PIONEERING EXPERIMENT
Optical Coherence Tomography
Figure 1. The 2017 Russ Prize honoring the teams of Professors Fujimoto from the Massachusetts Institute of Technology( MIT) and Fercher from the Institute of Medical Physics( IMP), Medical University of Vienna, for their pioneering experiments in OCT. Adapted with permission from The National Academy of Engineering.
Over the three decades that followed, several technologic breakthroughs have significantly contributed to the clinical utility of OCT and its broader adoption in a multitude of medical fields.
Major milestones in the development of OCT
OCT has evolved from its initial inception using the time delay between the reference arm and the sample arm, known as time-domain OCT( TD-OCT), to its advanced approach known as Fourier-domain OCT( FD- OCT) in which the interference between light reflecting from all layers of the sample and the reference arm can be registered separately for individual wavelengths. By combining this information from all wavelengths of the source and performing the Fourier analysis of the spectrum, one can decode the reflectivity strength of individual layers, defined as amplitude scan( A-scan), thereby forming the basis for the 3D image reconstruction in FD-OCT. This concept was first introduced by Fercher et al. in 1995 [ 3 ] and was later developed into two approaches: the first approach, termed spectral-domain OCT( SD-OCT), using a line-camera to simultaneously register all wavelengths of the source( i. e., the interference spectrum) [ 4 ]; and the second approach, termed sweepsource OCT( SS-OCT), in which individual wavelengths are registered sequentially using a point detector and the full interference spectrum is achieved by sweeping across individual wavelengths of the source
[ 5 ]. It is worth noting that the clinical adoption of TD-OCT systems was challenging( Zeiss, who was the first company to commercialize OCT systems almost abandoned this market segment) until the advantageous imaging sensitivity and speed of FD-OCT over the state-of-the-art TD-OCT was demonstrated by Leitgeb et al. in 2003 [ 6 ], marking a paradigm shift in the field of OCT and contributing to its broader adoption in clinical care. It is also worth mentioning that while SD- OCT has enabled the finest depth sectioning of tissue by broadening the light source, SS-OCT has enabled the deepest penetration in tissues owing to its primary range of operation within the longer wavelengths as well as the fastest time-delayed based point recording of the interference spectrum. Also, the advanced version of TD-OCT known as full-field OCT( FF- OCT) has enabled the fastest enface visualization of different layers of tissues with a transverse resolution close to that of confocal microscopy.
Figure 2. Comparison between TD-OCT, SD-OCT, and SS-OCT. Adapted with permission from [ 1 ]; Rothenbuehler, S. P., Malmqvist, L., Belmouhand, M., Bjerager, J., Maloca, P. M., Larsen, M., & Hamann, S., Comparison of spectral-domain OCT versus swept-source OCT for the detection of deep optic disc Drusen. Diagnostics, 12( 10), 2515( 2022); and Nathans, J., Seeing is believing: The development of optical coherence tomography. Proceedings of the National Academy of Sciences, 120( 39), e2311129120( 2023).
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