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)