义齿三维模型构建中海量点云数据模型的精简方法

背景：义齿三维模型的构建始终是义齿计算机辅助设计与制造系统的核心,而构建的三维模型的精度与效率又与对海量的原始点云数据进行精简密切相连。现有的点云精简技术,并不能完全满足义齿处理的特殊要求。目的：针对义齿的原始点云数据进行精简,以便以最少的点,实现最有效的表达。方法：通过三维栅格法对海量的原始离散点云数据进行拓扑重建形成一个个栅格,然后在每个栅格内拟合出平面和二次曲面,并利用弦差法来评定哪些栅格内点需要保留,哪些栅格内点需要继续八叉树剖分,从而保留曲率较大处的点云及其曲率和法向量。结果与结论：结果表明,该算法简单高效,所提出的点云精简算法适用于逆向工程中测量所得的散乱无序的点云数据,能够对大量密集数据进行直接而有效的精简,并且对曲率变化大且含有较多细节特征的区域保留更多的细节信息。提示通过3DSS-STD-Ⅱ型结构光三维扫描仪得到的义齿点云数据在λ=0.7,ε=0.0003mm,时的精简效果最好。     

利用激光扫描与CAD-CAM技术进行面部三维重建

目的:为临床医学个体化三维建模需要,提出用激光扫描仪与计算机辅助设计与制造技术反求面部表面轮廓的方法。方法:采用激光扫描系统对人体面部石膏模型进行扫描,通过获得的点云文件建立面部三维模型,将三维模型同原始点云数据进行偏差分析。结果:对由不同数量曲面片重建的模型与原始点云进行比较分析,精度可达0.01～0.20mm,最大误差主要集中在面部曲率较大处,结果满足整形设计的临床需求。结论:利用激光扫描与CAD/CAM技术进行面部三维模型重建,可以满足整形设计的临床需求。     

消融导管三维定位导航系统中心脏多腔模型的构建

背景:目前,心内膜三维标测系统的研制是国内外医学工程学界的一个热点。国外已经推出了适用于临床的系统,但是其价格极其昂贵;国内还只是处于研究阶段,因此,在心内膜三维标测系统的国产化方面任重道远。目的:采集分析消融导管三维定位导航系统的建模数据,并重构心内膜三维几何模型。方法:心内膜三维建模数据的采集直接关系到建模质量的高低。以离散点的形式将数据采集过程进行可视化,可获得全面和有效的建模数据点集;通过对所得的点云数据进行凸包运算,获取数据集的表面极值点,构造用边界表示的多面体模型;最后以生成的多面体为基本体素,采用离散法造型完成建模过程。并使用可视化类库VTK,成功的重构了心内膜三维模型。结果与结论:仿真实验表明,实验提出的方法可以较全面、高效地采集建模数据,重构心内膜三维模型。提示重构的心内膜三维几何模型可以为消融导管三维定位导航系统提供技术支持。     

三维二值掩模零化法结合体素绘制法切割及渲染CT与MRI体积数据

目的 观察三维二值掩模零化法联合体素绘制法对于切割及渲染CT与MRI体积数据的作用。方法 利用Matlab软件将DICOM形式头颈部CT、腹部平扫CT、正常头部MR T1WI及脑胶质瘤T1WI转化为体积数据，以半自动或手动方法获得相应掩模图像；基于原始掩模以零化法在x、y、z轴方向对体积数据进行正向与反向切割，结合三维体素绘制法对切割后的模型进行渲染。结果 所获目标结构原始掩模可在体素绘制后突出显示CT及MRI数据中的相应解剖结构。基于体素绘制法实现了以三维二值掩模零化法与原体数据点积后切割模型，结合手动输入不同阈值进行切割可显示脑胶质瘤及其毗邻结构。结论 三维二值掩模零化法联合体素绘制法可结合不同层面CT与MRI体积数据显示其内部结构。     

消融导管三维定位导航系统中心脏多腔模型的构建

背景：目前,心内膜三维标测系统的研制是国内外医学工程学界的一个热点。国外已经推出了适用于临床的系统,但是其价格极其昂贵;国内还只是处于研究阶段,因此,在心内膜三维标测系统的国产化方面任重道远。目的：采集分析消融导管三维定位导航系统的建模数据,并重构心内膜三维几何模型。方法：心内膜三维建模数据的采集直接关系到建模质量的高低。以离散点的形式将数据采集过程进行可视化,可获得全面和有效的建模数据点集;通过对所得的点云数据进行凸包运算,获取数据集的表面极值点,构造用边界表示的多面体模型;最后以生成的多面体为基本体素,采用离散法造型完成建模过程。并使用可视化类库VTK,成功的重构了心内膜三维模型。结果与结论：仿真实验表明,实验提出的方法可以较全面、高效地采集建模数据,重构心内膜三维模型。提示重构的心内膜三维几何模型可以为消融导管三维定位导航系统提供技术支持。     

