Abstract
目的
评价锥形束CT(cone beam computed tomography,CBCT)在不同分辨率设置下检测牙根外吸收的可靠性,为临床选择恰当的CBCT扫描参数设置提供依据。
方法
收集51颗离体单根前磨牙,分别在其舌面的根颈、根中和根尖部随机制备洞型,直径为1 mm,深度分别为0.1、0.2和0.3 mm,共153个位点(其中随机选择51个位点未制备洞型)用以模拟牙根外吸收的微小缺损。将离体牙置于人干下颌骨内,采用ProMax 3D和 DCT PRO型CBCT机分别对样本进行容积扫描;两名观察者评估图像,进行受试者工作特征(receiver operating characteristics,ROC)曲线分析,得到曲线下面积(Az值),用于评估不同分辨率设置下该设备检测牙根外吸收的可靠性。
结果
ProMax 3D CBCT可提供高(high)、正常(normal)和低(low)3种分辨率模式扫描,其Az值分别为0.867、0.703和0.665(P<0.05)。DCT PRO CBCT提供了两种分辨率参数设置,组合可得到4种曝光模式,分别为正常质量+正常分辨率(normal quality + normal resolution)、正常质量+高分辨率(normal quality + high resolution)、高质量+正常分辨率(high quality + normal resolution)和高质量+高分辨率(high quality + high resolution), 其Az值分别为0.527、0.725、0.743和0.794(P<0.05)。
结论
分辨率设置可显著影响锥形束CT检测牙根外吸收的可靠性,在不增加患者辐射剂量的前提下,采用计算机辅助后处理的方法可显著提高锥形束CT的诊断能力。
Keywords: 牙根吸收, 锥形束CT, 分辨率
Abstract
Objective
To evaluate the diagnostic accuracy of cone beam computed tomography (CBCT) with different resolution settings in detecting the simulated external root resorption defects.
Methods
External root resorption defects were simulated in 51 human single rooted premolar teeth. Ca-vities simulating root resorption defects of 1 mm in diameter and 0.1 mm, 0.2 mm, and 0.3 mm in depth were drilled in the cervical, middle and apical thirds of lingual surfaces of the teeth. In addition to the 51 locations as controls, a total of 102 cavities were obtained in the present study. Specimens were placed in a human dry mandible and scanned by ProMax 3D and DCT PRO CBCT with different resolution settings, respectively. The three-dimensional CBCT images were evaluated by two experienced observers. The data were analyzed with receiver operating characteristics (ROC) analysis. ROC curves were generated and the area under ROC curve (Az) was employed to express the diagnostic accuracy.
Results
The diagnostic accuracy (Az value) of ProMax 3D CBCT with high, normal and low resolution settings were 0.867, 0.703 and 0.665 (P < 0.05), respectively. Defects with depths of 0.2 mm and 0.3 mm were easier to be detected than those with depths of 0.1 mm (P < 0.05). The images obtained by high resolution mode scanning had obvious advantages in detecting smaller defects (depth 0.1 mm and 0.2 mm). The DCT PRO CBCT provided 4 resolution settings including normal quality + normal resolution, normal quality + high resolution, high quality + normal resolution and high quality + high resolution. The Az values for those 4 resolution settings were 0.527, 0.725, 0.743, and 0.794 (P < 0.05), respectively. Similar to ProMax 3D CBCT, the scanning mode with high resolution played a better role in detecting the defects with depth of 0.1 mm. Except for the scanning setting mode with normal quality + normal resolution, the other three modes could well be evaluated for the defects with depth of 0.2 mm and 0.3 mm.
Conclusion
It is concluded that the diagnostic ability for external root resorption of CBCT could be affected by resolution settings. Computer-aid imaging method can improve the CBCT diagnostic accuracy for external root resorption without increasing the radiation dose level during CBCT scanning.
