Locate the Object: April 2015 – ANSWER 2

Now for the answers from Tuesday.

Where is the radiolucent circle (between premolars) in relation to the mandibular left first premolar (#21)?

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).


locate the object April 2015 DrGstoothpix

The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the radiolucent circle. On these radiographs, we will use the mandibular left first premolar (#21).

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 canine/first premolar periapical radiograph and moving to the mandibular left premolar 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 premolar periapical radiograph), we need to compare the image movement of the radiolucent circle versus the mandibular left first premolar (#21) to see which object moved more anterior following point 2 listed above.

The mandibular left first premolar (#21) appears to be more anterior on the mandibular left premolar periapical radiograph meaning it is farther from the image receptor compared to the radiolucent circle.

This gives us an answer of the radiolucent circle being to the lingual of the mandibular left first premolar (#21).

SLOB (Same-Lingual, Opposite-Buccal)

We will use the same objects as above (unknown object = radiolucent circle and fixed object = mandibular left first premolar (#21)).

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

On the mandibular left premolar periapical radiograph determine what direction does the radiolucent circle appear to have moved in relation to the mandibular left first premolar (#21) – 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 radiolucent circle is to the lingual of mandibular left first premolar (#21).

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

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