骨盆及髋臼三维有限元模型单元选择及构建生物力学意义

目的:寻求骨盆及髋臼三维有限元模型构建中三维有限元模型单元的选择以及构建,为骨盆、髋臼生物力学研究及手术仿真模拟提供精确模型。方法:在获得的骨盆及髋臼三维CT图像数据矩阵中移动立方体,骨骼内部采用六面体单元,骨骼表面采用MarchingCube算法,建立四面体代替等值面,构建骨盆及髋臼三维有限元模型单元。结果:综合MarchingCube算法和传统方法构建三维有限元模型,所构建模型表面光滑连续,不仅能够分析骨骼表面应力应变分布,同时保留内部应力应变分布的特点。结论:结合传统三维有限元模型构建方法,在模型表面使用四面体单元,模型内部采用六面体单元,构建表面连续平滑的骨盆和髋臼有限元模型,不仅能够反应内部应力应变分布,而且能够模拟分析骨骼表面应力应变分布。     

基于通用CAD技术的三维假牙模型重建

背景:由于全口假牙没有牙齿作为牙托的支撑,压力全部到牙龈上面,假牙活动咀嚼食物时不可避免对作为牙托支撑的牙龈进行磨损,患者也经常出现牙龈发炎和假牙托的金属过敏症状,因此为患者安装假牙前,医生必须先测试其对假牙材料的适应性。目的:对经历磨合期后的假牙进行三维模型重建。方法:使用激光扫描仪等数字采集设备采集了经过磨合期的全口假牙三维数据,利用逆向软件进行减少噪声点、数据滤波、数据精简及对缺失的数据进行修补等预处理并封装为三角曲面,通过Pro/E等三维设计软件实现模型的重建。结果与结论:通过激光扫描仪、CT等数字采集设备采集经过磨合期的全口假牙三维数据,利用逆向软件对采集到的数据进行预处理包括减少噪声点、数据精简、数据滤波及对缺失的数据进行修补等并提取曲线模型,最后在Pro/E等三维设计软件中实现了模型重建。重建的模型可为假牙的制作提供数字信息,也可以通过快速成型得到假牙的复制品。逆向重建的经过磨合期的全口假牙,不仅节约了时间,减少医生的劳动强度,还降低患者对材料过敏的风险,缩短患者与假牙的磨合期,减少患者的痛苦。     

基于改进移动立方体算法的三维重建

目的 探讨基于改进移动立方体算法的腹部器官CT图像三维重建效果。方法 提出一种基于区域增长法的通用树结构和移动等值点法的自适应改进移动立方体算法,先进行医学图像分割,选取种子点后标记出与阈值相交的所有体元;创建通用树结构,将相交体元插入子节点中,确定基于通用树的顶点索引方式;通过移动等值点合并共面三角形,简化等值点信息的获取。基于1名志愿者的腹部CT图像,采用传统移动立方体算法和改进移动立方体算法构建肾脏三维模型,并比较其效果。结果 与传统算法比较,改进的移动立方体算法生成的三角面片个数减少39.20%,算法执行效率提高37.59%,三维模型表面平滑逼真,局部细节真实性较好。结论 基于改进移动立方体的算法可更快速精确地实现CT图像腹部器官三维重建。     

基于逆向工程的三维仿生膝关节建模

背景:膝关节置换后,要求患肢达到负重,伸屈及旋转活动,稳定性好.这就要求假体的几何形状要与人体相匹配并使其植入后与人体运动规律相一致,才能使假体与周围保留的软组织协调运动,这一切都得依赖于精确的膝关节假体设计与制造.目的:利用计算机辅助技术设计个性化人工膝关节.方法:利用三维镭射扫描和计算机辅助设计技术对人体膝关节进行扫描并对所输出的点云数据进行处理,利用工程软件Geomagic和UG进行膝关节三维实体模型重建.结果与结论:得到了非常精确的膝关节三维模型.为中国人工仿生膝关节的数字化制造,虚拟装配,应力分析,运动分析及模拟手术提供了精确的模型基础.     

