CCD Cameras in Ophthalmology

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Background of Ophthalmology Imaging

Ophthalmic imaging is a specialized form of medical imaging dedicated to studying and treating eye disorders. Ophthalmic diagnostic imaging integrates science and art to provide images to record the progress of treatment and for research and teaching purposes in ophthalmology.

Ophthalmic imaging is a vital part of the work involved in all ophthalmic departments. Ophthalmic imaging technicians perform several types of procedures based on clinicians’ specific requirements.

With the advent of digital methods, there have been tremendous changes in ophthalmic imaging over the last few years. Now, it is possible to send generated images to the desk of each ophthalmologist from the eye departments.

The images could be those taken from digital cameras, scanning laser ophthalmoscopy (SLO), visual fields, visual electrophysiological investigations, ultrasound/echography, corneal topography, operating room photography, corneal and external photography, and electrophysiology/electroretinography.

In addition to these, there are many other ophthalmic imaging techniques, including those listed below;

• Fundus photography - In this technique, a camera is used to capture an image of the retina, the eye’s back surface
• OCT scan - Light rays are used to scan the retina to measure its thickness
• Slit lamp photography - The front part of the eye is imaged with a camera. This includes the eyelids, the iris (colored part of the eye), and the cornea (front window of the eye)
• FFA - An image of the retina is taken using the fundus camera and a yellow contrast dye is injected by a nurse
• ICG - This procedure is used to obtain an angiogram of the choroid, which is the layer of connective tissue and blood vessels between the retina and sclera (white of the eye)

In the UK, the Ophthalmic Imaging Association (OIA) aims to establish working relationships with other like-minded organizations such as the Ophthalmic Photographers Society (OPS), AHPO, and AOSP.

OIA is also a member of the Federation for Healthcare Science. Its key objective is to bring together subject experts to further the knowledge of Ophthalmic Photography.

CCD Camera Technology for Ophthalmology Imaging

A charge coupled device (CCD) is an integrated circuit that is etched onto a silicon surface to form pixels or light-sensitive elements. Photons on the silicon surface produce charge, which is read by electronics and converted into a digital copy of the light patterns impinging on the device.

CCDs are available in different types and sizes, and are used in several applications from cellular phone cameras through to sophisticated scientific applications.

Digital camera systems that integrate various CCD detector configurations are the most widespread image capture technology used in advanced optical microscopy.

The sensitized film in the digital cameras is replaced with a CCD photon detector, a thin silicon wafer split into a geometrically regular assortment of thousands or millions of light-sensitive regions that trap and store image data in the form of localized electrical charge that differs based on the incident light intensity.

The uneven electronic signal linked with each pixel of the detector is read out quickly as an intensity value for the equivalent image location, and after digitization of the values, the image can be reconstructed and displayed on a PC monitor almost immediately.

One of the biggest advantages of digital image capture in optical microscopy is that it allows microscopists to instantly establish whether a preferred image has been effectively recorded. This advantage is valuable, considering the experimental complexities surrounding several imaging situations and the temporary nature of processes that are frequently analyzed.

Scientific-grade CCD cameras demonstrate amazing spatial resolution, dynamic range, acquisition speed, and spectral bandwidth. Certain CCD systems possess high light sensitivity and light collection efficiency, and therefore require a film speed rating of about ISO 100,000 to generate images of comparable signal-to-noise ratio.

The spatial resolution of existing CCDs is similar to that of film, however their light intensity resolution is one or two orders of magnitude higher than that realized by video cameras or film.

High-performance CCD sensors often have considerable quantum efficiency into the near infrared spectral region. Since the linear response capacity of CCD cameras spans across a broad range of light intensities, it results in excellent performance and provides these systems with quantitative capabilities as imaging spectrophotometers.

A CCD imager is made up of numerous light-sensing elements arranged in a 2D array on a thin silicon substrate. Silicon’s semiconductor properties facilitate the CCD chip to capture and seize photon-induced charge carriers under suitable electrical bias conditions. Each pixel is defined in the silicon matrix by an orthogonal grid of thin transparent current-carrying electrode strips, or gates, deposited on the CCD chip.

Image generation with a CCD camera can be categorized into four major functions or stages:

• Charge generation via photon interaction with the photosensitive region of the device
• Collection and storage of the liberated charge
• Charge transfer
• Charge measurement

Traditionally, CCDs designed for scientific imaging purposes have used larger photodiodes than those meant for industrial and consumer applications.

New design improvements in many high-performance scientific-grade cameras have made it possible to use large arrays with smaller pixels. Large arrays with several million pixels can deliver high-resolution images of the entire field of view.

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Imaging Technology Cameras and Sensors

Lt365R

The Lt365R from Lumenera is an innovative, high performance USB 3.0 CCD camera. It is user-friendly and is based on the leading edge ExView HAD II Quad Tap sensor technology to provide high sensitivity and high quality imaging in a compact camera packed with features.

The Lt365R has low noise electronics, ensuring clear and sharp images rendering details with remarkable accuracy. Its excellent responsiveness makes the Lt365R a good choice for near-infrared (NIR) imaging.

