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. 2019 Aug 18;51(4):753–757. [Article in Chinese] doi: 10.19723/j.issn.1671-167X.2019.04.028

扫频光学相干断层扫描根管内窥影像系统的建立及其在根裂诊断的应用

Construction of swept source optical coherence tomography imaging system for root canal endoscopy and application in diagnosis of root fractures

Li-yuan QI 1, Chen CHEN 2, Lan JIANG 2, Jia-nan LI 3,△, Yu-hong LIANG 1,4,△
PMCID: PMC7433476  PMID: 31420635

Abstract

Objective

To construct swept source optical coherence tomography (SS-OCT) imaging system for root canal endoscopy,and to evaluate the accuracy of diagnosing artificial root fractures.

Methods

An ultra-high-speed (40 kHz) swept laser source was developed based on the piezoelectric tuning filter and the Fourier domain mode locking (FDML) swept laser technology (patent number: 200620135940.2). Ultra-miniature gradient index lens technology (patent number: 201320241218.7) was used to create a thin endoscopic probe with a diameter of 0.86 mm for real-time image transmission. The SS-OCT light source had a wavelength of 1 310 nm and a bandwidth of 100 nm. The axial and transverse image resolutions were 15 μm and 25 μm,respectively. Artificial fractures were created on human mandibular premolars with single root and the premolar roots were prepared to 41 horizontal sections (1 mm thick). 27 root sections with fractures (width: 52-284 μm) and 14 the sections without fractures were observed under an optical stereomicroscope with a cold light source as the gold standard. The horizontal root sections were scanned by self-developed SS-OCT imaging system for root canal endoscopy with a central wavelength of 1 310 nm and bandwidth of 100 nm. The data were reconstructed with 30 μm thick slices at an interval of 30 μm. Two observers,a radiologist and an endodontist,were trained and independently evaluated all the reconstructed images blindly. The diagnostic performance of SS-OCT imaging system was calculated,and statistical analysis was performed.

Results

SS-OCT root canal endoscopic imaging system composed of high-speed swept laser source, fiber coupler, endoscopic probe, reference arm and differential detector. Root sections could be scanned by SS-OCT and imaged in realtime at a depth of 1 to 2 mm. The kappa value for interobserver agreement was 0.792,and the intraobserver agreement was 1.000 and 0.709 for two observers respectively. All of 27 fractured root sections and 12 of 14 root sections without fractures were accurately diagnosed while 2 unfractured root sections were misdiagnosed. The sensitivity was 1.000 and the specificity was 0.857 for diagnosis of artificial root fractures by SS-OCT. The positive predictive value (PPV),negative predictive value (NPV) and the overall accuracy rate were 0.931,1.000 and 0.951 respectively.

Conclusion

The swept source optical coherence tomography imaging system for root canal endoscopy is a promising imaging method for observing root fractures.

Keywords: Swept source optical coherence tomography (SS-OCT), Root canal endoscopy, Artificial root fractures

扫频光学相干断层扫描(swept source optical coherence tomography,SS-OCT)成像技术是将光学相干技术与激光扫描共焦技术相结合起来的一种层析成像技术[1],具有无创、高分辨率(5.3~25.0 μm)、透照、实时成像的优点[2],近年来被广泛应用于眼科、皮肤科以及心血管病学等生物医学领域[3,4]。有研究尝试应用OCT技术体外观察口腔软硬组织[5,6,7],诊断龋齿[8],评估剩余牙本质厚度[9]和充填体的边缘适应性[10],以及诊断根裂等[2,7,11-12]。根裂的早期诊断在临床上具有挑战性,临床最常用的诊断根裂的辅助手段是X线检查技术[如二维X线片、三维锥形束CT(cone beam computed tomography,CBCT)], 除了存在着一定的辐射风险外,还受到影像重叠、分辨率较低的限制[13,14],诊断效力受到影响。内镜和牙科显微镜均是通过光学反射辅助观察根管内壁的表面形貌[15],但不能透过牙体组织检查管壁内部结构。因此探索无创、便于椅旁操作且能够透照根管内壁结构的手段来辅助诊断根裂具有重要临床意义。迄今为止,利用OCT技术诊断根裂的研究较少见,有学者应用OCT对牙齿外壁进行透照检查即外窥扫描[2,11-12],但临床应用可行性差;Shemesh等[7]尝试利用OCT对根管内壁进行内窥扫描检查,该研究使用的OCT内窥探头为无外被装置的裸光纤,较易发生折断。本课题组与中国科学院西安光学精密机械研究所合作,旨在自主研发和构建扫频光学相干断层扫描根管内窥影像系统,并探索该系统应用于诊断模拟根裂的效力。

