Optical Measurement of Glucose Content of the Aqueous Humor

Dan Daly and Graeme Clark, Lein Applied Diagnostics

pdf version of problem description

Background

Many diabetics must measure their blood glucose levels regularly to maintain good health (Appendix 1). In principle, one way of measuring the glucose concentration in the human body would be by measuring optically the glucose content of the aqueous humor in the eye.

Lein Applied Diagnostics wish to assess how feasible this is,

1. purely by a system using a linear confocal scan (Appendix 2) with an LED source, as described below; and

2. by supplementing such a system with other suitable measurements.

The sensitivity of the refractive index of the aqueous humor to the glucose concentration is of the order of one part in 105 for a change in glucose concentration of 5mg/dl, and concentrations of between 50mg/dl and 400mg/dl need to be detected reliably.

The use of a confocal scanning technique enables one to measure the optical depth of the aqueous humor to this required accuracy. The optical depth, D, is given by L/n where L is the physical depth of the anterior chamber and n is the refractive index of the aqueous humor. This direct measurement cannot be made in practice as the real depth of the anterior chamber changes due to corneal swelling and accommodation of the ocular lens.

The Problems

1. Is it possible to use other information obtainable from the confocal microscopy to resolve this point. In particular the measurement detects retro-reflections from the front and back of the cornea, and from the front and back of the lens, in addition to the measurements giving the location of the various surfaces. Do these retro-reflections provide the necessary information?

2. If the scan can only tell us the optical depth, what else could be measured that would enable the refractive index to be obtained to the required accuracy? In particular, can this be achieved by any of (or some combination of) the following:

   1. Taking measurements at different wavelengths of light. Two wavelengths allow the measurement of the dispersion of the aqueous humor, which is a function of the glucose concentration.

   2. Taking several measurements, say one when the subject is focusing on infinity and one when he or she is focusing in the near-field.

   3. Use of polarization (since glucose is optically active).

   4. Use of spectroscopic techniques.

   5. Other suggestions

The appendices are contained in the pdf problem description.