Malgorzata A. Kowalska

Ultrasonic In Vivo Measurement of Ocular Surface Expansion

Our aim was to ascertain whether the ultrasonic measurement of longitudinal corneal apex displacements carried out in a proper headrest is a credible method of ocular pulse (OP) detection. To distinguish between longitudinal movements of the eye globe treated as a rigid body and ocular surface expansion caused by the variations of the eye-globe volume, two ultrasound distance sensors were applied to noninvasively measure displacements of cornea and sclera. The same sensors were used to examine the influence of the anterio-posterior movements of a fixed head on the registration of corneal apex pulsation. In both experiments, ECG signals were synchronically recorded. Time, spectral, and coherence analyses obtained for four healthy subjects showed that the ocular surface expansion due to pulsatile ocular blood flow (POBF) is the main component of longitudinal corneal displacement. Ocular surface pulsation is always affected by the head movement. However, there exist some unique properties of signals, which help to distinguish between head and eye movements. A rigid headrest and a bite bar are required to stabilize the head during OP measurement. Ultrasonic technique enables noninvasive and accurate in vivo measurement of corneal pulsation, which could be of interest for indirectly estimating intraocular pressure propagation and POBF component.

Phase dependencies between longitudinal corneal apex displacement and cardiovascular signals: is the ocular pulse influenced by the electrical activity of the heart?

The aim was to establish phase relationships between the principal harmonic, related to the heart rate, of synchronically registered longitudinal corneal apex displacement (LCAD), blood pulsation (BP) and electrical heart activity signals in a group of healthy subjects.

Custom designed dynamic videokeratometer

The human eye is a complex dynamic system that undergoes fine rotations and deformations. Usually information about the eye globe deformation and micromovements are averaged and lost when using commercial measuring devices, although they provide important information about the eyes physiology and the visual process. We have constructed a dynamic topographer from an existing Placido projection head and a video camera. The algorithms of calibration, data processing and topographic reconstruction are also explained in this paper. With our system, the CCD parameters and the algorithms can be controlled offering many possibilities for eye researchers. Obtained results show that the system is reliable for measuring eye dynamics. Some applications of the device are also outlined.

Comparison of high-speed videokeratoscopy and ultrasound distance sensing for measuring the longitudinal corneal apex movements.

Two different methods to measure binocular longitudinal corneal apex movements were synchronously applied. High‐speed videokeratoscopy at a sampling frequency of 15 Hz and a custom‐designed ultrasound distance sensor at 100 Hz were used for the left and the right eye, respectively. Four healthy subjects participated in the study. Simultaneously, cardiac electric cycle (ECG) was registered for each subject at 100 Hz. Each measurement took 20 s. Subjects were asked to suppress blinking during the measurements. A rigid headrest and a bite‐bar were used to minimize undesirable head movements. Time, frequency and time‐frequency representations of the acquired signals were obtained to establish their temporal and spectral contents. Coherence analysis was used to estimate the correlation between the measured signals. The results showed close correlation between both corneal apex movements and the cardiopulmonary system. Unraveling these relationships could lead to better understanding of interactions between ocular biomechanics and vision. The advantages and disadvantages of the two methods in the context of measuring longitudinal movements of the corneal apex are outlined.

Clinical utility of spectral analysis of intraocular pressure pulse wave.

BackgroundTo evaluate the clinical utility of spectral analysis of intraocular pressure pulse wave in healthy eyes of a control group (CG), patients having glaucomatous optic disc appearance or ocular hypertension, and patients with primary open angle glaucoma or primary angle closure glaucoma.MethodsThis is a prospective study that enrolled 296 patients from a single glaucoma clinic. Age matched CG consisted of 62 individuals. Subjects underwent comprehensive clinical diagnostic procedures including intraocular pressure (IOP) measurement with dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT). DCT time series were analyzed with custom written software that included signal preprocessing, filtering and spectral analysis. An amplitude and energy content analysis, which takes into account non-stationarity of signals but also provides methodology that is independent of IOP and ocular pulse amplitude (OPA) levels, was applied. Spectral content up to the 6th harmonic of the pressure pulse wave was considered. Statistical analyses included descriptive statistics, normality test, and a multicomparison of medians for independent groups using Kruskal-Wallis test.ResultsGAT IOP showed statistical significance (Kruskal-Willis test p < 0.05) for three out of 10 considered multiple comparisons, DCT IOP and OPA showed statistically significant results in five and seven cases, respectively. Changes in heart rate and central corneal thickness between the groups were statistically significant in two cases. None of the above parameters showed statistically significant differences between CG and the suspects with glaucomatous optic disc appearance (GODA). On the other hand, spectral analysis showed statistically significant differences for that case.ConclusionsSpectral analysis of the DCT signals was the only method showing statistically significant differences between healthy eyes and those of GODA suspects.

