Panoramic radiography

Question 1

describe panoramic radiography

Answer 1

• designed to provide a clear view of the entire maxillomandibular region
• proprietary (trade names) - Orthopantomogram (OPT/OPG), Panorex

Question 2

What is the problem of superimposition?

Answer 2

Radiographs produced by passing X-ray beam through everything between X-ray source & receptor so all these structures will be overlaid on the 2D image, potentially obscuring each other. [sinuses, nose, tongue etc all in the same area]. This is less of an issue with intra-oral radiographs as X-ray source & receptor can both be placed close to area of interest (therefore fewer structures captured).

Question 3

What is the solution - tomography?

Answer 3

Allow “slices” of the subject to be viewed separately

Question 4

Describe tomography in medical imaging

Answer 4

1) Conventional - one slice (mostly outdated in medicine except for panoramic radiographs)
2) Computed - multiple slices (all others)

Question 5

Can be produced using different phenomena?

Answer 5

• X-rays - e.g. panoramic radiographs, CBCT, CT
• Radio waves - e.g. MRI
• Gamma rays - e.g. SPECT (Nuclear Medicine)
• Positrons - e.g. PET (Nuclear Medicine)

Question 6

Why panoramic radiography?

Answer 6

Form of conventional tomography which was developed to capture a curved slice aligned with the “horseshoe” shape of the jaws and then displayed as a 2D flat image

Question 7

describe exposure technique

Answer 7

• the area of interest is exposed sequentially from one side to the other over an extended time (e.g. 14 seconds)
• this contrasts with intra-oral radiographs for which the area of interest is exposed uniformly in a split second (e.g. 0.2 seconds)
• this greatly alters how the image is affected by patient movement

Question 8

describe conventional tomography technique

Answer 8

panoramic radiography involves a modified version of linear tomography.

linear tomography captures a single, flat slice by moving the X-ray source & receptor past the area of interest during the exposure (should understand linear to grasp panoramic)

Question 9

Describe linear tomography - principle of slice formation

Answer 9

X- ray source moves in one direction while receptor moves in opposite direction.

Structures in a “focal” slice remain projected onto same point of receptor.

Structures outside this slice are continually projected onto different points of receptor

Answer 10

Structures in the “focal” slice will appear clearly and be distinguishable on the image (the tooth). This plane is in the ‘focal trough’.

Structures outside this slice will appear faint & spread out across the image (tongue and lips are stretched out and not that clear) The further out, the worse the effect

Answer 11

Focal trough in Panoramic radiography = the tomographic slice of interest = jaws (mandible and teeth)

Structures outside this slice appear faint, out of focus and should not obscure the region of interest.

Slice (focal trough) is curved because of complex rotational movements of the X-ray source & receptor around the patient as they are moving around the head.

The point of rotation is not constant and mimics shape of the “average” mandible.

Patients with “non-standard” arch size/shape may not conform → reduced image quality.

Question 12

What are focal trough limitations ?

Answer 12

Ectopic teeth may be far enough out of focal trough so they appear “missing”.

Question 13

What is the aim of the orthogonal program?

Answer 13

Aims to provide an optimal view of the dentition

x-ray beam angulation changed to be more orthogonal (i.e. closer to 90 degrees) to the teeth.

Orthogonal should be used as a default choice unless clinical reason to change it

Question 14

What is the advantages of orthogonal program?

Answer 14

• reduces overlap of the teeth to aid assessment of approximal caries
• particularly improves view of premolars (where dental arch curvature can be pronounced)
• Improves angulation to more accurately represent interdental periodontal bone levels

Question 15

What is the disadvantages of orthogonal program ?

Answer 15

• Distorts rest of skeleton to varying degrees (e.g. maxillary sinuses, mandibular rami)
• typically, a narrower field of view so may miss condyles at edge of image

suitable for cases requiring only caries &/or periodontal bone loss assessment

Question 16

describe magnification of panoramic radiographs

Answer 16

• within the focal trough the image is magnified by 25%
• structures lingual to the focal trough are magnified more
• structures buccal to the focal trough are magnified less
• effect is emphasised as distance from focal trough increases but distortion also occurs

Question 17

describe distortion

Answer 17

degree of magnification in the horizontal plane is much more sensitive to changes in position (due to the continuous rotation of the machine)

Question 18

Why are teeth wider if lingual and narrower if buccal?

Answer 18

relates to how close they are to the rotating X-ray source. Remember that the beam sweeps from one side of the jaws to the other but the X-ray source is always lingual to the focal trough

Question 19

Describe vertical projection

Answer 19

Structures positioned further away from receptor will be projected further up on the image - due to the angulation of the beam
- always angled slightly upwards
- typically, 8 degrees above horizontal

Question 20

panoramic vs periapical radiographs

Answer 20

Panoramic can capture entire dentition in one image.

Able to image non-dental areas e.g. rami, condyles, maxillary sinuses.

Lack of intra-oral holders benefits some patients e.g. gaggers, trauma cases, young children.

Worse clarity, lower spatial resolution (not as sharp), more superimposition, more artefacts.

Longer exposure time - Increased risk of patient movement,

Higher radiation dose per image - Approximately 5x more for a “full” panoramic radiograph compared to a periapical.

Panoramic vs “full mouth” periapical radiographs:

Consider difference in radiation dose, time taken to obtain images, information obtained, technical difficulty, etc… likely similar dose but time taken to take 10 periapicals will be longer

Ultimately the choice depends on which will give the best “clinical benefit to radiation harm” ratio.

Question 21

describe field limitation

Answer 21

instead of doing a full size panoramic radiograph, whenever appropriate the area should be reduced in order to reduce dose

Question 22

What are the main components of panoramic machine?

Answer 22

• X-ray tube head
• receptor (usually digital)
• control panel
• patient - positioning apparatus - holds patient in proper place

Question 23

What does preparation consist of?

Answer 23

Remove metal foreign bodies from head and neck e.g. piercings, glasses, dentures, necklaces, hairclips, position patient in machine -
- set machine at correct height
- keep neck as upright as possible
- position head using positioning apparatus
- patient holds handles for stability
(advise patient(keeping anatomy as still as possible))
- tongue to roof of mouth
- stand still
- do not talk or swallow

Question 24

What would you advise patient?

Answer 24

• Tongue to roof of mouth
• stand still
• do not talk or swallow

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