Dental Radiology Flashcards
what are x-rays a form of?
electromagnetic radiation
what are the properties of electromagnetic radiation?
No mass
No charge
Travel speed of light
Travel in vacuum
what are the 7 main groups on the EM spectrum?
Divided into 7 main groups
* Gamma ray (shortest wavelength, highest frequency, highest energy)
* X - ray
* Ultraviolet
* Visible
* Infrared
* Microwave
* Radio (opposite gamma)
how is frequency described?
how many times waves shape repeats per unit time
what are the units of frequency and what does it mean?
- In hertz
- One hertz = one cycle per second
what is the description of wavelength?
distance over which wave shape repeats
what is the formula for speed?
Speed = frequency X wavelength
What is the speed of EM?
speed of light
what happens if frequency increases?
wavelength decreases
how are photons known?
o Em radiation involves the movements of energy as packets of energy known as photons
what is photon energy measured in?
o Energy usually measured in electron volts, eV
what are photon energies in x-rays? (number)
124eV
what are the 2 types of x-rays
Hard (higher energies)
* Able to penetrate human tissues
Soft (lower energies)
* Easily absorbed
what are properties of x-rays?
Form of EM radiation
* No mass
* no charge
* very fast
* travel in vacuum
undetectable to human senses
man made
cause ionisation
describe the basic production of x-rays?
electron fired at very high speed
collision – kinetic energy converted to EM radiation and heat
photons aimed at a subject
what is the central nucleus made up of?
- Protons
o Number of these = atomic number (Z) - Neutrons
- Mass number (A) = no. protons + neutrons
what is the innermost electron shell?
K
what are the max no. electrons a shell can hold
o K= 2, L= 8, M=18, N=32
how are orbiting electrons held within their shells?
electrostatic force
what is description of binding energy?
Binding energy = additional energy required to exceed electrostatic force and remove an electron from shell
what shell has the highest binding energies?
K shell
what does a more positive charged nucleus mean?
greater electrostatic force
what is the enrgy requeired to move an electron to a more outer shell?
Diff in biding energies of 2 shells, if to a more inner shell energy is released
what is current described as?
Flow of electric charge, usually by movement of electrons
what is unit of current?
- Measure of how much charge flows past a point per sec
what are the descriptions of Direct and alternating current?
- Direct current (DC) = constant unidirectional flow
- Alternating current (AC) = flow repeatedly reverses direction
what is voltage described as?
Diff in electrical potential between 2 points in an electrical field
what does a transformer do?
Alter the voltage and current from one circuit to another
what are the 2 separate transformers required for an x-ray unit?
step up and step down transformer
what does a step up transformer do?
- Increase potential diff across x-ray tube
what is the voltage of a step up transformer?
usually 60-70 kV
what is the current reduced to in a step up transformer?
milliamps (mA)
what is description of a step down transformer?
- Decrease potential diff across filament
what does x-ray production require?
requires a unidirectional current
what doe x-ray units powered by?
mains electricity (AC)
what do x-ray unit generators do? and what is process known as?
modify AC so it mimics a constant DC
process known as recitfiication
what is intensity proportional to?
Proportional to current in filament (mA) and potential diff across x-ray tube (kV)
what is intensity of beam inversely proportional to?
o Intensity of beam inversely proportional to square of distance between x-ray source and the point of measurement
o Therefore doubling distance will quarter dose
explain basics of collision between electrons and atoms?
o Electrons accelerated towards atoms at high speed
o Collison. Kinetic energy of these electrons -> EM radiation (ideally x-ray photons) + heat
o X-ray photons aimed at subjected
what are x-ray tube components?
- glass envelope
- cathode (-ve)
- anode (+ve)
what is purpose of glass envelope?
o Vacuum inside
o Air tight enclosure
in cathode what is passed through wire?
Low-voltage, high current electricity passed through wire
how are electrons released from atoms in wire for cathode?
thermionic emission
greater current in filament in cathode means what?
Greater current in filament -> greater heat + greater no. electrons
what is cathode filament made of?
tungsten
what are properties of filament in cathode?
- High melting point
o Can withstand high temps - High atomic number (Z=74)
- Malleable
what does focusing cup in cathode do?