Keywords: Root resorption, Cone beam computed tomography, Resolution
牙根外吸收是口腔临床常见的一种疾病,它是因破骨和(或)破牙骨质细胞活跃而造成的牙根表面硬组织不可逆性的丧失[1]。牙外伤、牙髓及牙周慢性炎症、埋伏牙压迫及某些口腔颌面部肿瘤等均可引起牙根外吸收,临床较常见的是正畸治疗引起的牙根外吸收[2,3,4]。发生牙根外吸收的患牙通常没有明显的临床症状,临床检查难以及时发现[5]。根尖片是检查牙根外吸收常用的影像学方法,但是二维影像的重叠、变形等特点影响其诊断准确性[6,7]。
三维成像技术可为牙根外吸收的诊断提供更加准确、丰富的信息,并可在三维方向上对病变的部位、范围、严重程度进行评估,从而协助口腔医师早期发现、诊断疾病,制定正确的治疗计划,以达到最大限度保留受累牙齿的目的[8]。锥形束CT(cone beam computed tomography,CBCT)是上世纪90年代末出现的一种专用于诊断口腔疾病的三维成像技术,与多层螺旋CT(multi-slice computed tomography,MSCT)相比,CBCT具有辐射剂量相对较低、空间分辨率高、成本低等优点,尤其适用于硬组织疾病的检查[9],已有研究表明CBCT检测牙根外吸收的可靠性高于MSCT[10]。随着技术的发展,现有的CBCT设备可提供多种扫描参数设置,临床医师为获取高清晰的图像,大多倾向于选择高分辨率模式进行扫描,然而,高分辨率模式往往意味着高辐射剂量[11],且高分辨率模式能否提高牙根外吸收的诊断准确性,尚需要研究证实。本研究旨在评价CBCT在不同分辨率模式设置下检测牙根外吸收的可靠性,为临床选择恰当的扫描模式设置提供依据。
1. 材料与方法
1.1. 离体牙洞型制备
选择外科拔除的没有龋坏及充填体且牙根完整的离体单根前磨牙51颗,去除牙冠,将牙根清洁干净,包埋在甲基丙烯酸树脂内,暴露牙根舌面。
用电子机床分别在牙根舌面的根颈、根中和根尖部制备圆柱形洞型,直径为1 mm,深度分别为0.1 mm、0.2 mm、0.3 mm。在总共153个位点中随机选择51个位点,设为对照组(未制备洞型)。
1.2. 洞型深度和直径的检验
在体视显微镜下,使用日本三丰公司生产的C112AM型数显千分表测量所制备洞型的实际深度及直径,确保实验洞型的准确性。
1.3. 样本的CBCT扫描
将样本置于人干下颌骨的牙槽窝中,分别使用芬兰普兰梅卡公司生产的ProMax 3D型CBCT机和韩国Vatech公司生产的DCT PRO型CBCT机对样本进行扫描;ProMax 3D CBCT可提供高(high)、正常(normal)、低(low)3种分辨率模式扫描。DCT PRO型CBCT提供了两种分辨率参数设置,组合可得到4种曝光模式,分别为正常质量+正常分辨率(normal quality + normal resolution)、正常质量+高分辨率(normal quality + high resolution)、高质量+正常分辨率(high quality + normal resolution)和高质量+高分辨率(high quality + high resolution)。两种设备的扫描参数设置见表1。
1.
ProMax 3D 和 DCT PRO CBCT的扫描参数设置
Scanning parameter settings for ProMax 3D and DCT PRO CBCTs
| CBCT | Scan protocol | Voltage /kV | Current /mA | Exposure time /s | Reconstruction time/s | Voxel size /mm |
| NQ + NR, normal quality + normal resolution; NQ + HR, normal quality + high resolution; HQ + NR, high quality + normal resolution; HQ + HR, high quality + high resolution. | ||||||
| ProMax 3D | High | 84 | 16 | 12 | 150 | 0.16 |
| Normal | 84 | 16 | 12 | 90 | 0.16 | |
| Low | 84 | 8 | 2.8 | 60 | 0.32 | |
| DCT PRO | NQ + NR | 90 | 7 | 15 | 90 | 0.3 |
| NQ + HR | 90 | 7 | 15 | 120 | 0.3 | |
| HQ + NR | 90 | 7 | 24 | 90 | 0.15 | |
| HQ + HR | 90 | 7 | 24 | 120 | 0.15 | |
1.4. CBCT影像的判读
采用盲法,由两名资深口腔放射医师作为观察者,分别对CBCT影像进行评估并记录结果。观察者均经过培训并能熟练掌握CBCT操作软件。采用的显示器为Dell公司22英寸(1英寸=2.54 cm)的E228WFP型平板显示器,分辨率为1 680×1 050像素。牙根外吸收缺损的判定标准是CBCT影像中显示牙根外表面有低密度阴影。要求观察者按照以下的分级标准对每个位点的牙根外吸收缺损作出评估:(1)肯定没有缺损;(2)可能没有缺损;(3)不确定是否有缺损;(4)可能有缺损;(5)肯定有缺损。每名观察者前后评估两次,中间间隔2周以上。
1.5. 统计学分析
使用SPSS 13.0统计软件,以体视显微镜下直接测量的结果作为金标准,对观察者的评估结果进行受试者工作特征(receiver operating characteristics,ROC)曲线分析,计算得到ROC曲线下面积(the area under the ROC curve,记为Az值),计量资料以均数±标准差表示,采用one-way ANOVA检验进行统计学分析,P<0.05认为差异具有统计学意义。
2. 结果
2.1. ProMax 3D CBCT检测牙根外吸收的可靠性
ProMax 3D CBCT 3种分辨率设置下扫描获取到的影像示例见图1, 高、正常、低扫描模式下,该CBCT诊断牙根外吸收的Az值分别为0.867±0.032、0.703±0.030、0.665±0.055(F = 38.956,P < 0.05,图2)。
1.
