A novel device for the measurement of intraocular pressure, ocular rigidity and pulsatile blood flow.
The device consists of an opto-mechanical head comprising a deformation sensor and a force sensor. The pressure as well as the rigidity of the eye are calculated based on force-deformation measurements.
Principle of Operation
The deformation sensor is an optoelectronic device consisting of a beamsplitter optically bonded to a plano-convex lens, an illuminating fiber and a receiving fiber.
Figure 1 a. Deformation sensor. BS: Beam splitter, L: Lens, F1 and F2 illuminating and receiving optical fibers respectively.
Light emerging from the illuminating fiber is directed towards the eye, partially transmitted through the beamsplitter and partially reflected from the concave surface of the lens. The tips of the fibers are appropriately mounted (in optically conjugate planes) so that light reflected from the lens is coupled to the receiving fiber. (The intensity distribution is shown at a plane P)
When the lens touches the cornea of an eye, partial matching of the refractive index changes dramatically the intensity of the reflected light that is coupled to the receiving fiber.
Figure 1 b. Deformation sensor in contact with an eye.
The intensity of the light received by the receiving fiber has been calibrated against the diameter (expressed as area) of the deformation zone using cadaver corneas.
Figure 2. Calibration of the deformation sensor. (Deformation diameter is expressed as area).
From the deformation diameter, the displaced volume (V) can be calculated. This volume is used for the calculation of the pressure-volume relationship of the measured eye.
Figure 3. Deformation (indentation) of the cornea displaces a volume of aqueous.
The deformation sensor, is mounted on a force sensor (load cell). During the measurement, the deformation sensor is pressed against the cornea and for each deformation the corresponding force is recorded.
As the deforming surface is not flat, the force-area relationship is non-linear. In order to estimate the intraocular pressure from force- deformation area measurements, an empirical look-up table has been created based on experimental (cadaver eyes) measurements. (See following picture).
Figure 4. Nomogram for the conversion of force - area measurements to intraocular pressure. During the measurement, data points for both force and area are acquired at a rate of 400 samples/sec. For each pair of points, an IOP value is calculated.
Figure 5. Photograph of the experimental prototype, shows the deformation sensor mounted on the force sensor.
Figure 6. The system consists of the “head” (deformation sensor-force sensor) mounted on a slitlamp mount, electronics for readout of the sensor data, USB computer interface and laptop computer.
Figure 7. The software acquiring and analyzing the data is developed in LabView.
Each measurement takes a few seconds of contact between the sensor head and the cornea, during which the head is slowly pressed against it. Examples of data acquired are shown below.
Figure 8. Light collected by the receiving fiber.
Figure 9. Force corresponding to the deformation in figure 8.
The data in figure 10 are fitted with a function with the form
where Po is the estimated intraocular pressure before contact and K is ocular rigidity. this fitting process is the measurement as it returns the 2 parameters of interest: Intraocular pressure (resting) and the ocular rigidity coefficient.