It is suitable for numerous applications such as tolling, high-speed inspection, traffic, and machine vision. For highly demanding life science applications such as ophthalmology, slide scanning, and digital pathology, a scientific-grade variant is available. The Lt365R can be customized to be compatible with OEM designs.

Lw565

Lumenera’s Lw565 has a 5.0 MP Sony Super HAD ICX655 CCD sensor. It can be used in various scientific and industrial applications that demand high sensitivity and clear color reproduction such as ophthalmology, machine vision, 3D biometrics, traffic, and low light imaging applications.

The Lw565 is available in color or monochrome models, and includes features like interline transfer and a progressive scan 5.0 MP imager with a global electronic shutter.

It provides vivid color response (color model), and superior NIR sensitivity (monochrome model). There is an option to select 8 or 14-bit pixel data, and the system is based on Lw camera platform with 32 MB of RAM. Complete SDK is also available.

Lumenera’s Color Correction Matrices

Attaining a precise image of the retina and its various elements are one of the biggest challenges faced by manufacturers. The retina is made up of several similar colors that are visible to the human eye such as oranges, yellows, reds, and pinks.

However, these colors are quite difficult to replicate in an image. In order to better contrast the definition of the retina, exact color rendition of the various color hues is necessary.

To meet these color reproduction requirements Lumenera designed Color Correction Matrices (CCMs) for retinal imaging. The custom CCMs were built to better define and contrast red, orange, yellow, and pink colors. The customizations generated a much cleaner image appropriate for retinal imaging.

The combination of the optimized CCMs with the high dynamic, high sensitivity range camera proved highly effective and economical for retinal imaging.

Lu575

The Lu575 from Lumenera is a 5.0 multi-megapixel USB 2.0 camera. It has color or monochrome, progressive scan sensor, and 7 fps at full 2592X1944 resolution with full sub-window control.

There is a snapshot mode for use with strobe, and GPI/O for control of peripherals and synchronization of lighting (4in/4out). The Lu575 is designed with auto white balance and auto exposure.

Slit Lamp Microscopy

Kappa’s camera solutions offer vivid and high-resolution images of the eye that are ideal for use in LASIK procedures. The cameras have a compact construction that enables ergonomic handling of the split lamp.

Kappa also offers compact device solutions for retrofitting and OEM modules to be incorporated directly into the slit lamp. They are designed with comprehensive signal processing, a variety of interfaces, and a selection of sensors from VGA to full HD to high- resolution 5 MP in different design environments to provide flexibility for numerous applications.

Kappa’s camera solutions provide precise documentation of the course of the disease and progress of treatment. As the high-resolution cameras provide brilliant images of the eye, etiopathology and progress of treatment can possibly be documented.

Optical Coherence Tomography for Retinal Scanning

Optical coherence tomography (OCT) is a method to aquire sub-surface images in opaque material. It is a non-invasive near-infrared optical imaging technique and uses the interference patterns from near-infrared reflections within tissues to provide cross-sectional images up to many millimeters deep.

OCT is used in ophthalmology, especially in retinal imaging as it has the following advantages:

• There is no ionizing radiation
• No need to prepare sample or subject
• Live sub-surface images at near-microscopic resolution
• Instant, direct imaging of tissue morphology

OCT is excellent in measuring retinal thickness, which makes it helpful for diseases that cause fluid buildup, such as diabetic macular edema (DME), and retinal vein occlusion (RVO).

OCT requires near-infrared wavelengths of light to be able to penetrate tissues. Teledyne DALSA provides CMOS (complementary metal oxide semiconductor) and CCD image sensors and cameras that are sensitive in this spectrum.

This, combined with Teledyne’s low noise and high speed operation help ophthalmological applications to be performed successfully.

DALSA founder Dr. Savvas Chamberlain pioneered the development of CMOS and CCD image sensors, which are two different technologies used for capturing images digitally. Each technology has its own special strengths and weaknesses, providing benefits in various applications.

Both CMOS and CCD image sensors transform light into electric charge and process it into electronic signals. These imagers rely on the photoelectric effect to produce electrical signal from light.

Today, the benefits of CCD versus CMOS imagers is a topic of much debate and a definitive answer has not been arrived yet. However, with the promise of higher integration for compact components and lower power consumption, a great deal of investment has been made to develop CMOS imagers for cellular phones, which hold the largest image sensor application in the world.

This, along with enormous investment led to significant improvements in image quality. In terms of line scan imagers and high volume consumer area, it can be assumed that CMOS imagers surpass CCD imagers. However, the need to image in the near infrared spectral region can make CCDs a better choice for certain area and line scan applications.

Conclusion

Over the past few years, ophthalmic imaging has advanced considerably with the introduction of many digital methods and systems. CCDs offer technicians a high level of sensitivity, dynamic range, and linearity. Most CCDs provide a quantum efficiency of about 80%.

The mixture of high-dynamic and high-sensitivity range camera, customized CCMs and camera settings, triggering capability, first-hand support, and a specific form factor has provided manufacturers with an economical solution that offers excellent color rendition needed for recording, documenting, storing, and sharing images.