X-ray interactions Flashcards
What do kV and mA control during x-ray production?
-KV: controls the accelerating voltage between anode and cathode. increasing kV shifts spectrum to right, increased max/effective energy/increased total no of photons
-mA: controls the tube current (energy and quantity of electrons flowing between anode and cathode). increases quantity of photons
What effect does filtration have on the beam produced?
moves spectrum to right, increases minimum and effective energy, reduces no of photons.
Brief overview of x-ray production
-Tungsten filament within cathode is heated until incandescent –> allows thermionic emission.
-Electrons are focussed via the cup and attracted to the anode target (tungsten) while travelling through a vacuum.
-High frequency generators are used to maintain a near constant waveform (allows increase x-ray output and lower pt dose)
-Bremsstrahlung and characteristic radiation occurs to produce photons.
What is the focusing cup made up of? What kind of material is it
Molybdenum, negatively charged (repels electrons). Has very high melting temp and very poor thermionic emitter.
Anode: material, Z number, features
-Tungsten rhenium target (high Z number)
-Rhenium helps with longevity
-Molybdenum disk and stem for poor heat transfer. Can have silver lubricant/blackened copper rotor.
Whaat is the Tungsten K edge (to produce characteristic radiation)?
70kV
How is heat removed from the x-ray?
Removed by conduction through oil and radiation through glass vaccuum.
What are the types of x-rays produced at the anode?
Characteristic and Bremsstrahlung
What is Bremsstrahlung radiation? % of rays produced
-Braking of electron near nucleus released produces x-ray.
-80% of rays produced this way.
What is the effect on Bremsstrahlung radiation when you change the following features:
-kV
-mA
-Filtration
-Increasing kV: increases quantity, average energy and minimum energy
-Increasing mA: increases quantity of photons only
-Filtration: reduces quantity of photons, average energy and min energy.
How is atomic number linked to Bremsstrahlung radiadtion?
The higher the atomic number, the more protons in nucleus, the more radiation produced. (proportional)
What is characteristic radiation?
-Occurs at anode
-Incoming electron interacts with inner shell electron of target material and the electron is ejected.
-Creates a vacancy, which is filled by dropping down of an upper shell electron. This dropping down releases characteristic radiation.
When can characteristic radiation also be emitted (in tissues)
After photo-electric effect occurs, if the energy is higher than the binding energy of that shell.
Name 5 factors that affect the x-ray spectrum
- Filament current (mA) - affects quantity of electrons
- tube potential (kV): increases quality, quantity and max energy
- Filtration: filters lower energy electrons via photo-electric effect, decreases quantity if photons, increases average energy .
- Anode target: higher atomic Z, more Bremmstrahlung occurs (more quantity).
- Generator waveform: increases quality and quantity of electrons
Name the 3 types of interactions between x-rays and tissues
- Transmission
- Attenuation: absorption of x-ray (photoelectric effect)
- Scattering: deflection of photon (Compton, elastic and pair production)
Describe photo-electric effect (PEE). How does this contribute to patient dose?
-Photon hits inner shell electron and is completely absorbed + a photo-electron escapes (low energy <1 keV, will be attenuated in tissues and not contribute to image)
-Ejected electron creates a vacancy and a higher electron will come down and release characteristic radiation.
-End product of PEE: ion, low energy x-ray and photo-electron.
What 3 factors affect the probabilities of PEE?
- higher atomic number (Z) (eg iodine): more PEE
- Higher tissue density (p): more PEE
- Higher energy (E): less PEE b/c more likely to pass straight through without interacting with the matter.
Probility = pZ^3 / E^3
What is a K edge and why is it useful?
-Energy level at which the K-shell electron will be released (binding energy of the electron)
-Below this range: cannot release the electron and t/f no PEE occurs.
-At the edge: increase likelihood of the PEE to occur and can use this to our benefit at diagnostic ranges to increase contrast between iodine and surrounding tissues.
What is Compton scatter mechanism?
-Incident x-ray interacts with outer shell electron –> photoelectron is released and the x-ray scatters (not all energy is given to photoelectron).
-The angle of theta will be proportional to energy of new photoelectron (larger scatter angle = more energy conferred to photoelectron) and the less energy the Compton scatter has.
*Increasing theta decreases Compton scatter energy
What is the effect of atomic number, electron density, mass density on Compton scatter?
-Unaffected by atomic number
-Affected by density of the tissue and electron density.
What contribution to image does the Compton effect provide? (to contrast/noise/image quality/patient dose)
-Contributed to patient dose
-Reduces image quality
-Decreases contrast and increases noise
What is Elastic/Rayleigh’s scatter? When does it occur/does it contribute to patient dose?
-Xray interacts with all electrons in the atom –> all the electrons oscillate temporarily –> when they rest they release x-ray radiation at an angle.
-Occurs at lower diagnostic ranges, when photon energy is less than the binding energy. All energy is conserved.
-Doesn’t contribute to patient dose
What is HVL? What is the effect with beam hardening?
-Thickness (distance) of tissue required to halve the number of photons in a beam
-Is inversely proportional to LAC: as LAC increases (removes a large proportion of x-rays quickly) - HVL decreases
-Increases with beam hardening.
(beam hardening only occurs if the beam is not mono-energetic)
What is the LAC? Equation?
-Determines the amount of x-rays removed from a beam as it travels a set distance through tissue
-0.693/HVL
What is the MAC? How is it related to LAC and density? What factors affect MAC?
MAC = LAC/density, t/f is independent of density.
-only Z and E affect MAC.
-Described the attenuation of a monochromatic beam per unit mass
