radiology Flashcards
what is an alpha particle composed of?
2 protons and 2 neutrons
how far can an alpha particle travel?
few inches
what is a beta particle composed of?
electron
how far can a beta particle travel ?
few feet
what is the difference between atoms and ions?
atoms have equal numbers of protons and electrons, ions do not
when radiation passes through matter, it ionises atoms and deposits energy locally, what is the approximate quantity of energy?
35eV
what is the most significant effect of ionising radiation ?
DNA damage - can be seen on faulty repair of chromosome breaks
radiation can damage DNA directly and indirectly - describe the direct effect
radiation interacts with atoms of a DNA molecule or other important cell components
radiation can damage DNA directly and indirectly - describe the indirect effect
radiation interacts with water in the cell producing free radicals which can cause damage
what are free radicals?
highly unstable high reactive molecules
which radiation usually is the cause of double strand breaks in DNA?
alpha
the biological effect of radiation will depend on a number of factors, list 4
type of radiation
amount
time over which dose is received
tissue or cell type irradiated
what is the effect of radiation dose on cell repair capacity?
low dose rate - less damaging and cells can repair
high dose rate - DNA repair capacity likely to be overwhelmed
which tissues are at higher risk of cancer following large radiation exposures?
oesophagus
thyroid
breast
tissue radio sensitivity is dependent on two factors which are
the function of the cells making up the tissue
if the cells are actively dividing
which type of cell are very radiosensitive?
stem cells - divide frequently
which cells are less sensitive to radiation?
differentiated as the do not divide
true or false
the more rapidly a cell is dividing, the more radiosensitive
true
give an example of highly radiosensitive tissue
bone marrow
lymphoid tissue
GI
give an example of moderately radiosensitive tissues
skin
lung
lens
which tissues are least radiosensitive
bone and cartilage
connective tissue
CNS
what are the 4 possible outcomes of radiation hitting the cell nucleus?
no change
dna mutation - repaired
dna mutation - cell death
dna mutation - cell survives but is mutated
what is the units of absorbed dose?
Gray Gy
what does absorbed dose measure?
the energy deposited by radiation
what is the equivalent dose?
absorbed dose multiplied by a weighting factor depending on the type of radiation
what are the units for equivalent dose
sieverts Sv
what is the weighting factor for alpha particles?
20
what is the weighting factor for beta gamma and x-rays
1
what is the LNT model (Linear no threshold model) used for ?
to estimate the long term biological damage from radiation
what are the assumptions of the LNT model?
damage is directly proportional to radiation dose
radiation is always harmful with no safety threshold - several small exposures have same effect as one large
what is the risk of cancer from an intra-oral x-ray?
less than 1 in 10,000,000
what is the associated risk of cancer of a dose of 1mSv?
1 in 20,000
how can you reduce damage to CR plates?
insert between two plastic sheets
clean and replace damaged detectors
what are the adult diagnostic reference levels DRLs for intra-oral examinations?
0.9mGy for digital sensore
1.2mGy for phosphor plates and film
what are the current DLRs for child intra oral examinations?
0.6mGy digital sensors and 0.7mGy phosphor plates and film
what are the elements of radiation protection philosophy?
justification
optimisation
dose limitation
what does justification mean in radiology?
must have sufficient benefit to individual or society to offset detriment
how do we optimise radiographs ?
ALARP
individual doses and people exposed should be As Low As Reasonable Practicable
dose optimisation is a legal requirement, how do we ensure dose is optimised?
ALARP
use rectangular collimators
use E speed film
kV range 60-70
focus to skin distance >200mm
circular collimators increase radiation dose by how much?
40%
how far should the controlled area extend from X-ray tube and patient?
1.5m
what are the two types of radiation effects?
deterministic
stochastic
what are deterministic effects of radiation?
tissue reactions
severity of effect is related to dose received
only occur above certain dose
what are stochastic effects of radiation?
probability of occurrence is related to dose received
when will you see deterministic effects of radiation?
several days after exposure
what is a lethal dose of radiation?
6Sv to whole body
stochastic effects can be subdivided into two categories, what are they and describe them?
somatic - disease or disorder e.g. cancer
genetics - abnormalities in descendants
what are the possible consequences of radiation during pregnancy?
radiation exposure could damage or kill enough of the cells for embryo to resorb
what dose of radiation will have lethal effects on an embryo immediately after implantation?
100mGy
what dose of radiation will have lethal effects on an embryo during organogenesis?
> 250mGy
what are some sources of natural background radiation?
radionuclides in air - radon gas 50%
cosmic rays 12%
external gamma radiation 13%
internal radionuclides from diet
air travel
what is the estimated natural background radiation dose?
2.2mSv
what is the lifetime risk of cancer of intra oral X-ray?
1 in 10 million-1 in 100 million - negligible risk
what is the effective dose in intra oral x-rays?
0.005mSv
what plate size would you use for anterior periapicals?
0
4what plate side would you use for bitewings and posterior perioapicals?
2
what plate size would you use for occlusal radiographs?
4
digital images can be made more detailed, higher resolution and accurate by increasing what?
pixelation
what are the disadvantages of higher pixelation?
each digital image will require more storage space so increased costs
limitations on how small pixels can be due to manufacturing challenges
how many bits are radiographs typically processed in?