仿真环境下医学器官的三维有限元建模方法

背景:通过医学影像图片数据进行三维重建的模型主要应用于医学临床分析,而采用激光扫描进行逆向重建时建立的模型主要用于生物力学分析。生物力学模型建立的精确程度,直接影响分析结果。目的:分析采用医学影像图片数据和三维激光扫描重建医学器官模型的两种方法及意义。方法:①通过人体CT断层扫描医学影像图片数据,并运用医学重建软件进行三维重建。②采用三维激光扫描仪对医学模型进行扫描得到点云数据,再利用逆向处理软件实现医学器官模型的逆向重建。结果与结论:两种方法都可以建立符合人体解剖结构、可进行生物力学仿真计算的几何模型和有限元模型。医学影像CT/MRI数据重建的三维模型,能够真实再现被扫描对象的表面特征及内部结构,该模型为临床辅助诊断、手术规划、整形、假肢设计及解剖教学等方面提供了可靠的参考模型。三维激光扫描所得点云数据进行逆向重建,具有测量精度高、速度快,能够反映所测标本的表面形态,该模型可在交通运输、军事领域中因发生钝性冲击对人体内部器官造成的损伤进行计算机仿真分析。     

TSegNet: An efficient and accurate tooth segmentation network on 3D dental model

Automatic and accurate segmentation of dental models is a fundamental task in computer-aided dentistry. Previous methods can achieve satisfactory segmentation results on normal dental models; however, they fail to robustly handle challenging clinical cases such as dental models with missing, crowding, or misaligned teeth before orthodontic treatments. In this paper, we propose a novel end-to-end learning-based method, called TSegNet, for robust and efficient tooth segmentation on 3D scanned point cloud data of dental models. Our algorithm detects all the teeth using a distance-aware tooth centroid voting scheme in the first stage, which ensures the accurate localization of tooth objects even with irregular positions on abnormal dental models. Then, a confidence-aware cascade segmentation module in the second stage is designed to segment each individual tooth and resolve ambiguities caused by aforementioned challenging cases. We evaluated our method on a large-scale real-world dataset consisting of dental models scanned before or after orthodontic treatments. Extensive evaluations, ablation studies and comparisons demonstrate that our method can generate accurate tooth labels robustly in various challenging cases and significantly outperforms state-of-the-art approaches by 6.5% of Dice Coefficient, 3.0% of F1 score in term of accuracy, while achieving 20 times speedup of computational time.     

塔中奥陶系碳酸盐岩凝析气藏综合分类及开发技术对策

在三维地震区块数据的采集处理和工区管理中, 地震点的逻辑—实测坐标是仿射变换关系。传统方法是在逻辑网格中取3个拐角地震点来确定, 在实践中发现其对异常情况的处理缺少严格的数学基础。本文提出基于地震点的残差矢量距离满足柯西分布的假设, 运用稳健估计算法来求解该变换关系。对正交和斜交网格两种情况分别处理, 可通过迭代法得到满意的解。通过对算法的分析可知, 对K个地震点其时间和空间的复杂度都为 O(K)。在网格仿射变换的求解公式推导过程中使用几何代数语言, 其解析式具有直观的几何意义。因对逻辑网格的线性变换和原点平移这两组参数的成功分离, 作数值计算时能做并行处理, 并给出了在2×108个点级内的优化算法。     

Classifying airborne bathymetry data using the Random Forest algorithm

The information on the topography of the seabed included in data from airborne lidar bathymetry can be used for the detection of changes occurring at the bottom of the basin and for the detection of objects deposited on it. Processing of full waveform data enables obtaining data on the water surface and the identification of the underwater situation. The classification process based on the Random Forest (RF) algorithm is presented using data from lidar bathymetry. The classification was performed in two independent approaches using input vector consisting of 16 features. In the first approach, the entire point cloud was classified, in the second the point cloud did not contain points of the water surface. In the classification, traits based on the full waveform and resulting from point cloud geometry were used. The quantitative efficiency of classification was verified through error matrices. The obtained efficiency (100% water surface, 99.9% seabed and 60% objects) of the point classification of the objects enables the possibility for using the RF algorithm for detection of objects on the seabed. In comparison to Support Vector Machines, the RF algorithm has better results in the detection of points on the objects in point cloud with water surface.     

Estimating diameter at breast height using personal laser scanning data based on stem surface nodes in polar coordinates

ABSTRACT

Personal laser scanning (PLS) has shown great potential in diameter of breast height (DBH) estimation. Compared with the current laser scanning technique, PLS is hardly restricted by occlusion and trafficability, and produces omnidirectional point cloud data. However, the DBH estimation accuracy using PLS data was not higher in the previous research. One of the leading sources of bias is overlapping and inaccurately co-registered point cloud fragments. Points of these fragments can be far from the stem surface, which leads to a large bias in DBH estimation. In this letter, a novel DBH estimation method using PLS data, the stem surface node method (SSN) is proposed. To reduce the impacts of the inaccurately co-registered points far from the surface, SSN attempts to use a number of stem surface nodes in polar coordinates instead of the whole stem point cloud to estimate DBH. Six plots with 247 stems under three different working conditions were selected to evaluate the method. Compared with the DBH estimation method using direct circle fitting, SSN achieved a total error reduction of 62.64% and 38.35% for bias and root mean squared error, respectively. These findings confirm that SSN is generally reliable and adaptive for different kinds of working conditions.     

