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Bioptigen imaging systems

Bioptigen imaging systems are based on the technology of Optical Coherence Tomography (OCT). OCT is analogous to ultrasound, but uses light waves instead of sound waves. Light backscattered from within a sample is processed to develop a high-resolution, depth-resolved image suitable for analyzing internal microstructure, in vivo, without physical contact. With appropriate lateral scanning, two-dimensional and three-dimensional images with resolution better than 10 micrometers are acquired rapidly and non-invasively.

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Figure 1: Principles of Optical Coherence Tomography

The longitudinal profiling capability of OCT is based on the science of low coherence interferometry. In low coherence interferometry, wide bandwidth light is split and sent along two paths. Light that is reflected or backscattered from a subject sample along one path combines, or interferes, with light reflected from a known reference along another path. This interference signal is collected on a photodetector; the signal strength is directly related to light absorption and scattering properties of the sample precisely at the point where path lengths to the sample and to the reference are matched. The axial (depth) resolution depends critically on the bandwidth of the source light. Very fine depth discrimination requires a specialized wide-bandwidth source.

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Figure 2: Low-Coherence Interferometry – Time Domain technology

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Figure 3: Low-Coherence Interferometry – Fourier Domain technology

Bioptigen OCT systems utilize a narrow single-mode beam from a wide bandwidth light source to probe the structure of a sample. A single line image of the internal structure of a sample is commonly known as an A-scan, in analogy to ultrasound. By scanning the single-mode beam laterally in on dimension, a two-dimensional B-scan is acquired. A B-scan provides an informative depth-resolved image along a slice without ever making a cut. A series of B-scans can be acquired to develop a three-dimensional image suitable for visualizing internal structures, layers, and volumes with high resolution in all three dimensions.

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Figure 4: Representative A-scan of finger

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Figure 5: Representative B-scan of finger

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Figure 6: Representative volume reconstruction of finger

Bioptigen currently develops OCT systems in two different wavelength regions. Mid-infrared systems operating at a 1310 nm center wavelength are preferred for highly-scattering subjects, such as biological tissues, where maximum depth penetration is desired. Near-infrared systems operating at an 820 nm center wavelength are preferred where scattering is less, absorption by water is higher, or where higher resolution images are desired. Because of the shorter wavelength, the resolution is generally superior at 820 nm, but there is a tradeoff between image brightness, imaging depth, and resolution that must be considered for each application. For biological samples, the general rule is that ophthalmic retinal imaging uses 820 nm because the light beam must travel through a significant length of water in the aqueous before reaching the retina. Conversely, measurements of the epidermis or mucosal tissue benefit from the increased penetration afforded by the 1310 nm wavelength.

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Figure 8: Comparison of images of a finger obtained at 1310 nm and 820 nm