- (-) charged -> repels electrons released at filaments
- Shaped to focus on small point on anode target
what is focusing cup in cathode made of?
molybendum
what does target in anode produce?
photons
what is target in anode made of?
tungsten
what is focal spot on target?
Focal spot = precise are on target where electrons collide and x-rays are produced (i.e x-ray source)
what is a problem with focal spot angulation? and how does focal spot size matter with this?
heat
Less 1 percent of kinetic energy from electrons is converted to x-ray photons. (almost all into heat)
Smaller focal spot size = greater image quality but great heat concentration
what is solution to problem for focal spot in target?
angled target?
how does an angled target help in producing x-rays?
Increase actual surface area where electrons impact = better heat tolerance
Reduces the apparent SA from where beam is emitted = smaller penumbra effect
what is heat dissipating block made of?
copper
what is cathode anode relationship?
o High voltage electricity passed through x-ray tube
- High potential diff between (-) cathode and
(+) anode
o Electrons released at filament are repelled away from cathode and attracted to anode
- Accelerate to very high speed over very short
distance
* Up to half speed of light
-Greater potential diff -> greater acceleration ->
greater kinetic energy
Electrons have high kinetic energy when colliding with anode target
what does step down transformer decrease potential difference to?
10V
what is electron volts (eV) used to measure?
o Units used to measure kinetic energy gained by electrons as they accelerate from cathode to anode
what is penumbra effect?
o Blurring radiographic image due to focal spot not being a single point (but rather a small area)
Minimised by shrinking size of focal spot
what does collimator do?
reduces pt dose
why should you use rectangle collimators?
o Can reduce SA irradiated by almost 50 percent
what does aluminium filtration in tubehead do? and what are the thicknesses required?
Removes lower energy (non-diagnostic) x-rays from beam
Min thickness aluminium required
* <70kV -> 1.5mm
* >= 70kV -> 2.5mm
what does spacer cone dictate?
Dictate distance between focal spot and pt
what are specific sizes required for focus to skin distance?
<60 kV -> 100mm
>= 60 kV -> 200mm
what does altering focus skin distance do?
- Altering fsd will affect degree of divergence of x-ray photons in beam
o Increasing fsd reducs divergence of beam so therefore reduces magnification of image
what are the diff types of interactions that produce x-rays?
continuous radiation interactions
characteristic radiation interactions
what is description of continuous radiation interactions?
Bombarding electron passes close to target nucleus, causing to be decelerated and deflected -> lost kinetic energy released as x-ray photons
how is max energy achieved in continous radiation interactions?
- Max energy achieved when electron collides directly with nucleus and stops completely
what is description of characteristic radiation interactions?
bombarding electron collides with an inner shell electron and either
* Displaces it into a more peripheral shell – excitation
* Removes it completely – ionisation
what happens when an electron ‘drops’ to a lower shell?
- When an electron ‘drops’ to a lower shell it loses energy which is emitted as a photon of specific energy
what are the comparison between continous and characteristic interactions?
Continuous
* Produces continuous range of x-ray photon energies
* Max photon energy matches peak voltage
* Bombarding electron interacts with nucleus target atom
Characteristic
* Produces specific energies of x-ray photon, characteristic to element used for target
* Photon Energies depend on binding energies of electron shells
* Bombarding electron interacts with inner shell electrons of target atom
how can photons in a diagnsotic x-ray beam interact with matter?
transmission – passes through matter unaltered
absorption – stopped by matter
scatter – changes direction
what does attenuation mean?
reduction in intensity of beam
what does scattered plus absorbed mean?
attenuation
what are number of photons in x-ray beam affected by?
current in filament (mA)
greater mA –> geater no. photons
hwo does attenuation indirectly lead to radiographic image?
o diff tissues/materials have varying degrees of attenuation which determines how many photons reach the receptor
o minimal attenuation -> black
o partial attenuation -> grey
o complete attenuation -> white
what is photoelectric effect?
complete absorption
- * photon in beam interacts with inner shell electron -> results in absorption of photon -> creation of photoelectron
what does absorption by photoelectric effect lead to?
absorption by photoelectric effect prevents x-ray photons reaching receptor -> leads to lighter area on image
what is compton effect?
