Lecture 2: radiographic techniques and image evaluation Flashcards
What are the 3 criteria (involving film, focus, object and distance) that must be met to produce a perfect image?
1- long FOD (focus to object distance)
2- long FFD (focus to film distance)
3- short OFD (object to film distance)
What are the 7 geometry criteria to produce an accurate radiographic image?
1- X-ray photons should be emitted from an infinitely small point source
2- The object should be parallel to the image recording plate
3- The focus should be as far from the object as possible (long FOD)
4- The focus should be as far from the film as possible (long FFD)
5- The object and film should be as close to possible as each other (short OFD)
6- The central ray of the x-ray beam should be directed to the centre of the object and recording plate
7- The central ray should be perpendicular to the image recording plate
What would happen if you moved the object further from the film?
- magnification (theoretically)
- unsharpness (blurring)
What are some reasons why a film won’t fit into a patient’s mouth?
- gagging
- shallow palate
- short frenulum
- recent trauma/surgery
What 4 positioning lines/planes are used to assist in the positioning of the patient’s head?
- Median Sagittal Plane
- Frankforts plane
- Upper positioning line
- Lower positioning line
Where does the Median Sagittal Plane run?
- It runs vertically, directly down the middle of the face.
- It divides the face into equal left and right halves
Where does the upper positioning line run?
- It runs horizontally, from the ala of the nose to the top of the tragus of the ear.
- It is parallel to the occlusal plane
Where does the lower positioning line run?
- It runs horizontally, from the canthus of the mouth to the top of the tragus of the ear.
- It is parallel to the occlusal plane when the mouth is open
Where does Frankforts plane run?
- It runs horizontally, from the lower orbital margin to the top of the tragus of the ear.
- It is parallel to the hard palate
What is the role of the positioning lines?
- They act as external markers for internal structures
- They aid in the accurate positioning of the patients head for radiographs
What technique must be used for intra-oral radiography with film holders?
paralleling technique
What are the 4 reasons for using the paralleling technique for intra-oral radiography?
1- it always gives an accurate image (no distortion)
2-the technique is reproducible
3-there will be fewer repeats due to fewer errors
4-the use of rectangular collimators becomes more practicable
When using ring film holders, what other side can the upper right be used for?
lower left
When using ring film holders, what other side can the upper left be used for?
lower right
What are the names for bitewing holders?
- Rinn Dentsply
- Kerr-Hawe (Kwik Bit)
When taking bitewings, what distance must always exist between the X-ray tube and the patient’s skin?
20cm
What are 3 advantages of using a rectangular collimator?
- reduces the effective dose, 40-50%
- improves contrast
- improves sharpness (fine detail)
What is the purpose of the ‘blip’ on the film?
Informs us which side of the mouth is left/right
For bitewings, where does the blip go?
towards the mesial end
For peri-apicals, where does the blip go?
towards the biting surface (crown of tooth)
What axis do bitewing/PA films have?
- central long axis
- central short axis
What is the required distance of film from the biting surface?
3-4mm
How much apical tissue should be seen in addition to the crowns in a bitewing?
4mm
For a bitewing, how is the film placed?
- In landscape mode
- The central long axis will coincide with the occlusal surface
- the posterior border will include the last interproximal space
What are the 6 things to examine when evaluating a radiograph?
1- area 2-projection 3- density 4- contrast 5- sharpness (level of fine detail) 6- any artefacts on the image
What is the correct setting of the focal trough?
2/3
How can you see if the focal trough was set correctly?
Check the anterior teeth (1-3), they should have the same level of detail as the premolars and molars.
How can you determine if the tongue wasn’t to the roof of the mouth?
A dark shadow of air will be seen below the hard palate
How can you check if Frankforts Plane was set correctly?
The hard palate will appear as a single line,
if two lines appear then Frankforts Plane was not horizontal
How can you check if the Median Sagittal Plane was set correctly (vertical)?
This is to determine if the head was tilted.
-If the condyles from the TMJ are visible, measure the distance from them to the top of the film. If the measurements are equal there is no tilting. If the measurements are different the Median Saggital Plane was tilted.
How can you determine if the head was rotating to the left or right?
Measuring the width of the mandibular ramii.
If they appear the same width, there has been no rotation. If they are different widths, there has been rotation
How can you check if the occlusal plane is correct?
The mouth should appear slightly upturned.
If it appears smiling or frowning it is incorrect
How many focus points does an OPG have?
it has 3
- Two side pivot points to provide a sharp image of the left and right sides
- One at the front that is adjustable to accomodate different bite profiles
How can the focal trough of an OPG be adjusted?
- Patient can be moved in and out by using the chin rest
- The focal point itself can be moved
What does OPG stand for?
orthopantomography
What are the different grades for radiography and what is their criteria?
There are 3 grades:
1- perfect, no errors of position or exposure (GDC recommends 70% should be in this category)
2- some errors, the image is still diagnostic (GDC recommends 20%)
3- errors render the image non diagnostic (GDC recommends 10%)
What technique is used in OPG radiographs?
tomography
Give some examples of artefacts
tongue stud
scratches on film
earrings
hair clips
What fine details can be observed to assess the level of sharpness?
- bony trabeculae of alveolar bone
- lamina dura
- ligament canal (most enduring) this is seen as a dark line just in front of the lamina dura
How can the vertical angle be assessed for molars?
if correct: the molar cusps would superimpose on each other and the crown of the tooth would appear in profile.
If incorrect: The cusps are separated and the occlusal surfaces are seen, you can ‘peer into’ the bitting surface of the tooth.
-No part of the tooth should be in the bite block: this is a geometric effect of the vertical angle being incorrect
How can the vertical angle be assessed for anterior teeth?
Changing the vertical angle would move the image up or down the film.
- The biting surface of the tooth must be 3mm from the edge of the film.
- If the tooth is more than or less than 3mm from the edge of the film, the vertical angle is incorrect. T
-No part of the tooth should be in the bite block: this is a geometric effect of the vertical angle being incorrect
In the bisecting technique, where is the x-ray directed to?
it is directed perpendicular to the bisecting plane
Where is the bisecting plane?
The imaginary line drawn between the tooth and the film.
How can you measure the Median Sagittal Plane was set correctly if the condyles aren’t visible?
If you can’t see the condyles, draw a line along the hard palate and compare it to a second line drawn across the centre of the plate. If they are both parallel, there is not tilting of the Median Sagittal Plane
What would have happened to the patient if the left ramus appeared wider than the right on the radiograph?
-If the left ramus is wider than the right, it means the head is rotated towards the left- since the left side has moved further away from the focus and so it has become magnified.
What does R1, R2, R3 and R4 mean in terms of radiographic diagnosis?
R1: radiolucency confined to early enamel lesion
R2: radiolucency in enamel up to EDJ (late enamel lesion)
R3: radiolucency in enamel and outer half of dentine
R4: radiolucency in enamel and inner half of dentine
What does a good diagnostic test have?
- high specificity
- high sensitivity
What does high sensitivity mean?
high proportion of true positives found (carious area)
What does high specificity mean?
high proportion of true negative identified (carious free area)
