Panoramic Radiography Flashcards
What is tomography
method of radiography displaying details of a selected plane (layer/slice) within the body
Wha tis the image layer/focal trough
layer in the patient that contains the structures of interest that are demonstrated with sufficient resolution to make them recognisable
How are structures at other depths to image layer seen
not clear
What happens in linear tomography
the X-ray tube is moved in a straight line in one direction while the film moves in the opposite direction
Where are objects that are not in the focal plane projected in linear tomography
to continually changing points on the film
Where are objects that are in the focal plane projected in linear tomography
Objects in the focal plane are projected to the same point of film
Describe the focal trough in panoramic radiographs
- Contains all teeth
- Structures above and below the teeth
- Structures superficial and deep to the teeth provided they are close
- Distant structures are not clear
Is the rotation centre fixed
no
it continually changes
What is layer formation a product of
○ Movement of the X-ray source and therefore X-ray beam through the desired objects, the teeth
○ Movement of the X-ray image receptor (film or digital receptor) through the X-ray beam at the correct speed so that desirable objects will be recorded as clear images
What will objects outside the desired image layer be portrayed as
either distorted unsharp images or be imperceptible
What is the impact of different size perimeters
- Aka the distance from the rotation centre
the further from the rotation centre, the faster the beam passage around the circumference has to be
What does it mean for speed that the posterior teeth are further from the rotation centre
○ This means there is faster beam passage through teeth
○ The image receptor must also move fast to match
What does it mean for speed that the anterior teeth are closer to the rotation centre
○ This means there is slower beam passage through the teeth
○ Image receptor movement becomes slower to match and prevents distortion
What is the beam direction
lingual to buccal
What does the distance from the rotation centre to the teeth affect
The width of ‘layer in focus/focal trough’
Horizontal distortion if patient in incorrect position
Ghost images
What is the width of the layer dependent on
Width of the X-ray beam (same throughout so not really a variable)
Distance to rotation centre
What is the effect on the width of the layer if it is closer to the rotation centre (ant teeth)
narrower layer
What is the effect on the width of the layer if it is further from the rotation centre (post teeth)
Further away (posterior teeth) = wider layer
How is patient positioned
- Patient positioned within gantry containing X-ray source and image receptor; standing or seated (any metal objects removed prior to this)
- Frankfort plane horizontal (lower border orbit to upper border external auditoary meatus)
Patient remains stationery and the gantry moves
- Frankfort plane horizontal (lower border orbit to upper border external auditoary meatus)
How is the X-ray beam angled
upwards - 8 degrees (due to curve of Monson)
Why must the speed of beam through teeth and image receptor through the beam be synchronized
to produce an accurate image (formation of correct image layer) dependent on correct patient position and machine adjustments
What should px be asked to remove
- Ask them to remove glasses, necklaces, jewellery, dentures, appliances in their mouth, metal hair accessories and provide them with containers
Where should the vertical canine be
should shine on the upper canine: if not compensate using the appropriate adjustment button
What are limitations of DPT
positioning difficulties
long exposure time
narrow width in focus anteriorly
horizontal distortion
Which patients occlusion result in position difficulties
§ Class II Div 1
§ Class III
How does class II Div 1 result in positioning difficulties
□ Means that the upper incisors are significantly infront of the lower, it may not be possible to get the teeth into the required grooves
How does classIII result in positioning difficulties
□ Lower infront of upper
□ Not possible to get teeth into required position as they cannot move their jaw further back
How does long exposure time create difficulties
hard staying still
What does the narrow width in focus anteriorly cause limitations
Structures not shown are not necessarily not present
What can movement during exposure result in
Can sometimes see a wavy border of the mandible, showing there has clearly been movement
If the patient’s canine is behind the canine guide line what does this result in
they are closer to x-ray source than machine expects
speed of beam is slower through teeth as they are closer to rotation centre
if not compensated will mean
that image receptor is too fast and the image is magnified horizontally
If the patient’s canine is in front of the canine guide line what does this result in
they are closer to the x-ray source than the machine expects
speed of beam is faster as they are further form rotation centre
if not compensated then image receptor is too slow and image of teeth is reduced in width horizontally
What are common objects that cause ghost images
○ Earrings
○ Metal restorations
○ Anatomical features - especially opposite side of the mandible e.g ramus
○ Soft tissue calcifications, e.g lymph nodes, salivary calcification
What are general points about ghost images
○ Always higher due to negative vertical beam (-8 degrees) e.g if it is an earring then will be reflected higher up
○ Always horizontally magnified
○ Change in antero-posterior position (usually further forward)
Can interfere with diagnosis but not always
How are ghost images formed
○ The X-ray tube start position directs beam posteriorly towards opposite TMJ region
○ The tube moves round behind the patient’s head
○ When the image of premolar region is created, beam is coming from posterior point on opposite side
Ghost images are usually more anterior than real image
What are applications of DPT in dentistry
- Development of dentition (ortho)
- Developmental and acquired anomalies
- Caries, pulpal and perio disease (some limitations for caries due to overlap of teeth but if not able to take bitewings this is an alternative)
- Pathological jaw lesions
- Trauma - fractures of mandible
How is dose reduced in DPT extra orally
- Extra oral
○ Collimation (panoramic programme selection)
○ Rare-earth screens: system speed 400 or greater
○ Digital
-
How is dose reduced in DPT intra orally
Intra-orals ○ 60-70kV ○ Rectangular collimation ○ E or F-speed film Digital and if not then fast speed film
Summarise DPT
- 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 lecture on radiographic anatomy)
