Retinal Birefringence Scanning

 

Our laboratory has been developing novel technology for detecting accurate eye alignment directly, by exploiting the birefringence (property that changes the polarization of light) of the uniquely arranged nerve fibers (Henle fibers) surrounding the fovea (the small area of the retina, responsible for sharp central vision). Retinal birefringence scanning (RBS) is a technique that uses the changes in the polarization of light returning from the eye to detect the projection into space of the array of Henle fibers surrounding the fovea. In RBS, polarized near-infrared light is directed onto the retina in a circular scan, with a fixation point in the center, and the polarization-related changes in light retro-reflected from the ocular fundus are analyzed by means of differential polarization detection. Due to the radially symmetric arrangement of the birefringent Henle fibers, a characteristic frequency appears in the obtained periodic signal when the scan is centered on the fovea, indicating central fixation. By analyzing frequencies in the RBS signal from each eye, the goodness of eye alignment, and thus strabismus, can be measured. In preliminary studies and in an early prototype, RBS has demonstrated reliable and non-invasive detection of foveal fixation, as well as detection of eye misalignment.

Retinal birefringence scanning is innovative technology developed by two colleagues (Drs. David Guyton and David Hunter) in the Johns Hopkins Department of Ophthalmology. When I joined the department in 2000, I added to their expertise my experience – in electronics, optoelectronics, signal processing, computer programming, and modeling – to design and build the first prototype of a vision screener that employs retinal birefringence scanning for strabismus detection. Clinical testing of that device validated the principle of operation and showed remarkable performance in some patients. But technical limitations that caused relatively high optical noise (contamination of the signal with unwanted light) in the instrument, as well as significant variation of the signal level among individuals, thwarted its commercial development, and the device did not reach clinical application. We now propose to build on the lessons learned from that early work, which involved studying the physiological phenomenon, making quantitative assessments, studying the opportunities, and developing basic technologies.

The principle of RBS is explained in more detail in the following illustrations:

 

The fovea is the most sensitive part of the retina. The object of fixation is projected onto it.

 

With proper central fixation, the point of fixation should be projected in the center of the vovea.

Detecting central fixation is done by checking the location of the fovea

 

The location of the fovea can be detected by scanning around its presumed center.

 

When illuminated with polarized light (such as the light coming from a low-power laser), the Henle fibers surrounding the fovea change the polarization state of light and create a bow-tie pattern.

 

 

The simplest and fastest way to check whether the eye is fixating properly is to use circular scanning around the expected center of the fovea.

 

Here is exactly how it works:

The basic principle of RBS

Device functioning is based on detecting the location of the polarization cross which cross coming from the retinal nerve fibers which exhibit optical birefringence, especially the radial array of Henle fibers, containing neurotubules whose diameter is comparable to the wavelength of the near-infrared light. Birefringence is a property of materials to change the polarization state of light. So, we need a simple and fast method to tell us whether the fovea is where it should be during central fixation.

With the eye fixating properly (central fixation), the fovea is encircled by the scanned annulus. The concentric circle of light falls entirely on the radial array of Henle fibers and generates a signal which is twice the scanning frequency. With other words, we have doubling of the scanning frequency, if the fovea is where it should be.

By arranging for both eyes to view the same target, and by separately detecting the signals emerging from the two eyes, we can detect proper fixation by both eyes simultaneously, and thus detect proper alignment of the two eyes.

 

The basic principle of RBS is illustrated in the figure above. More precisely, due to the strict radial symmetry of the Henle fibers about the fovea, retinal birefringence causes the polarization state of linearly polarized light to change at “whole multiples” of the frequency of the circular scan, such as at twice the scanning frequency as described in detail previously.

While automated screening tools exist, very few are effective in truly detecting where the eyes are looking. The novel technology we are developing will tell when both eyes are looking in the desired direction by detecting the actual fovea of each eye – the part of the retina that we aim at objects when we look at them. With a hand-held instrument from 12 inches away, we scan the retina with a spot of polarized near-infrared light in a circle, a technique we call “retinal birefringence scanning” (RBS). Our Pediatric Vision Screener will combine a new version of RBS for detecting strabismus accurately, with added technology for assessing proper focus of both eyes simultaneously, and optimized electronics and signal processing to ensure high-quality signals. By enabling early identification and treatment of children at risk for amblyopia, this instrument has the potential to prevent lifelong disability from a readily treatable condition.

 

The simplest method to detect the central and para-central frequencies is to use the Fast Fourier Transform.

 

Alignment detection

For each eye, if the scanned circle of polarized near-infrared light is centered on the fovea, the signal of the returning light has a strong frequency component that is a specific “multiple of half” frequency of the scan frequency, determined by the fractional frequency of the HWP, indicating central fixation. Spectral analysis by means of the Fast Fourier Transform (FFT) for each eye reveals whether a subject is fixating on the target with one eye, both eyes, or neither eye.

 

Binocular fixation and alignment detection

We also need to assess proper fixation of both eyes simultaneously:

The position of the fovea is checked simultaneously for both eyes, both fixating on the same target.

