Locate the Object: February 2015 – 18 ANSWER 2

Now for the answers from last weeks post.

Where is the radiopaque entity (orange arrow) in relation to the mandibular right second premolar (#29)?

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).

image shift 7-1


The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the radiopaque entity. On these radiographs, we will use the mandibular right second premolar (#29).

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 right molar periapical radiograph and moving to the mandibular right canine/premolar periapical radiograph, the horizontal angle decreases meaning the source of radiation (tubehead) moves anteriorly. According to point 1 above, this means the images move posterior.

Looking at the second radiograph (mandibular right canine/premolar periapical radiograph), we need to compare the image movement of the radiopaque entity versus the mandibular right second premolar (#29) to see which object moved more posterior following point 2 listed above.

The radiopaque entity appears to be more posterior on the mandibular right canine/premolar periapical radiograph meaning it is farther from the image receptor compared to the mandibular right second premolar (#29).

This gives us an answer of the radiopaque entity being to the facial/buccal of the mandibular right second premolar (#29).

SLOB (Same-Lingual, Opposite-Buccal)

We will use the same objects as above (unknown object = radiopaque entity and fixed object = mandibular right second premolar (#29)).

Next, we need to determine which direction we are moving from the mandibular right molar periapical radiograph to the mandibular right canine/premolar periapical radiograph and the answer would be – mesial.

On the mandibular right canine/premolar periapical radiograph determine what direction does the radiopaque entity appear to have moved in relation to the mandibular right second premolar (#29) – 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 – opposite and the acronym states that opposite is buccal, so the radiopaque entity is to the buccal of mandibular right second premolar (#29).

There we have it. The last case of last month. I’ll keep posting cases as I find fun ones for you to practice on. If you have any questions or comments, please leave them below. Thanks and enjoy!

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