OCT
PIONEERING EXPERIMENT
Figure 2 presents the three main configurations of OCT along with illustrations of their imaging capabilities. Another major milestone in OCT was the introduction of OCT angiography( OCTA) for non-invasive assessment of vasculature in living tissues, which was up to then performed with the traditional invasive fluorescein angiography. While blood flow evaluation was first demonstrated using Doppler TD-OCT, the advanced imaging speed of FD-OCT had made possible fast volumetric imaging of tissues with repeated scans, enabling enface visualization of the vasculature network in the eye and brain [ 7 ]. This milestone provided another paradigm shift in the clinical utility of OCT, adding functional imaging capabilities to the structural OCT imaging modality, and enabling to assess disease progression and response to treatments such as anti-vascular endothelial growth factor( anti-VEGF) in a multitude of vascular retinal diseases, including diabetes retinopathy, age-related macular degeneration, and glaucoma.
Contribution of advanced technologies to OCT development
From TD-OCT to SD-OCT and SS-OCT, OCT owes its rapid development and adoption, not only to scientific innovations, but more importantly to advances in technologies such as fiber optic telecommunication, and photonics, including broadband sources, fast tunable sources, and sensitive detectors. 1. The development of single-mode fibers in the 1980s has enabled OCT systems to move from free-space interferometers using bulk optical components to fiber-based interferometers, yielding compact and stable imaging systems while also opening new opportunities for OCT integration into catheters and endoscopes.
2. OCT systems initially relied on ultrafast femtosecond lasers to achieve high axial resolution imaging, making OCT systems expensive and less user-friendly as femtosecond lasers are challenging to operate and maintain. The development of robust broadband superluminescent diode( SLD) light sources has enabled more compact and cost-effective systems, thereby facilitating the broader adoption and clinical utility of OCT.
3. The development of Fourier domain mode locking technologies for tuning SS-OCT sources as well as microelectromechanical systems vertical-cavity surface emitting laser( MEMS-VCSEL) have enabled SS-OCT to reach a multi-megahertz A-scans / sec speed, significantly boosting the development of intraoperative OCT and OCTA and their clinical utility.
4. While initial OCT systems relied on Silicon( Si) based charged couple device( CCD) detectors with limited sensitivity at longer wavelengths, the development and adoption of Indium Gallium Arsenide( InGaAs) based complementary metal-oxide-semiconductor( CMOS) detectors with higher sensitivity at longer wavelengths has enabled the expansion of OCT in imaging highly scattering tissues with improved light penetration and imaging depth.
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