Imaging geometry

Question 1

What is Frankforts plane?

Answer 1

Line from lower orbital margin to top of tragus of the ear. (Parallel to the hard palate).

Used for OPG and lateral Cephalometry

Question 2

Upper positioning line?

Answer 2

Line from top of tragus of the ear to alla of nose

Used for all paralleling techniques

Question 3

Lower positioning line?

Answer 3

Top of tragus of the ear to canthus of the mouth (parallel to lower occlusal plane).

Question 4

Median sagittal plane?

Answer 4

Line directly down the middle of the face to divide the face into 2 equal parts.

Question 5

Why is it important to always use the paralleling technique?

Answer 5

• Give accurate image
• Technique is reproducible
• Fewer need for repeats due to errors
• More practical when using rectangular collimeter

Question 6

Describe the technique to carry out bitewings

Answer 6

• Patient seated erect
• Median sagittal plane vertically postioned
• Upper positioning line horizontally positioned
• Blip mesially @ biting surface
• Film/ phosphor plate positioned palatally or lingually
• Central short axis for posterior bitewings and central long axis for anterior bitewings coincident with central point of the image
• Lower edge of the film is 3mm below the bite surface
• Central long axis coincident with occlusal surface and posterior border will include last interproximal space

Question 7

Why should the cone is placed in contact with the ring of the holder?

Answer 7

Ensures that the central ray is perpendicular to the centre of the film.

Question 8

What are the conditions needed to produce an accurate image?

Answer 8

• X-ray photons emitted from infinitely small point source
• Object parallel to imaging recording plate
• Recording plate and object should be closed to eachother
• Focus should be as far away from the object as possible
• Focus should be as far away from the recording plate as possible
• Central ray of x-ray beam should be directed to the centre of the object and recording plate
• The central ray should be perpendicular to the image recording plate.

Question 9

What happens when FOD is increased

Answer 9

Image expands but less sharp as unable to create x-rays from infinitely small source.

Question 10

What happens when the OFD is long?

Answer 10

theoretically increase magnification and increases unsharpness due to decreased divergence.

Question 11

What happens when the central ray is perpendicular to film but not object?

Answer 11

Foreshortening

Question 12

What happens when the central ray is perpendicular to the object but not the film?

Answer 12

Elongation

Question 13

Why is it important that the beam is 20cm from the patients face?

Answer 13

if more = film underexposed

if less = won’t get magnfication and sharpness

Question 14

Why do we get blurring?

Answer 14

Increase OFD means x-rays coming from infinitely small point source but a relatively large area so x-rays at the edge of focus will pass at the same of object making images of the same part in a different place. Thus blurring gets bigger.

Question 15

Why does the film and object need to be parallel?

Answer 15

distorts scale (foreshortening and elongation)
Image will still be accurate as long as they are parallel !

Question 16

When can bisecting angle be used?

Answer 16

Anatomical difficulties: shallow palate/ tongue tied

Recent trauma/ surgery

Question 17

What effect does incorrect vertical angle have on the bitewing image?

Answer 17

With paralleling technique the Image will still be accurate as central ray is at the centre (perpendicular) of the object & plate

Posteriors= cusps are separated and occlusal surface is
visible
Anteriors = won’t change the appearance of biting surfaces as they’re a flat plane

Question 18

What effect does an incorrect central ray have on the image?

Answer 18

some areas won’t be exposed

Question 19

What effect does incorrect lateral (horizontal) angle have on the image?

Answer 19

Overlap thus cannot see interproximal spaces.

Question 20

How would you ensure that the film is parallel?

Answer 20

By utilising the full length of the bite block

Question 21

How can the vertical angle be assessed?

Answer 21

checking the distance between the bite surface and edge of the film is 3mm. If more or less the angle is incorrect.

Question 22

How do you assess density?

Answer 22

Directly exposed areas are black and there is enough detail to see bony detail or maxilla and mandible.
can see apical areas in PA and bony crests in BW

Question 23

How do you assess the contrast ?

Answer 23

Differentiate between different density of areas. E.g. pulp cavity, dentine and enamel are seperate entities.

Question 24

How do you check for sharpness (level of fine detail)?

Answer 24

Sufficient detail for adequate diagnosis. Check bony trabecular pattern, lamina dura, canal for periodontal ligament

Question 25

If the hard palate appears as double line on an OPG, what does this mean?

Answer 25

Frankforts plane was not positioned horizontally.

Question 26

If 3-3 are not as sharp as the premolars on an OPG what does this mean?

Answer 26

The teeth were not in the focal trough in the 2/3 region

Question 27

What happens if the maxiallary teeth overlap the mandibular teeth in an OPG?

Answer 27

The patient was not biting edge to edge on the biting block.

Question 28

How do you check if the median sagittal plane is correct and not tilted?

Answer 28

Measure condyles should be equal distances from the top of the film on the left and right.

Question 29

If the condyles are not shown on the OPG how else can you assess the correct orientation of the median sagittal plane?

Answer 29

Draw a line between the extreme edges of the hard palate this line should be parallel to a line drawn across the centre of the plate.

Question 30

If a patient was looking to the left what side would the ramus of the mandible look to be larger?

Answer 30

Left - the side the patient looks towards is the side that would appear larger thus the median sagittal plane is rotated.

Question 31

If an OPG showed that the palate as a double line what was wrong with the positioning?

Answer 31

Frankforts plane was not horizontal.

Question 32

In an OPG what has likely to have happened when the palate is a double line on one side and single on the other?

Answer 32

Rotation or tilting of the head

Question 33

In an OPG what is indicated when there is air (translucency) below the palate?

Answer 33

The tongue was not in the roof of the mouth.