Ratheesh K M, Seah L K, Murukeshan V M
Center for Optical & Laser Engineering, School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore.
Phys Med Biol. 2016 Nov 7;61(21):7652-7663. doi: 10.1088/0031-9155/61/21/7652. Epub 2016 Oct 14.
The automatic calibration in Fourier-domain optical coherence tomography (FD-OCT) systems allows for high resolution imaging with precise depth ranging functionality in many complex imaging scenarios, such as microsurgery. However, the accuracy and speed of the existing automatic schemes are limited due to the functional approximations and iterative operations used in their procedures. In this paper, we present a new real-time automatic calibration scheme for swept source-based optical coherence tomography (SS-OCT) systems. The proposed automatic calibration can be performed during scanning operation and does not require an auxiliary interferometer for calibration signal generation and an additional channel for its acquisition. The proposed method makes use of the spectral component corresponding to the sample surface reflection as the calibration signal. The spectral phase function representing the non-linear sweeping characteristic of the frequency-swept laser source is determined from the calibration signal. The phase linearization with improved accuracy is achieved by normalization and rescaling of the obtained phase function. The fractional-time indices corresponding to the equidistantly spaced phase intervals are estimated directly from the resampling function and are used to resample the OCT signals. The proposed approach allows for precise calibration irrespective of the path length variation induced by the non-planar topography of the sample or galvo scanning. The conceived idea was illustrated using an in-house-developed SS-OCT system by considering the specular reflection from a mirror and other test samples. It was shown that the proposed method provides high-performance calibration in terms of axial resolution and sensitivity without increasing computational and hardware complexity.
傅里叶域光学相干断层扫描（FD - OCT）系统中的自动校准功能，使得在诸如显微手术等许多复杂成像场景中，能够实现具有精确深度测距功能的高分辨率成像。然而，由于现有自动校准方案在其过程中使用了功能近似和迭代操作，其精度和速度受到限制。在本文中，我们提出了一种用于扫频光源光学相干断层扫描（SS - OCT）系统的新型实时自动校准方案。所提出的自动校准可在扫描操作期间执行，无需用于生成校准信号的辅助干涉仪以及用于采集校准信号的额外通道。该方法利用对应于样品表面反射的光谱分量作为校准信号。从校准信号中确定表示扫频激光源非线性扫描特性的光谱相位函数。通过对获得的相位函数进行归一化和重新缩放，实现了精度更高的相位线性化。直接从重采样函数中估计对应于等距相位间隔的分数时间索引，并将其用于对OCT信号进行重采样。所提出的方法能够实现精确校准，而不受由样品的非平面形貌或振镜扫描引起的光程长度变化的影响。通过考虑来自镜子和其他测试样品的镜面反射，使用自行开发的SS - OCT系统对所提出的想法进行了说明。结果表明，该方法在不增加计算和硬件复杂性的情况下，在轴向分辨率和灵敏度方面提供了高性能校准。
