3rd year Flashcards
principles of radiation protection
justification
optimisation
dose limitation
optimisation
ALARP
dose limitation - who is it for?
radiation workers and public
not pts - by justifying you are saying that the dose is worth the benefit
source
x-ray machine
produces xrays
image receptor
digital - direct/indirect
film
screen-film combinations
processing
conversion of latent image into permanent visible image
digital or chemical
what energy source do xray machines use?
domestic electricity supply
converts to high voltage (to produce X-rays)
potential range
60-70kV (pan higher)
which part of the xray machine creates X-rays?
tube
what is a radiographic image?
pictorial representation of part of body
record of pattern of attenuation of xray beam after it has passed through matter
= absorption and scatter events
BWs include?
distal of canine posteriorly to include all CPs
one per side unless all premolars and molars
true (CS) occlusal
plan view of a section of mandible/FOM
2 types of occlusal
true
oblique
ceph
view of facial bones
incs ST profile
properties of xrays
senses: not perceptible
- need warning signals - sound (law), light
EM radiation
direction of travel
- straight, diverging beam
- inverse square law - area measured at end point larger the further you get from a source
photographic
interaction with matter
- no effect e.g. air
- complete absorption - white
- absorption and scatter - beam has its direction changed
ideal projection geometry
image receptor and object in contact and parallel
parallel beam of xrays
xray beam perpendicular to object plane and image receptor
- image size identical to object size
problems with projection geometry
image receptor and object not in contact
- tooth supported by bone so can’t contact all of it
beam of X-rays not parallel
- divergent
xray bean central ray may/may not be perpendicular to object plane and image receptor
image size not identical to object size due to magnification - divergent beam
projection geometry - 2 solutions
paralleling technique: image receptor and object parallel (but not touching)
bisecting angle technique: image receptor and object partially in contact, and not parallel to each other
paralleling technique
object and image receptor parallel so positioned some distance apart - not in contact
only central ray truly perpendicular - divergent beam
focus
where X-rays produced
short FSD
bad as produces extensive magnification of the image as diverging beam
FSD
where X-rays are produced to skin of pt
why should you use a long FSD?
reduce magnification as near parallel xray beam
at least 20cm
reasons for using film holders and BADs
dose reduction
better quality
fewer rejects so fewer retakes
components of film holders
bite block
BAD and rod
image receptor support
blue
anterior PAs
yellow
posterior PAs
red
BWs
green
endo
assembly of film holders
look through ring - should see image receptor support right in middle
if not then wrong - get ‘coning off’ - only part of image has radiograph
collimation
“restriction of CS area of beam”
controlling size and shape of xray beam, narrows it so less divergent
should be provided on new equipment and retro-fitted to existing equipment
circular or rectangular diaphragm
what material is used for collimation and why?
lead - v good at absorbing xrays
which type of collimation is ideal and why?
rectangular - 30% dose reduction compared to circular
curve of Spee
AP
curves up posteriorly
produces a happy smile
curve of Monson
BL
influences technique e.g. pan vertical angle is negative to occlusal plane -8 degrees
International Commission for Radiological Protection
international, independent, non-gov
recommendations and guidance on radiation protection
volunteer members
basic principles ICRP
justification
- sufficient benefit to individuals/society to offset any detriment
optimisation
- magnitude and number of persons exposed ALARP
dose limitation
- so no-one receives an unacceptable level of exposure
International Atomic Energy Authority
publish regulations based on ICRP - designed to be used as a template for radiation safety legislation globally
European Commission used it as a basis for Basic Safety Standards Directive
UK then required to put recommendations into law
what do IRR17 and IRMER17 come under?
HandS at work act
IRR17
occupational exposures and exposure of general public inc staff
IRMER17
medical exposures of patients (and some other groups)
who is IRR17 enforced by?
HSE
IRR17 employer and employee responsibilities
employer - responsible for compliance arrangements
employee - responsible for following safety arrangements
give some components of IRR17
staff training risk assessments dose limitation - ALARP dose limits RPAs RPSs controlled areas set of local rules
IRR17 licensing
employer must obtain registration from HSE for use of xrays
1 - answer Qs on compliance arrangements
2 - pay £25
IRR17 who is responsible for compliance?
NHS
private - owner responsible as employer
RPA
a person meeting HSE requirements to advise on radiation safety
- get certificate issued by ‘RPA2000’ (renew every 5yrs)
give some aspects that an employer should consult an RPA on
designation of areas
prior examination of plans for installations and acceptance into service of safety features and warning devices
regular equipment checks
periodic testing of safety features and warning devices
radiation risk and dose assessments
investigations
contingency plans
training for staff
basic radiation safety measures
ant specific requirements for that workplace
basic understanding of risks and awareness of regulations
annual radiation dose limits
radiation workers = whole body limit 6mSv/year (unclassified staff)
public = whole body limit 1mSv/year
carers and comforters
individuals ‘knowingly and willingly’ exposed to ionising radiation through support and comfort of those undergoing exposure
not doing so as part of their employment
often friends/relatives
Radiation risk assessment
what safety features are required?
what level of radiation exposure could staff receive?
controlled area
space nobody should be in while taking radiograph unless absolutely necessary
who will advise if need plasterboard/lead in walls?
RPA
IO controlled area
1.5m from xray tube and within primary beam
CBCT controlled area
entire room
controlled area regulations
need signage - entire room - entrance leads directly in set off local rules appoint RPS to oversee arrangements
who enforces IRMER17?
