Before starting to use the image shift principle it is important to know/remember two key points
- Images move in the opposite direction from the movement of the source.
- Images of objects farther from the image receptor will move more (aka objects (images) more facial/buccal will appear to move more).
The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the pin in the second premolar. On these radiographs, we will use the gutta percha in the root canal space of the second premolar. The most obvious angle change is to the horizontal angle. Starting with the canine periapical radiograph and moving to the premolar periapical radiograph, the source of radiation (tubehead) moves posteriorly increasing the horizontal angle. According to point 1 above, this means the images move anterior.
Looking at the second radiograph (premolar periapical radiograph), we need to compare the image movement of the pin versus the gutta percha in the root canal space of the second premolar to see which object moved more anterior following point 2 listed above.
The pin appears to be more anterior on the radiograph meaning it is farther from the image receptor compared to the gutta percha in the root canal space of the second premolar. This gives us an answer of the pin being to the buccal/facial of the gutta percha in the root canal space of the second premolar.
SLOB (Same-Lingual, Opposite-Buccal)
The first thing to do is again pick an object with a known fixed location – the gutta percha in the root canal space of the second premolar.
The second thing to do is determine which direction we are moving from the canine periapical radiograph to the premolar bitewing radiograph and the answer would be – distal.
Lastly, on the premolar bitewing radiograph determine what direction the pin appears to have moved in relation to the gutta percha in the root canal space of the second premolar – mesial. Here is where the acronym comes into play. Did the unknown object move in the SAME direction as the radiographs or in the OPPOSITE direction? Our answer is – opposite and the acronym states that opposite is buccal, so the pin is to the buccal of the gutta percha in the root canal space of the second premolar.
Another case will be coming in two weeks. If you have any questions or comments, please let me know below. Thanks and enjoy!