partial absorption and scatter
- * Photon in beam interacts with outer shell electron -> results in partial absorption and scattering of photon -> creation of recoil electron
in compton effect incoming photon is what?
much greater than binding energy
what is electron ejected in compton effect called?
recoil electron
how is direction of scatter in compton effect dictated?
o Higher energy photons are defected more forward -> ‘forward scatter’
o Lower energy photons are deflected more backward -> ‘back scatter’
- majority of scatter from beam is forward
what does scatter in compton effect lead to?
o Causes darkening of image in wrong place
o Results in ‘fogging’ of image -> reduces image contrast/quality
how is scatter reduced?
collimation
what is effect of radiation dose from the 2 effects?
Increase pt dose but is necessary for image formation
Increase pt dose but scattered photons do not contribute usefully to image
what is the effect of lowering kV on x-ray unit?
o Lower x-ray tube pd (kV) leads to ->
o Overall lower energy photons produced leads to ->
o Increase photoelectric effect interactions leads to ->
o Increase contrast between tissue with diff Z BUT
o Increase in dose absorbed by pt
effect of raising kV on x-ray unit?
o Higher x-ray tube pd (kV) leads to ->
o Overall higher enrgy photons produced leads to ->
o Decrease photoelectric effect interactions (+ increase in forward scatter) leads to ->
o Decrease dose absorbed by pt BUT
o Decrease in contrast between tissues with diff Z
what is the comparison between 2 interactiosn adn 2 effects
interactions
- occur x-ray tube
- electrons interact with tungsten atoms
- lead to production of x-ray photons
effects
- occur in pt/receptor/shielding
- x-ray photons interacting with atoms
- lead to attenuation of beam
what is the approx energy each ionisation process will deposit?
35eV
what causes damage in indirect dna damage?
free radicals
what are factors of biological effect of dna damage?
Type radiation
Dose
Dose rate
* Low rate – cells can repair damage before further damage occurs
* High - dna repair capacity likely overwhelmed
Tissue or cell type irradiated
what does tissue radiosensitivty depend on
- Function of cells that make up tissues
- If cells are actively dividing
what are possible outcomes after radiation hits a cell nucleus?
- DNA mutation
o Mutation repaired
Viable cell
o Cell death
Unviable cell
o Cells survives but is mutated
Cancer - no change
what is units of absorbed dose?
gray
how is equivalent dose calculated and what is its units?
- Absorbed dose multiplied by a weighting factor depending on type of radiation
- For beta, gamma and x-rays the weighting factor is 1. For alpha particles it is 20
- Units – sieverts (Sv)
what are the 2 types of radiation effects?
deterministic effects
stochastic effects
describe deterministic effects?
- Tissue reactions
- Only occur above a certain (threshold) dose
- Severity of effect is related to dose received
- Unusual to see in radiology although possible in high dose areas such as interventional radiology
- Effects will show a few days after exposure
describe stochastic effects?
- Probability of occurrence is related to dose received
- No known threshold for stochastic effects
o There is no dose below which effects will not occur - Cannot predict if these effects will occur in an exposed individual or how severe
o Likelihood of effect increases as dose increases - Effects can develop years after exposure
stochastic effects are subdivided into 2 categroies?
o Somatic – results in disease or disorder
o Genetics – abnormalities in descendants
what is effective dose from an intra oral x-ray
0.005mSv
how far away is controlled area?
1.5m from x-ray tube
describe radiation protection philosphophy?
o Justification
Must have sufficient benefit to individuals or society in order to offset detriment
o Optimisation
Doses and no. people exposed should be as low as reasonably practicable (ALARP), taking into account economic and social factors
o Dose limitation
System of individual dose limits so that the risks to individuals are acceptable
describe dose optimisation?
Dose ALARP
* Still maintain image quality
* Circular collimated have been shown to increase dose by 40 percent
o Must use rectangular collimators
Pt doses can be reduced in a variety of methods
* Use E speed film or faster
* Use a kV range 60-70
* Focus to skin distance should be >200mm
* Use rectangular collimation
what are diagnostic reference levels?
o Legislation requires employers to have established dose levels for typical exams for standard sized patients
o Comparative standard used in optimisation
o Compared to national ref levels
o Individual x-ray units are compared to DRLs and national ref levels
Enables identification of units giving higher doses
what are current DRL’s for intra oral exams?