8
8 binary digits so 2 to power of 8 so 256 shades of grey
what does PACS stand for?
picture archiving and communication system
what is the purpose of identification dot on receptors?
to aid orientation
give an example of digital x-ray receptors
phosphor plate
solid state sensor
what are some advantages and disadvantages of phosphor plates?
thinner, lighter
wireless
latent image
variable room light sensitivity so risk of impaired image
what are some advantages and disadvantages of solid state sensors?
no issues with room light control
more durable and replace less often
more expensive
bulkier and rigid
wired
what’s inside an intra oral film packet?
radiographic film
protective paper - protects from light exposure, damage and saliva
lead foil to absorb excess X-ray photos
outer wrapper - prevents ingress of saliva
what photons is radiographic film sensitive to?
x-ray photons and visible light photons
what are the advantages of digital radiography?
no need for chemical processing
may storage and archiving
images can be integrated into patient records
easy transfer of images
can be manipulated
what are the disadvantages of digital radiography?
worse resolution
requires diagnostic level computer monitors
risk of data corruption
hard copy generally lower image quality
what are the advantages and disadvantages of self developing film?
no dark room or processing facilities required
faster
poorer image
image deteriorates
no lead foil
expensive
what affects the reaction time of developing film?
time, temperature and concentration
what are the potential causes of a pale image on film?
exposure issue - radiation exposure factors are too low
developing issue - film removed from solution too early
solution too cold or too dilute
opposite results in darker image
film image is greenish yellow or milky, what has happened?
inadequate fixing meaning non-sensitised tissues are left behind
what does the fixing process of film processing involve?
chemical reaction which removes non-sensitised crystals and hardens the remaining emulsion
what is the structure of radiographic film?
transparent plastic base
adhesive attaches emulsion to plastic base
emulsion
protective coating of clear gelatin - sheilds emulsion
what is radiographic emulsion composed of?
silver halide crystals embedded in a gelatin binder
what is the equivalent of pixels in film radiography?
microscopic silver halide crystals
how do silver halide (usually silver bromide) crystals react with x-ray?
become sensitised upon interaction with x- ray photons
what happens to sensitised and non-sensitised crystals during the processing of film?
sensitised crystals are converted to particles of black metallic silver (dark parts)
non-sensitised crystals are removed (light parts)
what are the fasted films in order of fastest to slowest?
F 20% faster than E
E twice as fast as D
D
what is the purpose of intensifying screens?
used with indirect film to reduce radiation dose
but also reduce detail
what does film processing do to the film?
converts it from latent invisible image to visible permanent image
what are the steps of film development?
developing
washing
fixing
washing
drying
what occurs during the developing stage of film processing?
converts sensitised crystals to black metallic silver particles
what occurs during the fixing stage?
removes non-sensitised crystals and hardens the emulsion containing the black metallic silver
what is the purpose of lead foil in the film packet?
absorbs some excess photons - those continuing past the film and those scattered by patient tissues
what is the effect of film speed on radiation required to achieve an image?
increased speed means less radiation is required to achieve an image
larger crystals mean faster film but poorer image quality
give examples of extra-oral radiographs
panoramic
cephalometric - lateral and posterior-anterior
oblique lateral
direction of beam in lateral radiograph
side of head
direction of beam in postero-anterior radiograph
starting posteriorly and passing anterior
angulation of beam in oblique radiograph
not perpendicular to head
angulation of beam in a true radiograph
perpendicular to head
where is the Frankfurt plane?
infraorbital margin and superior border of internal auditory meatus
how many degrees difference is there between orbitomeatal line and Frankfort plane?
10
how many degrees difference is there between orbitomeatal line and Frankfort plane?
10
where is the orbitomeatal line?
connects outer can this and centre of external auditory meatus
what are the clinical applications of cephalometry?
orthodontics and orthognathic surgery
monitor changes
root length, locating un-erupted teeth
what is a lateral cephalograph and what can be seen?
standardised true lateral radiograph
teeth, facial bones and soft tissues, sinuses
cephalographs must be standardised and reproducible, how do we achieve this?
cephalostat - ensures standardised positioning of equipment and patients head
what is the function of cephalostat?
to make cephalographs reproducible and standardised
how does a cephalostat standardise radiographs?
avoids discrepancies between radiographs taken years apart
reduces magnification and distortion of image
includes ear rods and forehead support to hold head at correct angle and stabilise head
what is the standardised distance between the receptor and x-ray focal spot for a cephalograph?
1.5m - 1.8m to reduce magnification
soft tissues show up poorly when exposure settings are optimised for hard tissues, what is the solution?
use software to enhance soft tissues post-exposure
place aluminium wedge filter in unit to attenuate beam where it exposes soft tissues
how do cephalographs meet ALARP legislation?
thyroid collar almost always
triangular collimation to reduce exposure of cranium
field of view should not be bigger than what is clinically required
what are the two lateral extra oral radiographs in dentistry?
lateral cephalogram
oblique lateral
what are the benefits of lateral oblique radiographs?
view of posterior jaws without superimposition of contralateral side
good if pt cannot tolerate intra-oral or won’t fit in OPT
what are some indications of oblique lateral radiography?
pathology assessment
un-erupted teeth position
fractures
lesions e.g. tumours
poorly cooperative patients
involuntary movements
how thin are slices on a CBCT?
0.4mm or thinner
parallax horizontal tube shift options
2 periapicals
2 bitewings
2 oblique occlusifs
parallax vertical tube shift options
panoramic and oblique occlusal
panoramic and lower periodical
