is this different to characteristic radiation produced at the x-ray tube?
Within the x-ray tube, characteristic radiation is generated by the interaction of high energy electrons with tightly bound electrons in the target material.
When talking about x-ray interactions with mtter, characteritic radiaiton is generation by the interaction of x-ray photons with tightly bound electrons in tissue.
The characteristic x-ray produced by the photoelectric effect have such low energy (as the K-shell binding energy of soft tissue elements H C O N is about 1kEV)), as such are absorbed almost immediately and have no role in image formation.
Also, the energy difference is more likely to be released i nthe form of an Auger electorn, rather than a characteristic x-ray.
Advantages of PE
- Images have excellent image quality, does not produce scatter radiation)
- Enhances natural tissue contrast
- PE magnifies difference in tissue composed of different elemnts due to Z3 dependence (particularly useful in mammography)
- Contrast agents
- High Z (barium 56 iodine
- Lead protection
Disadvantages of PE
- As all energy is absorbed, results in higher dose
- Can minimise the PE by using a high energy technique (i.e. high kVP, e.g. CXR)
Compton Scattering
The Compton effect refers to interactions between an x-ray photon with an outer shell (loosely bound or free) electron. The photon causes ionisation of the atom and scatters in a different direction with less energy.
Compton scatter is the predoiminant interaction of x-ray photons with soft tissue in general radiology above 26 kEv
Compton effect is inversely proportional to the x-ray energy and independent of atomic number as it
only involves free electrons whose binding energy is negligible. However, as photon energy increases, the rate of photoelectric absorption decreases more than the Compton effect. Hence, at high photon energies, the Compton effect still predominates. Photoelectric absorption predominates in high-atomic-number materials, e.g. lead and contrast medium. Hence the Compton effect predominates in low-atomic-number materials e.g. air, water and tissue.
Factors that determine amount of scatter:
- kVP – high kVP
- Decreasing the kVP will however result in increased patient absorbed dose and decrease in beam penetrance, possibly resulting in loss of contrast resolution
- part thickness – thick sections
- field size – large field size (most important factor)
Impact of scattered radiation
Forward scatter will decrease the contrast resolution of the image (not the noise).
Increase absobrbed dose, especially at the ski netntrance
Scatter reduction
- Reduce irradiated volume by using collimation
- Reduce patient thickness if applicable, by using compression in mammography
- Reduce kVP increased contrast resolution produced by increasing PE effect, however, this results in higher patient absorbed dose
- Anti-scatter grid
- Air-gap technique
Linear Energy Transfer (LET)
The linear energy transfer refers to the amount of energy that an ionizing particle transfers to the material traversed per unit distance. A high LET will attenuate radiation more quickly
Linear attenuation co-efficient: the fraction of photons removed fro ma beam per unit thickness (per cm)
Linear attenuation depends on characteristic of the medium (atomic nmber Z and density p) and x-ray photon beam.