I don't think that OCT measurement needs correction.

Really in Hyperopia optic disc is smaller, but OCT is an objective method in contrast to IOP measurement which needs correction taken into account thickness of the cornea

OCT images are reconstructed based on the optical path lengths (or apparent lengths) of time-of-flight interference signals along the scan axis (A-scan) and then 'stitched' together with adjacent lateral scans (B-scan). Images do not assume changes of refractive indices through different compartments of the eye (e.g. cornea, aqueous humor, lens, vitreous humor, etc.). Assuming the B-scans are close to being telecentric (no curvature), you may attempt to modify the OCT images to be more physically representative by segmenting the compartments, assigning known refractive indices to them, and then re-adjusting the dimensions along each A-scan line. With this process, the optic disc size and curvature/shape should be closer to physical.

Just my two cents.

Kin

Hi, Dr. Julio Gonzales!

That's such a good question. In my opinion a correction is not necessary, although a perfect assessment demands quite a few points to consider:

1) OCT is optical technology whose laser crosses different refraction indexes;

2) A good parameter may be comparing OCT measures with necropsy data; there may be some papers on this matter; I think that this measure depends much more on transparency than on other factors.

Regards,

Honassys

Thank very much both of you, Chan and Honassys:

Probably, no correction is neccesary.

Best regards.

Julio.

Hi

It depends on what system you are using. Most clinical systems assume an axial length or 23-24mm, others suggest an optimum acquisition distance from the retina and optimise the reference arm. Usually the systems that do this are the ones that combine retinal photography in to the same module. Research systems allow manual adjustment of the reference arm length and power. Refractive status is a means to optimise the image rather than incorporate it in to a calibration model.

From my experience using the acquisition angle of the system to estimate a visual angle negates the need to rely on axial length and makes refractive status irrelevant with respect to OCT imaging.

Regards

Ravi

An OCT system has a defined system axial length of approximately 24 mm. Imaging an eye that is shorter or longer than the system axial length requires correction in order to obtain correct measurements.

Here is a list of useful references, where the first one includes the formula:

1: Bennett AG, Rudnicka AR, Edgar DF. Improvements on Littmann's method of determining the size of retinal features by fundus photography. Graefes Arch Clin Exp Ophthalmol. 1994 Jun;232(6):361-7. PubMed PMID: 8082844.

2: Rudnicka AR, Burk RO, Edgar DF, Fitzke FW. Magnification characteristics of fundus imaging systems. Ophthalmology. 1998 Dec;105(12):2186-92. PubMed PMID: 9855145.

3: Rudnicka AR, Frost C, Owen CG, Edgar DF. Nonlinear behavior of certain optic nerve head parameters and their determinants in normal subjects. Ophthalmology. 2001 Dec;108(12):2358-68. PubMed PMID: 11733285.

4: Garway-Heath DF, Rudnicka AR, Lowe T, Foster PJ, Fitzke FW, Hitchings RA. Measurement of optic disc size: equivalence of methods to correct for ocular magnification. Br J Ophthalmol. 1998 Jun;82(6):643-9. PubMed PMID: 9797665; PubMed Central PMCID: PMC1722616.

Best regards,

Zoran

Dear Popovic, thanks very much for so many references.

I see that you think that the behavior of OCT is similar to the behavior of retinographies and that it also need correction.

Best regards.

Julio.