Fig 1.
Schematic diagram of the measurement setup.
A) Setup used in the phantom experiments and B) Schematic diagram of the porcine eye measurement setup. An aluminum plate featuring a 2 mm size hole confined the shock wave excitation to a small area on the phantom/eye surface. The shock wave excitation generated a surface wave (here greatly exaggerated) on the polymer membrane/cornea and a laser Doppler vibrometer detected its passing.
Fig 2.
Schlieren-microscope video of an electric spark -induced shock wave imaged with a high-speed camera (frame rate: 391 kfps).
The yellow arrow indicates the wave front of the shock wave, emanating from the electric discharge (blue arrow).
Fig 3.
Spectral content of measured surface waves at 15 mmHg IOP.
The propagating waves on the eye phantom (blue) feature a frequency of 3.4 kHz, while those travelling on a porcine eye (red) have a center frequency of 2.4 kHz.
Fig 4.
Propagating membrane waves on the eye phantom.
A) Detected signals (vertically offset for demonstration) at different IOP values measured with iCare TA01 tonometer on the phantom. The red line is a guide to the eye and demonstrates the time of arrival of a specific peak of the wave packet as a function of IOP. B) Comparison between the pressure values obtained with our method and those obtained by the iCare TA01 tonometer. The result suggests a change in speed of sound of the wave travelling on the membrane when the IOP is altered.
Fig 5.
Repeatability of the method within one porcine eye.
(left) 2000-point Saviztky-Golay filtered raw signals measured at different IOPs in chronological order (top to bottom in the figure). IOP values, as determined by the manometer, in mmHg marked on the right next to the signals (darker color represents higher IOP). (right) Time of flight determined manually from the signals (highest peak) as a function of IOP.
Fig 6.
A) Time of flight as a function of iCare IOP value, B) Estimated IOPs as a function of manometer (water column) values obtained with the method introduced in this study, C) Bland-Altman plot of our IOP estimate against manometer (water column) readings for the three pressure up-down repeats within one eye. Four different eyes are marked with different colors and shapes. The phantom measurements (Fig 4), analyzed with the same envelope function as the porcine eyes, are marked in the same figure with red circles (A). The fit (B) yields a reproducibility coefficient (RPC) of 5 mmHg and a summed squared of residuals (SSE) of 2.5 mmHg (C).