 

 

In the 2002 prototype of the pediatric vision screener this is accomplished via the following binocular design:

 

 

More on Retinal Birefringence Scanning, including improved technologies:

1.      Gramatikov, BI, Irsch, K, and Guyton, D; "Optimal timing of retinal scanning during dark adaptation, in the presence of fixation on a target: the role of pupil size dynamics.  Journal of Biomedical Optics, 2014, 19(10), 106014. doi:10.1117/1.JBO.19.10.106014.

http://biomedicaloptics.spiedigitallibrary.org/article.aspx?articleid=1921066%20&journalid=93

2.      Irsch,K., Gramatikov,B.I., Wu, Y. K., Guyton,D. L. A new pediatric vision screener employing polarization-modulated, retinal-birefringence-scanning-based strabismus detection and bull’s eye focus detection with an improved target system: Opto-mechanical design and operation.  Journal of Biomedical Optics, 2014, Jun 1;19(6):67004. doi: 10.1117/1.JBO.19.6.067004.

http://biomedicaloptics.spiedigitallibrary.org/article.aspx?articleid=1881172

3.      Gramatikov, B.   Modern Technologies for retinal Scanning and Imaging. An Introduction for the biomedical Engineer.”  An invited review, Biomedical Engineering OnLine, 2014, 13:52; DOI: 10.1186/1475-925X-13-52. Published April 29, 2014. 

http://www.biomedical-engineering-online.com/content/13/1/52

  1. Irsch, K, Gramatikov, B.I., Wu, Y-K, Guyton, D.L.  Improved eye-fixation detection using polarization-modulated retinal birefringence scanning, immune to corneal birefringence. Optics Express,  22(7):7972-7988, published on 18 March 2014, DOI:10.1364/OE.22.007972, http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-7-7972

http://www.opticsinfobase.org/oe/fulltext.cfm?uri=oe-22-7-7972&id=282349

5.      Gramatikov, B.  Detecting fixation on a target using time-frequency distributions of a retinal birefringence scanning signal. BioMedical Engineering OnLine 2013, 12:4. Published on May 13, 2013.    doi:10.1186/1475-925X-12-41

http://www.biomedical-engineering-online.com/content/12/1/41.

6.      Gramatikov,B., Irsch,K., Müllenbroich,M., Frindt,N., Qu,Y., Gutmark,R. Wu,Y.-K., Guyton, D.   A Device for Continuous Monitoring of True Central Fixation Based on Foveal Birefringence.  Annals of Biomedical Engineering, Vol. 41, Issue 9 (1 Sept 2013), pp. 1968-1978.  Also Epub ahead of print: May 4, 2013. DOI: 10.1007/s10439-013-0818-2.  PMID: 23645511.

http://link.springer.com/article/10.1007%2Fs10439-013-0818-2

7.      Irsch, K., Gramatikov, B., Wu, Y.-K., Guyton, D.    Modeling and minimizing interference from corneal birefringence in retinal birefringence scanning for foveal fixation detection.  Biomed Opt Express. 2011 Jul 1;2(7):1955-68.

http://www.opticsinfobase.org/boe;  http://www.ncbi.nlm.nih.gov/pubmed/21750772

8.      Agopov M., Gramatikov B.I., Wu Y.K., Irsch K, Guyton D.L.   Use of retinal nerve fiber layer birefringence as an addition to absorption in retinal scanning for biometric purposes.  Applied Optics, 2008, Mar 10; 47 (8):1048-53,

http://www.ncbi.nlm.nih.gov/pubmed/18327275

https://www.osapublishing.org/ao/abstract.cfm?URI=ao-47-8-1048

9.      Gramatikov B.I.,  Zalloum O.H.Y., Wu Y.K.,  Hunter D.G.,  Guyton D.L.      A Directional Eye Fixation Sensor Using Birefringence-Based Foveal Detection.   Applied Optics, Vol. 46, Issue 10, pp. 1809-1818,  April 2007,

https://www.osapublishing.org/vjbo/fulltext.cfm?uri=ao-46-10-1809&id=130944

10.  Gramatikov B.I.,  Zalloum O.H.Y., Wu Y.K.,  Hunter D.G.,  Guyton D.L.      Birefringence-based eye fixation monitor with no moving parts.  Journal of Biomedical Optics, 2006, 11(3), May-June, pp. 034025-1 - 034025-11,

 http://www.ncbi.nlm.nih.gov/pubmed/16822074

http://www.sciencedirect.com/science/article/pii/S0886335004009484

  1. Hunter D.G., Nassif D.S., Piskun N.V., Winsor R., Gramatikov B.I., Guyton D.L.   Pediatric Vision Screener 1: instrument design and operation.   Journal of Biomedical Optics, 2004 Nov;9(6):1363-1368.

http://biomedicaloptics.spiedigitallibrary.org/article.aspx?articleid=1101874

  1. Nassif D.S., Piskun N.V., Gramatikov B.I., Guyton D.L., Hunter D.G.    Pediatric Vision Screener 2: pilot study in adults.  Journal of Biomedical Optics, 2004 Nov;9(6):1369-1374,

http://www.ncbi.nlm.nih.gov/pubmed/15568960