1. 资料与方法

1.1. 扫频光学相干断层扫描根管内窥影像系统的构建(图1)

1.1.1 扫频激光光源探测系统 探测系统包括高速扫频激光光源、光纤耦合器、样品臂(内窥探头)、参考臂、差分探测器。基于压电调谐滤波器并应用傅里叶域锁模技术自主研发40 kHz超高速扫频激光光源(专利号200620135940.2),其中心波长为1 310 nm,带宽100 nm。扫描图像的横向分辨率为25 μm,纵向分辨率为15 μm。扫频光源发出的探测光经过光纤耦合器分光后进入样品臂(50%)和参考臂(50%)。样品臂的前端连接内窥光纤探头,探头利用超微型梯度折射率透镜(专利号201320241218.7)和封装工艺制作,直径为0.86 mm。光纤探头由外部旋转回撤装置驱动实现360°环形扫描成像,转速为80~100 r/s;样品臂中的探测光由内窥探头聚焦后入射到样品中,然后发生背向散射被探头接收返回样品臂。参考臂中的探测光聚焦到反射镜后原路返回,参考臂可以自由调节光程以匹配样品臂探头端的光程。因此,两部分返回的光在光纤耦合器中发生干涉,最后被差分探测器接收实现信号探测。此外,扫频光源还引出一路触发信号传输到数据采集卡中实现同步采集。

1.1.2 图像采集和处理系统 系统包括高速GPU(graphic processing unit)算法模块、计算机。差分探测器接收到的样品干涉信号在高速GPU算法模块中被高速采集卡采集后经处理由波长域信号转换到波数域信号,然后经过插值运算和傅里叶变换实现光谱信号到空间域信息的转换,得到样品的实时重建图像并传输至计算机,重建的层厚为30 μm,层间距为 30~125 μm。计算机(人机界面)可以实现系统的操作和控制,以及实时图像预览、图像质量评估等。

1.1.3 马赫-曾德干涉仪(Mach-Zehnder interfero-meter,MZI) 系统采用MZI干涉仪实现系统监控和光谱定标,并同时对采集到的波长域信号进行实时校准,保证稳定性。

1.2. 根裂样本制备

选取完整的人离体下颌单根前磨牙22颗,于釉牙骨质界(cemento-enamel junction,CEJ)处截冠,根管使用ProTaper Universal镍钛系统、#2 Gate-Glidden bur(直径0.90 mm, Dentsply Maillefer,Ballaigues,Switzerland)扩大以保证内窥探头(直径 0.86 mm)顺利通过。制作根裂:将修整(去除尖端的球形结构,保留颈部的楔形结构)后的#1/2球钻(直径0.5 mm)(登士柏,美国)伸入根管内,敲击球钻末端施加垂直向压力直至立体显微镜(ZOOM-630E,长方光学仪器有限公司,上海)下观察到有根管内壁裂出现,排除完全裂者。包埋:牙根表面涂布氰基丙烯酸粘接剂(爱必达胶粘剂有限公司,广东)后包埋于化学固化型义齿基托树脂(贺利式古莎齿科公司,上海),使用低速盘踞(SYJ-150,沈阳科晶自动化设备公司)水冷却下(转速200 r/min)制备得到高度为1 mm的牙根横断面切盘。再次应用立体显微镜(×40)观察样本,排除仅有外壁裂(内壁无裂)的样本后,来自于18颗下颌离体前磨牙的存在根管内壁裂(n=27)和无根裂(n=14)一共41个待测样本,采用二维图像处理系统(Image Measure,CF-2000C, 长方光学仪器有限公司,上海)拍摄并保存图像(PSD格式)。亚甲蓝染色后,立体显微镜下(×40)使用平台测微尺(1 mm;0.01 mm)(C1,微域光学仪器有限公司,长沙)定标,应用Image J 1.28软件(National Institutes of Health, Bethesda, MD)对根裂宽度进行测量,记录每个样本最宽位点的宽度。

