X-ray Production Flashcards
3 stages of X-ray production
- Electrons accelerated towards atom at very high speed
- On collision, kinetic energy of electrons converted to heat and EM radiation (X-ray photons)
- X-ray photons aimed at subject
Main components of X-ray unit
Tubehead, collimator (cap to aim X-ray beam), positioning arm, control panel and circuitry
Components of X-ray tube
Glass envelope (vaccum inside)
Cathode (negative)
Anode (positive)
Describe 2 main parts of cathode (negatively charged side of X-ray tube)
Filament- Coiled metal wire points toward anode, heats up until thermionic emission where electrons are released. Made of Tungsten
Focusing cup- metal plate around filament. Negatively charged to focus electrons to anode. Made of molybdenum.
Describe cathode-anode relationship
High voltage passes through, electrons released at filament are repelled, attracted to anode creating big potential difference.
Greater PD, greater acceleration, greater kinetic energy
How does the X-ray tube get the voltage it needs
Transformers take mains electrical supply (220-240V) and convert by changing voltage and current
Name two types of transformer in X-ray tube head
Step up- takes mains and increases PD across X-RAY TUBE to 60-70 thousand volts
Step down- Decreases PD across FILAMENT to about 10V
Define electron volts (eV)
1 eV= Kinetic energy gained by one electron moving across PD, from cathode to anode, of 1V
(PD is 70kV, each electron will have 70keV of kinetic energy)
Main features of target of anode
Metal block bombarded by electrons
Produces photons and heat
Made of tungsten (high melting point)
Off angle ( not facing cathode straight on)
Has focal spot (made by focusing cup) where electrons hit and X-RAYS ARE PRODUCED
Describe heat-dissipating block of anode
Where target is embedded
Heat produced by target dissipates into block by thermal conduction (REDUCES RISK OF OVERHEATING)
Made of copper (high melting point and thermal conductivity)
What is penumbra effect
Blurring of radiograph due to focal spot not being a single point but rather a small area
Small focal spot= reduced penumbra effect
What is the adaptation within the X-ray tube to reduce penumbra effect
Angled target of anode
Main features of glass envelope
Air tight-contains cathode and anode
Maintains a vaccum
Made of leaded glass
APART from un-leaded window-> X-ray photons travel in desired direction
X-ray tubehead main features
Metal (Lead) shielding, absorbs X-rays -> window where X-ray beam exits
Aluminium filtration
Oil surrounds X-ray tube for dissipation
Space cone
Need for aluminium filtration?
Remove lower energy X-ray beams (low energy photons increase dose but do not contribute to image)
Minimum thickness of aluminium required for aluminium filtration
<70kV= 1.5mm
>70kV= 2.5mm
N.B most modern equipment operates from 60-70kV
Function of spacer cone
Maintains distance between focal spot and patient
Known as FOCUS TO SKIN DISTANCE
Also indicates direction of beam
Focus to skin distance requirements
< 60 kV 100mm
>60 kV 200mm (MODERN EQUIPMENT)
Usually dot on X-ray tube head to indicate focal point
3 fates of X-ray photons in xray tubehead
Attenuated by lead shielding (of glass, tube head, spacer cone)
Attenuated by aluminium filtration (too low energy)
Exit tubehead to form X-ray beam
Main features of Collimator
Lead attached to end of spacer cone
Reduces pt dose
Crops X-ray beam to match shape and size of receptor
Standard in new equipment and fitted in old
FUNCTION of collimator
Reduces pt dose up to 50% of original area
Changes from 60mm diameter circular beam to 45x35mm rectangular beam (size 2 receptor)
Also reduces SCATTER
Features of control panel
On/off
Timer
Exposure time selector and presets
Warning light/noise
Kilovoltage selector
What are the two X-ray producing electron interactions
Continuous radiation interactions (most)
Characteristic radiation interactions
Describe continuous radiation interaction
Electron passes close to nucleus (tungsten nucleus in target of anode)
Electron decelerates
Kinetic energy lost
This is released as X-ray photons
In continuous radiation interactions, why are there X-ray photons produced at different energies?
Closer to nucleus> more deceleration and deflection> more energy released> higher energy photon
MORE LOWER ENERGY PHOTONS AS MORE ELECTRONS DO NOT COME CLOSE TO NUCLEUS OF TUNGSTEN ATOM
Describe characteristic radiation interaction
Electron from cathode displaces electron in tungsten atom or removes it completely
Electrons rearrange and as an electron drops from outer shell to inner shell, releases a photon of a specific energy