HIS
inspectors
who does IRMER17 apply to?
pts as part of diagnosis/tx health screening research asymptomatic individuals carers and comforters individuals undergoing non-medical imaging using medical equipment
RPS
ensures regulations and training are followed
non-medical imaging using medical radiological equipment which does not confer a health benefit to the individual exposed
health assessment - employment - immigration - insurance radiological age assessment identification of concealed objects within body
Employer’s procedures
set out how regulations are complied with
14 procedures
- pt identification
- entitlement of staff
- info provided to pts e.g. poster - benefits and risks
duty holders
referrer
practitioner
operator
employer
referrer
refer for imaging
clinically justify, pt details
practitioner
justification (and authorisation)
benefits vs risks
no recent radiographs
ALARP
operator
(authorise)
check pt demographics, ALARP, takes exposure, processes and reports
anyone who carries out practical aspects that can affect pt dose
employer
legal person, safety
make sure equipment in line with IRR99
staff follow regs
what is justification based on?
history and clinical exam
process of justification
info from referrer, consider: objectives of exposure, efficacy, benefits and risks of available alternative techniques benefits (diagnostic/therapeutic) - individual - society detriment to individual characteristics of individual involved
justify then authorise - record
justification can be a 2 step process
written justification guidelines prepared by practitioner
authorisation as justified by operator at time of exposure
justification - refer back to referrer
insufficient info
not justified
clinical evaluation
legal requirement
each exposure outcome
- can’t be justified if known a CE won’t be performed
Referrer’s responsibility
medical physics expert
advice on exposure factors and equipment-related matters
optimisation
IRMER17 ALARP responsibility - Practitioner and Operator considerations - investigations and equipment - exposure factors - QA - assessing pt dose - adherence to DRLs
QA of radiation equipment
test regularly
- working
- expected dose level
routine local tests - staff who normally operate equipment
physics tests - every 1-3yrs by specialist staff
which legislations outline tests required and recommended freq?
IPEM 91
CoR
NRPD
DRLs
guideline dose levels for “standard sized” pts undergoing typical exams
can use as a benchmark against local and national practice
some equipment displays ‘dose indicator’ after exposure - compare against DRL
checked during QA tests
risk assessments used to evaluate control measures§
occlusal radiographs IR
7x5cm
3x4 size 2 PA in children
which arch are true CS occlusals taken for?
lower
oblique occlusal indications
PA type assessment where PA not possible = trismus/gag
pathology too large to be seen on a single PA
- bigger IR, can see all 4 incisors
trauma - fractures
- easier to bite gently than on hard plastic PA
localisation using parallax
bisecting angle technique
IR and object partly in contact but not parallel
IR and object close together at crowns but apart at apices
- distance depends on part of mouth
do you need IR holders for bisecting angle technique?
no
bisecting angle technique - vertical angle selection
bisect angle between long axis and image receptor
central ray at 90 degrees to bisector - correct length of image
correct due to identical triangles
bisecting angle technique - xray beam at 90 degrees to LA of tooth
elongated image
vertical angulation too small
bisecting angle technique - xray beam at 90 degrees to plane of IR
foreshortened image
vertical angulation too large
bisecting angle technique - how much image receptor beyond incisal edge?
2-3mm
bisecting angle technique - how is the angle adjusted to adapt to the incisor angulation?
proclined - increase
retroclined - decrease
head position for maxillary occlusal
ala-tragus line parallel to floor
for a seated upright pt
head position for mandibular occlusal
corner of mouth - tragus line parallel to floor
for a seated upright pt
centring point
where the central ray enters the body
what should the horizontal angle be?
90 degrees to line of arch to avoid overlaps
centring points - PAs
maxilla - on ala-tragus line
mandible - 1cm above lower border of mandible
centring points - oblique occlusals
maxilla - 1cm above ala-tragus line (collimator just above bridge of nose)
mandible - through lower border of mandible
how are PP protected?
cardboard/plastic
what does the orientation of IR depend on?
size of mouth and pt tolerance
oblique occlusals - guideline vertical angles
upper anterior (standard) - 60
occlusal centred on canine 55
premolar 50
molar 45
lower anterior occlusal 40 (to occ plane)
lower occlusal centred laterally 35
teeth become more upright as go back to molars - why you drop by 5 each time
indications for a mandibular true occlusal
detection of SM duct calculi
- concentric growth or conforms to duct
- unless advanced imaging indicated
- assessment of BL position of UE teeth
- evaluation of pathological BL expansion
- horizontal displacement of fractures
nowadays more CBCT
true/CS occlusal
occlusal or PA size IR
plan view
when beam is through LA of a tooth
only do L jaws - get a poor image for upper
mandibular true occlusals positioning
IR transverse in occlusal plane OR lengthwise over region of interest
head tipped back as far as is comfortable
x-ray beam directed at 90 degrees to IR in midline or through region of interest
errors in pan radiography
pt prep exposure positioning processing film handling
DPT
method of radiography displaying details of a selected plane (layer/slice) within the body
image layer/focal trough
a layer in the pt that contains structures of interest that are demonstrated with sufficient resolution to make them recognisable, whilst structures at other depths (superficial and deeper) are not clearly seen
contains all teeth, structures above and below
- close superficial and deep
- distant structures not clear
impact of different size perimeters i.e. distance from rotation centre
further from the rotation centre the faster the beam passage around the circumference
larger circle equivalent to further from rotation centre = faster speed
linear tomography - principle of layer formation
xray source L to R
receptor R to L
objects not in focal plane projected to continually changing points on film
object in focal plane projected onto same point of film
what makes layer formation happen
movement of xray source (therefore beam) through teeth
movement of receptor through xray beam at the correct speed so desirable objects (teeth etc) will be recorded as clear images
objects outside the desired layer will be portrayed as either distorted unsharp images, or be imperceptible
layer position and speeds - posterior teeth
posterior teeth further from their rotation centre
- faster beam passage through teeth
- IR movement also fast to match
layer position and speeds - anterior teeth
anterior teeth closer to their rotation centre
- slower beam passage through teeth
- IR movement becomes slower to match and prevent distortion
xray beam panoramic
vertical narrow beam
passes through pt from lingual to buccal
xray tube head rotates around back of pt
xray beam angled upwards at -8 (due to curve of Monson)
IR rotates around front of pt, and passes through xray beam
panoramic - movement
wavy lower border of mandible
ghost images - common objects
earrings
metal Rxs
anatomical features - esp opp side of mandible
ST calcifications e.g. LNs, salivary calcification
what happens to the rotation centre?
it changes continuously
what does the distance from rotation centre to teeth affect?
with of layer in focus/focal trough
horizontal distortion if pt in incorrect position relative to machine focal trough
ghost images
what is the width of the focal trough/layer in focus dependent on?