ProMax 3D CBCT 3种分辨率设置下扫描离体牙样本所获取到的影像示例
Images reconstructed from ProMax 3D CBCT scanning with different resolution settings
A, high resolution; B, normal resolution; C, low resolution.
2.
ProMax 3D CBCT 3种分辨率设置下检测牙根外吸收的ROC曲线和Az值
ROC curves and Az value obtained from combined observer performance of ProMax 3D CBCT with different resolution settings
ProMax 3D CBCT 3种分辨率模式设置下检测不同深度缺损的Az值见表2。与深度0.1 mm相比,深度0.2 mm和0.3 mm的缺损更易被发现。对于检测较浅的缺损(深度0.1 mm、0.2 mm), 高分辨率模式扫描获取的图像具有明显优势。
2.
ProMax 3D CBCT 3种分辨率模式设置下检测不同深度缺损的Az值
Az values from ProMax 3D CBCT scanning with different resolution settings for defects in different depths
| Depth/mm | High | Normal | Low |
| 0.1 | 0.793 | 0.589 | 0.562 |
| 0.2 | 0.914 | 0.762 | 0.695 |
| 0.3 | 0.919 | 0.798 | 0.776 |
2.2. DCT PRO CBCT检测牙根外吸收的可靠性
DCT PRO CBCT 在4种分辨率设置下扫描获取到的影像示例见图3。正常质量+正常分辨率、正常质量+高分辨率、高质量+正常分辨率和高质量+高分辨率扫描模式下,该CBCT诊断牙根外吸收的Az值分别为0.527±0.043、0.725±0.057、0.743±0.030、0.794±0.022(F=63.961,P<0.05,图4)。
3.
DCT PRO CBCT 4种分辨率设置下扫描离体牙样本所获取到的影像示例
Images reconstructed from DCT PRO CBCT scanning with different resolution settings
A, normal quality+normal resolution; B, normal quality+high resolution; C, high quality+normal resolution; D, high quality+high resolution.
4.
DCT PRO CBCT 4种分辨率设置下检测牙根外吸收的ROC曲线和Az值
ROC curves and Az value obtained from combined observer performance of DCT PRO CBCT with different resolution settings
DCT PRO CBCT在4种分辨率模式设置下检测不同深度缺损的Az值见表3。与ProMax 3D CBCT类似,对于检测深度0.1 mm的缺损,高分辨率模式更有优势;对于检测深度0.2 mm和0.3 mm的缺损,除了最低分辨率的正常质量+正常分辨率模式外,其他3种模式均可作出较好的评价。
3.
