Locate the Object: September 2015 ANSWER 4

Now the answers from last Thursday.

Where is the resorptive lesion on the mandibular left lateral incisor (#23)?

Image shift

Before starting to use the image shift principle it is important to know/remember two key points

  1. Images move in the opposite direction from the movement of the source.
  2. Images of objects farther from the image receptor will move more (aka objects (images) more facial/buccal will appear to move more).

lto - September 2015


The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the lesion. On these radiographs, we will use the root canal space of the mandibular left lateral incisor (#23).

The next step is to determine what angle change is obvious between the two radiographs? Positive vertical angle, negative vertical angle or horizontal angle.

The most obvious angle change is the horizontal angle between the two radiographs. Starting with the mandibular left central incisor and lateral incisor periapical radiograph and moving to the mandibular left canine periapical radiograph, the horizontal angle increases meaning the source of radiation (tubehead) moves posteriorly. According to point 1 above, this means the images move anterior.

Looking at the second radiograph (mandibular left canine periapical radiograph), we need to compare the image movement of the lesion versus the root canal space (#23) to see which object moved more anterior following point 2 listed above.

The root canal space (#23) appears to be more anterior on the mandibular left canine periapical radiograph meaning it is farther from the image receptor compared to the lesion.

This gives us an answer of the resorptive lesion being to the lingual of the root canal space (#23).

SLOB (Same-Lingual, Opposite-Buccal)

We will use the same objects as above (unknown object = lesion and fixed object = root canal space (#23)).

Next, we need to determine which direction we are moving from the mandibular left central incisor and lateral incisor periapical radiograph to the mandibular left canine periapical radiograph and the answer would be – distal.

On the mandibular left canine periapical radiograph determine what direction does the lesion appear to have moved in relation to the root canal space (#23) – distal.

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 – same and the acronym states that same is lingual, so the resorptive lesion is to the lingual of  the root canal space (#23).

If you have any questions or comments, please leave them below. Thanks and enjoy!

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