width of xray beam - same throughout
distance to rotation centre
- closer to rotation centre (anteriors) = narrower layer
- further away (posteriors) = wider layer
pan limitations
pts occlusion
long exposure time (up to 16s)
big shoulders
if you can’t see it, it doesn’t mean its not there - width of layer in focus
horizontal distortion
positioning difficulties
narrow width in focus anteriorly - miss some
about ghost images
always higher due to - vertical beam angulation -8 degrees
always horizontally magnified
change in AP position - usually further forward
can interfere with diagnosis - but not always
formation of ghost images
xray tube start position directs beam posteriorly towards opposite TMJ region
tube moves round behind pts’ head
when image of premolar region is centred beam is coming from a more posterior point on opp side
ghost images usually more anterior than real image
EO dose reduction in pan
collimation - pan programme selection
rare earth screens: system speed 400 or greater
digital
IO dose reduction
60-70kV
rectangular collimation
E or F speed film
digital
pan - what must be synchronised to produce an accurate image?
speed of beam through teeth and IR through beam
pan magnified horizontally
pts C behind C guide line (closer to xray source than machine expects)
speed of beam slower through teeth as closer to rotation centre
if not compensated, IR too fast and image magnified horizontally
pan reduced in width horizontally
pts C in front of CGL (further from xray source than machine expects)
speed of beam faster through teeth as further from rotation centre
- if not compensated, IR too slow and teeth reduced in width horizontally
pan uses
development of dentition pathological jaw lesions mandibular fractures developmental and acquired abnormalities surgery = evaluation and review (caries, pulpal, PDD)
EM radiation
flow of energy created by simultaneously varying electrical and magnetic fields - schematically represented as a sine wave (up and down movement in its energy)
travels as “packets” of energy known as photons
properties of EM radiation
no mass
no charge
travels at speed of light 3 x10⁸ ms-1
can travel in a vacuum
freq
cycles/secs Hz
EM spectrum
different properties dependent on energy, wavelength, frequency
- same type of radiation
gamma, xray, UV, visible, IR, microwave, radiowave
radiowave
longer WL
lower freq
lower energy
gamma ray
shorter WL
higher freq
higher energy
amplitude
distance from midline
freq definition
how many times the wave’s shape repeats per unit time
Hz
- 1 = 1 cycle/sec
wavelength
the distance over which the wave’s shape repeats itself
m
speed equation
speed = freq x wavelength
BUT speed of all photons is constant 3x10⁸ms-1
so if freq increases then WL must decrease and vv
energy directly proportional to freq
photon energy
EM radiation involves the movement of energy as photons
eV
1eV
enery (in J) gained by one electron moving across a potential difference of 1V
properties of xrays
form of EM radiation
undetectable to human senses
man-made
- y rays identical except that they occur naturally (and generally have higher energies)
cause ionisation
- what causes damage to human tissues
- i.e. displacement of electrons from atoms/molecules
The atom (Bohr model)
nucleus - protons. +. 1. - neutrons. neural. 1 shells (orbiting) - electrons. - negligible (0)
electron shells
electrons spin around the nucleus in discrete orbits/shells
- cannot exist between these shells
innermost shell K, then L, M, N, O etc
e-s try to fill spaces available in inner shells first
why are X-rays called this?
because of their unknown nature
xray photon energies
124eV to 124KeV
hard xrays
higher energies
able to penetrate human tissues
soft xrays
lower energies
easily absorbed
what type of X-rays does medical imaging mostly use?
hard X-rays e.g. >5KeV
basic production of xrays
can use tungsten
electrons fired at atoms at v high speed
on collision, the KE of these electrons is converted to EM radiation (ideally X-rays) and heat (side product)
xray photons aimed at a subject
nucleus
collection of nucleous
- protons and neutrons have similar mass
- overall + charge
atomic number (Z)
number of protons
unique to each element
mass number (A)
P + N
max number of e- in each shell
2n²
- shell number
how are orbiting electrons held in their shell?
by electrostatic force
negative charge of electrons attracted to + nucleus
number of electrons
determines chemical properties of an atom
“ground state” - neutral e=p
ionisation
removing or adding e
binding energy
additional energy required to overcome the electrostatic force and remove an e from its shell
increased binding energy
closer e to nucleus = greater electrostatic force and therefore binding energy
- K shell highest BE
more positively charged nucleus (i.e. higher Z) greater electrostatic force
what happens if you lose an electron from an inner shell?
an outer shell e will move in
formula to remove an e
if the energy input ≥ BE
current
flow of electric charge, usually by the movement of e
SI unit for charge
amp, A
- measure of how much charge flows past a point per sec
two directions of current
DC
AC
as long as the e are moving you will be producing energy
DC
constant unidirectional flow e.g. batteries
AC
flow repeatedly reverses direction
number of complete cycles (reverse and reverse back) per unit time is the freq
SI unit is Hz (cycles per sec)
voltage
difference in electrical potential between 2 points in an electrical field
related to how forcefully/fast a charge (e) will be pushed through an electrical field
SI unit voltage
Volt, V
potential difference
voltage
electron movement between shells
the specific amount of energy required to move an e to a more outer shell = the difference in BEs of the 2 shells
if an e drops to a more inner shell then this specific amount of energy is released
- possibly in the form of xray photons (if sufficient energy)
dental xray unit components
tubehead collimator positioning arm control panel circuitry
mains electricity supply
AC
220-240V
rectification of current
xray production requires a unidirectional current
- but xray units are powered by mains electricity (AC)
have generators which modify the AC so that it mimics a constant DC
- rectification
dental xray unit electricity requirements
DC (rectification)
requires 2 different voltages
- one as high as 10000s V (firing electrons fast)
- one as low as 10V (create electrons to be fired)
transformers
alter the voltage (and current) from one circuit to another
2 separate transformers required for xray unit
mains to xray tube (cathode - anode)
mains to filament
step-up transformer
increase potential difference across xray tube
usually 60-70kV
current reduced to mA
step-down transformer
reduce potential difference across filament
10V
10amps
xray beam intensity
quantity of photon energy passing through a CS area of the beam per unit time
increase number and/or energy of photons = increased intensity
proportional to current in filament (mA) and PD across xray tube (kV)
a particles
stopped by paper
B particles
stopped by Al
y rays
reduced by thick lead
how are a, B, y different to X-rays?
