Carolyne Dion

Development of a novel instrument to measure the pulsatile movement of ocular tissues

We demonstrate an optical instrument that can measure the axial displacement of different eye tissues, including the cornea and the fundus. The instrument is based on spectral-domain low-coherence interferometry, which extracts displacement information from sequential axial scans of the eye with 100 Hz sampling frequency and with a precision of 400 nm. Longitudinal retinal and corneal movements were successfully measured in vivo in live rats, and Fourier analysis of the signal revealed the signature of the respiratory and cardiac cycles at 1.0 and 3.5 Hz, respectively. The effective amplitudes of retinal and corneal displacements at the cardiac frequency were found to be about 1.10 and 1.37 mum, respectively. The synchrony and direction of these two movements relative to the systole and diastole were found to be nearly the same. This novel instrument can be applied to assess biomechanical properties of the eye, which could be important for early diagnosis and for understanding the pathophysiology of glaucoma and other ocular diseases.

Measurement of Ocular Fundus Pulsation in Healthy Subjects Using a Novel Fourier-Domain Optical Coherence Tomography

PURPOSE Anomalies in the pulsatility of the eye have been associated with many types of ocular pathology. Estimation of ocular pulsatility is usually obtained by measuring the variation in the intraocular pressure using tonometry-based instruments. In this work, the authors present and demonstrate the applicability of a novel and noninvasive Fourier-domain optical coherence tomography (FD-OCT) system to measure pulsatile ocular tissue movements. METHODS The authors simultaneously measured the longitudinal movement of the cornea and the retina driven by the cardiac cycle in 21 healthy volunteers using their custom-made FD-OCT. They calculated the corresponding fundus pulse amplitude (FPA), which is the variation in the distance between the cornea and the retina. RESULTS It was found that in young, healthy subjects, the cornea and the retina move axially during the cardiac cycle, with almost equal amplitude but with a phase difference ranging between 1° and 20°. The measured FPA was found to be mostly due to the relative phase difference between corneal and retinal movements, and frequency analysis revealed the presence of the harmonics of heartbeat. The root-mean-square values for cornea, retina, and FPA movements were found to be 28 ± 9 μm, 29 ± 9 μm, and 4 ± 2 μm, respectively. The dominant frequency component in corneal and retinal movement was found to be the second harmonic of the heartbeat. CONCLUSIONS The technique described here is useful for a precise description of FPA and the movement of ocular tissues. Further investigations and technical improvements will be beneficial for understanding the role of choroidal pulsation in the pathophysiology of ocular diseases.

Spectral-domain phase microscopy with improved sensitivity using two-dimensional detector arrays

In this work we demonstrate the use of two-dimensional detectors to improve the signal-to-noise ratio (SNR) and sensitivity in spectral-domain phase microscopy for subnanometer accuracy measurements. We show that an increase in SNR can be obtained, from 82 dB to 105 dB, using 150 pixel lines of a low-cost CCD camera as compared to a single line, to compute an averaged axial scan. In optimal mechanical conditions, phase stability as small as 92 μrad, corresponding to 6 pm displacement accuracy, could be obtained. We also experimentally demonstrate the benefit of spatial-averaging in terms of the reduction of signal fading due to an axially moving sample. The applications of the improved system are illustrated by imaging live cells in culture.

Pulsatile Movement of the Optic Nerve Head and the Peripapillary Retina in Normal Subjects and in Glaucoma

