| Computer digital simulation is a kind of mathematical modeling technique bycomputer. To establish a digital three-dimensional model on the basis of the CT orMRI data of human body and computer simulation technique, which can providemedical teaching platform for the anatomy teaching, physiological simulation andsurgical operation training. According to patient's digital simulation model, clinicaldoctors can make the personal diagnosis, preoperative program and surgicaloperation simulation.The finite element analysis of the human body structure has major significance.The medical finite element analysis need to resolve two important problems: to getaccurate organization structure geometry models and assign a precise material forevery kind of tissue. The CAD software can not design anatomic structure modelwhich meets the demand because of the complexity of anatomic structure and thearbitrarily of the curve surface. Under reverse engineering method we can only getthe roughly outside profile of anatomy structure, but not the internal structure of thehuman anatomy. The heterogeneity of the organism structure material distribution andnonlinearity of various kinds' material distribution make current finite element pre-processing software not given the precise material assignment to the differenthuman tissue. So the assigned material can not express the material attribution ofhuman tissue.Materialise's Interactive Medical Image Control System (Mimics) is a software of3D image production and edit processing which was exploited by Belgian Materialisecompany. Making use of the Mimics we can input various scan data (CT, MRI),convert them to precise 3D numerical model for anatomy teaching, surgicaloperation simulation and finite element research and carry out large-scale datatransformation on PC. The Mimics can rapidly transform scanned data to CAD,FEA, RP data, then output them to RPM(fast prototype manufacturing), CAD(Computer Aided Design), CAE(Computer Aided Engineering), CAM(computeraided manufacturing), FEA(finite element analysis). The Mimics can also provide akind of efficient method of material assignment base on gray scale value which canassign precise material to 3D model. The mesh re-divide function of FEA moduleoptimizes the imported data to the largest degree, assigns material to body meshbase on HounsField unit of scan data, then carries out finite element analysisthrough various different output interface software.Phillips Brilliance 64 is the most advanced multi-tunnel spiral CT which can get64 layer thin layer images through 64 detection gateways at the same time; moreoverthe 0.625 mm isotropy resolution makes the reconstruction of 3D image almostperfect.Digital Image and Communication in Medicine (Dicom), is the standard ofmedicine digital image storage and correspondence. It is the international standardshould be complied in the construction of integrated medical information system andmedicine image file and transmission system. The establishment of the Dicom 3.0standard provides a united standard for transmission medical image and various digital information between the PC. Because no any conversion of image in preparinghandle, the Dicom format reduce a lot of loss in information.In this research, we combined the Philips Brilliance 64 rows spiral CT thinlayer scanning technique with the Dicom 3.0 standard, made use of the advantageof Mimics in the digital simulation and finite element modeling to explore the bestway of rebuilding the 3D model of various kinds' human body tissue structure andfinite element analysis.Part One: Fast Reconstruction of the Human Skeleton Finite Element ModelBased on CT ImagesObjective: To investigate an easier and more precise method of reconstructionof the human skeleton finite element model based on CT Images.Methods: In advantage with the 0.625 mm isotropous imaging resolution scantechnique of Philips/Brilliance 64 rows spiral CT, we got 118 layers of2-Dimensional CT pictures of human atlas and axis. Mimics 9.1 read these originalDicom' format pictures in and extracted each deck contour line automatically afterdefined bone tissue threshold. Every layer of picture would be processed by partingedge of it, editing it selectively, repairing the hole of it and removing the redundantdata from it, then the atlas and axis skeleton 3D finite element geometry modelwould be obtained and saved as the ANSYS surface mesh document, Which couldbe directly introduced into Ansys 10.0 software to establish 3D finite element bodymesh.Results: A 3D finite element model of atlas and axis was established. It haslifelike appearance, can be arbitrarily rotated or zoomed and displayed with colorful, transparent or arbitrary combination of image. Therefore, it has a bettervisual effect. Through this model, we can observe the inner structure relation of eachcomponent from any plane of different cross-section, the 3D geometric model of atlasand