all produced by radioactive decay of unstable atoms - unlike X-rays which are directly man-made
xray beam
made up of millions of xray photons directed in the same general direction
photons effectively travel in straight lines but diverge from the xray source - do not travel in parallel
divergence of xray beam
dose decreases with distance from xray source - beam diverges so not all xray photons irradiating them
inverse square law
intensity of xray beam is inversely proportional to the square of the distance between the xray source and the point of measurement
intensity proportional to 1/distance squared
so doubling the distance will quarter the dose
when are X-rays produced?
when fast moving electrons are rapidly decelerated
xray tube head components
filament - cathode transformer target - anode target surround evacuated glass envelope (shielding) filtration collimator spacer cone
filament - cathode
negative
tungsten
filament circuit (step-down transformer): low voltage, high current
tungsten
W
Z = 74
mp 3410 degrees - can reuse and it won’t disintegrate/melt - it will retain its integrity
filament function
cathode
low voltage current passed through filament circuit
filament heats up to incandescence
e form a cloud around filament
operating potential
new equipment should operate within range 60-70kV
affects
- how X-rays will interact with matter
- pt dose
transformer - why hollow centre?
so X-rays can go through it so they don’t interact with it
step-up transformer process
240eV domestic input
60-70KeV high voltage output
huge attraction of e (mA) from cathode towards anode (target)
flow of e about 7-15mA
want to pull e over towards positive side of xray tube - need high V
target - anode
positive
tungsten
effective area 0.7mm²
20 degree slope i.e. not parallel to filament - increases efficiency
referred to also as a focus or focal spot
target interactions
heat production 99% - inefficient
xray production <1%
- continuous spectrum (energy output)
- characteristic spectrum (characteristic to tungsten - material where interactions are happening)
target interactions - xray production: continuous spectrum
incoming e passes close to nucleus of a target atom
e rapidly decelerated and deflected
amount of deceleration and deflection proportional to E loss
E loss in the form of EM radiation has a continuous spectrum of energies
max E is applied kV e.g. 70
what is continuous spectrum also known as?
Bremstrahlung/braking/white radiation
target interactions: heat production
incoming e
- deflected by a cloud of outer shell tungsten e or collides with an outer shell e displacing it
small loss of energy (E) - in the form of HEAT
removed through copper block, oil then air
target surround
tungsten target set into a block of copper which is v good at conducting heat away
copper Z = 29 mp = 1080 degrees - effective heat conductor
why are low energy X-rays not useful?
don’t have enough energy to get through the tissues and produce an image
describe the appearance of the graph for continuous spectrum of xray energies
number of photons (intensity) - y axis photon energy (KeV) - x axis straight line decreasing (linear) to 70
characteristic spectrum of tungsten xray energies
characteristic radiation of tungsten has values of approx
- 8kV - L shell
- 58kV - K shell
- 68kV - K shell
filtration
get rid of low energy X-rays that you don’t want
material used for filtration
Al Z = 13
- high energy X-rays will get through
1. 5mm ≤ 70kV
2. 5mm > 70kV
what does the spacer cone do?
control the target FSD
where is FSD measured between?
external marker and pt end of cone
FSD distances
100mm <60kV
200mm ≥ 60kV
target interactions - xray production - characteristic spectrum
incoming e collides with an inner shell target e
target e displaced to an outer shell or completely lost from atom
target atom unstable
orbiting e rearranged to fill vacant orbital slots to return atom to neutral state
difference in E between orbits is released as characteristic radiation, of known E values
same mechanism as PE absorption
glass envelope
evacuated glass
vacuum prevents risk of interaction of electrons with air atoms prior to meeting target
shielding
lead Z=82
- high atomic no means good absorber of X-rays
to ensure dose rate in vicinity not >7.5u Sv h -1
collimator
lead
circular or rectangular diaphragm
max bean diameter 60mm at pt end of spacer cone
what does a long FSD reduce?
magnification
xray photons traversing tissue may:
pass through unaltered - no energy loss
change direction with no energy loss (scatter)
change direction losing energy (scatter and absorption)
be stopped, depositing all energy within tissue (absorption)
production of a radiographic image
xray photons pass from tube, and some through pt to reach image receptor
interaction with different tissues alters number of photons exiting pt
- diff spread of energy levels
variation in number of photons reaching IR produces radiographic appearance of different tissues
attenuation
reduction in number of photons (X-rays) within beam
occurs as a result of absorption and scatter
affects number of photons reaching IR
effect of photon absorption on image
all photons reach film - black
partial attenuation - grey
complete attenuation - white
principal interactions of diagnostic X-rays in tissue
photoelectric effect - absorption
Compton effect - scatter and absorption
what does PE effect result in?
complete absorption of photon energy - photon does not reach film
PE effect
xray photon interacts with inner shell e (usually K)
photon has energy just higher than the binding energy of electron
- only happens if this is true
xray photon disappears
most of photon energy is used to overcome BE of e, remainder gives e KE
electron is ejected (photoelectron)
atom has ‘hole’ in electron shell: + charge
ionised atom is unstable
e drops from outer shell, filling void
diff in energy between 2 levels is emitted as light/heat (characteristic radiation)
- to the elements and the shells
outer voids filled by ‘free’ e
effect of PE absorption on image
complete absorption of photon
prevents any interaction with active component of IR
image appears white if all photons involved, grey if some photons not involved
occurrence of PE absorption proportional to:
atomic number cubed (Z³)
1/photon energy³ (1/kV³)
density of material
PE absorption - atomic numbers and cubes
relatively small differences in Z result in large differences in PE absorption
- good differentiation between tissues
Compton effect - first stage
xray photon interacts with loosely bound outer shell e
photon energy considerably greater than e BE
e ejected taking some of photon energy as KE: recoil e
atom is then positively charged
Compton effect - what happens to excess energy in the original photon?
following collision, photon has lower energy (longer wavelength)
called a scatter photon
undergoes a change of direction - related to how much energy it has lost
Compton effect - following scatter events
atomic stability regained by capture of free electron
recoil electron can interact with other atoms in tissue
scatter photon, dependent on energy and position of bound electron involved, can be involved in more Compton or PE interactions
Compton effect - what happens to scattered photons?
