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BDS3 - radiology > intra-oral techniques - paralleling > Flashcards

intra-oral techniques - paralleling Flashcards

1
Q

what is the ideal projection geometry

A
  • image receptor and object in contact and parallel
  • parallel beam of x-rays
  • x-ray beam perpendicular to object plane and image receptor
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2
Q

what does the ideal projection geometry lead to

A

image size identical to object size

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3
Q

do we get ideal projection geometry

A

no

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4
Q

why don’t we get ideal projection geometry

A
  • image receptor and object can’t be in contact
  • beam is divergent/not-parallel
  • have magnifications
  • central ray may not be perpendicular to image receptor and object
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5
Q

how can we fix the problems with projection geometry (what 2 techniques)

A
  • paralleling technique

- bisecting angle technique

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6
Q

what is the paralleling technique

A
  • image receptor and object parallel but not in contact
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7
Q

what is the bisecting angle technique

A
  • image receptor and object partially in contact, and not parallel to each other
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8
Q

what happens in paralleling technique

A
  • image receptor some distance away from object

- central ray is perpendicular to long axis of tooth, but outer rays not quite perpendicular due to divergent beam

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9
Q

what does the divergent beam cause

A
  • undesired magnification

- due to image receptor and object being some distance apart

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10
Q

how can the magnification be reduced

A
  • use of a long focus to skin distance instead of short
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11
Q

what is focus-skin distance

A
  • measured on machine

- distance between where x-rays produced to skin

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12
Q

what distance should the fsd be to reduce magnification

A
  • at least 20cm
  • can be longer as long as it is over 20 cm
  • measure on mark on outside of tube to patient end of spacer cone
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13
Q

how should the beam aiming device be placed

A
  • beam aiming device of film holder should always be close to, but not actually touching patient
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14
Q

why should rectangular collimation be combined with beam-aiming devices and film holders

A
  • causes dose reduction
  • improved quality
  • fewer rejects
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15
Q

what are the 3 colours for film holders

A
  • blue
  • yellow
  • red
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16
Q

what is a blue film holder for

A
  • anterior periapicals
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17
Q

what is a yellow film holder for

A
  • posterior periapicals
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18
Q

what is a red film holder for

A
  • bitewings
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19
Q

what makes up a film holder

A
  • bite block, beam aiming device and rod
  • beam-aiming device is circular part that slides up and down connecting rod
  • beam-aiming device has rectangular guides
20
Q

what can happen if film holders are not assembled correctly

A
  • coning off

- image in part of view but then rest of image is just white

21
Q

how can we prevent coning off

A
  • by using rectangular beams to match up with image receptor
22
Q

what colour are Endodontic film holders

A
  • green
23
Q

what is different about Endodontic film holders

A
  • patient can’t bite tooth to tooth so need something to keep teeth apart as have files in them
  • has a basket to support instrument heads or gutta-percha points
24
Q

what is collimation

A
  • controlling shape and size of x-ray beam
25
Q

what is rectangular collimation

A
  • rectangular shaped x-ray beam
  • use this as we have rectangular shaped image receptors
  • modern machines comes with it
26
Q

what is a collimator made of

A
  • lead as it is good at absorbing x-rays

- circular or rectangular diaphragm

27
Q

what is the law for the maximum beam diameter at patient end of spacer cone

A
  • maximum beam diameter for circular beam is 60mm at patient end of spacer cone
28
Q

what must the beam size be for rectangular

A
  • 40mm x 50mm
  • greater reduction in x-rays at patient end
  • need more care to line everything up well
29
Q

what are the different sizes of film holder and image receptor for

A
  • size 0 -> vertical anteriors, blue holder

- size 2 -> horizontal posterior, yellow holder

30
Q

how is the film holder placed in the mouth

A
  • patient needs to open mouth wide
  • tilt film holder and put it in mouth and position bite block against teeth of interest as then have some stability
  • long axis of film holder parallel to long axis of teeth and parallel to line of arch
  • bite block against teeth
  • cotton roll on opposite side to stabilise
  • patient close mouth gently but firmly
  • then slide beam aiming deceive up so close to but not touching patient
31
Q

where is the cotton wool roll placed

A
  • if upper periodical then cotton wool goes below bite block
  • if anterior periodical then cotton wool placed on occlusal
32
Q

how is the x-ray mating positioned

A
  • correct orientation to match image receptors, and spacer cone close to beam aiming device
  • align tube head, check from 2 directions
  • when align X-ray tube head need to check that it is aligned with connecting rod
33
Q

what is the whole paralleling technique summary

A
  • image receptor and object parallel but not in contact
  • image receptor and object distance apart (can cause magnification)
  • use long fsd (at least 20mm)
  • use film holders
  • stabilise with cotton wool between bite block and teeth in opposite arch
34
Q

what do bitewings show

A
  • side teeth (premolars and molars)
  • symmetry of upper and lower teeth
  • minimal overall of adjacent teeth
  • inter-dental bone
35
Q

how many bitewings will you need on one side of a patient

A
  • if got all premolars and molars then might need 2 to avoid overlap
  • if missing some teeth then 1 would be enough
36
Q

what size film holders are used for bitewings

A
  • size 2 for adults

- size 0 for small children

37
Q

what orientation do we usually do for bitewings

A
  • horizontal

- can do vertical where extensive loss of supporting bone due to perio disease

38
Q

how is the bitewing positioned

A
  • position against lower teeth parallel to the arch
  • put it in and manoeuvre it
  • want bite block against occlusal surface of lower teeth
  • front edge of film packet mesial to canine/premolar contact
  • surface of image receptor parallel to line of teeth
39
Q

how is bitewing taken

A
  • patient bite together on film holder (keep biting to keep in place)
  • rectangular collimator orientation correct
  • hold rod and slide beam aiming device up
  • spacer cone to beam aiming device
  • check alignment of tube head from 2 directions (in front and look down)
40
Q

what are key points of bitewings

A
  • image receptor parallel to line of arch
  • central ray at 90 degrees
  • vertical angle controlled by film holder
  • vertical angle +5-10 degrees
41
Q

can you do bitewings without film holders

A
  • yes

- use little tabs that stick onto image receptor but have to figure out vertical angle

42
Q

can you do periapicals without film holders

A
  • no
43
Q

what is the curve of Spee

A
  • antero-posterior
  • curves up posteriorly
  • produces a happy smile
44
Q

what is the curve of Monson

A
  • bucco-lingual
  • influences x-ray technique
  • controlling position of film holder and determining vertical angle, may need to angle tube down slightly (panoramic vertical angle is negative to occlusal plane - minus 8)
45
Q

how do you if you are looking at upper or lower molars

A
  • if can see bone between roots then looking at the mesial and distal roots so much be lower as they have two buccal roots
46
Q

how do you know if you are looking at radiograph right way

A
  • use curve of Spee

- want a happy smile, not a sad smile

BDS3 - radiology (19 decks)

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