1.Key Laboratory of Space Photoelectric Detection and Perception (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology, Nanjing 210016, China 2.Department of Applied Physics, College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China
Fund Project:Project supported by the Fundamental Research Funds for the Central Universities, China (Grant Nos. NS2020067, NJ2020021) and the Open Project Funds for the Key Laboratory of Space Photoelectric Detection and Perception (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology, China (Grant No. NJ2020021-5)
Received Date:22 January 2021
Accepted Date:02 March 2021
Available Online:07 June 2021
Published Online:05 August 2021
Abstract:In this paper we present a miniaturized pre-calibration based forward-viewing Lissajous scanning fiber probe for endoscopic optical coherence tomography (OCT). The probe is based on an asymmetric fiber cantilever driven by the piezoelectric bender to realize the two-dimensional (2D) Lissajous scanning, which can realize a relatively large scanning range under a low driving voltage. A capillary metal tube is mounted at the end of the main fiber to reduce the resonant frequency of the fiber cantilever. The relationship between the filling rate and the side-lobe number of the Lissajous scanning pattern is studied, and a method of selecting the orthogonal resonant frequency of the Lissajous scanning is proposed. Through the numerical simulation by COMSOL software, the structural parameters of the asymmetric fiber cantilever are determined. The orthogonal resonant frequencies of the asymmetric fiber cantilever are 169 Hz and 122 Hz. The lengths of the main imaging fiber, the auxiliary fiber and the metal capillary tube are 15.94 mm, 4.49 mm and 2 mm, respectively. The probe is fully packaged in a metal tube for endoscopic imaging. The focal spot and the working distance are 25 μm and 5 mm, respectively. The field of view is larger than 1.5 mm × 1.5 mm. The total rigid length and the outer diameter of the probe are 35 mm and 3.5 mm, respectively. The stability and repeatability of the Lissajous scanning trajectory, and the imaging stability with the rotation of the probe are investigated and verified. The probe is incorporated into a 50 kHz swept source OCT system. The axial resolution of the endoscopic OCT is 10.3 μm, and the imaging frame rate is 1 FPS (frames per second). The maximum signal-to-noise ratio of the imaging system is 110 dB. The imaging performance of the probe is validated by the 2D en-face and three-dimensional volumetric OCT imaging of the high scattering sample and the biological tissue. The probe can be used for the endoscopic imaging of the human tooth. From the result we can distinguish the dental enamel, dental essence and the dental calculus. The developed forward-viewing Lissajous scanning fiber probe is expected to be used in dental applications such as early calculus detection. Keywords:optical coherence tomography/ Lissajous scanning/ endoscopic imaging
图2(a)为实现Lissajous扫描所基于的非对称光纤悬臂的结构示意图. 用于扫描成像的主光纤近端固定在PZT双晶片上表面的中间位置, 远端附加一段毛细金属管. 附加的毛细金属管可降低非对称光纤悬臂的谐振频率以匹配SS-OCT系统的成像速度. 在PZT双晶片下表面边缘处固定一段附加光纤, 通过连接光纤黏接到主光纤, 组成刚性框架面BCDE. 图2(b)是非对称光纤悬臂的受力分析图, PZT双晶片提供的驱动力F垂直于其表面, 经非对称光纤悬臂分解为正交方向上的两个分力F1和F2, F1垂直于刚性框架面BCDE, F2在刚性框架面内. 将对应于非对称光纤悬臂正交谐振频率的正弦信号合成PZT双晶片的驱动信号, 用于驱动主光纤的自由端进行Lissajous扫描. 图 2 (a)非对称光纤悬臂结构示意图; (b)非对称光纤悬臂受力分析图 Figure2. (a) Schematic of the asymmetric fiber cantilever; (b) force analysis of the asymmetric fiber cantilever.