can travel in any direction
direction of scatter is affected by energy of scatter photon
- high energy - forward direction
- backward direction - low energy
full range of directions between the two extremes dependent on energy
probability of Compton effect occurring
proportional to density of material (e density)
independent of atomic number
not related to photon energy, although forward scatter more likely with high energy photons
effect of Compton scattered photons
scattered photons produced before the IR is reached, and scattered backwards, do not reach IR and do not contribute to the image
scattered photons produced beyond IR, and scattered back towards it, may reach IR producing darkening
- as their path is randomly altered they do not contribute useful info to the image
- results in fogging (general increased darkness) of image, reducing contrast (between adjacent materials) and image quality
reduction of scatter - methods
collimation - reduce area and vol irradiated
- reduce number of scattered photons produced as well as reducing pt dose
- smallest area compatible with diagnostic outcomes
lead foil within film packet prevents back scattered photons from oral tissues reaching film (also absorbs some of energy in primary beam)
- not used with digital receptors - inherently more sensitive so use lower dose anyway
effect of PE absorption on dose
deposition of all photon energy within tissue - increases pt dose but necessary for image quality
effect of Compton scatter on dose
deposition of some photon energy within tissue
adds to pt dose but doesn’t give useful info
may increase dose to operators (only if standing too close to pt)
effect of high kVp on image quality and pt dose
high tube kVp produces higher energy photons
PE interactions are reduced
contrast is reduced
dose absorbed by pt is reduced
no point reducing dose so much that image is of no diagnostic quality
absorption of photons more likely if:
object traversed has a high atomic number
object traversed is thicker
photon energy is lower
radiographic contrast
difference in density in light and dark areas of radiograph
image showing both light and dark areas with clear borders - high contrast - ideal
when is contrast greatest?
when difference in absorption by adjacent tissues is greatest
effect of low kVp on image quality and pt dose
low tube potential difference (kVp) produces lower energy photons
PE interactions are increased
contrast between different tissues increases
BUT dose absorbed by pt is increased
what is the chosen kVp a compromise between?
diagnostic quality of the image and dose
60-70kV
a particle
2p/2n
large particle, travels a few inches
most damaging type of radiation
B-particle
e-
v small particle, travels a few feet
y-ray
EM
high energy
travels long distances
ionising radiation
atoms have e=p, ions don’t
ionising radiation has enough energy to turn atoms into ions
- “knocks” away e orbiting the nucleus of an atom
interaction of radiation with tissues
when radiation passes through matter - ionises atoms along its path
each ionisation process will deposit a certain amount of energy locally, around 35eV
- greater than the energy involved in atomic bonds (4eV)
single strand break in DNA
can usually be repaired
double strand DNA breaks
more difficult to repair
usually result of a radiation
if the repair is faulty - can lead to mutations which can affect cell fct
factors affecting the biological effect of DNA damage
type of radiation
amount of radiation (dose)
time over which the dose is received (dose rate)
tissue or cell type irradiated
what is the most significant effect of ionising radiation?
damage to DNA
evidence of DNA damage
can be seen in faulty repair of chromosome breaks
direct effect DNA damage
radiation interacts with atoms of a DNA molecule or another important part of the cell
indirect effect DNA damage
radiation interacts with water in the cell, producing free radicals which can cause damage
free radicals are unstable, highly reactive molecules
dose survival curves
low doses of radiation produce less damage
linear relationship for a particles, which in turn kills more cells than a similar dose of X-rays would
dose rate
radiation delivered at a low dose rate is less damaging
cells can repair less serious DNA damage before further damage occurs
at high dose rates, the DNA repair capacity of the cell is likely to be overwhelmed
possible outcomes after radiation hits a cell nucleus
no change DNA mutation - mutation repaired - viable cell - cell death - unviable cell - cell survives but is mutated - cancer?
dose quantities - tissue cancer risk
following large radiation exposures - higher incidences of cancer in certain tissues
most medical exposures do not irradiate the body uniformly
- risk will vary depending on organ that receives the highest dose
what is tissue radio sensitivity dependent on?
the fct of the cells that make up the tissues
if the cells are actively dividing - the more rapidly a cell is dividing the greater the sensitivity to radiation
SCs and tissue radiosensitivity
exist to produce cells for another cell pop - divide freq, v radiosensitive
differentiated cells and tissue radiosensitivity
do not exhibit mitotic behaviour, less sensitive to radiation damage
highly radiosensitive tissues
bone marrow lymphoid tissue GI gonads embryonic tissues
moderately radiosensitive tissues
skin
vascular endothelium
lungs
lens of eye
least radiosensitive tissues
CNS
bone and cartilage
CT
dose
measure of amount of energy that has been transferred and deposited in a medium
why have additional dose units been defined?
to quantify the level of biological damage and the overall effect of the dose
severity of ionising radiation
effect of ionising radiation on tissue is greater than would be expected from amount of energy involved
what might heavily damaged cells be programmed to do?
die
absorbed dose
measures the energy deposited by radiation
Gy
= but different types of radiation can cause different levels of damage to tissues
equivalent dose
absorbed dose multiplied by a weighting factor depending on the type of radiation
B, y and X-rays - 1
a particles - 20
Sv
equivalent dose units
Sv
absorbed dose units
Gy
what does the LNT model estimate?