Phase dependencies between longitudinal corneal apex displacement of human eye and cardiovascular system

Intraocular pressure (IOP) varies quasi-periodically due to blood pulsation in vessels inside the eye globe. This variations cause the eye deformations and displacements of the outer surface of the eye. The aim of this paper is to calculate the correlation between longitudinal corneal apex displacement and cardiovascular activity. Using ultrasound transducer at sampling frequency of 100Hz we have measured longitudinal corneal apex displacement (LCAD) of the left eye for 5 subjects. Synchronically we have registered ECG and blood pulsation signals at the same sampling frequency. Cross-correlation function was applied to investigate dependencies between these signals. To find time shift between LCAD and ECG or pulse, the time window of 3 seconds length have been chosen from all signals and had been shifting with the step of 0.01 seconds from 0 to 7s. For each shift the cross-correlation function and its extrema were calculated in the window area. We have obtained information about extrema position of cross-correlation function and its stability in time for particular subjects. The time shift between LCAD and ECG or pulse is individual feature of each subject. Such calculations may lead us to better understanding of pulse propagation in human eye and creation a non invasive method of eye hemodynamics and ocular diagnosis.

Analysis of keratoscopic images for detecting fixational eye movements and ocular surface deformation

A sequence of videokeratoscopic images was registered using commercially available instrument E300 at a rate of 50 fps. During the 20 seconds measurement, subjects head was fixed strongly. Acquired images were analyzed for detecting fixational eye movements and corneal surface deformation. For this purpose two rings were extracted from each frame and the ellipses were fitted to them, using least square method. The time series of the ellipses geometrical parameters were considered: minor and major axes length as well as the ellipses center and the orientation. The frequency spectra of mentioned parameters were obtained by application of the Fast Fourier Transform. The longitudinal position of the corneal apex was controlled, thanks to the cone side viewer installed inside the videokeratoscope. The average amplitude of the variation of the ellipses axes length is around 20μm and of the orientation of the ellipse around 0,1 rad. In the signals frequency characteristics, appear the peak corresponding to the heart rate. No clear relationship was found between the variations of the fitted ellipse parameters and the longitudinal position of the corneal apex. The fixational eye movements were examined using two different methods. One of them consists of calculating the correlation function between the first and successive frame of the sequence and searching its maximum. The other is based on tracking the center of the ellipse fitted to particular ring of the videokeratoscopic image. The accuracy of the second method found to be higher. Simple methods proposed in this work can extend the application of videokeratoscopic measurements.

Correlations between Longitudinal Corneal Apex Displacement, Head Movements and Pulsatile Blood Flow

Determination of the phase dependencies between longitudinal corneal apex displacement (LCAD), anterio-posterior (AP) head displacements and pulsatile blood flow, using the cross-correlation function, was the main goal of our investigation. We have proposed a noninvasive method to measure signals LCAD and head movements using two ultrasonic distance sensors. Synchronically, the blood pulsation was registered with pulsoxymeter. We calculated the phase relationship between particular signals for the first four harmonics associated with the heart rate. In this paper we presented results obtained for the 3rd harmonic because of the highest value of the coherence function. We applied the time window in calculation of the correlation function due to the nonstationary nature of analyzed signals. The length of this window was selected for each harmonics. It allowed to observe more details in phase shift variations in time. Results show that the time shift between LCAD and pulse, as well as between head movements and pulse exist. We can notice more clear relationship for pair of signals: AP head displacements and blood pulsation. Obtained data are not sufficient enough to explain influence the blood pulsation on the ocular pulse. But proposed method might be helpful to acquire information about this phenomenon.