根据悬臂谐振理论, 单根光纤悬臂的谐振频率可由公式$f = \dfrac{\beta }{{4{\text{π}}}} \cdot \dfrac{r}{{{l^2}}} \cdot \sqrt {\dfrac{E}{\rho }}$确定[11]. 其中, E表示光纤截面的刚性扰度; l表示光纤悬臂长度; r表示光纤半径; ρ表示光纤纤芯密度; β为零阶振动模态对应的常数, 取值为3.52. 光纤的谐振频率与光纤悬臂长度l的平方成反比, 与光纤半径r成正比. 光纤的E, ρ, r都是常量, 因此通过选择合适的悬臂长度就能得到预期的谐振频率. 根据图1(c)的结果, 当波瓣数为291时填充率可达到100%, 因此选择122和169 Hz作为正交谐振频率. 为了精确确定非对称光纤悬臂的结构参数, 基于COMSOL仿真与数值模拟研究了非对称光纤悬臂的正交谐振频率与其结构参数的关系. 图3(a)展示了在COMSOL中模拟的探头结构, 主光纤前端的毛细金属管长度设定为2 mm, 主光纤长度取值范围设定为14—17 mm, 附加光纤取值范围设定为3—5 mm. 数值模拟得到的主光纤和附加光纤长度与正交谐振频率的依赖关系如图3(b)所示. 曲面上的红线和黑线分别代表了选取的正交谐振频率122 Hz和169 Hz的等值线, 两条等值线投影的交点对应着可实现相应正交谐振频率的非对称光纤悬臂结构参数, 即主光纤长度为15.94 mm, 附加光纤长度为4.49 mm. 图 3 (a) COMSOL中仿真的非对称光纤悬臂结构示意图; (b)主光纤和附加光纤长度与正交谐振频率的关系图 Figure3. (a) Simulated probe structure in COMSOL; (b) the relationship between the length of the main fiber and the auxiliary fiber and the orthogonal resonance frequency.
3.实 验全封装的Lissajous扫描光纤探头的结构示意图如图4(a)所示, 各部分使用紫外固化胶黏结固定. 使用0.25节距的GRIN透镜用于聚焦从主光纤出射的成像光束. 非对称光纤悬臂及GRIN透镜封装在外径为3.5 mm的金属管内, 探头总刚性长度为35 mm. 图4(b)是全封装探头的实物照片. 探头的驱动信号由频率分别为122和169 Hz的两个正弦信号组成. 图 4 (a)全封装的Lissajous扫描光纤探头结构示意图; (b)全封装探头的实物照片 Figure4. (a) Schematic of the fully packaged Lissajous scanning fiber probe; (b) photograph of the fully packaged probe.
4.结 果经实验测量, 全封装Lissajous扫描光纤探头的工作距离为5 mm, 焦点直径为25 μm. 图6(a)展示了实测的振幅频率响应曲线, 正交谐振频率分别为122和169 Hz, 与模拟值一致. 驱动信号的电压为1 V时, 预标定的Lissajous扫描轨迹重建结果如图6(b)所示. 经GRIN透镜聚焦后的光学扫描范围为1.76 mm × 1.67 mm, 探头的光学放大率为2.7倍, 主光纤末端的扫描范围为651 μm × 618 μm. 图 6 (a) Lissajous扫描光纤探头的振幅-频率响应曲线; (b)预标定的Lissajous扫描轨迹重建结果 Figure6. (a) Amplitude-frequency response curves of the Lissajous scanning fiber probe; (b) the reconstructed Lissajous scanning trajectory by pre-calibration.
为了研究Lissajous扫描光纤探头扫描的稳定性与可重复性, 通过预标定系统多次独立采集扫描轨迹的位置信息数据. 图7(a)—(f)展示了独立采集的6次扫描轨迹的前1500个位置信息数据的重建结果, 其中红色*号代表了扫描轨迹的起始位置. 由图7可见, 多次独立采集的扫描轨迹起始位置相同, 扫描路径一致. 计算了多次独立采集的位置信息数据与预标定位置信息数据对应坐标间的差值, 其差值最大值为13 μm, 方差约为0.016. 验证了探头扫描具有良好的稳定性和可重复性. 图 7 (a)—(f) 6次独立实验的前1500个点的扫描轨迹重建结果 Figure7. (a)–(f) Reconstructed scanning trajectory of the first 1500 points from the 6 independent experiments
为了进一步验证Lissajous扫描光纤探头成像的旋转稳定性, 将探头接入实验室搭建的SS-OCT系统. 以探头中轴线为旋转轴分别旋转0°, 90°, 180°和270°, 对1元硬币上的字母A进行OCT成像. 图像重建采用预标定的扫描轨迹位置信息. 图8(a)是用相机拍摄的1元硬币及字母A的照片, 图8(b), (c), (d), (e)分别为探头在旋转0°, 90°, 180°, 270°状态下采集重建的OCT表面成像结果. 在探头绕自身中轴线旋转不同角度下采集到的字母A的OCT数据均能正确重建, 验证了探头成像具有良好的旋转稳定性. 图 8 (a) 1元硬币及字母A的照片; (b)—(e)探头旋转0°, 90°, 180°和270°对字母A的OCT表面成像结果 Figure8. (a) Photograph of the 1 Yuan coin and the letter A; (b)–(e) the en-face OCT images of the letter A with the probe rotating to the angle of 0°, 90°, 180° and 270°.