the long term biological damage from radiation
LNT model assumptions
damage directly proportional (linear) to radiation dose
radiation always harmful with no safety threshold
response linearity - several small exposures would have same effect as one large exposure
the effective dose is directly proportional to the risk of cancer
deterministic effects
tissue reactions
can only occur above a certain (threshold) dose
severity of the effect is related to the dose received
unusual to see in radiology although possible in high dose areas e.g. interventional radiology
often effects won’t show immediately but several days after exposure
e.g. erythema, tissue damage, skin injury
stochastic effects
no known threshold - no dose below which the effects will not occur
cannot predict if these effects will occur in an exposed individual or how severe they will be
- likelihood of the effect occurring increases as the dose increases
effects can develop years after exposure
lethal dose
6Sv to whole body
subdivision of stochastic effects into 2 categories
somatic - disease/disorder e.g. cancer
genetics - abnormalities in descendants
pregnant pts
don’t need to take into account for dental X-rays because the dose to the foetus is so low
foetus must not be irradiated inadvertently nor should the xray beam be directed towards pts abdo
- if this is unavoidable then a protective lead apron (0.25mm) must be worn
effects of radiation during early pregnancy
radiation exposure could damage/kill enough of the cells for the embryo to undergo resorption lethal effects induced by doses of 100mGy before or immediately after implantation of the embryo into the uterine wall during organogenesis (2-8wks post-conception) when the organs are not fully formed, doses >250mGy could lead to growth retardation doses for these abnormalities are >1000x greater than an IO
sources of natural background radiation
cosmic rays internal radionuclides from diet radionuclides in air e.g. radon external y radiation e.g. soil, rocks and building materials air travel
estimated annual background radiation dose
2.2mSv
UK pop split of natural and artificial radiation
84% natural (50% of this radon gas)
16% artificial
IO xray effective dose
0.005mSv
lifetime risk of cancer 1 in 10m to 1 in 100m
negligible risk
protection of staff - dose limits to body
employee 20mSv
U18 trainee 6mSv
other 1mSv
protection of staff - controlled area
should extend at least 1.5m from the xray tube and pt
xray beam should always be directed away from staff
dose optimisation
ways to reduce pt dose use E speed film or faster (fewer xray photons required) use a kV range of 60-70kV FSD >20cm rectangular collimation
images with minor artefacts or non-uniformities should be saved
refer to these if suspected artefact in clinical image
can also be used for training purposes
CBCT
sectional images
thin slices, usually 0.4mm or thinner
DRLs
established dose levels for typical examinations for standard sized pts
a comparative standard that is used in optimisation compared to NRLs
individual xray units compared to DRLs and NRLs
- identify units giving higher doses
current DRLs for IO exams - adult
- 9mGy digital
1. 2mGy PP and film
current DRLs for IO exams - child
- 6mGy digital
0. 7mGy PP and film
what do digital and film radiography differ in?
how the xray beam is dealt with after it has interacted with the pt
ie how it is captured, converted into an image, stored
size 0
ant PAs
size 2
BWs, post PAs
size 4
occlusals
receptors
digital - PP - SSS film - direct action film - indirect action film
conversion of xray shadow into image
when xray beam passes through an object some of the xray photons are attenuated “xray shadow”
image “info” held by photons after an xray beam has passed through an object
image receptor detects this xray shadow and uses it to create an image
digital radiography - xray shadow to digital image
detector measures the xray intensity at defined areas (arranged in a grid)
each area given value relating to xray intensity
- typically 0-255
- 0 - high intensity
each value corresponds to a different shade of grey
- 0 = black
- 255 = white
the digital image
displayed as a grid of squares - pixels
each pixel can only display one colour at a time
the more pixels you have the more detailed/accurate your image can be
number of pixels
more pixels = better detail = higher resolution
will provide a more diagnostic image up to a limit
- eventually will not provide any meaningful clinical benefit
need more storage space - increased costs
digital receptors limited in now small they can make the pixels because of manufacturing issues (film - substrate of microscopic crystals - don’t have to create a grid)
greyscale bit depth
radiographs typically processed in at least 8 bits
refers to the number of different shades of grey available
8 binary digits = 2⁸ = 256 shades of grey
manipulating digital images
software can be used to copy, resize and alter images contrast/windowing negative emboss magnify
management of digital images: PACS
Picture Archiving and Communication System
storage and access to images
archives for storage and retrieval
viewing digital radiographs
env - subdued lighting, avoid glare monitor - clean - adequate display resolution - high enough brightness level - suitable contrast level
format for digital images - DICOM
Digital Imaging and Communications in Medicine
international standard format for handling digital medical images
- used to transmit, store, retrieve, print, process and display images
essentially alternative to jpeg etc
allows imaging to work between diff software, machines, manufacturers, hospitals and countries without compatibility issues
stores other important data alongside image e.g. pt ID, exposure settings, date
SMPTE test pattern
Society of Motion Picture and Television Engineers
can be used to assess the resolution, contrast and brightness of your monitor
types of digital IO receptor
SSS e.g. CMOS sensor
PP e.g. PSP plates
PP after taking xray
not connected to a computer
after receptor is exposed to xrays it must be put in a scanner and ‘read’ to create final image
image creation using PPs - in mouth
receptor exposed to xray beam
phosphor crystals in receptor excited by the xray energy - create a latent image
image creation using PPs - in scanner
receptor scanned by a laser
laser energy causes the excited phosphor crystals to emit visible light
light is detected and creates visible image
(phosphor plate scanners are connected to computer)
types of SSS
CCD (charge-coupled device)
CMOS (complimentary metal oxide semi-conductor)
SSS creation of image
connected to computer
- usually wired but can be wireless
latent image created and immediately read within the sensor itself
- final image created virtually instantly
SSS components
back housing and cable electronic substrate CMOS imaging chip fibre-optic face plate scintillator screen front housing
SSS - CMOS imaging chip
light converted to electrical signals
SSS - scintillator screen
emits light when xrays hit
identification dot
located in corner of receptor to aid orientation of image
only effective if receptor was positioned correctly during exposure
BWs - dot to top
PAs - dot towards crown
cross-infection control
IO receptors have single use covers to prevent saliva contamination e.g. adhesive sealed plastic covers for PPs, long plastic sleeves for wired SSSs
receptor still disinfected between uses
need to dry after disinfect - bubbles on receptor will show on image
why should you hold receptor by edges not by flat surfaces?