Adult:
* 0.9 mGy – digital sensors
* 1.2 mGy – phosphor plates and film
Child
* 0.6 mGy – digital sensors
* 0.7 mGy – phosphor plates and film
what are diff types of digital x-ray receptors?
phosphor plate
solid state sensor
how are phosphor plate images created?
o Within patients mouth
1 receptor exposed to x-ray beam
2 phosphor crystals in receptor excited by x-ray energy, resulting in creation of a latent image
o Within scanner
3 receptor scanned by a laser
4 laser energy causes excited phosphor crystals to emit visible light
Light is detached and creates digital image
what are 2 types of solid state sensors
charge couple device
complimentary metal oxide semi conductor
what are phosphor plate sizes?
E.g size 0 (anterior periapicals)
Size 2 (bitewings, posterior periapicals)
Size 4 (occlusal radiographs)
describe conversion of x-ray shadow into image?
o When Beam passes through an object some photons are attenuated creating an ‘x-ray shadow’
o X-ray shadow is basically the image ‘formation’ held by the x-ray photons after an x-ray beam has passed through an object
o Image receptor detects this shadow and uses it to create an image
what is the format for Digital image?
Digital imagine and communications in medicine
what is management of digital images?
picture archiving and communication system
how to view digital radiographs?
o Environment
Subdued lighting
Avoid glare
o Monitor
Clean
Adequate resolution
High enough brightness
Suitable contrast
explain diffs between phosphor plates and solid state sensors?
o Phosphor plates
Thinner, lighter and flexible
Wireless -> more stable (more comfortable)
Variable room light sensitivity -> risk of impaired image
Latent image needs to be processed in scanner separately
Handling similar to film
o Solid-state sensors
Bulkier and rigid
Usually wired
Smaller active area (for same physical area of receptor)
No issues with room light control
Arguably more durable -> replaced less often
More expensive
how is film speed affected?
Greater speed -> smaller radiation required to achieve an image
Affected by no. and size of silver halide crystals
* Larger crystals -> faster film but poorer image quality
what are diff methods of film processing?
- Manual
- Automated
- (self-developing films)
steps to film processing?
- 1 developing
o Converts sensitised crystals to black metallic silver particles - 2 washing
o Removes residual developer solution - 3 fixing
o Removes non sensitised crystals
o Hardens emulsion (which contains the black metallic silver) - 4 washing
o Removes residual fixer solution - 5 drying
o Removes water so that film is ready to be handled/stored
describe manual (or wet) cycle?
Person dips film into diff tanks of chemicals
* At precise conc/temps
* For specific periods of time
* Washes film after each tank
Must be carried out in dark room with absolute light tightness and adequate ventilation
describe the processing issue of fixing?
Involves reaction to remove non-sensitised crystals and hardens remaining emulsion
Inadequate fixing means non-sens crystals left behind
* Image green-yellow or milky
* Image becomes brown over time
describe processing issue of washing?
Developing and fixer solution will continue to act if not washed off
digital vs film?
o Digital
Advantages
* No need for chemical processing
* Easily storage and archiving images
* Easy back up images
* Images can be integrated into pt records
* Easy transfer images
* Images can be manipulated
Disadvantages
* Worse resolution -> risk pixelation
* Requires diag-level computer for opimal viewing
* Risk of data corruption
* Hard quality prints have poor image quality
* Image enhancement can create images misleading
common types of extra oral radiographs?
o Panoramic
o Cephalometric radiographs
Lateral
Postero-anterior
o Oblique lateral radiographs
o Skull radiographs
Occipitomental
Postero-anterior skull/mandible
Reverse towne’s
True lateral
describe angulation of beam?
- True = perpendicular to head
- Oblique = not perpendicular to head
what are examples of reference lines/planes?
Mid sagittal plane - line down middle face
Interpupillary line – connects both pupils
Frankfort plane – connects infraorbital margin and superior border of external auditory meatus
Orbitomeatal line – connects outer canthus and centre of EAM
* A.k.a canthomeatal line or radiographic baseline
what is cephalometry?
o Measurement and study of the head
what is lateral cephalogram used for in ortho?
