Advancing Optical Coherence Tomography: Achieving High Depth Resolution with Discontinuous Bandwidth

Optical coherence tomography (OCT) has revolutionized the field of medical imaging, offering a noninvasive and high-resolution imaging technique for visualizing microscopic cross-sectional structures within transparent and semitransparent objects. With its ability to provide detailed volumetric images, OCT has become a valuable tool in medical diagnostics, material science, artwork examination, lens manufacturing, and nondestructive testing. The introduction of phase-sensitive detection in OCT has further expanded its capabilities, allowing for the detection of small changes in physical quantities and providing functional information. Regrettably, phase-sensitive OCT is plagued by three problems which are poor depth resolution, low signal-to-noise ratio in strain imaging, and inaccurate phase measurement. In a new study published in the peer-reviewed journal Optics Express by Professor Yulei Bai, Shuying Cai, Shengli Xie, and Professor Bo Dong from Guangdong University of Technology, presented a novel method for achieving high depth resolution in OCT under a discontinuous bandwidth condition using a real-valued iterative adaptive approach (RIAA).

Depth imaging in OCT is based on Fourier transform (FT), and the depth resolution is inversely proportional to the main lobe width of the OCT signal in the depth domain. The limited bandwidth of the light source used in OCT leads to a broadening of the main lobe, thereby reducing the depth resolution. To overcome this limitation, some research teams explored new methods to expand the bandwidth of OCT signals. Techniques such as using supercontinuum light sources and ultrabroad bandwidth thermal light sources have shown promise in improving depth resolution. However, hardware limitations in certain situations, such as mode hops of semiconductor lasers or restricted bandwidth of amplified spontaneous emission spectra, prevent the acquisition of a large continuous bandwidth. This necessitates the use of OCT spectra with a discontinuous bandwidth, resulting in the estimation of OCT signals with abnormal sidelobes and compromised depth resolution.

The authors addressed the challenge of abnormal sidelobes in OCT signals estimated under a discontinuous bandwidth condition. They proposed a method based on RIAA, an algorithm known for its high-frequency resolution and ability to suppress sidelobes. By utilizing a weighted matrix to suppress the abnormal sidelobes caused by wavelength gaps, the RIAA approach allows for high-resolution measurements in OCT. The researchers conducted advanced experiments to validate the effectiveness of RIAA in sidelobe suppression and achieved excellent depth resolution compared to other methods such as FT and random-sampling FT (RSFT).

For instance, to validate the performance of RIAA in sidelobe suppression and high depth resolution, the researchers measured a single-reflector OCT spectrum and estimated its amplitude in the depth domain using various methods. Their results clearly demonstrated that RIAA outperformed other approaches in suppressing abnormal sidelobes and achieving superior depth resolution. Furthermore, cross-sectional images and phase-difference maps of different samples, including a light-cured polymer droplet, a silicone rubber film, and a human tooth with composite filling, were measured and compared. This comparison not only confirmed the practical value of RIAA in tomographic measurements but also highlighted its efficacy in dealing with OCT signals with varying wavelength gaps.

The authors showcased a significant advancement in achieving high depth resolution in OCT under a discontinuous bandwidth condition. By employing the RIAA approach, they successfully suppressed abnormal sidelobes induced by wavelength gaps, thereby enabling high-resolution measurements. The experimental results, including the validation using a single-reflector OCT spectrum and the measurement of various samples, demonstrate the superior performance of RIAA in sidelobe suppression and its practical value in tomographic imaging. This new method holds promise for further developments in OCT research and its widespread application in medical diagnostics and other fields.

As OCT continues to evolve, innovative techniques like the RIAA approach will contribute to enhancing its capabilities, improving imaging quality, and enabling more precise and accurate diagnostics. The findings of the new study serve as a stepping stone towards realizing even higher resolution and functional imaging in the future. With ongoing research and advancements, optical coherence tomography will continue to play a pivotal role in revolutionizing medical imaging and beyond.