axis was processed with high quality mesh partition after it's triangular patchsurface mesh precision was reduced, then was introduced into the ANSYS fortransformation into the finite element volumetric mesh based on which we can selectthe suitable unit and assign the material to it for further loading and solution.Conclusion: The applications of thin layer CT scanning technique ofPhilips/Brilliance 64 rows spiral CT and the Dicom 3.0 standard can make theestablishment of digital model in the 3D finite element more accurate. The Mimicssoftware greatly improved the efficient in establishing the finite element geometry ofhuman bones.Part Two: Digital Simulation of the Skeletal System of "Chinese Virtual PersonMale No 1" and the Initial Establishment of Whole Body Skeletal DatabaseObjective: To investigate a method to reconstruct the 3D digital model of wholehuman skeletal system base on CT section images of the "Chinese Virtual personmale NO 1" in order to provide a 3D database of whole human skeleton for the digitalanatomy teaching, physiological simulation, surgical plan and training, and finiteelement analysis.Method: The whole body of "Chinese Virtual person male NO 1" can bescanned by Siemens Somatom Plus 4 spiral CT machines from seven sequence whichare head, neck and chest, abdomen and upper limb, pelvis and hand, lower limb,knee joint and foot. All layer distance were 1.0 mm, the picture pixel size of thehead sequence, neck and chest sequence, abdomen and below part sequence was 0.441 mm, 0.965 mm, 0.977 mm. the whole body was divided as 1721 slice. Wedirect input every two-dimensional Dicom format picture of each CT sequence in theMimics, then original scanning parameter will be set automatically and the imagewill be handled through two times inside interpolation. After bone tissue thresholdwas defined, the 3D model of whole body skeleton would be obtained throughsemiautomatic interactive square means. Each sequence of the whole skeleton wouldbe saved and output as the STL format document, we import another 6 STL documentin the pelvis and hand sequence, used Reposition function to matched-pairs, andgot the whole digital model of the skeletal system of "Chinese Virtual person maleNO 1" after Boolean calculation. Marked and divided the whole bone except thehead, hand and feet, saved the outside contour line model of each bone as separateSTL document and displayed it with different color, we established an initial skeletaldatabase of whole body.Results: The construction of whole digital model of the skeletal system of"Chinese Virtual person male No 1" showed display of single or arbitrarycombination of each bone and created the initial skeletal database of whole body.Every component of this rebuilding digital model can be output for further research.In Mimics, all data can be converted to IGES format which defined as standard dataformat by Mimics, and then output for CAD(Computer Aided Design) and RP(rapidprototyping), or converted into volumetric mesh for finite element analysis in thefinite element software such as Ansys, Nastran, Patran, Abaqus etc.Conclusion: Base on the two-dimensional spiral CT section Images of "ChineseVirtual person male No 1", Mimics software can create the high precise 3D model ofskeletal system according to the threshold of bone tissue. Each component of themodel can be output for CAD, RP and FEA. Part Three: Reconstruction of the Digital Model and Three-DimensionalVisible Research of Lumbar Motional SegmentsObjective: To investigate a method to reconstruct the three-dimensional visibledigital model of lumbar motional segments based on CT Images.Methods: 156 layers of 2-Dimensional pictures of L4-L5 were takencontinuously by 64 row spiral CT. The scanning condition was bone tissue window,picture pixel distance was 0.365mm and layer distance was 0.5nm. The Dicom'sformat data of these pictures was read into the Mimics. After set the primitive CTscanning parameter, handled the picture twice interpolating and defined bone tissuethreshold value, the software automatically formed the outside contour line of bonetissue in every deck and divided the picture of lumbar 4-5, which was edited,repaired the hole, removed the redundant spot and data, and 3D calculated afterregion growing, for building the 3-dimensional geometric model, it includesbuilding the 3-dimensional model of the two cartilage endplate and the annularfibrosus, nucleus pulpous between its after the threshold of each kind of soft tissuewas defined; building the 3-dimensional model of 6 kinds of ligaments afterparameterization. Every part of the reconstructed digital model could be used to FEA.We made the finite element mesh optimization to L5 and import to FEA software totest the model.Results: We established a three-dimensional digital model of lumbar motionsegments which includes two vertebral bodies, cortical bone, cancellated bone,endplate, annulus fibrosus, nucleus