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根据第二节中介绍的系统原理的搭建实验平台如图1(b)所示。文中使用中心波长1060 nm,带宽30 nm的定制扫频光源(AXSUN-1060),光源频率3 kHz相干长度为69 mm,光源输出最大功率30 mW。采用外部搭建的重采样K-trigger标定光路对OCT信号进行重构,实现眼轴及眼前节的实时成像。
该系统使用离体仍具生理活性的鱼眼进行实验,得到图7(a)所示的原始图像,实现生物眼的眼轴及眼前节的大量程成像。对图7(a)按照上节方法进行增强处理后得到信号明显增强、噪声大幅度抑制且边缘轮廓清晰的图像,如图7(b)所示。角膜表面、晶状体前后表面及眼底各层结构均清晰可见。
由于实验中所用离体生物眼的各层折射率与相对位置未知,因此对等比例人眼模型(Ocular,OEMI-7)成像,验证该系统测量结果的准确性。
为解决系统因扩大干涉范围所引起的纵向分辨率降低、减小扫描过程中由于扫描振镜机械振动造成纵向伪影和展宽的问题,实验中控制扫描振镜在0.1 cm的横向范围内进行扫描,以眼轴位置为中心,前后各取5组A-scan信号,生成如图8所示的二维结果,通过各层数据均值作为各层结构的实际位置对实验结果进行校准。图8中各层信号从左至右分别对应的眼球内部结构依次为角膜前、后表面,晶状体前、后表面以及眼底。
文中将实验所得数据点数依据眼内各组织折射率折算为相应生物学尺寸,从而得到各层组织参数。已知空气中光程差改变所对应数据点数的变化
${R_{air}}$ ,根据人眼各层结构所对应的点数${D_{air}}$ 与折射率${n_i}$ 可知各结构的实际尺寸${D_n} = {D_{air}}/\left( {{R_{air}} \cdot {n_i}} \right)$ ,人眼内角膜、前房、晶状体、玻璃体及视网膜的折射率分别为1.387、 1.342、1.415、 1.341、 1.380。根据参数计算出样品各层结构尺寸如表1所示。Measurement CCT/mm AD/mm LT/mm VT/mm AL/mm Sample size 0.33 3.06 4.06 18.97 26.42 Result without calibration 0.35 3.08 4.07 19.10 26.60 Result after calibration 0.32 3.07 4.07 19.00 26.46 Error value 0.01 0.01 0.01 0.03 0.04 Table 1. Comparison between the measurement of the parameters of each layer of the eye axis and the actual sample size
该系统通过对多组A-scan数据求取均值的方法校准可精确得出等比例眼模型眼轴的各层参数,表1中分别展示了校准前后的各结构测量尺寸。与模型实际尺寸进行对比可知,本系统对角膜厚度(CCT)、前房深度(AD)、晶状体厚度(LT)、玻璃体厚度(VT)等组织的测量误差分别为0.01 mm、0.01 mm、0.01 mm和0.03 mm,眼轴测量误差为0.04 mm,测量重复性满足临床医学成像的要求。
参考文献[13]使用可变光程的参考臂并拼接成像,系统的测量误差和实时性均低于该系统;参考文献[14]中双波段双聚焦OCT在三维成像方面取得了较好的效果,但成像误差为0.77 mm,成像时间10 s,远超过该系统提出的方法;参考文献[28]方法误差为0.02 mm,但耗时较长。通过表2中各分段检测方法对比可知,该系统可以稳定实现对生物眼轴的实时精确测量,各层结构平均测量误差小于0.02 mm,眼轴平均测量误差0.04 mm,成像时间0.45 s,兼具测量误差小与成像速度快的优势,在无需对各结构进行长度拼接的情况下,成像范围可覆盖全眼。
Source Method Measurement
error/mmImaging dimension Number of
layersTime/s Tao[13] Reference arm with variable optical path 0.45 2D 5 0.9 Shanghai Jiao Tong University[14] Dual band dual focus OCT 0.77 2D/3D 5 10 Marco[28] Reference arm with variable optical path 0.02 2D 5 3.4-4 This system Large-scale measuring system 0.04 2D 5 0.45 Table 2. Comparison results with traditional segmented measurement system
Integrated imaging system of eye axis and anterior segment based on SS-OCT
doi: 10.3788/IRLA202049.0413006
- Received Date: 2019-12-11
- Rev Recd Date: 2020-01-24
- Publish Date: 2020-04-24
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Key words:
- swept-source optical coherence tomography /
- eye axis measurement /
- anterior imaging /
- k
-domain resampling / - image enhancement
Abstract: A system of Swept-Source Optical Coherence Tomography for synchronous measurement of the eye axis with large imaging depth range and anterior segments was proposed to solve the problem that traditional ophthalmic axis measuring instruments can not achieve the functions of large imaging depth in axial measurement and imaging on anterior simultaneously. The proposed system increased the imaging depth by designing a infrared swept source with wide range. Then a calibrated optical path was built for k-domain resampling to improve the imaging quality in large range interference. Thus the high precision axial information and the anterior segment image could be obtained simultaneously. To solve the problem of low SNR of OCT images in this system, an edge-preserving denoising algorithm based on anisotropy filtering was proposed to suppress the speckle noise, which effectively improved the image contrast. Using the equal-proportion human eye model and vitro fish eye imaging respectively, the experimental result shows that the imaging depth of the system can reach 69 mm. The 12 mm anterior segment can be completely scanned in transverse direction, while the axial five-layer structure is accurately measured. The average measurement error of axial length is 0.04 mm, and the imaging time is about 0.45 s, which fulfil the real-time requirements of clinical medical application.