Dental panoramic tomography 2 Flashcards
What is a tomogram?
- Radiograph showing a slice or section of tissue in focus
- Tissues either side of slice are exposed to radiation but are not clearly seen on resulting film
- tissues seen are within focal trough or place
- multiple tomograms of sequential planes would build up to give 3D image (e.g. CT)
Describe tomography
- conventional tomography is rarely used nowadays
- linear tomography is simplest
- DPT is a form of tomography that allows us to bring the teeth and supporting structures into focus
- other tissues are blurred and not clearly seen but contribute to the image we look at
- this reduced the sensitivity of the technique, but our eyes and brain may not be aware of this
describe narrow beam tomography
- synchronised movement of tube head and film/sensor
- rotate round patient in horizontal plane
- circular path
- single centre of rotation
- focal trough is an arch of circle
- of limited use for dentistry as the dental arch is not part of the circle
Describe DPT
- Dental arch elliptical shaped
- focal trough is horseshoe shaped
- complex rotation - usually within two centres of rotation
- x-ray tube rotates behind the patient
- sensory or cassette move in front of patient synchronised with tube head
what are the disadvantages of intensifying screens?
- light is emitted in all directions
- light affects larger area of film than a single photon
- image quality (fine detail) is not as good as direct action film
What is different about digital radiography?
Indirect action film and intensifying screen replaced by phosphor plates or solid-state sensor/CCD
What are the disadvantages of DPT?
- Lack of fine detail
- superimposition of other soft and hard tissues, air shadows
- patient must be correctly positioned for optimal image quality
- exposure time up to 16 seconds
- patient co-operation required
- magnification of image due to object/receptor distance
Notes about lead apron?
- Current guidance states no justification for routine use of lead aprons in dental radiography
- Does not protect from internal scatter
- Interferes with image on DPT as can be projected onto image and may also catch the tube as it rotates
Describe focal trough
We try and position the patient so that their teeth are in the middle of the focal trough - with the patient biting into the groove of the bite block. The patient brings their mandible forwards into an edge-to-edge incisal relationship - which is impossible to do if you have a class III incisal relationship
describe DPT and caries diagnosis
- Not the “gold standard”
- frequently requested when strong gag reflex
- poor fine detail
- overlap, particularly in premolar regions
- superimposition of anatomy/air (lip shadow)
- some have suggested DPT may be better for occlusal caries diagnosis (especially in molars)
Describe caries in children
- tendency for caries to be approximal
- by the time cavitation has occurred there is pulp involvement
- panoramic often has overlap of contact points - better with bitewings
What should be noted about radiographic requests?
- DPTs need to be justified
- should aid diagnosis and treatment planning
- referrer should know how image is acquired
- patient must be able to co-operate
- clinician must be able to interpret the image
- principles of ALARP should always be followed