scratches/tears
fingerprints
bending/creases
importance of careful handling
certain types of damage will impact every subsequent image obtained from that receptor
- reduce diagnostic value and may render receptor unusable
advantages of PPs
thinner and lighter
(usually) flexible - good if limited mouth opening - can bend a bit going into mouth
wireless - more stable and comfortable
disadvantages of PPs
variable room-light sensitivity
- risk of impaired image
- left in sunlight too long can bleach sensor
latent image needs to be processed in scanner separately
handling similar to film
SSS disadvantages
bulkier and rigid
usually wired
sealer active area (for same physical area of receptor)
£££
SSS advantages
no issues with room light control - light can’t get through hard plastic
arguably more durable - replaced less often
components of radiographic film packet
protective black paper
lead foil
outer wrapper
film
components of radiographic film packet - protective black paper
protects film from light exposure, damage by fingers and saliva
components of radiographic film packet - lead foil
absorbs some excess xray photons - not contributing to image, will just enter pt and cause damage
components of radiographic film packet - outer wrapper
prevents ingress of saliva
indicates which side of the packet is the front
components of radiographic film packet - film
material in which the actual image is formed
sensitive to both xray photons and visible light photons
photons interact with emulsion on film to produce latent image which only becomes visible after chemical processing
radiographic film structure
transparent plastic base
adhesive
emulsion
protective coating of clear gelatin
radiographic film structure - transparent plastic base
supports the emulsion
radiographic film structure - adhesive
attaches the emulsion to the plastic base
radiographic film structure - emulsion
layered on both sides of the plastic base
silver halide crystals embedded in a gelatin binder (usually silver bromide)
microscopic crystals
what become the ‘pixels’ of the final image
- film generally higher resolution than digital
radiographic film structure - protective coating of clear gelatin
shields the emulsion from mechanical damage
film - silver halide crystals mechanism of action
usually silver bromide
become sensitised upon interaction with xray (and visible light) photons
- change slightly and become excited
during processing - sensitised crystals converted to particles of black metallic silver (dark parts of final image)
non-sensitised crystals removed (light parts of final image)
film speed
relates to amount of xray exposure required to produce an adequate image
increased speed - reduced radiation required to achieve an image
what is film speed affected by?
number and size of silver halide crystals
- larger crystals - faster film but poorer image quality
what film speed should be used?
the fastest film which still provides satisfactory images
comparing E and D film
E is twice as fast as D
- therefore requires half exposure time - half radiation dose
- most commonly used film
comparing E and F film
F is 20% faster than E
- 20% reduction in exposure time (and dose)
requires automated processing - not everyone has this in practice
if changing to a different film speed must either:
convert settings on xray unit (by a qualified technician)
install a filter to absorb part of the primary xray beam
lead foil
in packet, lying behind the film
absorb some excess xray photons
- those in primary beam continuing past the film
- those scattered by pts tissues and returning back to film
embossed pattern to highlight (on image) if receptor was placed wrong way round
- embossed so you don’t think too low exposure used and you repeat
what are intensifying screens used alongside?
special “indirect action” film for EO radiographs e.g. pan, ceph
- too bulky for IO use
effect of intensifying screens
reduce radiation dose
but reduce detail - slightly fuzzier as it is being spread out by the cone of visible light
why are intensifying screens becoming less commonplace?
digital receptors more common
how intensifying screens work
“indirect action” film placed inside cassette with an intensifying screen on either side
screens release visible light upon exposure to xrays - this visible light creates latent image on film
- designed only to interact with visible light photons
film processing
steps which convert the invisible latent image to a permanent visible image
need controlled, standardised conditions to ensure consistent image quality
methods of film processing
manual
automated
(self-developing)
film processing common steps
1 - developing 2 - washing 3 - fixing 4 - washing 5 - drying
film processing step 1
developing
converts sensitised crystals to black silver particles
film processing step 2
washing
removes residual developer solution
film processing step 3
fixing
removes non-sensitised crystals
hardens emulsion (which contains the black silver)
film processing step 4
washing
removes residual fixer solution
film processing step 5
drying
removes water so that film is ready to be handled/stored
manual (wet cycle) - what happens?
person dips film into different tanks of chemicals
- at precise concs/temps
- for specific times
- washes film after each tank
manual (wet) cycle requirements
dark room with absolute light-tightness
adequate ventilation
how long does manual (wet) cycle take?
about 20mins
difference in steps between manual and automated cycle
extra washing step in manual
in automated - sponge rollers squeeze developer solution out of film (instead of washing with water)
automated cycle
machine exposed film goes in at one end = processed film comes out the other developer fixer wash dryer
pros and cons of automated cycle
faster (5mins)
more standardised/controlled than manual
avoids need for dark room
more £££
opening a film packet for automated processing
disinfect surface of packet and wipe off
hold packet under hood of process or unit
peel back flap of outer wrapper
fold back lead foil
pull back paper flap
hold film by edges (not surfaces) and slide out
insert film into processer slot/shelf
self-developing films
not recommended
give tube a squeeze and the chemicals go up to where the film is
self-developing films pros
no darkroom or processing facilities required
faster e.g. 1min
self-developing films cons
poorer image quality image deteriorates more rapidly over time no lead foil easily bent difficult to use in positioning holders relatively £
potential causes of pale image
exposure issue - radiation exposure factors too low developing issue - film removed from solution too early - solution too cold - solution too dilute/old (opp will result in dark)
processing issues - washing
developer and fixer solution will continue to act if not washed away/off
- fixer - looks like bubbles
- developer - black spots
film storage
takes up room
- have to keep films for 11yrs for medico-legal
need to be accessible and safe from damage
require a reliable organisation system
- to allow images to be found easily
- to reduce risk of images being lost/mixed up
processing issues - developing
chemical reaction
- sensitised silver halide crystals to black silver (oxidised in air)
reaction affected by time, temp and solution conc
developer solution oxidises in air
- becomes less effective over time
- needs to be replaced regularly (irrespective to how many films have been developed)
processing issues - fixing
chemical reaction - removes non-sensitised crystals and hardens the remaining emulsion
inadequate fixing - non-sensitised crystals left behind
- image greenish-yellow or milky
- image becomes brown over time
advantages of digital
no need for chemical processing easy storage and archiving of images easy back-up of images images can be integrated into pt records if digital easy transfer/sharing of images images can be manipulated
disadvantages of digital
worse resolution - risk of pixelation
- digital perfectly good nowadays
requires diagnostic-level computer monitors for optimal viewing
risk of data corruption/loss (solved by backing up)
hard copy print outs generally worse quality
image enhancement can create misleading images
caries diagnosis methods
visual - smooth and occlusal radiography elective temporary tooth separation FOTI electrical methods laser fluorescence calcivis - detects Ca2+ loss from demineralising tooth surfaces
caries clinically vs on radiograph
always larger clinically than on radiograph
cervical burnout
phenomenon caused by relative lower xray absorption on the M/D aspect of teeth, between the edge of the E and the adjacent crest of alveolar ridge
B-L dimension of tooth less at IP area - diff amount of xray energy getting through
because of the relative diminished xray absorption, appear relatively radiolucent with ill-defined margins
may mimic root surface caries
- should be detectable clinically
exposure-dependent
saucer-shaped radiolucencies
PD assessment - selection criteria recommendations
radiography secondary to clinical exam and full mouth PD assessment
pocketing 4-5mm: horizontal BWs
pocketing ≥6mm: vertical BWs and PAs if bone not shown
irregular: may supplement with PAs
panoramic useful for overview of all teeth, supplemented by PAs if required or full PAs
PAs for suspected endo-perio lesions
which wall of MS don’t you see on a pan?
