X-ray Production Flashcards
What are the basic principles of x-ray production?
- Electrons accelerated towards atoms at very high speed
- On collision, the kinetic energy of these electrons is converted to heat & electromagnetic radiation (ideally X-ray photons)
- The X-ray photons are aimed at a subject
What are the main components of the dental x-ray unit?
- Tubehead (with X-ray tube)
- Collimator
- Positioning arm
- Control panel
- Circuitry
What is in the x-ray tube?
Glass envelope
* Vacuum inside
Cathode (-ve)
* Filament
* Focusing cup
Anode (+ve)
* Target
* Heat-dissipating block
What is a cathode filament and what does it do?
a coil of wire that is heated to produce electrons in an x-ray tube
Low-voltage, high-current electricity passed through wire
* Heats up until incandescent (~2200 °C)
* Electrons released from atoms in wire by thermionic emission
* “Cloud” of electrons forms around cathode
incandescent: emitting light as a result of being heated.
How are the electrons released from atoms?
thermionic emission
(the process of releasing charged particles, also known as thermions, from a hot metal surface)
What does an increase in current in the filament mean?
increase in heat and electrons
What is the filament made out?
tungsten
What are the qualities of tungsten?
- High melting point (3422°C)
- High atomic number (Z = 74) - lots of electrons per atom
- Malleable
What is the focusing cup, what is it made of and what does it do?
Metal plate shaped around filament
* Negatively charged → repels electrons released at filament
* Shaped to focus the electrons at a small point on the anode target
Made of molybdenum
* High melting point (2623°C)
* Relatively poor thermionic emitter
What does an increase in potential difference increase?
increase in acceleration
increase in kinetic energy
How do the electrons travel?
Electrons released at filament are repelled away from the cathode (filament & focusing cup) & attracted to the anode (target)
* Accelerate to a very high speed over a very short distance (up to half the speed of light)
What do transformers do?
ransformers take mains electrical supply (220-240V) & convert it by changing the voltage & current
What are the two transformers present in the tubehead?
what do they each do?
- Step-up transformer
increase potential difference across X-ray tube to 60,000-70,000V - Step-down transformer
decrease potential difference across filament to ~10V
What is a electron volt?
Unit used to measure the kinetic energy gained by electrons as they accelerate from cathode to anode
1 eV = kinetic energy gained by 1 electron moving across a potential difference of 1 volt
What is the anode made of?
tungsten
What is the anode and what does is produce?
Metal block bombarded by electrons
* Produces photons (& lots of heat)
What is the focal spot?
prescise area on target where electrons collide & X-rays are produced (ie. the X-ray “source”)
What is the role of the heat disspitating block that the target is embedded in?
what is it made of?
Heat produced in target (by electron collisions) dissipates into this block by thermal conduction
- Reduces risk of overheating which may damage target
Made of copper
* High melting point (1085°C)
* High thermal conductivity
What is the penumbra effect?
Blurring of radiographic image due to focal spot not being a single point (but rather a small area)
* Minimised by shrinking size of focal spot
What is the problem with a small focal spot?
- <1% of kinetic energy from the electrons is converted to X-ray
photons whereas ~99% is converted to heat - ↓ focal spot size = ↑ image quality but ↑ heat concentration
How can the heat problem be solved in small focal spots?
Angled target
* Increases the actual surface area where electrons impact
* ↑ better heat tolerance
* Reduces the apparent surface area from where the X-ray beam is emitted
* ↓ penumbra effect
What is the glass envelope and what is it’s role?
Air-tight enclosure
* Supports cathode & anode
* Maintains a vacuum
* Electrons able to travel from cathode to anode unhindered by gas molecules
What is the glass envelope made of?
Leaded glass to absorb X-ray photons
* Except for an un-leaded window
* Only the X-ray photons traveling in the desired direction can escape from the X-ray tube
What are the main components of the tubehead?
- x-ray tube
- metal shielding
- alumination filtration
- oil
- spacer cone
What does the metal shielding do?
usually lead
absorbs x-rays
window where x-ray beam exits
What is the role of the oil?
dissipates heat produced by x-ray tube by thermal convection
What does filtration do and how?
Removes lower energy (non-diagnostic) X-rays from beam
* Low energy photons would all be fully absorbed by patient’s tissues
& increase patient dose but not contribute to image
Aluminium (of adequate thickness) is able to absorb these photons so that the resulting X-ray beam contains mostly diagnostic X-ray photons
What is the minimum thickness of aluminium required for <70kV and >70kV?
