NIR cameras: A perfect solution for retinal imaging

The medical industry is highly reliant on imaging-based diagnostic tools due to the capacity of these instruments to offer critical insights into several diseases.

Advances in medical imaging have enabled medical professionals to accurately and rapidly detect a number of diseases, visualize interior structures invisible to the naked eye and make more informed decisions around treatment planning and monitoring.

Ophthalmology imaging applications, in particular, have attracted significant attention in recent years as a result of technological developments in the evaluation of ocular and orbital diseases.

For example, the ophthalmoscope is regarded as one of the world’s most popular medical devices, despite there being several other discoveries and inventions in the field since its invention.

Recent developments in imaging techniques, such as fluorescence angiography, optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO), have significantly enhanced the ophthalmology field, with healthcare professionals and patients benefiting from the resulting advances in rapid growth diagnosis and care.

This article explores one of the key diagnostic techniques in ophthalmology: retinal imaging. This technique uses a CMOS sensor and a camera with excellent sensitivity in the near infrared (NIR) region to capture clear images of the retina.

Understanding retinal imaging

Retinal imaging is a non-invasive technique able to produce high resolution images of the retina, optic nerve and blood vessels in the eye.

Retinal imaging is able to replace a number of routine eye tests, supporting ophthalmologists to make more precise diagnoses of ocular diseases and abnormalities, for example, glaucoma, macular degeneration and diabetic retinopathy.

One exemplary retinal imaging method is fundus photography. This technique is frequently used for diagnosis and documentation or for the rapid examination of the retina and the eye’s internal structures.

There is an increasing need for portable, compact fundus cameras able to provide high resolution images for near-patient testing.

Conventional fundus photography involves the patient being given mydriatic eye drops to dilate their pupils – a process that is time-consuming and uncomfortable for the patient.

Contemporary non-mydriatic fundus imaging techniques have successfully streamlined this imaging process, removing the need for pupil dilation. These fundus cameras boast high sensitivity and the ability to perform NIR imaging, enabling the capture of retinal images with reduced turnaround time and no loss of detail.

Benefits of NIR-based retinal imaging

It has been estimated that more than 4.1 million people over the age of 40 suffer from diabetic retinopathy in the United States alone. NIR imaging is an essential tool in the early diagnosis of diabetic retinopathy as well as a range of ocular diseases, including macular degeneration, vascular occlusions and melanomas.

NIR imaging offers a non-invasive means of detecting alterations and mapping pathological changes in the retinal pigments. As this technique uses illumination, it is able to avoid back reflection from the cornea while simultaneously capturing the retina.

Near infrared light is more apparent than visible light when examining sub-retinal characteristics. Using this method, the optic nerve head, choroidal vessels and retinal vessels appear dark, while sub-retinal deposits appear thickened and bright.

The contrast and visibility of features rises as the wavelength is increased from 795 nm to 895 nm.

The use of a NIR fundus camera allows the interior of the human eye to be imaged via multi-spectral NIR image capture. The use of retinal imaging with spectral reflectance supports the detection and identification of retinal conditions.

It is also able to provide useful details on both diabetic retinopathy and melanomas, making it ideal for the diagnosis of potentially fatal diseases.

Current trends in fundus retinal imaging see the use of portable, affordable cameras with high quality sensors able to facilitate high resolution output. All of these features are critical in ensuring accurate and timely diagnosis.

High performance CMOS – A replacement for CCD technology

The camera sensor is key to the performance of ophthalmologic devices. Clinicians depend on high quality and high-resolution image data for better diagnosis and proposing a suitable treatment plan.

Two types of sensor technology are available on the market: charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS) sensors.

CCD sensors are employed in a range of scientific applications which involve light-sensitive pixels being converted into signals, offering the ability to capture images with low noise.

Cameras equipped with CCD sensors have seen extensive use in scientific imaging, and these cameras are available with larger photodiodes for use in ophthalmologic imaging applications.

CMOS sensors have recently become a more preferred choice over CCD sensors. This shift has primarily been a result of CMOS sensors’ cost advantages.

While earlier CMOS technology was known to produce images with noise, more recent models employ a parallel A/D architecture to increase frame rates with correlated double sampling before and after each A/D, rather than just before the A/D. This significantly improves the devices’ signal-to-noise ratio (SNR).

CMOS sensors have historically utilized front-illuminated pixels, employing metal wiring on silicone. However, these wires reflect a portion of the incoming photons, reducing the sensor’s sensitivity to light.

It became possible to partially counteract this photon reflection by using micro lenses to redirect incoming photons away from the wiring and towards the pixel’s photosensitive area.

CMOS technology has since evolved from front-illumination to back-illumination. Back-illumination sees the sensor being turned around, with any wiring or circuitry with the potential to impede incoming photons being shifted to the reverse of the sensor.

The image below highlights the key difference between front- and back-illuminated CMOS sensor technologies.

These advances have significantly improved the image quality achievable using CMOS sensors, allowing CMOS sensors to take their place as the natural choice over CCD sensors, particularly in the medical and life sciences fields.

Comparison of front illuminated and back illuminated CMOS sensor technologies.

Figure 1. Comparison of front illuminated and back illuminated CMOS sensor technologies.

Image Credit: e-con Systems

Retinal imaging with the IMX462-based See3CAM_CU27

The See3CAM_CU27 from e-con Systems™ is a Sony STARVIS IMX462-based USB3 camera that has been specifically designed for use in medical point-of-care devices.