1.3. 应用SS-OCT根管内窥影像系统扫描待测样本

扫描参数为波长1 310 nm,带宽100 nm,横向分辨率25 μm,纵向分辨率为15 μm。扫描时内窥探头穿过根管,驱动马达带动探头旋转回撤实现连续的360°环形扫描(图1), 回撤速度为10 mm/s;转速为100 r/s,扫描在水相中进行。扫描数据被系统重建为横断面图像后输出(BMP格式), 图像重建层厚30 μm;层间距30 μm。

1.

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扫频光学相干断层扫描根管内窥影像系统示意图

Schematic representation of swept source-optical coherence tomography (SS-OCT) imaging system for root canal endoscopy

1.4. 培训和一致性检验

图像判读前对观察者进行培训,2名观察者(1名口腔放射科医师、1名牙科牙髓科医师)判读SS-OCT重建图像,第一次判读后计算观察者之间的一致性,1周后重复读片,计算观察者自身一致性。

1.5. 图像判读

2名观察者在安静的暗室使用31.21 cm×17.56 cm(14 in, 1 in=2.54 cm)的分辨率为1 920×1 080的电脑显示屏 (U410 uq, Microsoft Vista OS; ASUS,中国台湾) 独立盲法的判读SS-OCT重建图像。判读结果包括有无根管内壁裂和根裂位置,观察者进行两次判读,两次间隔1周,判断不一致的图像经组内讨论后取得一致意见。

根裂判读标准:当出现从根管壁向牙本质内延伸的“亮线”或“暗线”或两条“亮线”间有暗影时则判读为根裂(图2)。

2.

SS-OCT扫描重建图像显示根管内壁裂

Reconstructed images of SS-OCT with artificially fractured root sections

A and B, a “bright line”(white arrow) or 2“bright lines” with a “void” between them(green arrow) and a “dark line”(blue arrow)extending from the canal were considered as root fractures.

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1.6. 统计学分析

应用SPSS 20.0统计软件进行分析,采用Cohn’s Kappa评价2名观察者自身及观察者之间的一致性,计算Kappa值;以立体显微镜(组织学)检查作为金标准,计算应用SS-OCT发现根管内壁裂的准确性、灵敏度、特异度,阳性预测值和阴性预测值。

2. 结果

两位观察者自身一致性的Kappa值分别为1.000和0.709,两位观察者之间的Kappa值为0.792。

41个待测样本立体显微镜下显示有根裂和无根裂的样本比例为27 ∶14(约为2 ∶1),根裂宽度52~284 μm,最宽处均位于距离根管内壁1 mm以内。

扫频光学相干断层扫描影像系统应用于根管内窥诊断模拟根裂发现,全部27个根裂样本均被正确判读为有根裂,内壁裂的判读部位与实际立体显微镜上的金标准标识部位全部一致(100%),判读的裂纹发生部位在SS-OCT图像上显示为一条线状亮影(图3)。14个无根裂的样本有12个被正确判读(85.7%), 特异度为0.857, 2个无根裂样本被判读为有根裂发生(14.3%), 为假阳性,阳性预测值、阴性预测值分别为0.931和1.000。本实验条件下,应用SS-OCT根管内窥影像系统正确诊断39个样本,准确性为0.951。

3.

待测样本立体显微镜检查图像(×15倍)(左)及SS-OCT扫描重建图像(右)

Root sections revealed by optical stereomicroscope image(×15, left)and SS-OCT reconstructed image (right)

A, fractures were visible on both images(white arrow);B, no fracture was detected on either image;C, a histological section without fractures was misdiagnosed as a bright line(yellow arrow)on SS-OCT reconstructed image.

