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Messages - Grant

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The Tear Film Quality map shows the tear film surface quality (TFSQ) over the surface of the eye at a given point in time.   This is basically a measurement at each point on the eye of local variability in the ring width.  Typically local tear film break down is characterized by a large variability in the ring width.

Mathematically the TFSQ at a point is calculated by finding the standard deviation (SD) in the widths of the surrounding rings 10 degrees either side of the point.  The TFSQ  at the point is then calculated as percentage of the square of the SD normalized by the average width (W) between the rings:

TFSQ = 100 * (SD / Average W)^2

A local TFSQ value of greater than 0.3 typically indicates that there is some tear film breakup going on in that area.

The TFSQ Area attribute (which is graphed over time in the bottom graph for Tear Film Exams) is the percentage of the eye, within a 7mm chord, that has a local TFSQ value greater than 0.3 (ie is in breakup).  So a value of 5 in the graph indicates that 5% of the eye surface has tear film breakup.

The Auto Tear Film Breakup Time (TBUT) is calculated by finding the first two consecutive frames where the TFSQ Area exceeds 5%.

The Tear Film Breakup map shows the time (in seconds) until the TFSQ exceeds 0.3 for each point on the cornea (ie the time till breakup).   It is useful for understanding how the tear film breakup progresses over time.

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.

Feature Discussions & Suggestions / Re: tear film analysis
« on: May 15, 2015, 10:20:54 am »
Firstly you need to be running Version 6 of Medmont Studio.  You can capture a single exam as normal and see the tear film quality map for a single E300 exam by selecting "Tear Film Quality" from the Display > Map Type ribbon menu.   

If you want to look at changes in tear film (ie breakup) then you can use the capture dialog to capture a video of the patient.  To do this select the Capture Mode > Video and then use the up down arrows to set the Video Settings > Frames Per Second (if you type into the up down box then make sure you hit enter or move the cursor to the max duration box to ensure the frame rate setting is saved).  High frame rate video requires a large amount of storage so I would recommend that you set the Frames per second to a maximum of 4.   

Align the patient as per normal and then instruct them to blink twice and start the video capture by clicking on the "Start" button or pressing the F9 function key.  Keep the patient aligned (and still) while capturing video.  Once the capture is completed click the Analyse All button to analyse the exams.   You can then save the video exams using the "Save All" button and view them in Studio.   Select the Tear Film Quality map type and you can use the Video ribbon bar to play, pause and step through the recording.

In the attribute pane there are two new attributes for Tear Film Surface Quality (TFSQ and TFSQ Central).   TFSQ is an average of the Tear Film Quality calculated over a 7mm chord while TFSQ Central is calculated only over the central 2mm chord.   If these attributes aren't displayed on the Data Pane (under the exam) then you can use the "Arrange Attributes" button (with the up down arrows on the left of the Data pane) to add them.

Are you comparing the sag between central exam from a composite and the composite - or comparing a different primary gaze topography?   If you are comparing a primary gaze exam that is included in the composite then it is possible that the axes of measurement is not exactly aligned which will result in some differences in sag (which is measured relative to the axes) that will be more pronounced toward the periphery.   

In theory the composite exam should be more accurate then a single exam particularly in the periphery.  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.

Yes you should change both to 5.   You should however create a new attribute (eg Corneal_Angle_at_6mm_0) rather than just editing the 5mm one.  This is because each attribute has a unique ID and so just changing the definition may product unexpected results when importing/exporting between databases.

Feature Discussions & Suggestions / Re: Kids and topography
« on: May 28, 2012, 04:34:00 pm »
Don't worry too much about where they are looking while you get the alignment close (rings centered and focus bar indicator central).   Once these are right and the topographer is capturing images then ask them to look at the central green ring and do the final alignment.

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