The IMX462 sensor offers excellent sensitivity and picture quality in both the NIR and visible light regions.

See3CAM_CU27 represents an excellent choice of camera solution for a wide range of medical point-of-care and ophthalmological devices. The camera offers five key features which make it ideal for this field:

  • High sensitivity
  • Spectral bandwidth
  • High frame rate
  • Low noise
  • Dynamic range

High sensitivity

The See3CAM_CU27’s high sensitivity and use of large pixels allow it to deliver high quality image output in low light conditions. This is also due to its use of back-side illuminated (BSI) pixels and low dark current.

The camera’s high degree of quantum efficiency (QE) supports its ability to efficiently convert photons to electrons, improving the accuracy of clinical observations by facilitating the capture of retinal images with less noise.

Spectral bandwidth

While the majority of ophthalmologic devices utilize NIR lighting for retinal imaging, the IMX462 camera in the See3CAM_CU27 offers high enough spectral bandwidth to enable clinicians to capture superior NIR images of the retina’s structure.

The camera offers high levels of quantum efficiency in both the visible and near infrared spectral regions, while the photodiode section of the pixel well is deeper than that of other BSI sensors.

This allows photons with longer wavelengths to penetrate deeper into the substrate, considerably increasing the sensor’s sensitivity to NIR and red light.

The sensor also displays almost equal peak sensitivity to NIR light and visible light, with the peak NIR quantum efficiency at a wavelength of 800 nm - as high as the peak QE of light in the visible wavelength regions.

High framerate

Thanks to the IMX462 sensor, the See3CAM_CU27 is able to deliver a framerate of over 100 fps –ideal for ophthalmology imaging. This high frame rate is able to facilitate the automated specimen scanning of retinal images, enabling the accurate diagnosis of a range of diseases.

Low noise

The See3CAM_CU27 offers super high conversion gain (SHCG), allowing it to deliver very low read noise at high gain. This feature also helps to improve SNR when the camera is working in low light conditions.

Dynamic range

The maximum number of electrons per pixel is referred to as the ‘saturation capacity’ or ‘full well capacity’.’ A sensor’s dynamic range is represented by the ratio of full well capacity to sensitivity threshold.

The greater a CMOS’ saturation capacity, the higher the dynamic range value.

The IMX463-based See3CAM_CU27 offers a high dynamic range and can adapt well to varying light conditions – features that make it ideal for retinal imaging.

See3CAM_CU27 versus standard monochrome cameras

As monochrome cameras do not include RGB filters, they are typically more sensitive to photons. This results in a higher QE. The See3CAM_CU27’s NIR performance offers significantly improved results, however. These results are highlighted in the images below which shows the performance difference between See3CAM_CU27 and another monochrome camera output take at the same intensity and exposure values.

Image captured with See3CAM_CU27 at 850 nm.Image captured with 5MP monochrome camera at 850 nm.

Figure 2. (left) Image captured with See3CAM_CU27 at 850 nm.

Image Credit: e-con Systems

Figure 3. (right)Image captured with 5MP monochrome camera at 850 nm.

Image Credit: e-con Systems

Image captured with See3CAM_CU27 at 940 nm.Image captured with 5MP monochrome camera at 940 nm.

Figure 4. (left) Image captured with See3CAM_CU27 at 940 nm.

Image Credit: e-con Systems

Figure 5. (right) Image captured with 5MP monochrome camera at 940 nm.

Image Credit: e-con Systems

Due to their wide-ranging and beneficial features, CMOS-based vision systems are becoming increasingly popular in the medical and life sciences fields, particularly for ophthalmology applications. The See3CAM_CU27 also represents the ideal option for retinal imaging.

e-con Systems™ continues to support its customers to enhance their ophthalmology device performance.

Acknowledgments

Produced from materials originally authored by Vinoth Rajagopalan, https://www.linkedin.com/in/vinothrajagopalan, from e-con Systems.

About e-con Systems

e-con Systems™ is a leading OEM camera manufacturer with 18+ years of experience and expertise in embedded vision. It focuses on delivering vision and camera solutions to industries such as medical, retail, and industrial. The company’s wide portfolio of products includes MIPI camera modules, USB 3.1 Gen 1 cameras, GMSL cameras, stereo cameras, etc. It has built over 250+ product-based solutions and shipped millions of cameras around the globe. What sets the company apart is its deep expertise in building customized product designs while ensuring rapid prototyping and custom modifications in hardware and software. e-con Systems™ has close partnerships with Sony, ON Semiconductor®, Omnivision, NVIDIA, Xilinx, Socionext™, Cypress, Connect Tech, Toradex, Variscite, Toshiba, Diamond Systems, Avermedia and Texas Instruments.

Giving sight to medical applications:

e-con Systems™ offers end-to-end camera solutions to meet the needs of the medical and healthcare industry. It has a strong foothold in the medical device industry – having empowered clients to integrate unique camera solutions for medical and life science applications in ophthalmology, dentistry, dermatology, laboratory equipment, microscopy, assistive technology, point-of-care technology etc. e-con Systems also has a solid track record of working with the medical device industry leaders such as Thermofisher, Idexx, Perkin Elmer, Amwell, Welch Allyn, Seegene and others.”


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Last updated: Dec 10, 2021 at 10:20 AM

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