Now onto the answers for the October 2013 Locate the Object. I will be going over both image shift and SLOB (Same-Lingual, Opposite-Buccal) to determine the location of the left lateral incisor in relation to the left central incisor.
Image shift
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 circular radiopaque entity inferior to the mandibular central incisors. On these radiographs, I am using the right central incisor. The most obvious angle change is to the horizontal angle. Starting with the central incisors periapical radiograph and moving to the left central/lateral incisor 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 (left central/lateral incisor periapical radiograph), we need to compare the image movement of the circular radiopaque entity versus the right central incisor to see which object moved more anterior following point 2 listed above.
The circular radiopaque entity appears to be more anterior on the radiograph meaning it is farther from the image receptor compared to the right central incisor. This gives us an answer of the circular radiopaque being to the buccal/facial of the right mandibular central incisor.
SLOB (Same-Lingual, Opposite-Buccal)
The first thing to do is again pick an object with a known fixed location – the right central incisor.
The second thing to do is determine which direction we are moving from the central incisors periapical radiograph to the left central/lateral incisor periapical radiograph and the answer would be – distal.
Lastly, on the left central/lateral incisor periapical radiograph determine what direction the circular radiopaque entity appears to have moved in relation to the right central 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 circular radiopaque entity is to the buccal of the right central incisor.
Another case will be coming next month, November 12th. If you have any questions or comments, please leave them below. Thanks and enjoy!