lat wall (see post wall)
PD radiography techniques
if pan chose orthogonal projection (P4)
beam angulation crucial
horizontal angle 90 degrees to line of arch
- avoids overlaps of adjacent teeth
vertical angle 90 degrees to LA of tooth
pockets may be difficult to show - consider GP point
clinical pocket depth exam crucial
EO radiography definition
xray source and IR outside pt
lateral radiography types
true
oblique
true lateral radiography
film and MSP are parallel and xray beam is perpendicular to both
oblique lateral radiography
film and MSP are not parallel
xray beam is not perpendicular to either, but oblique to both
OM line
RBL
outer canthus of eye to centre of EAM
Frankfort plane
superior border EAM to lowest point of IO rim
difference between RBL and FP
10 degrees
cephalometric radiography
standardised and reproducible form of skull/facial bones radiography
used in ortho
lateral or PA projections
indications for lat ceph
orthognathic surgery - pre-op assessment and post-op review implant planning - historically - anterior mandible - CS image - now often CBCT
lat ceph distances
source to pts MSP = 152.4cm (5ft) in traditional equipment
image receptor to MSP: manufacturer dependent, fixed or adjustable
effect of anode-object distance on magnification
longer - less difference in magnification as less divergent xray beam
lateral views
lat ceph
lat oblique (mandible)(OJ)
- only shows you one side of pt
bimolar - both sides on one receptor
lat ceph
true lateral view of facial bones, base of skull and upper cervical spine
also shows paranasal sinuses and nasopharyngeal STs
ortho radiographs - lat ceph
pts with skeletal vertical or AP discrepancy
need fixed/fct appliance therapy, for labiolingual movement of incisors
requiring orthognathic surgery and ortho
- do CBCT now instead don’t do both
set up of lat ceph equipment
cephalostat (free standing/attached to pan machine)
ear rods
CCD/CMOS sensor or cassette holder (PP or intensifying screens)
(anti-scatter grid - but higher dose to pt - not often used)
lat ceph collimation
height and depth of field of view or triangular - adjustable, by programme or visual
positioning and preparation for lat ceph
select
press button to line up xray tube head and cephalostat with receptor
hinge nasion rest up and sideways, nasion marker
thyroid collar on
FP horizontal
MSP vertical and parallel to casette
MSP correct distance from cassette if adjustable
teeth together - in centric occlusion or as requested
ear rods in EAM - move symmetrically
nasion support in space
programme selection (height and width adjustment) or move triangular collimation
automatic exposure adjustment or Al ST filter, preferably pre-pt
magnification scale
automatic facial contour in direct digital machines OR Al wedge filter - ideally at tube head
- allow you to see STs?
why can successive lat ceps be used to analyse changes?
fixed distances so subsequent images will always be able to be directly comparable
oblique lateral
film and MSP not parallel
xray beam not perpendicular to either MSP or film
EO view of jaws - R and L sides separately
uses dental or EO xray set
limited use now due to pan
indications for lateral oblique
generally same as for pan but particularly when pan not available or possible e.g. handicapped pt
no longer done at GDH
subjective quality rating - excellent
no errors of pt prep, exposure, positioning or film handling
target >70%
subjective quality rating - diagnostically acceptable
some errors
does not detract from diagnostic utility
target <20%
subjective quality rating - unacceptable
errors make film diagnostically unacceptable
needs retaken
target <10%
reject analysis
unacceptable radiographs and reasons why?
may be multiple errors - don’t stop looking when you find one
what must PA contain?
must contain full length of roots
ideally full crown - but not critical as can examine it clinically
bone around the roots
what must BWs contain?
from mesial of first premolar distally to last tooth
U and L teeth equally
critical - to see ADJ
desirable - no overlap
- but some overlap (as long as you can see ADJ) doesn’t make it 3
QA - prev dental guidance notes definition
to ensure consistently adequate diagnostic info whilst radiation doses are controlled to be ALARP
“the establishment of procedures, at every stage of image formation and utilisation, to ensure optimum image quality and max acquisition of info”
QA - stages involved
selection criteria - right view when needed
production of xrays - correct kV
image geometry - film holders with BAD
image receptor - fastest film/digital
image processing - test tool
image viewing - light box or monitor quality
reject analysis
coning off
incorrect film holder assembly or collimator orientation
density variation
exposure factors
object factors
processing factors
viewing facilities
monitoring processing
process test film daily
compare with reference film
reference film options
clinical film
standard object e.g. extracted tooth
ideal object e.g. Al or Pb step foil wedge - test tool and film - use BW exposure, in contact
wooden spatula
darkroom and daylight loading processors
cleanliness stock control - organisation light tightness - room and cassettes safelights - coin test replenishment
quality of original - viewing conditions
dim room
transmitted light restricted to film
film sensitive to xrays and pressure
image viewing influenced by:
quality of original
equipment
env
knowledge base
dark crescents
nail pressure
safelight testing
need to ensure processing in safe env that doesn’t damage the film
in dark, place coins at intervals on an EO film
cover completely with card
turn on safelights
uncover each coin
- at 30/10s intervals, leaving last coin covered
- which is the first coin to be seen
elongated teeth - occ and PAs
error is decreased vertical angulation - how the beam is related to the occ plane
shortened teeth - occ and PAs
error is increased vertical angulation
pale IO image
too long fsd or inadequate development (development creates the dark bits)
IRR99 - 5 advised safety features
controlled area warning sign for controlled area sign lights up when equipment on light and audible sound during exposure exposure with continuous pressure only exposure stops automatically