Assess skeletal discrepancy when appliance used for labio lingual movement of incisors
Aids location and assessment of unerupted, malformed and misplaced teeth
Indicates upper incisor root length
what are oblique lateral radiographs used to assess?
- Dental pathology
- Unerupted teeth
- Mandibular fractures
- Lesions
describe need for radiographic localisation>
o to determine location of structure or pathological lesion in relation to other structure
what are the options for views at right angles in relation to radiography localisation?
o OPT and lower true occlusal
o Paralleling periapical and lower true occlusal
o CBCT
describe methods of radiographic localisation?
o Normally 2 views required
o Views at right angles in their projection geometry
o Views with any diff projection, provided the diff is known
o With aid of opaque objects (e.g gutta percha point)
o Anatomical knowledge crucial
describe parallax and how it work?
o An apparent change in position of an object, caused by a real change in the position of the observer
o How it works:
Imagine looking in a cupboard and objects appear to change their relationship to each other and you
what are prallax localisation options?
o Horizontal tube shift – equivalent views
2 periapicals
2 bitewings
2 oblique occlusals
o Vertical tube shift – diff views
OPT and oblique occlusal
OPT and lower (bisecting angle) periapical
describe parallax localisation?
o 1. Take 2 radiographs of same object
o 2. Identify direction tube movement
o 3. Arrange on viewing box
o 4. ? is this correct
who do you need input from with regards to the quality assurance program?
o input from medical physics expert (role defined in IRMER17)
what are things to check for a digital image receptor?
1. Receptor itself
* Visible damage to casing/wiring
* Check if clean
2. Image uniformity
* Expose receptor to unattenuated beam and check if image is uniform
3. Image quality
* Take radiograph of test object and assess against baseline
how to tell if phosphor plates are damaged?
Scratches -> white lines
Cracking from flexing -> network white lines
Delamination -> white area around edge
* i.e separation of phosphor layer from base plate
how to tell if solid state sensors are damaged?
sensor damage -> white squares
how to tell if film images are damaged?
damage often appears as black marks due to sensitisation of radiographic emulsion
may appear white if emulsion is scraped off
how to test image quality?
o Step wedge
Type test object used to check quality/contrast
Exposed to normal clinical exposure and compared to baseline
* Baseline – must be able to differentiate all 6 steps
* Carried out regularly
what are the 3 p[arts of assessing clinical image quality?
1. Image quality rating
* Grading each image
2. Image quality analysis
* Reviewing images to calculate ‘success rate’ and identify any trends for suboptimal images
* Carried out periodically
3. Reject analysis
* Recording and analysis each unacceptable image
what is the recommended QA system and what are the quality ratings?
o FGDP
Quality rating
* Diag acceptable (‘A’)
* Diag not acceptable (‘N’)
what is diag acceptable position factor of bitewings?
Show entire crowns of upper and lower teeth
Include distal aspect of canine and mesial aspect of last standing tooth
* May require more than 1 radiograph
Every approximal surface shown at least once without overlap (where possible)
* May be impossible if crowding
what is diag acceptable position of periapicals?
Shows entire root
Shows periapical bone
Shows crown
what most all radiographic images be in order to be diag acceptable?
o Must have adequate contract, sharpness, and resolution as well as minimal distortion
what are some potential causes to a too dark or pale image?
o Exposure factors
Incorrect setting, pt tissues too thick, fault x-ray timer
o Developing factors (film)
Incorrect duration, incorrect temp, incorrect conc
o Viewing factors
Inappropriate light source (film), inappropriate display screen (digital), excessive environmental light
what are some potential faults of an image?
- too dark or pale
- poor contrast
- unsharp
- distorted
- over-collimated
- receptor marks
what makes better heat tolerance?
Increase SA of place electron interact
What makes a smaller penumbra effect?
Reduction in apparent SA where beam is emitted
What is excitation of an electron?
When it moves to a more peripheral shell
What is effective dose and how is it calculated?
- it is calculated using equivalent dose times tissue weighting factor
what is delamination?
White area around edge
what collimation is used for lateral cephalogram and what does it do?
triangular collimation
reduce exposure of cranium
