Radiotherapy Equipment Flashcards
What are the differences between diagnostic and therapeutic X-Ray systems?
Diagnostic -
- Rotating anode at step angle (6-15deg)
- Adjustable light-beam diaphragm/collimators
- Target at positive voltage
- Typically fixed filtration
Therapeutic:
- Fixed target at shallow angle (30-40deg)
- Interchangeable applicators
- Target at ground (0V)
- Interchangeable filter
Why is Tungsten used as the cathode in a therapeutic X-Ray set?
High melting point.
High atomic number - low binding energy of outer shell electrons.
By which process are electrons released from a tungsten filament?
Thermonic Emission
How does the filament current affect the intensity of the x-ray beam?
As the filament current increase numer of electrons accelerated per unit time increases.
X-Ray intensity is the number of photons of a given energy per unit area per unit time.
More electrons make more photons, more photons give more total-energy to beam.
How is the intensity of a beam proportional to the voltage across the tube?
Proportional to kV^2
Why is a non-rotating anode feasible in a therapeutic X-Ray set?
Small focal spot (required for sharp images) not needed. Energy can be deposited over a larger area.
What percentage of electrons produce either characteristic X-Rays or bremsstrahlung at therapeutic energies?
1%
Why are additional filters added to the end of the X-Ray set?
Harden the beam by removing unwanted low-energy “soft X-rays” that only contribute to skin dose.
What are two sources of inherent filtration in a therapeutic X-Ray set?
Filtration from the anode itself.
Filtration through Beryllium window.
What functions do the applicators provide?
Collimation of the therapeutic beam.
Keeps Fixed SSD.
What are the differences between at Co-60 Unit and a Linac?
Co-60: - Fixed energy (1.17 & 1.33MeV) - Single modality - Cheaper - Specialised Systems - Gammaknife Linac: - L-Shaped gantry - Multi-Modality and varying energy - Better penetration than Co-60 - Specialised systems - Tomotherapy, Cyberknife.
What are the main components of a Linac gantry?
- Modulator
- Electron gun
- Microwave Generator
- Accelerating Waveguide
- Bending Magnet
What does a modulator primarily consist of?
- Pulse forming Network
- Thyratron (high-speed switch)
What are the main components of the electron (triode) gun?
- Heater (element)
- Cathode
- Grid
- Anode
What shape is the cathode and what is the reason for this?
Concave shape focusses electron emissions
What is the function of the grid?
High-speed switching of +/- potential
Pulses adjusted to phase match electron injection with microwaves
What are the two devices used to generate microwaves in a linac?
Magnetrons and Klystrons