paralleling Flashcards
what is the ideal projection geometry
image receptor and object in contact and parallel
parallel beam of X-rays
X-ray beam perpendicular to object plane and image receptor
- image size identical to object size
cannot happen
- tooth in isolation, image receptor directly behind, 3 X-ray beams come in parallel to each other and perpendicular to long axis of tooth
projection geometry problems
image receptor and object NOT in contact
beam of X-rays NOT PARALLEL
X-ray beam central ray MAY or MAY NOT be perpendicular to object plane and image receptor
image size NOT identical to object size DUE to MAGNIFICATION - DIVERGENT BEAM
2 solutions to problems in projection geometry
paralleling technique
- image receptor and object parallel
bisecting angle technique
- image receptor and object partially in contact and not parallel to each other
paralleling technique
image receptor and object parallel but not in contact
X-ray beam central ray perpendicular to long axis of tooth and image receptor, but outer rays not quite perpendicular due to divergent beam
divergent X ray beams due to
image receptor and object some distance apart
- potential for undesirable magnification
Short focus to skin distance (fsd) – measured on machine
- spacer cone now long focus skin distance
- sticks out some way to help focus beams
focus-skin distance
use long X-ray focus - skin distance (fsd) to reduce magnification
at least 20 cm – part of regulations with 60 or 70kV
N.B. Beam-aiming device of film holder should always be close to, but not actually touching patient
location of focus
use long X-ray focus - skin distance (fsd) to reduce magnification
- at least 20 cm,
- may be 30cm
longer distance = more parallel rays
Measure from mark on outside of tube head (here a red dot) to patient end of spacer cone
3 reasons why rectangular collimation should be combined with beam-aiming devices and film holders
dose reduction
improved quality
fewer rejects
2 types of film holders
rinn film holders (current)
endodontic film holders
rinn film holders
currently used
colour coded
- blue - anterior
- yellow - posterior
- red - bitewing
components (all autoclavable)
- image receptor support – perpendicular part at end
- bite block – rigid section by image receptor support
- beam-aiming device with rectangular guides – circular, slides, marks to show where corners of rectangle of X-ray machine head should line up
- connecting rod – different for colours
components of rinn film holders
image receptor support – perpendicular part at end
bite block – rigid section by image receptor support
beam-aiming device with rectangular guides – circular, slides, marks to show where corners of rectangle of X-ray machine head should line up
connecting rod – different for colours
all components are autoclaveable
blue rinn holder
anterior
yellow rinn holder
posterior
red rinn holder
bitewing
endodontic film holders
green
image receptor support (receptor vertical or horizontal)
bite block
beam-aiming device
connecting rod – no colour
autoclaveable
PLUS basket to support instrument heads or gutta percha points, unable to bite
film holder assembly
critical
correct = good result
- see support of image receptor in middle of circle
incorrect = coning off
retangular collimations
should be provided on new equipment, and retro fitted to existing equipment.
collimator made of
lead
- circular or rectangular diaphragm
prevents X-rays escaping (lead absorbs)
max. beam diam. of circle
60mm at patient end of spacer cone
beam size for circular
no greater than 60mm diameter at patient end of spacer cone
area = 2828 sqmm
beam size for retangular
2000 sqmm
30% reduction compared to circular
- requires greater care on lining up
how to take a periapical
select film holder and image receptor
- size 0 vertical anteriors (blue holder)
- size 2 horizontal posteriors (yellow holder, larger)
assemble correctly
position against teeth of interest, parallel to line of arch and long axis of tooth
- keep beam aiming ring far away so not to hit pt
bite block against teeth
- stability to film holder
- long axis parallel to long axis of teeth
cotton roll on opposite side of biteblock to stabilise
beam-aiming device to patient; close to but not touching
rectangular collimator:
- correct orientation to match image receptor, and spacer cone close to beam-aiming device
align tube head
- check from 2 directions
side and underneath
paralleling technique summary
image receptor and object parallel but not in contact
image receptor and object some distance apart
- potential for undesirable magnification
use long X-ray focus - skin (fsd) distance to reduce magnification
- at least 20 cm
requires use of film holders
stabilise with cotton roll between bite block and teeth in arch opposite to that being X-rayed
tongue and X-ray
move out of way as will interfere with image
bitewings show
side teeth (premolars and molars)
symmetry of upper and lower teeth
minimal overlap of adjacent teeth
inter-dental bone
one or two per side
how to take bitewings
select film holder (red holder) and image receptor
- size 2 adults
- size 0 small children
long axis horizontal
assemble correctly
position against lower teeth, parallel to line of arch
front edge of film packet mesial to canine/premolar contact
patient to bite together
beam-aiming device to patient – close not touching
rectangular collimator orientation correct
Spacer cone to beam-aiming device
align tube head - check from 2 directions
- front and downwards
critical points for taking bitewings
image receptor parallel to line of arch
central ray at 90 degree
vertical angle controlled by film holder
no film holder – is possible
- determine vertical angle + 5-10 degrees relative to occlusal plane - Monson
occlusal curve of spee
antero-posterior
- curves up posteriorly
- produces a happy smile
tell orientation of X rays
also Bone between root – therefore mesial and distal root, thus lower
occlusal curve of Monson
bucco-lingual
influences X-ray technique e.g.bitewings and panoramic radiography (Panoramic vertical angle is negative to occlusal plane: minus 8 degrees)