PURPOSE To measure the pulsatile movement of neuroretinal tissue at the optic nerve head synchronous with the cardiac cycle. METHODS We used a noninvasive imaging device based on Fourier domain low-coherence interferometry to measure the pulsatile movements of the optic nerve head, peripapillary retina, and cornea with submicron accuracy along a line across the fundus. We also measured the change in the Axial Distance between the peripapillary Retina and the base of the optic disc Cup (ADRC) during the cardiac cycle. Twelve normal subjects and 20 subjects with open-angle glaucoma were tested. RESULTS In normal subjects, the mean fundus pulsation amplitude (defined as the fundus movement minus the simultaneous corneal movement) were 13.0 ± 2.5 μm, 9.0 ± 2.1 μm, and 8.7 ± 2.9 μm at the base of the optic nerve head cup, the nasal peripapillary retina, and the temporal peripapillary retina, respectively, compared with 16.7 ± 6.8 μm, 17.3 ± 10.9 μm, and 12.7 ± 6.2 μm for the corresponding values in the glaucoma group (P = 0.26, P = 0.008, and P = 0.12, respectively). The mean changes in ADRC during the cardiac cycle in normal subjects were 10.7 ± 2.1 μm and 11.6 ± 1.8 μm for the nasal and temporal side of the optic disc, respectively, compared to 14.9 ± 5.6 μm and 14.0 ± 4.9 μm in glaucoma subjects (P = 0.03 and P = 0.10, respectively). CONCLUSIONS There was an approximately 11-μm pulsatile change in the ADRC in normal subjects, and on the nasal side of the disc, this amount was significantly greater in glaucoma patients.

Analysis of Pulsatile Retinal Movements by Spectral-Domain Low-Coherence Interferometry: Influence of Age and Glaucoma on the Pulse Wave

Recent studies have shown that ocular hemodynamics and eye tissue biomechanical properties play an important role in the pathophysiology of glaucoma. Nevertheless, better, non-invasive methods to assess these characteristics in vivo are essential for a thorough understanding of degenerative mechanisms. Here, we propose to measure ocular tissue movements induced by cardiac pulsations and study the ocular pulse waveform as an indicator of tissue compliance. Using a novel, low-cost and non-invasive device based on spectral-domain low coherence interferometry (SD-LCI), we demonstrate the potential of this technique to differentiate ocular hemodynamic and biomechanical properties. We measured the axial movement of the retina driven by the pulsatile ocular blood flow in 11 young healthy individuals, 12 older healthy individuals and 15 older treated glaucoma patients using our custom-made SD-OCT apparatus. The cardiac pulse was simultaneously measured through the use of an oximeter to allow comparison. Spectral components up to the second harmonic were obtained and analyzed. For the different cohorts, we computed a few parameters that characterize the three groups of individuals by analyzing the movement of the retinal tissue at two locations, using this simple, low-cost interferometric device. Our pilot study indicates that spectral analysis of the fundus pulsation has potential for the study of ocular biomechanical and vascular properties, as well as for the study of ocular disease.

Laser-Based Single-Axon Transection for High-Content Axon Injury and Regeneration Studies

The investigation of the regenerative response of the neurons to axonal injury is essential to the development of new axoprotective therapies. Here we study the retinal neuronal RGC-5 cell line after laser transection, demonstrating that the ability of these cells to initiate a regenerative response correlates with axon length and cell motility after injury. We show that low energy picosecond laser pulses can achieve transection of unlabeled single axons in vitro and precisely induce damage with micron precision. We established the conditions to achieve axon transection, and characterized RGC-5 axon regeneration and cell body response using time-lapse microscopy. We developed an algorithm to analyze cell trajectories and established correlations between cell motility after injury, axon length, and the initiation of the regeneration response. The characterization of the motile response of axotomized RGC-5 cells showed that cells that were capable of repair or regrowth of damaged axons migrated more slowly than cells that could not. Moreover, we established that RGC-5 cells with long axons could not recover their injured axons, and such cells were much more motile. The platform we describe allows highly controlled axonal damage with subcellular resolution and the performance of high-content screening in cell cultures.

Imaging of corneal incisions by second- and third-harmonic generation microscopy

Second and Third harmonic imaging were investigated to observe a corneal flap created by an ophthalmic knife or a microkeratome as it can be processed during a LASIK surgery.

High-resolution imaging of a corneal incision by second- and third-harmonic generation microscopy

We demonstrate high spatial resolution imaging of a stromal cut in the ex-vivo pig cornea, using second- and third-harmonic generation microscopy. From these images, we see in detail how the cut affects the corneal layers. In the beginning of the cut, the anterior layers, in which the blade is passing through, are disorganized, which could explain the shadows observed on the images. In the stroma, the cut can be imaged by third harmonic microscopy, probably due to the χ3 contrast. Although the current results were obtained from the healthy ex-vivo cornea, it already allows one to understand the effects of the cut on the tissue characteristics (such as scattering).