Laser applications and system considerations in ocular imaging

2022 Prentice Award Lecture: Advancing Retinal Imaging and Visual Function in Patient Management and Disease Mechanisms

SIGNIFICANCE

Patient-based research plays a key role in probing basic visual mechanisms. Less-well recognized is the role of patient-based retinal imaging and visual function studies in elucidating disease mechanisms, which are accelerated by advances in imaging and function techniques and are most powerful when combined with the results from histology and animal models.A patient's visual complaints can be one key to patient management, but human data are also key to understanding disease mechanisms. Unfortunately, pathological changes can be difficult to detect. Before advanced retinal imaging, the measurement of visual function indicated the presence of pathological changes that were undetectable with existing clinical examination. Over the past few decades, advances in retinal imaging have increasingly revealed the unseen. This has led to great strides in the management of many diseases, particularly diabetic retinopathy and macular edema, and age-related macular degeneration. It is likely widely accepted that patient-based research, as in clinical trials, led to such positive outcomes. Both visual function measures and advanced retinal imaging have clearly demonstrated differences among retinal diseases. Contrary to initial thinking, sight-threatening damage in diabetes occurs to the outer retina and not only to the inner retina. This has been clearly indicated in patient results but has only gradually entered the clinical classifications and understanding of disease etiology. There is strikingly different pathophysiology for age-related macular degeneration compared with photoreceptor and retinal pigment epithelial genetic defects, yet research models and even some treatments confuse these. It is important to recognize the role that patient-based research plays in probing basic visual mechanisms and elucidating disease mechanisms, combining these findings with the concepts from histology and animal models. Thus, this article combines sample instrumentation from my laboratory and progress in the fields of retinal imaging and visual function.

Ultralow energy photoacoustic microscopy for ocular imaging in vivo

SIGNIFICANCE

The development of ultralow energy photoacoustic microscopy (PAM) on the clinically relevant pigmented rabbit eye model paves a road toward translation of the emerging PAM technology in ophthalmology clinics.

AIM

Since the eye is particularly vulnerable to laser damage, we aim to develop an ultralow energy PAM system to significantly improve the laser safety of PAM by increasing the sensitivity of the system and reducing the incident laser energy for imaging.

APPROACH

A multichannel data acquisition circuit with two-stage signal amplification was specially designed, which, in combination with the application of 3 by 3 median filter in the spatial domain, significantly improved the signal-to-noise ratio of the PAM system. The safety of this system was validated by histopathology, fluorescein angiography, and fundus photography.

RESULTS

Experiments performed on pigmented rabbits demonstrated that, when using this ultralow energy PAM system, satisfactory image quality can be achieved in the eye with an incident laser fluence that is only 1% of the American National Standards Institute safety limit. Fundus photography, fluorescein angiography, and histopathology were performed after the imaging procedure, and no retinal or ocular damage was observed.

CONCLUSIONS

The proposed ultralow energy PAM system has excellent safety and holds potential to be developed into a clinical tool for ocular imaging.

Cones in ageing and harsh environments: the neural economy hypothesis

PURPOSE

Cones are at great risk in a wide variety of retinal diseases, especially when there is a harsh microenvironment and retinal pigment epithelium is damaged. We provide established and new methods for assessing cones and retinal pigment epithelium, together with new results. We investigated conditions under which cones can be imaged and could guide light, despite the proximity of less than ideal retinal pigment epithelium.

RECENT FINDINGS

We used a variety of imaging methods to detect and localise damage to the retinal pigment epithelium. As age-related macular degeneration is a particularly widespread disease, we imaged clinical hallmarks: drusen and hyperpigmentation. Using near infrared light provided improved imaging of the deeper fundus layers. We compared confocal and multiply scattered light images, using both the variation of detection apertures and polarisation analysis. We used optical coherence tomography to examine distances between structures and thickness of retinal layers, as well as identifying damage to the retinal pigment epithelium. We counted cones using adaptive optics scanning laser ophthalmoscopy. We compared the results of five subjects with geographic atrophy to data from a previous normative ageing study. Using near infrared imaging and layer analysis of optical coherence tomography, the widespread aspect of drusen became evident. Both multiply scattered light imaging and analysis of the volume in the retinal pigment epithelial layer from the optical coherence tomography were effective in localising drusen and hyperpigmentation beneath the photoreceptors. Cone photoreceptors in normal older eyes were shorter than in younger eyes. Cone photoreceptors survived in regions of atrophy, but with greatly reduced and highly variable density. Regular arrays of cones were found in some locations, despite abnormal retinal pigment epithelium. For some subjects, the cone density was significantly greater than normative values in some retinal locations outside the atrophy.

SUMMARY

The survival of cones within atrophy is remarkable. The unusually dense packing of cones at some retinal locations outside the atrophy indicates more fluidity in cone distribution than typically thought. Together these findings suggest strategies for therapy that includes preserving cones.

Non-mydriatic confocal retinal imaging using a digital light projector

A digital light projector is implemented as an integrated illumination source and scanning element in a confocal non-mydriatic retinal camera, the Digital Light Ophthalmoscope (DLO). To simulate scanning, a series of illumination lines are rapidly projected on the retina. The backscattered light is imaged onto a 2-dimensional rolling shutter CMOS sensor. By temporally and spatially overlapping the illumination lines with the rolling shutter, confocal imaging is achieved. This approach enables a low cost, flexible, and robust design with a small footprint. The 3^rd generation DLO technical design is presented, using a DLP LightCrafter 4500 and USB3.0 CMOS sensor. Specific improvements over previous work include the use of yellow illumination, filtered from the broad green LED spectrum, to obtain strong blood absorption and high contrast images while reducing pupil constriction and patient discomfort.

HeNe laser (633 nm)-coupled confocal microscope allows simulating magnetic resonance imaging/computed tomography scan of the brain and eye: a noninvasive optical approach applicable to small laboratory animals

Magnetic resonance imaging (MRI) and computed tomography (CT) are noninvasive medical imaging techniques used for the detailed visualization of internal organs of the human body. Because CT uses X-rays for imaging, there is a risk of radiation exposure. In contrast, MRI uses radiowaves and magnetic fields for imaging; thus, there are no reported biological hazards. However, neither MRI nor CT is suitable as a noninvasive imaging tool applicable in small laboratory animals such as zebrafish embryos or larvae. The recently established micro-CT scanner is only suitable for scanning adult fish and a staining procedure is required for imaging. In addition, CT-based scanning is generally more suitable for skeletal imaging but not for visualization of soft tissues because of its lower contrast. In this study, we evaluated whether 633 nm HeNe laser-coupled confocal microscope allows simulating MRI/CT scan and imaging soft tissues such as brain and eye in zebrafish embryos/larvae. We show that the 633 nm HeNe laser can penetrate well into intact brain and eye of zebrafish. It represents a noninvasive imaging method with high resolution while not requiring contrast agents, enabling the detection of differential signals from normal and pathological organs such as brain and eye.