DCT PRO CBCT 4种分辨率模式设置下检测不同深度缺损的Az值
Az values from DCT PRO CBCT scanning with different resolution settings for defects in different depths
| Group | Depth/mm | ||
| 0.1 | 0.2 | 0.3 | |
| Normal quality + normal resolution | 0.405 | 0.545 | 0.665 |
| Normal quality + high resolution | 0.539 | 0.82 | 0.846 |
| High quality + normal resolution | 0.646 | 0.761 | 0.846 |
| High quality + high resolution | 0.681 | 0.853 | 0.865 |
3. 讨论
传统二维口腔医学影像学手段对于牙根外吸收的诊断具有一定的局限性,由于牙根外吸收形成的是三维立体的缺损,根尖片显示的二维影像会因解剖结构的重叠而降低其诊断准确性,低估了牙根外吸收的发生率,许多病例直到病变晚期才被发现,导致治疗效果较差,严重时甚至需要拔除患牙。有文献报道根尖片识别牙根外吸收的假阴性率约51.9%,假阳性率约15.3%,尤其当病损位于颊侧或舌侧根面时,准确率更低[12],而对于深度小于0.3 mm的病变,根尖片很难发现[13]。
与二维影像技术相比,三维影像技术的应用可提高对牙根外吸收的诊断准确性[14]。螺旋CT和CBCT是常用于口腔颌面部疾病诊断的两种三维影像学设备,已有研究表明,CBCT检测单根牙舌面缺损的可靠性优于8层螺旋CT,检测小缺损的优势更明显[10],这表明CBCT是检测牙根外吸收的有效的影像学手段。由于CBCT设备可提供多种扫描模式,临床可以根据疾病种类、检查部位等选择合适的扫描参数,以达到检查目的和辐射剂量的优化。
本研究定量制备了牙根外吸收的体外模型,采用ROC曲线方法评价了两种CBCT设备不同分辨率模式下检测牙根外吸收的可靠性。对ProMax 3D的研究结果表明,检测牙根外吸收可靠性的顺序为高 > 正常> 低,说明可通过升高分辨率参数的方法来达到提高CBCT诊断准确性的目的。由于高模式相对正常模式并不增加病人的辐射剂量[11],因此建议选择该曝光模式来扫描牙根表面的可疑微小缺损。对DCT PRO的研究结果表明,正常质量+正常分辨率对牙根外吸收的诊断准确性较差,而其他3种模式的诊断准确性较高,但三者之间差异无统计学意义,临床可选择辐射剂量相对较低的分辨率模式进行扫描,以在不影响诊断的前提下尽可能地减低患者接受的辐射剂量。
计算机辅助图像处理的应用是CBCT技术发展的亮点[15]。本研究采用的ProMax 3D可提供3种分辨率设置模式,其中高和正常的曝光参数相同,只是在高模式中通过计算机对原始图像进行后处理,正是这种计算机辅助图像后处理方法的应用可显著提高CBCT检测牙根外吸收的可靠性。同样,在DCT PRO提供的4种分辨率设置模式下,也可实现不增加辐射剂量的前提下提高诊断准确性的目的。未来随着计算机辅助图像处理技术的发展,低剂量、高诊断准确性的CBCT设备有望应用于临床。
本研究所采用的ROC曲线分析是一种可靠的诊断试验评价方法,通过改变诊断阈值, 获得多对真(假)阳性率值,以假阳性率为横坐标, 真阳性率为纵坐标,绘制而成的曲线称之为ROC曲线[16]。以曲线下面积Az值反映诊断试验的价值,一般认为Az值为0.5~0.7时诊断价值较低;0.7~0.9时诊断价值中等;大于0.9时诊断价值较高。Az值因不受患病率和诊断阈值的影响,可对两个诊断试验的准确度进行综合比较,因而成为目前公认的诊断试验最佳评价指标。
为了尽量模拟真实CBCT扫描情景,本研究进行CBCT扫描时将离体牙样本置入人下颌骨的牙槽窝内,尽管如此,仍与患者体内的真实情况有差别。此外,人工制备的牙根舌面缺损外形过于规则,观察者更易通过缺损的外形作出判断,人为地提高了实验的灵敏度和特异度[17]。因此本研究结果尚不能完全代表真实的临床情况,所得结论尚需体内研究进一步证实。
综上所述,分辨率设置可显著影响锥形束CT检测牙根外吸收的可靠性,在不增加患者辐射剂量的前提下,采用计算机辅助后处理的方法可显著提高锥形束CT的诊断能力。
志谢:
感谢北京手表厂技术人员在制备洞型方面提供的帮助;感谢北京大学口腔医学院预防科司燕主任医师在统计分析方面提供的帮助。
Funding Statement
国家自然科学基金重点项目（11435007）; 北京大学口腔医学院科研基金（2011-7）
Supported by National Natural Science Foundation of China(11435007); and Scientific Foundation of Peking University School of Stomatology(2011-7)
Footnotes
The authors have declared that no competing interests exist.
作者已声明无竞争性利益关系。
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