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FINITE ELEMENT MODELING FROM CT SCAN DATAS. Diemente", B.S . , C.G. Lewis*, M.D. M.D. Nowak*+, D.SC.,and D.J. Leone+, PhD.

*Department of OrthopaedicsUniversity of Connecticut Health Center10 Talcott NotchFarmington, CT 0 6 0 3 2

+Department of Civil EngineeringUniversity of Hartford20 0 Bloomfield AvenueWest Hartford, CT 06117

AbstractWork done over the past twenty yearsestablishes finite element modeling (FEM) asa valuable tool for structural analysis inorthopaedic biomechanics. However, mostgroups build models by manual measurementand data entry, or by hand-digitizingradiographic images. This paper describesan automated method for building finiteelement models. The algorithm obtainsbinary image data from CT scans taken on anElscint Exel 2 4 0 0 Elite scanner, andmanipulates the data into a format acceptedby PATRAN. The data manipulation algorithmruns on a 2 8 6 pc, and PATRAN is used on a

VaxStation 3100. It took an experienceduser less than one hour to import over 750grid points into PATRAN.Utroduction

Finite element modeling is a veryvaluable tool for structural analysis inorthopaedic biomechanics. Since 1 9 7 2 , manyresearchers have built models to studyvarious physiological conditions [l].However, three-dimensional models sometimesrequire a prohibitive number of man hours tobuild, because of the complex geometriesencountered in the human anatomy. Thispaper describes a method of automating dataacquisition and entry, by using CT scandata.screen, every pixel of the image has anassociated gray level, correlating with thedensity of the scanned object [ 2 ] . Becausefinite element modeling software accepts(x,y,z) coordinate data, the CT scan graylevel data has to be manipulated into thecorrect format.The set of data described in this paperis a cylinder, with a smaller cylinder of adifferent density inserted in one end. Thisgeometry is a very simplified approximationof the femur/implant scenario.

When a CT scan is produced on a computer

MethodsFauioment; CT scan data was generated on anExel 2 4 0 0 Elite scanner (Elscint, Inc.,Haifa, Israel). The program for datamanipulation runs on a 2 8 6 pc. The pcprogram outputs the data onto a diskette,which the user copies onto a Vaxstation 3100IDigital Equipment Corp.) by running it inthe soft pc mode. PATRAN (PDA Engineering,Palo Alto, CA) also runs on the Vaxstation3100.

Object Definition: When the datamanipulation software on the pc firstgenerates the image from the CT scandata, each of the sixteen colors representsa range of gray level values. This imagemay be adjusted and redisplayed, until theuser produces a well-defined image ofbackground, bone, and prosthesis.Edae Detect on: Once the user defines theobjects, the algorithm sets all of thepixels that are in the gray ievel ranges forbackground to zero, and the pixels in thebone and prosthesis ranges to fifty and twohundred, respectively. By looking at thedifferences between pixels on each line, thealgorithm may detect edges if it findsnon-zero gradients. The x and y coordinatesof an edge point are the correspondingcolumn and row of the image.non-zero gradient if differences of zeroexist for five pixels before, and fivepixels after the non-zero gradient. Thespecific value of the gradient determinesthe interface type.Point Selection: In the next step, theprogram displays the edges on the screen,and prompts the user to enter the number ofpoints to be converted to the PATRANformat. The algorithm then chooses thespecified number of evenly spaced pointsfrom the edges by using the ratio of thetotal number of edge points to the number ofpoints to be selected.Data OutDut: inally, the software obtainsthe true x and y coordinates by multiplyingthe x and y coordinates of the selected edgepoints by the image scale. The scale, aswell as the z value for every CT scan slice,are part of the header information that isassociated with the CT scan gray leveldata. Once the user has extracted data forall of the CT slices, the algorithmconcatenates all of the coordinate data andoutputs it to a file. This file is in theneutral file format for "grids" (i.e.,points) accepted by PATRAN.

An edge is assumed to exist at a

DiscussionThe main advantage of using thisautomated technique to input points inPATRAN is that it saves time. Anexperienced user created the first modelwith this software by selecting twentypoints from the edges detected in fourteen

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CT scan slices, resulting in more than 750points. Data extraction took approximatelyone half hour, and it took only a fewseconds to import the file into PATliAN. Inaddition, this technique eliminates most ofthe potential for human error associatedwith data measurement and entry.The disadvantage of this system is thatthe model is only as accurate as the usersability to define the edges. If thematerials scanned are similar in density,the user will rely on his or her eye todetermine the ranges for the edges.However, this generally should not be aproblem for large objects.Work still to be done on this projectincludes creating a finite element model ofan epoxy model of the human femur with animplant, using the automated method. Thismodel will be compared to a finite elementmodel of the same object, created bydigitizing images produced by the CTscanner. In addition, models will be builtwith increased numbers of CT slices,resulting in an increased number of gridpoints, in an effort to get the results ofthe analyses to converge.

Ref ences111 R. Huiskes and E.Y.S . Chao, A Surveyof Finite Element Analysis in OrthopedicBiomechanics: The First Decade,J.Biomechanics. vol. 16, pp.385-409, no.6,1983.[ 2 ] A . Macovski, Medical Imauina. EnglewoodCliffs, New Jersey: Prentice-Hall, Inc.,1983, ch.7, pp.106-144.

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