What is the nature of X-rays?
- Electromagnetic energy
- Natural and produced by Sun
- Absorbed by atmosphere
- Travel in straight lines
What are the properties of X-rays?
- Electromagnetic
- X-ray beam has millions of photons of different energies
- Travel in straight line
- Energy carried can be attenuated by matter
- X-rays capable of ionizing matter and causing biological damage in living tissue
- X-rays undetectable by human senses
What is a photon?
Packet of energy (quantum)
What is the relationship between the wavelength and frequency?
Shorter wavelength:
- Higher frequency.
- Higher energy.
- More penetrating radiation.
Define wavelength
Distance between 2 similar points on 2 successive waves.
Define frequency
No. of waves that pass given point per unit of time
Define velocity
Speed of wave
How are X-rays produced?
- X-ray machine turned on and length of exposure selected with timer.
- Activate low-voltage circuit – heat filament.
- Activate high-voltage circuit – pull electrons across tube.
- Electrons cross tube, strike target and produce x-rays.
- The X-rays pass through the filter and collimator before exiting through the PID.
- X-ray production stops when exposure time ends – release exposure button.
Why is the X-ray production process inefficient?
Only 1% of interactions result in X-rays and rest produces heat.
What happens to excess heat produced by X-rays?
Excess heat absorbed by high tungsten target, copper sleeve and cooling from oil surrounding X-ray tube.
What is the kVp – kilovoltage peak?
Maximum voltage across the tube between anode and cathode (70K).
What is the eV – electron volt?
Force applied to electrons.
What is the keV – kiloelectron volts?
Measures kinetic energy of electrons (70keV).
Name 3 X-ray interaction reactions
- Heat producing reaction
- Bremsstrahlung/braking reaction
- Characteristic reaction
Explain the heat producing reaction
- Incoming electron is deflected by cloud of outer-shell tungsten electrons, with small loss of heat energy.
- Incoming electron collides with outer shell tungsten electron, displacing it to more peripheral shell (excitation) or displacing it from the atom (ionisation) – small loss of heat energy.
Explain the Bremsstrahlung/braking reaction
- High-speed electron from filament enters tungsten atom.
- Electron slowed down by +ve charge of nucleus – energy released as X-ray.
- Electron continues in different direction to interact with other atoms until all energy lost.
- Maximum energy:
- High-speed electron from filament enters tungsten atom – loses energy and disappears.
- X-ray produced has energy equal to that of high-speed electron (maximum energy possible).
Explain the characteristic reaction
- Electrons aim towards the nucleus and collide with an inner-shell tungsten electron (K/L/M shell) and displaces it (ejected electron).
- Electrons jump from one shell to another resulting with subsequent emission of X-ray photons with specific energies.
- Electrons jump so the atom returns to its neutral state.
- Photons that are only produced by X-ray tubes operating at no less than 70 kV (critical voltage (Vc)).
- K lines are of importance since L lines have too little energy.
What is attenuation?
Reduction in intensity of X-ray beam
Name the 4 types of interactions of X-rays at atomic level
- Pair production
- Coherent scatter
- Photoelectric effect
- Compton/incoherent effect
What is Pair production?
Pure absorption
What is Coherent scatter?
Pure scatter
Unmodified/Rayleigh scattering
How does Coherent scatter work?
- Low energy X-rays (< 10keV).
- Photons interact with matter without change in wavelength (unmodified).
- No ionisation no energy transferred.
- X-ray/photon is scattered in different direction unchanged – forward.
- Contributes to fogging.
What is photoelectric effect?
Pure absorption
How does the photoelectric effect work?
- Primary x-ray strikes an outer-shell electron, knocking it out of its orbit (ionisation).
- X-ray loses all of its energy and disappears.
