14 Dental Panoramic Radiography 1 Flashcards
What is the most common extra-oral radiograph in dentistry?
Panoramic radiographs
- DPT (dental panoramic tomogram)
- Orthopantogram (OPT/OPG- trade name)
What technique was developed as a solution to superimposition in extra-oral radiographs?
Tomography - developed to allow ‘slices’ of the subject to be viewed separately
What are the two types of tomography?
- conventional - one slice (DPTs)
- computed - multiple slices (CT, MRI)
Describe a panoramic radiograph?
Is a form of conventional tomography which was developed to capture a curved slice aligned with the “horseshoe” shape of the jaws
What is the basic technique for taking a DPT
The patient stands still in the middle of the machine
Controlled rotation of x-ray source and receptor around head during exposure
- both remain opposite each other but point of rotation constantly shifts
- x-ray source remains primarily behind
- receptor remains primarily in front
how long does a DPT take?
14 seconds
What is the relevance of the focal slice in tomography?
Focal slice = focal trough
Structures in the focal slice will appear clearly distinguishable on the image,
Whereas structures outside of the focal slice will appear faint and spread out further across the image (the further out, the worse the effect)
What can be a problem if a patient has a “non-standard” mandible size/shape?
The focal trough is curved as a result of complex rotational movements of the x-ray source and receptor around the patient.
This mimics the size of the average mandible
If the patients mandible is doesn’t fit the “standard”, parts may not be in the focal trough, therefore reduced image quality
What are the boundaries of the focal trough?
There are no defined boundaries, sharpness continually decreases as you move further away.
Why are the anterior teeth more likely to be blurry on a DPT?
The focal trough is thinner anteriorly
- related to the speed of rotation at this point
What is a limitation of the focal trough?
Ectotopic teeth may be far enough out of the focal trough, so appear as “missing”
What are ectopic teeth?
Teeth that develop in an abnormal position, and fail to erupt in their normal location in the mouth
What does the orthogonal programme do on a DPT?
The orthogonal programme alters the x-ray beam so that it is orthogonal (90degrees) to the buccal surface of the teeth. This aims to provide an optimal view of the dentition
What are the advantages to orthogonal program?
- Reduces overlap of the teeth to aid assessment of approximal caries (especially premolars)
- Improves angulation to more accurately represent interdental periodontal bone levels
What are the disadvantages to orthogonal program?
- Distorts the rest of the skeleton to varying degrees
- Typically a narrower field of view, so may miss condyles at edge of image
What is the orthogonal program suitable for?
For cases requiring only caries and/or periodontal bone loss assessment
Why are panoramic radiographs inherently magnified?
Due to divergence of the x-ray beam
Within the focal trough, by what % is the image magnified?
25%
Are structures more magnified if they are buccal or lingual to the focal trough?
Structures lingual to the focal trough are magnified more
Structures buccal to the focal trough are magnified less
The effect is more the further from the focal trough
Is the degree of magnification more sensitive vertically or horizontally to changes in position?
horizontally due to continuous rotation of the machine
How do teeth buccal to the focal trough appear?
narrower
How do teeth lingual to the focal trough appear?
broader
If a patient is too far back in the machine, how will the radiograph appear?
Blurry, teeth are broader than usual as they are lingual to the focal trough
Why will structures positioned further away from the receptor be projected further up on the image?
This is due to the angulation of the beam
- Always angled up slightly
- Typically 8degrees above horizontal
Some machines can vary this angle to help reduce superimpositon
What are the advantages to panoramic radiographs over periapical?
- Can capture entire dentition in one image
- Able to image non-dental areas
- Lack of intra-oral holders benefits some patients (gaggers, trauma cases, young children)
What are the disadvantages to panoramic radiographs over periapical?
- Worse clarity (lower spatial resolution, 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)
In a DPT you can get superimposition of the tongue, why is this not a problem on a periapical?
because for a periapical, the receptor is right next to the tooth, tongue is not in the way
What is the gross estimate of the effective dose (µSv) of a panoramic radiograph?
20µSv
What is the gross estimate of the effective dose (µSv) of a periapical/bitewing radiograph?
4µSv
What are the main components of the panoramic machine?
- X-ray tube head
- Recepter (usually digital)
- Control panel
- Patient positioning
What is the preparation steps for a DPT?
- Remove metal foreign bodies from head and neck (jewelry, glasses, dentures etc)
- Position patient in machine
- Set machine correct height
- Keep neck upright
- Position head using positioning apparatus
- Patient holds handles for stability
- Advise patient to :
- Tongue to roof of mouth
- Stand still
- Do not talk or swallow
What is the patient positioning apparatus in a dpt machine?
Bitepeg
- forces patient into edge-to-edge occlusion
- positions both arches in the focal trough
Light beam markers
- frankfort plane
- mid-saggital plane
- canine lines
Head grabber, chin rest etc
What is this “smiley appearance a result of?
If the patient tilts their chin down (frankfort plane)
What is this flat occlusion a result of?
If the patients chin is tilted up (frankfort plane)
What is this a result of?
If the patients head is rotated round slights, the mid-sagittal plane is not centred
(one side of the jaw is buccal to focal trough = narrower, and the other side lingual = broader)
What is this a result of?
Mid-sagittal plane is not vertical, tilted at an angle
What is this a result of?
Patient is not upright and is slumped over, spine is getting in the way
What can you do to reduce dose of a DPT
reduce field size to capture only the areas you need
During the scan, what is it important for the patient to do?
- stay still
- not talk or swallow
- keep tongue to roof of mouth
Movement artefacts can be affected by a number of factors, what kind of movement caused this?
Slow horizontal movement
Movement artefacts can be affected by a number of factors, what kind of movement caused this?
Mandible moving up and down
Movement artefacts can be affected by a number of factors, what kind of movement caused this?
very brief and sharp horizontal movement