为了验证所研制探头的成像性能, 应用基于Lissajous扫描光纤探头的内窥SS-OCT系统对生物组织进行了数据采集和图像重建. 首先实验中选取橘子果粒作为样品, 使用预标定的Lissajous扫描轨迹位置信息进行图像重建. 图9展示了重建的橘子果粒组织的OCT成像结果, 其中图9(a)为橘子果粒的实物照片, 红色方框为扫描光纤探头的成像范围. 图9(b)为橘子果粒组织的二维OCT横截面图像, 可以清晰分辨出橘子果粒组织内部的网格状细胞结构, 验证了研制的Lissajous扫描光纤探头具有良好的成像性能. 图 9 (a)橘子果粒的实物照片; (b)橘子果粒组织的二维OCT横向截面图像 Figure9. (a) Photograph of the orange grain; (b) two-dimensional OCT cross-sectional image of orange grain tissue.
应用基于Lissajous扫描光纤探头的内窥SS-OCT系统对带有牙结石的成人磨牙进行了内窥成像. 磨牙牙体由牙釉质和牙本质组成, 牙釉质是人体最坚硬、钙化程度最高的组织, 牙本质的钙化程度比牙釉质稍低, 其散射系数大于牙釉质. 牙结石由人日常饮食堆积在牙齿附近的食物残渣矿化形成, 其主要组成为磷酸钙. 图10(a)和图10(b)分别为重建的磨牙健康牙体区域的二维OCT横截面图像和三维OCT图像, 可以清晰分辨出牙釉质、牙本质等健康牙体内部的分层结构. 图10(c)和图10(d)分别为重建的牙结石区域的二维OCT横截面图像和三维OCT图像, 可以看出牙结石内部不存在类似健康牙齿的分层结构. 研制的Lissajous扫描光纤探头可用于区分健康牙体和牙结石结构. 图 10 (a)磨牙的二维OCT横截面图像; (b)磨牙的三维OCT图像; (c)牙结石的二维OCT横截面图像; (d)牙结石的三维OCT图像 Figure10. (a) Two-dimensional OCT cross sectional image of the health molar tooth tissue; (b) three-dimensional OCT image of the health molar tooth tissue; (c) two-dimensional OCT cross sectional image of the dental calculus; (d) three-dimensional OCT image of the dental calculus.
5.结 论提出了一种用于内窥OCT的小型化预标定Lissajous扫描光纤探头, 刚性长度为35 mm, 外径为3.5 mm, 工作距离为5 mm, 视场大小约为1.5 mm × 1.5 mm. 非对称悬臂结构可以有效地减少正交方向的机械耦合, PZT双晶片和光纤悬臂的组合具有偏转响应大和驱动电压低的优点. 预标定系统可以自由设计探头的谐振频率, 匹配不同扫描速度的SS-OCT系统和不同的视场大小. 利用预标定系统多次独立采集了扫描轨迹的位置信息数据, 验证了探头扫描轨迹的稳定性和可重复性. 研究了探头的旋转稳定性, 验证了探头的角度状态改变不会使扫描轨迹相对于预标定曲线产生明显偏离. 结合实验室搭建的50 kHz SS-OCT系统对牙齿进行了内窥OCT成像实验, 正确重建了牙齿和牙结石的OCT图像. 验证了在探头位置状态不确定的内窥成像环境下, 使用预标定的位置信息数据可以正确重建样品的OCT图像. 说明在内窥环境下探头的扫描轨迹相对于预标定曲线没有出现明显偏离. 研制的Lissajous扫描光纤探头有望用于牙结石检测等牙科应用领域.