Author Topic: Composite topography maps - best way to take  (Read 1292 times)

dmw69

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Composite topography maps - best way to take
« on: September 09, 2015, 06:17:59 pm »
In general placido disc topographers are most accurate closest to the topography axis.  The composite combines several exams weighting the average at each point toward the topography closest to that point (and therefore most accurate).   This is however dependant on the accuracy of the registration process.

I just stumbled across the quote above from Grant on an old topic on this forum. I had been taking composite topos by asking the patient to look just 4 rings up/down/left/right compared to the central map. This is usually just enough to put the outermost placido rings at the limbus - but the centre of these U/D/L/R maps is still less than 1mm from the actual corneal centre.

If the purpose of a composite map is to help with fitting contact lenses that might land at a 7-8-9mm chord, will we get better accuracy in this region of the cornea by capturing the U/D/L/R images further away (3.5-4.5mm away) from the corneal centre? In other words ask the patient to look much further away from the centre than just 4 rings?

thanks


Grant

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Re: Composite topography maps - best way to take
« Reply #1 on: August 31, 2017, 09:26:12 am »
While placido topography is more accurate toward the center, the drop off in accuracy for the E300 is very small.    It is more important that you have as large an overlap as possible between the captured images while still being able to capture rings right out to the limbus.   The new Composite Capture wizard in Version 6 makes getting this right much easier by providing guidance and auto capturing once the patient fixation is in the right area.