- <70kV→1.5mm
- ≥ 70 kV → 2.5mm
(modern equipment operates in the range of 60-70 kV)
What is the role of the spacer cone?
Dictates distance between focal spot (of target) & patient
* “Focus to skin distance” (fsd)
* Altering fsd will affect degree of divergence of X-ray photons in X-
ray beam (& affect intensity of X-ray beam)
* Increasing fsd reduces divergence of X-ray beam & therefore reduces magnification of image (but also reduces intensity of X-ray beam)
* A set distance helps ensure a consistent radiographic technique
What are the specific sizes required for fsd?
- <60kV→100mm
- ≥ 60 kV → 200mm (ie. modern equipment)
What is the fate of the x-ray photons emitted from the focal spot?
attenuated by lead shielding
attenuated by aluminium filtration
exit tubehead to form x-ray beam
What is a collimator, what is it’s role and what does it do?
Lead diaphragm attached to end of spacer cone
* Reduces patient dose
Crops X-ray beam to match size & shape of X-ray receptor
* Tubeheads inherently create a circular beam
* Collimators typically change circular cross-section to a rectangular cross-section
What is the beam area reduction when using size 2 receptors with collimators?
50mm x 40mm
What are the advantages/disadvantages of rectangular collimation?
advantage - can reduce effective dose to patient by approximately 50%
disadvantage - increases risk of collimation errors (but this can be minimised using good radiographic technique)
What does the control panel have?
- On/off switch & light
- Electronic timer
- Exposure time selector & presets
- Warning light & noise (for when X-rays are being generated)
- (Kilovoltage selector)
What are the consequences of electrons bombarding target?
- heat production (involve outer shell electrions of tungsten atoms at target)
- x-ray production (involve inner shell electrons and nuclei of tungsten atoms at target, less than 1% of interations)
How do the heat producing interactions occur?
Bombarding electron reaches tungsten outer shell electron & either:
- Comes into close proximity & is then decelerated & deflected - both negatively charged
- Collides & is deflected
Bombarding electron loses kinetic energy which is converted to heat
Heat energy dissipated
Tungsten target > copper block >oil in tubehead > air
How do the x-ray producing interactions occur?
2 ways
Continuous radiation interactions (majority)
Characteristic radiation interactions
What is continuous radiation?
Bombarding electron passes close to target nucleus, causing it to be rapidly decelerated & deflected → lost kinetic energy released as X-ray photons
* aka. Bremsstrahlung (ie. German for “breaking radiation”)
What is the continous radiation spectrum?
Photons produced over wide range of energies
* increase in proximity of electron to nucleus → increase in deceleration + deflection → increase in energy released
- Greater proportion of lower energy photons
When is the maximum energy achieved?
when electron collides directly with nucleus & stops completely
* Rare
* Numerically identical to the potential difference (ie. voltage) across the X-ray tube
* eg. 70 kV → 70 keV photon produced
What is characteristic radiation?
Bombarding electron collides with an inner-shell electron & either displaces it into a more peripheral shell (excitation) or removes it completely (ionisation)
The remaining orbiting electrons rearrange themselves to re-fill the innermost shells
* When an electron “drops” to a lower shell it loses energy which is emitted as a photon of specific energy
* Values depend on the element involved (eg.tungsten)
What is the characteristic radiation spectrum?
Photon energy equals the difference in the binding energies of the 2 shells involved (which are specific to that element)
Target made up of tungsten atoms
* K shell binding energy = 69.5 keV
* L shell binding energy = 10.2 keV
* M shell binding energy = 2.5 keV
Can dental x-ray tubes displace the k shell electrons?
dental X-ray tubes often operate at 70 kV so that bombarding electrons have sufficient energy (70 keV) to displace K shell electrons
Continous vs Characteristic
Continuous
* Produces a continuous range of X-ray photon energies
* Maximum photon energy matches the peak voltage
* Bombarding electron interacts with nucleus of target atom
Characeristic
* Produces specific energies of X-ray photon, characteristic to the element used for the target
* Photon energies depend on the binding energies of electron shells
* Bombarding electron interacts with inner- shell electrons of target atom
What is the x-ray beam made of?
continuous radiation + characteristic radiation - filtered photons
