2. How are x-rays produced? Flashcards
LOs
- Explain how x-rays are produced including the atomic interaction
- Describe the basic features of dental x-ray generating equipment
- Give details of current recommendations and regulations concerning x-ray equipment
- Explain the exposure variables kV and mA and time
x-ray machine structure?
Tubehead = main body of machine , hold to move the x-ray to different positions
Glass x-ray tube = where x-rays are produced
- cathode = filament (coil of wire) on one end
- anode = tungsten target set into copper block, on other end
- entire glass tube surrounded by oil , facilitates heat removal
Step up transformer = increases mains voltage of 240 volts to high voltage across glass tube , 60-70 kv
Spacer cone = indicates direction of beam + sets ideal distance to target
Where are X-rays produced ?
Occurs inside glass X-ray tube at atomic level
What are the two ends of a X-ray ?
Cathode end
- negatively charged
- filament made of tungsten wire
Anode end
- positively charged
-Tungsten target, set into large block of copper
function of tube head components
- glass x-ray tube
- step up transformer
- surrounding oil
- Cu block
- spacer cone
- where x-ray produced
- required to step up mains voltage of 240 volts to 60-70000
- facilitates removal of heat created during the production of x-rays
- conduct heat away from target quickly
5.
- device for indicating direction of beam
- setting ideal distance of x-ray tube’s target to skin of patients face
How do X-rays work ?
- Electric current flows through filament at cathode end + heats it up
- Electrons from tungsten wire are boiled + released from filament , cloud around end of filament
- Electrons need to travel from filament to target block
- Voltage applied , 60-70kV from step up transformer
- Electrons provided with energy allowing them to accelerate across gap to target
- Electron collision with target , produces heat which is conducted by copper block
- X-rays produced
What are the electron interactions at tungsten target ?
Heat production , conducted away by copper block
Produce X-rays
- most produced in one direction and pass down spacer cone
Some X-rays leave in other directions
- leakage of tube head
- emission during exposure
- hence must never directly hold tube head
Efficiency of X-ray process
Very inefficient
1% of e- energy used for X-ray
99% of e- energy lost as heat
Heat loss management of X-ray
Copper block
- good conductor of heat
Oil surrounding X-ray glass tube
-dissipates heat
1.
Importance of electron collision with tungsten nucleus
/
how are x-ray photons in dent produced?
- lower energy or higher energy photons more dangerous?
1.
Electron from filament is negatively charged
Accelerates towards positive tungsten nucleus
Causes deflection of electron + slows it down
Causes loss of energy + converted into X-ray photon
How close electron is travelling to the nucleus determines the degree of deflection and speed reduction
More the electron slowed down = bigger the deflection = more powerful X-ray photon lost
X-ray photons are all different + form a continuous spectrum
2.
lower energy photons = more dangerous = will stop in patients soft tissues + be absorbed
Features of photon energy from nucleus interaction
/
what does photon energy depend on?
Low energy photon have low penetrating power
- stop in patients soft tissues + absorbed
- more dangerous
High energy photons pass through patient and hit film
- more diagnostically useful
Photon energy depends on the energy/acceleration of incoming electron
Incoming electron energy depends on size of voltage applied
Lots of low energy photons
Very few high energy photons
Creates continuous spectrum
E. Max
60kv machine, e.max?
Max energy any one bullet can have
E.g for a 60 Kv machine , E.max is 60’000 electron volts
Higher the kV machine , the greater the energy the X-ray photons that are produced
Importance of electron collision with orbiting electron of tungsten
/
other ways x-rays can be produced if voltage applied is 70kv or more
Incoming e- from filament passes into tungsten atom
Interacts with orbiting e- causing it to be knocked out
Incoming e- is deflected
Displacement of e- from shell causes atom to be unstable
Electron from outer adjacent shell drops in to fill space
Moving between shells = dropping in energy levels
- causes energy to be emitted in form of X-ray photon
Creates cascade
Free e- will be captured to allow atom to become stable
These emitted X-rays form a characteristic spectrum
- only at certain specific energy levels
Features of photon energy from electron interaction
(70kv or more voltage)
Only X-ray photons of a specific size or energy produced
- not full range
Photon energies are characteristic of tungsten
- material of origin
Only produced at voltage of 70 kV or more
Lower kV doesn’t have enough energy to knock out inner e- from tungsten
Characteristic spectrum
(70kv or more voltage)
X-ray photons of specific energies are emitted following repositioning / re-arrangement of orbiting electrons within specific shells
How are low energy photons removed from beam before hitting patient ?
why is this useful?
Beam passed through aluminium sheet
Absorbs + filters out lowest energy photon
Low penetrating power so cannot pass through aluminium sheet
Allows less low energy photons to be absorbed by patients
Important radiation safety feature to reduced radiation dosage
Recommendation for thickness if aluminium filter
1.5 mm for X-rays operating at 70kV or less
2.5 mm for X-rays operating at 70 kV or more
Main controls of X-ray production via control panel/timer
Kv = kilo voltage
- usually at fixed level
MA = milli amperage
- usually fixed
Time
- can change time of exposure
- anatomical timer , chose time depending on tooth
What is the kV ?
Determines energy of the photons
Determines quality of X-ray beam
Higher the kV = greater energy given to e- = greater the energy of photons
Determines penetrating power
Affects film contrast
- increasing kV , difference between black and white less obvious
Higher kV beneficial to patient , less absorbed
(higher KV + more shades of grey)
(more photons = blacker)
What is mA and time ?
Together they determine quantity of X-ray photon produced
MA = current flowing through the X-ray tube
- determines number of e- that travels from filament to target
Time = how long exposure lasts
- determines total number of e- that travels along gap between filament and target
- Greater the number of e- leaving filament = greater no of photons produced at target
- more photons = more blackening + overexposure
- No of photons affects degree of blackening of film
Over exposure = dark
Under exposure = light
Equipment recommendations
Operating range : 60-70 kV
DC or AC potential output
Half wave rectification of AC output
Negative half of cycle removed
Electron flow becomes pulsatile
Inefficient , exposure times are longer
Each pulse has to start from 0 and reach peak operating conditions each time
Causes more lower energy photons
Cheapest + easiest
Single wave rectification of AC
Negative half of cycle converted into positive half
Simultaneous pulsatile current
1.
Three phase full wave rectification of AC
2.
AC Vs DC
1.
Creates ripple effect
2.
DC
= more efficient
= more expensive
= recommended by 2001 guidlines
Definition of focus to skin distance
Distance from focal spot on target ( point where X-ray produced )
To end of spacer cone that touches patients face
Fsd
Ideally should be long
Position of focal spot is marked on outer plastic covering of tube head
Why do we need a long fsd ?
Short fsd : beam diverges + a larger area of face is radiated
Long fsd : only central part of bean , smaller area of face radiated
Current recommendation for fsd
200 mm
Or long cone
What is the collimator , beam size and spacer cone?
Collimator = device that limits beam size
Spacer cone = determines distance of target to patient and directs beam
Beam diverges slightly when travelling from collimator through spacer cone
Recommendations for collimation + beam size
Rectangular collimation : 40mm X 50 mm
Maximum diameter of circular collimators = 60 mm
summary/ key points to know
- not all x-ray photons (bullets) are the same)
- lower energy photons = lower penetrating power + stop in patient soft tissues + absorbed
- high energy will pass through patient + hit film
- low energy photons = more dangerous
- high energy photons = more diagnostically useful
- overall photon energy = determined by energy of incoming e-
- incoming e- energy = determined by size of voltage applied (KV)