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3. 讨论

本研究构建了一套扫频光学相干断层扫描根管内窥影像系统,系统采用自主研发的扫频激光光源可实现40 kHz的超高速扫描,系统还使用MZI干涉仪来进行光谱定标和信号实时校准,实现系统监控,以保证系统的稳定性。

SS-OCT技术基于光的干涉原理,通过接收、处理组织内不同深度层面背向散射回来的光信号来对组织进行断层成像[16],具有无创、高分辨率、实时成像的优点,已经在眼科、心血管病学等生物医学领域中得到广泛应用[3,4]。目前,SS-OCT技术在口腔医学领域的应用尚处于探索阶段,本课题组前期研发的牙根外窥扫描SS-OCT系统可以正确地诊断牙根裂[12]。Shemesh等[7]将SS-OCT应用于内窥扫描,发现其对于诊断人工模拟根裂有较高的准确性。相比外窥扫描,应用SS-OCT内窥扫描牙齿将更具临床实用性,但是牙的根管系统是无让性的硬组织,且内径狭小(直径<1 mm)、形态不规则、往往存在弯曲,而内窥探头为精密的光学仪器,常见直径为2~3 mm,如何减小探头直径,怎么对易脆的探头进行保护,是技术突破的难点。中国科学院西安光学精密机械研究所李嘉男副研究员所在的OCT与生物光子学工程中心已经具备自主研发超微型光学相干断层成像探头、超高速扫频激光器等核心技术,并成功开发出应用于临床的心血管OCT内窥扫描系统。在此基础上,本研究与该中心合作研发出一套扫频光学相干断层扫描根管内窥影像系统。

根裂的早期诊断一直以来是口腔临床难题,X线检查技术,以及牙科显微镜和根管内镜等手段在一定程度上可以辅助诊断。根管内镜利用冷光源发出的可见光在管壁表面发生反射来实现实时成像,可以呈现根管壁的表面形貌。内镜可以伸入根管内检查,具有无创、可以椅旁操作的优点。但是由于内镜的探头价格昂贵、易脆,其开发应用受到局限[17]。不同于根管内镜,本研究构建的SS-OCT系统不仅显示管壁的表面形貌,还可以穿透表面揭示根管壁的内部结构。系统的内窥探头使用超微型梯度折射率透镜专利技术制作,直径最小为0.86 mm,可以进入根管一定深度。随着极细光纤的进一步开发和利用,SS-OCT内窥系统可能具有较好的临床应用前景。

本研究制作人工模拟根裂[18],共41个待测样本中,根裂与无裂样本比例约为2 ∶1,参考了同类根裂诊断试验研究的设置比例,具有可比性[7,19-22]。根裂发生时,牙体组织断开产生两个界面,探测光穿过界面时发生折射率突变,导致探测信号增强。根裂细窄时两界面紧密接触,SS-OCT重建图像上表现从根管壁向牙本质内延伸的一条“亮线”;根裂较宽则两界面存在间隙,表现为在两条“亮线”间有暗影;但若探测光平行于断裂界面,界面上不发生折射率突变,根裂则表现为一条由根管壁向牙本质内延伸的“暗线”(图2),据此标准,全部27个根裂样本均被判读为有根裂发生,无漏诊,内壁裂的判读部位与实际立体显微镜上的金标准标识部位一致,2个无根裂样本被判读为有根裂发生(14.3%),判读的裂纹发生部位在SS-OCT图像上显示为一条线状亮影(图3),可能根管内壁残留有强反射性的物质(如牙本质碎屑等),从而引起探测信号增强。应用构建的SS-OCT根管内窥影像系统正确诊断39个样本,准确性为0.951。本实验条件下,SS-OCT可以有效帮助诊断根管内壁裂(52~284 μm), 并提示根裂的部位和延伸方向。

本研究提示SS-OCT应用于观察根管内壁裂有一定的临床应用前景,同类根裂诊断试验研究中样本总量在22~80例[7,19-22]。本实验最终制备且纳入研究的样本数量为41例,实验结果具有一定代表性,后续研究需要扩大样本量来进一步验证SS-OCT对根裂的诊断效力。受到光纤探头的直径限制,研究仅观察了根管中上段的根裂,构建的SS-OCT系统的透照根管壁深度约为1~2 mm,模拟根裂起始于内壁,宽度在52~284 μm。SS-OCT对于更细小的根裂(小于50 μm)的诊断能力尚待进一步研究。

Contributor Information

李 嘉男 (Jia-nan LI), Email: jli@vivo.light.com.

梁 宇红 (Yu-hong LIANG), Email: leungyuhong@sina.com.

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