Panoramic Radiology

Question 1

Panoramic radiographs try to capture an image layer that includes the teeth. What is an image layer?

Answer 1

It is a layer in the patient that contains structures of interest that are demonstrated with sufficient resolution to make then recognisable, whilst structures at other depths (both superficial and deeper) are not clearly seen

Question 2

What is an image layer also known as?

Answer 2

A focal trough

Question 3

The principle of layer formation is what?

Answer 3

Linear tomography

Question 4

What is linear tomography?

Answer 4

Linear tomography is where the x-ray tube is moved in a straight line in one direction and the x-ray film is moved in a straight line in the opposite direction.

The things that we want to see are in the focal plane.

The objects in the focal plane are projected to the same point in the film so are clear.

Objects not in the focal plane are projected to continually changing points on the film.

Question 5

In the image layer for a panoramic radiograph, we want to achieve a layer that has what properties/composition?

Answer 5

• Contains all teeth
• Contains the structures above and below the teeth
• Contains the structures superficial and deep to the teeth provided they are close
• The distant structures are not clear

Question 6

Is the rotation centre static when taking a panoramic radiograph? Why?

Answer 6

no - it is continuously changing in order to capture what we want to see in the radiograph

Question 7

Describe the effect of the distance of the rotation centre from objects regarding the speed of the beam.

Answer 7

• The further away from the rotation centre the objects are then the faster the bean travels through them
• The closer the objects are to the rotation centre the slower the beam moves through them

Question 8

Why is the speed of the beam/rotation centres relevant to the image receptor in panoramic radiology?

Answer 8

The image receptor needs to match the changes in speed of the beam (changing rotation centres and distance from objects)

This is why it is important that we position the patient correctly (so the machine can automatically make these changes)

Question 9

Describe the speed of the X-ray beam through the different teeth in panoramic radiographs. (and relative position of rotation centre)

Answer 9

Posterior teeth are further away from their rotation centre so the beam travels faster through them

Anterior teeth are closer to the rotation centre so the beam passes through more slowly

Question 10

The distance from the rotation centre to the teeth can affect what?

Answer 10

• the width of the layer in focus
• horizontal displacement if patient in incorrect position, relative to machine focal trough
• Ghost images

Question 11

How should the Frankford plane be positioned when taking a panoramic radiograph?

Answer 11

Horizontal the the ground

Question 12

In what direction does the beam pass?

Answer 12

From lingual to buccal (the x-ray tube rotates around the back of the head)

Question 13

What is the angulation of the x-ray beam and why?

Answer 13

negative 8 degrees (curved 8 degrees up)

done because of the curve of Monson (the cusps of the buccal molars higher than the lingual cusps)

Question 14

What are the limitations of panoramic radiology?

Answer 14

• Patients occlusion
  
  • Need to bite into a certain peg and if patient’s teeth don’t meet then this will be difficult
• Long exposure time (up to 16secs)
  
  • Nervous or other special needs patients may find this difficult
• Horizontal distortion
• Positioning difficulties
  
  • Big shoulders – machine very close to shoulders so may not fit
  • Class II Div 1 o Class II – can’t move lower jaw back
  • Very young children, elderly and obese patients
• Narrow width in focus anteriorly
  
  • If you can’t see something (e.g. a certain structure) it doesn’t necessarily mean it is not present due to the width of the layer in focus
• Movement

Question 15

What guide line is there to aid with positioning of the patient?

Answer 15

The canine guide line (a light that shines that should shine on the upper canine)

Question 16

What must be done if the guide line doesn’t shine on the upper canine?

Answer 16

Must be compensated for (so that the beam and image receptor are synchronised and an accurate image is produced)

Question 17

Describe what it means if the patient’s canine is behind the guide line and how this will impact the image.

Answer 17

• the teeth are close to the x-ray source than the machine expects and the speed of the beam will be slower through the teeth as they are closer to the rotation centre
• If this is not compensated for then the image receptor will move too quickly and the image will be magnified horizontally Note how in the picture the centrals look wider than normal.

Question 18

Describe what it means if the patient’s canine is in front of the guide line and how this will impact the image.

Answer 18

• teeth are further from X-ray source than machine expects and the speed of the beam will be faster through teeth as further from rotation centre
• If not compensated for then the image receptor will move too slowly and the image of the teeth will be reduced in width horizontally

Note how narrow the incisors appear.

Question 19

What are the uses of panoramic radiology in dentistry?

Answer 19

• Development of the dentition (often in relation to ortho) • Developmental and acquired anomalies • Caries, pulpal and periodontal disease - more for perio but should look for caries if a panoramic radiograph has been taken • Pathological jaw lesions • Surgery – evaluation and review • Trauma – fractures of the mandible

Question 20

How can you reduce the dose when taking an OPT?

Answer 20

only take a radiograph of what is required. E.g. half mouth.

Question 21

Summary

Answer 21

• Panoramic radiography is a sectional image • Patient positioning is crucial for clear images • Recognition of ghost images is important • The anatomical features are determined by the ‘view’ of the patient (explained in a later lecture)

Question 22

What are some features of ghost images?

Answer 22

• are always higher due to -ve beam angulation (8 degrees)
• always horizontally magnified
• Change in the antero-posterior position (usually further forward)
• Can interfere with diagnosis but not always