Development of an automated individual tree detection model using point cloud LiDAR data for accurate tree counts in a Pinus radiata plantation

We develop and evaluate a new individual tree detection (ITD) algorithm to automatically locate and estimate the number of individual trees within a Pinus radiata plantation from relatively sparse airborne LiDAR point cloud data. The area of interest comprised stands covering a range of age classes and stocking levels. Our approach is based on local maxima (LM) filtering that tackles the issue of selecting the optimal search radius from the LiDAR point cloud for every potential LM using metrics derived from local neighbourhood data points; thus, it adapts to the local conditions, irrespective of canopy variability. This was achieved through two steps: (i) logistic regression model development using simulated stands composed of individual trees derived from real LiDAR point cloud data and (ii) application testing of the model using real plantation LiDAR point cloud data and geolocated, tree-level reference crowns that were manually identified in the LiDAR imagery. Our ITD algorithm performed well compared with previous studies, producing RMSE of 5.7% and a bias of only ?2.4%. Finally, we suggest that the ITD algorithm can be used for accurately estimating stocking and tree mapping, which in turn could be used to derive the plot-level metrics for an area-based approach for enhancing estimates of stand-level inventory attributes based on plot imputation.     

激光扫描并三维重建对重庆地区正常数字化模型的测量

背景:研究采用三维测量仪和计算机辅助系统分别对中国人正常牙齿颊侧和舌侧表面形态进行了三维测量,证实了中国人牙齿各数据与国外数据相比,有明显差异。目的:旨在获得重庆地区正常数字化模型牙体形态的矫正学数据资料,结合Andrews、国人测量结果及Ormco舌侧托槽数据,比较不同人种,不同地区牙体在唇(颊)侧,舌侧的形态差异,为个性化矫正系统提供测量依据。方法:激光扫描并三维重建45副重庆地区未经矫正治疗的正常数字化模型,通过计算机辅助系统精密测量唇(颊)侧及舌侧各4项测量指标:牙冠轴倾角、转矩角,凸距和磨牙补偿角。结果与结论:重庆地区正常牙体的矫正学测量结果特点:①牙冠轴倾度角:上前牙,尖牙较白种人直立;双尖牙,磨牙近中倾斜明显。②牙冠转矩角:上前牙较唇倾,磨牙舌倾较白种人明显,但倾斜度均小于国人。③牙冠凸距:上下磨牙凸距比白种人大,同时,尖牙凸距比国人大。④磨牙补偿角:上颌磨牙补偿角较白种人偏小,与国人大致相似。结果表明,重庆地区正常的牙齿形态与白种人,国人有较大的差别,统计显示有其自身特征。测量方法可为直丝弓矫治技术,舌侧矫治技术提供准确的牙体矫正学所需数据,有助于制订个性化的矫正方案,帮助正畸医生更全面和客观地评估不同地区、不同人种的牙畸形特征。     

Vibratory stimulation for the relief of pain of dental origin

The present paper describes the effect of vibratory stimulation on pain of dental origin in 36 patients. The patients were from a clinic for dental surgery and all had suffered pain from pulpal inflammation, apical periodontitis or postoperative pain following extraction of an impacted wisdom tooth for more than 2 days. Vibration at 100 Hz was applied to various points in the facial region or the skull. All the patients except three experienced an effective reduction of the intensity of the pain. In the patients who experienced pain reduction there was usually a best point at which vibration had a greater pain alleviating effect than at other points. At some points the stimulation added to the pain. In 16 patients the stimulation caused a reduction in pain intensity of 75–100%; out of these 12 patients reported a complete relief of pain.     

Lossless progressive compression of LiDAR data using hierarchical grid level distribution

This letter considers a new approach for the lossless progressive compression of light detection and ranging (LiDAR) data stored within a LAS file (public file format for the interchange of three-dimensional point cloud data), which is used for storing the results of LiDAR scanning. The presented method builds a hierarchical data model for arranging LAS points into different levels in one pass. The higher levels are compressed using variable length and arithmetic coding, whilst the lower levels apply a prediction model of the non-progressive compression schema. The order of the points, as captured by the LiDAR scanner, has to be preserved within each level as better compression ratios are achieved in this way.