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 left central incisor. On these radiographs, we will use the left lateral incisor. The most obvious angle change is to the horizontal angle. Starting with the central incisors periapical radiograph and moving to the canine 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 (canine periapical radiograph), we need to compare the image movement of the left central incisor versus the left lateral incisor to see which object moved more anterior following point 2 listed above.
The left central incisor appears to be more anterior on the radiograph meaning it is farther from the image receptor compared to the left lateral incisor. This gives us an answer of the left central incisor being to the buccal/facial of the left lateral incisor.
SLOB (Same-Lingual, Opposite-Buccal)
The first thing to do is again pick an object with a known fixed location – the left lateral incisor.
The second thing to do is determine which direction we are moving from the central incisors periapical radiograph to the canine periapical radiograph and the answer would be – distal.
Lastly, on the canine periapical radiograph determine what direction the left central incisor appears to have moved in relation to the left lateral incisor – 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 left central incisor is to the buccal of the left lateral incisor.
Another case will be coming next month. If you have any questions or comments, please let me know below. Thanks and enjoy!