pulposus and six kinds of ligaments. The modelcan be output to the CAD, RP and FEA for further research.Conclusion: The application of thin layer CT and the Dicom standard made theestablishment of digital model more accurate. We can establish more commodious human skeleton and various structures of soft tissue more conveniently through theMimics software and also can output digital model for further research.Part Four: Reconstruct the 3D Finite Element ModelOf Prosthetic Disc Nucleus ReplacementObjective: To establish a three-dimensional finite element model of prostheticdisc nucleus of L4/L5 lumbar motion segment to provide a standard mathematicalmodel for further lumbar biomechanic research.Methods: A healthy volunteer and a patient of Prosthetic disc nucleus(PDN-SOLO-7) replacement for each accepted the Philips Brilliance 64 rows spiralCT scanning to obtain data. The healthy volunteer was scanned continuously followcross section from the superior borders of L4 lumbar to the inferior borders of L5lumbar which scanning condition was bone tissue window, the slice increment was0.5 mm and the picture pixel size was 0.365mm, total 156 layers. The patient ofprosthetic disc nucleus replacement was scanned continuously follow cross sectionfrom the superior borders of L1 lumbar to the coccyx which scanning condition wassoft tissue window, the slice increment was 0.329mm and the picture pixel size was0.282mm, total 735 layers. The scanning data was stored with Dicom 3.0 standards.L4/L5 lumbar motion segment geometric model was created in the Mimics 10.11software. In the Mimics software, according to the prosthesis' height 8.5 mms,anteroposterior diameter 14.7 mms, transverse diameter 25 mms after PDN-SOLO-7applliance hydration, we created the 3-dimensional geometric model ofPDN-SOLO-7 type of prosthetic disc nucleus by parameterization, stored and exportwith STL triangular patch document, created a three-dimensional finite elementgeometric model of prosthetic disc nucleus after imitate the lumbar vertebraeintervertebral discs removal and Posterior PDN Approach (PPA) by using the software's reposition function. The front view of PDN-SOLO-7 appliance was locatedon central of intervertebral space; the lateral position of it was located on the frontand middle 1/3 of intervertebral space according to the surgical operation design. Thesurface mesh of each component was handled through accuracy down sharply andoptimizes quality to largest degree, then was Output as the document of the Ansyssurface mesh and was converted to volumetric mesh in the ANSYS PRODUCTS 11.0.Volumetric mesh of Lumbar L4-L5 was import into Mimics again for materialassignment which can be divided into cancellous bone, cortex bone, lamina ofvertebral arch and pedicle of vertebral arch four regions according to the grey scalevalue, we assigned the proper material to the corresponding volumetric mesh afterconvert HounsField unit to density value according to empirical formula of lumbarvertebrae, defined Young's modulus and Poisson's ratio for each material. TheYoung's modulus and Poisson's ratio of other component and the PDN-SOLO-7 wasdefined directly. Then we established a three-dimensional finite element model ofPosterior PDN Approach (PPA) of L4/L5 lumbar motion segment in advantage withthe high assemble technique of finite element in the ANSYS software.Results: We established three-dimensional geometry, finite element model ofL4/L5 lumbar motion segment, PDN-SOLO-7 and Post-PDN Approach (PPA). ThePost-PDN Approach (PPA) includes 92 516 tetrahedron elements, 153 963 node, itis constituted by lumbar vertebrae L4-L5, annulus fibrosus, the top and bottomlamina terminalis, seven ligaments and Prosthetic disc nucleus and simulated by 3Dten node SOLID92 solid unit. The PDN-SOLO-7 model includes 3 288 tetrahedronelements and 5 495 node. The model of L4/L5 lumbar motion segment includes63 839 tetrahedron elements, 99 139 node. Six series of ligaments are anteriorlongitudinal ligament, posterior longitudinal ligament, ligamentum flavum,interspinal ligaments, supraspinal ligament and a pair of intertransverse ligaments. These Seven ligaments includes 11 669 tetrahedron elements, 23 857 node, and thetotal surface area of ligaments is 7 624 mm~2. Superior and inferior articular surface ofthe articular process, PDN-SOLO-7 appliance and top-bottom lamina terminaliswere processed as 3D contact model, flexibility contact, face-face contact element.Conclusion: The application of 64 rows spiral CT scanning can obtain theaccurate data of lumbar vertebrae geometry. Mimics software can provide moreprecise model through the calculation base on threshold and the parameterizationmodeling. Making use of its precise material assignment according to gray levelvalue, we can establish the finite element simulation model of lumbar motionsegment and PDN displacement. |
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