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
2
Q
what does the ideal projection geometry lead to
A
image size identical to object size
3
Q
do we get ideal projection geometry
A
no
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
5
Q
how can we fix the problems with projection geometry (what 2 techniques)
A
- paralleling technique
- bisecting angle technique
6
Q
what is the paralleling technique
A
- image receptor and object parallel but not in contact
7
Q
what is the bisecting angle technique
A
- image receptor and object partially in contact, and not parallel to each other
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
9
Q
what does the divergent beam cause
A
- undesired magnification
- due to image receptor and object being some distance apart
10
Q
how can the magnification be reduced
A
- use of a long focus to skin distance instead of short
11
Q
what is focus-skin distance
A
- measured on machine
- distance between where x-rays produced to skin
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
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
14
Q
why should rectangular collimation be combined with beam-aiming devices and film holders
A
- causes dose reduction
- improved quality
- fewer rejects
15
Q
what are the 3 colours for film holders
A
- blue
- yellow
- red
16
Q
what is a blue film holder for
A
- anterior periapicals
17
Q
what is a yellow film holder for
A
- posterior periapicals
18
Q
what is a red film holder for
A
- bitewings
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
what is rectangular collimation
- rectangular shaped x-ray beam
- use this as we have rectangular shaped image receptors
- modern machines comes with it
26
what is a collimator made of
- lead as it is good at absorbing x-rays
| - circular or rectangular diaphragm
27
what is the law for the maximum beam diameter at patient end of spacer cone
- maximum beam diameter for circular beam is 60mm at patient end of spacer cone
28
what must the beam size be for rectangular
- 40mm x 50mm
- greater reduction in x-rays at patient end
- need more care to line everything up well
29
what are the different sizes of film holder and image receptor for
- size 0 -> vertical anteriors, blue holder
| - size 2 -> horizontal posterior, yellow holder
30
how is the film holder placed in the mouth
- 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
where is the cotton wool roll placed
- if upper periodical then cotton wool goes below bite block
- if anterior periodical then cotton wool placed on occlusal
32
how is the x-ray mating positioned
- 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
what is the whole paralleling technique summary
- 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
what do bitewings show
- side teeth (premolars and molars)
- symmetry of upper and lower teeth
- minimal overall of adjacent teeth
- inter-dental bone
35
how many bitewings will you need on one side of a patient
- if got all premolars and molars then might need 2 to avoid overlap
- if missing some teeth then 1 would be enough
36
what size film holders are used for bitewings
- size 2 for adults
| - size 0 for small children
37
what orientation do we usually do for bitewings
- horizontal
| - can do vertical where extensive loss of supporting bone due to perio disease
38
how is the bitewing positioned
- 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
how is bitewing taken
- 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
what are key points of bitewings
- image receptor parallel to line of arch
- central ray at 90 degrees
- vertical angle controlled by film holder
- vertical angle +5-10 degrees
41
can you do bitewings without film holders
- yes
| - use little tabs that stick onto image receptor but have to figure out vertical angle
42
can you do periapicals without film holders
- no
43
what is the curve of Spee
- antero-posterior
- curves up posteriorly
- produces a happy smile
44
what is the curve of Monson
- 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
how do you if you are looking at upper or lower molars
- 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
how do you know if you are looking at radiograph right way
- use curve of Spee
| - want a happy smile, not a sad smile
