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Current medical practice poses a constant demand for development of non-contact, minimally invasive imaging techniques that can be used in clinics and hospitals for early diagnostics of various diseases and as an alternative to standard excisional biopsy. Optical coherence tomography (OCT) is a relatively novel imaging method that allows for non-invasive, cross-sectional imaging of the biological tissue structure with cellular level resolution and at depths of up to 1.5–2 mm below the surface. In addition, OCT can provide useful information about physiological processes in biological tissue by probing the birefringence properties, the rate of blood flow and the blood oxygen and/or glucose concentration levels in the imaged sample. In the past 10 years OCT has emerged as a viable imaging technique with great potential for clinical applications in ophthalmology, dermatology, cardiology and more.
The principle of operation of OCT is based on low coherence or white light interferometry. The original OCT technology, Time-Domain OCT (TD-OCT), uses primarily Michelson-type interferometer design, where one of the mirrors is substituted with the imaged object, and the interference fringe pattern is detected by a single photodiode. Cross-sectional 2D and 3D tomograms of the imaged object are obtained by scanning the reference mirror (imaging in depth, Z) and scanning the imaging beam in XY direction over the sample surface. High image acquisition rates are required for imaging large 3D volumes of tissue with high spatial resolution to suppress image artifacts arising from involuntary motion in living tissue. High frame rate also is required for imaging blood flow or fast developing physiological processes in biological tissue. In TD-OCT, the image acquisition rate, scanning range, and signal-to-noise ratio (SNR) as a function of imaging are determined by the scanning rate of the reference mirror, which is typically limited to maximum of approximately 15 kHz.
Fourier Domain OCT (FD-OCT) uses an alternative detection scheme based on a high-resolution spectrometer, coupled to a linear array camera, to remove the necessity for scanning the OCT reference mirror. Since the image acquisition rate of an FD-OCT system is directly related to the read-out rate of the camera, this detection scheme results in up to approximately 100 times improvement in the image acquisition rate and about 10-30 dB improvement in the SNR as compared to TD-OCT.
Because biological tissue is more transparent to light in the near-infrared (NIR) spectrum (1,000 nm-1,500 nm), a range also known as the short wave infrared (SWIR), many biomedical applications of the TD-OCT have been developed for these longer wavelengths. In the past five years there has been an increasing demand for high-speed, linear array infrared cameras to implement FD-OCT for these wavelengths. In addition to high data transfer rate, this application requires linear arrays with a large number of pixels, since the FD-OCT scanning range is directly proportional to the number of pixels of the camera.
Indium gallium arsenide (InGaAs) photosensors can provide optimum sensitivity in the 1,000 nm–1,600 nm wavelength range. Until recently, InGaAs line scan cameras, based on 1024 pixel linear arrays offered a maximum readout rate of approximately 4,000 lines per second (l/s), while the readout rate of shorter 512 pixel arrays was limited to 20,000 l/s. In the fall of 2007, a new 1024 pixel linear array design was developed by Sensors Unlimited, Inc, part of Goodrich Corp., that features a maximum readout rate of 47,000 l/s, combined with 2.5 times increase in the gain.