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Jin Woo Jung is a computer science graduate student who is doing research at the Minnesota Dental Research Center for Biomaterials and Biomechanics (MDRCBB), led by Professor Alex Fok (Restorative Sciences), in the School of Dentistry. He has been using MSI for about a year. Mr. Jung was a finalist in the poster competition at the 2013 MSI Research Exhibition with his poster, “Photo-realistic Rendering of Teeth and Restorative Bio-materials Using Monte-Carlo Photon Tracing.” He is in the research group of Associate Professor Gary Meyer (Computer Science and Engineering) and also works with Professor Ralph DeLong (MDRCBB), and Associate Professor Maria Pintado (MDRCBB). Mr. Jung sat down with MSI recently to discuss his research; Professors DeLong and Pintado also participated in the interview.
MSI: What kinds of research do you use MSI for?
Jin Woo Jung: I’m doing light transport simulations to reproduce the photo-realistic appearance of dental tissues and biomaterials on the computer screen. The light path within the volume can be modeled as random walks of photons, and Monte Carlo simulation is able to compute the outgoing radiance from the random walks. And then we synthesize the images that show the realistic translucent objects, using the information we calculate.
MSI: In your poster, you’re modeling materials that will be used in tooth restorations. Tell me what you did to get to this poster.
JJ: Obviously, restorative materials have to have very similar appearance to the tooth. As any dentist will tell you, that is a very difficult problem because the color of the restoration changes when it becomes part of the tooth. Furthermore, it is not easy for dentists to choose the correct shades of restorations for their patients. Well, I’m sure Professor DeLong and Professor Pintado will correct me if I’m wrong about the properties of teeth. A tooth is a heterogeneous object; it consists of three major components that are very different. From the outside working inwards, they are: enamel, dentin, and pulp. Overall a tooth is translucent. We all agree that it looks kind of yellow. That’s not because of the color of the enamel; it’s because of the color of the underlying dentin, which is yellow. The yellow passes through the enamel, which may add some white to the final appearance.
But if we can predict the appearance of the restorations in various situations through a simulation, we can assist the materials engineer to develop restorations with the correct appearances, and dentists to choose the right shades for the restorations. So, that’s why we started this research. First of all, we scanned real teeth using the micro-CT scanner at the MDRCBB, and extracted their geometric information; not only the surface geometry but also the density information inside the volume.
Maria Pintado: This geometric information is also used in my study and teaching on the anatomy of teeth. We have produced a self-learning software package called Tooth Explorer using the images created by Jin Woo.
JJ: Using Avizo and Hypermesh that MSI provides, we segmented the micro-CT data and reconstructed the surface information. We also obtained the optical characteristics of teeth and restorative materials from published papers. Applying all of this information to my simulations, I can render the appearance of the teeth on the computer screen. The picture at the end of the poster shows a rendition of the real teeth, including their surfaces and volumes. The simulated light has all the characteristics that come from real enamel and dentin.
I also simulated restorative materials. The restorative material I used was from 3M ESPE, a division of 3M that manufactures dental products and has a long-term collaboration with MDRCBB. They have the spectrophotometers to measure the optical characteristics of the material, such as scattering, absorption, and anisotropy. I modeled the restorative material to show how shade, color and translucency combined to give off overall appearance.
MSI: Now, you’re a computer science student, and you’re working with the Dental School. Can you talk a little bit about the interdisciplinary aspects of this work?
JJ: We are trying to solve a problem in the field of dentistry using computer graphics. The computer science aspects of this require the appropriate mathematics and optimal computing algorithms using the computational power of modern computers, which leads to a good approximation of the real world. We need to have data for the light scattering and the geometry of the teeth. The field of dentistry and related industries provide the data we can make use of. The simulations have some very practical ends in view that is, assisting the dentists in the real world in shade and color matching. The medical/dental industry is a good practical field that the computer science experts can apply their approaches. That’s how the two fields are working together.
Ralph DeLong: The way this research came about was that we have a problem. The problem is making realistic teeth out of, essentially, numbers. Jin Woo’s advisor in Computer Science [Associate Professor Gary Meyer] is interested in light transfer through materials, so the two requirements fit very well together. The third component of this is 3M. 3M manufactures restorative materials, and there’s always been a problem in dentistry of matching colors to the natural teeth, because if they don’t match, people aren’t happy. For years, dentistry has been trying to find a better way to do this. Right now, it’s still done by eye. You look at a tooth, compare it to a color guide and decide this color is close, so I’ll use it. Generally, you can get a good result. Now, you have things like CAD/CAM [computer-aided design/computer-aided manufacturing] coming into dentistry, where you’re milling restorations, and most of them don’t look very aesthetic. So, if you want to make an aesthetic restoration, you have to mill different materials and put them together. Also, for composite fillings, you take one color and you insert it into the tooth it may or may not give you a good match. If you really want aesthetics, you have to start using multiple colors and put them together that’s an art form. What we’d like to do is capture an image of the tooth, then use a computer using known optical properties of the restorative materials and the teeth, and our software tells us what restorative materials to use. This is a three-way junction between computer science, dentistry, and industry. Much of this development was foreseen in our work on an NIDCR [National Institute of Dental and Craniofacial Research] project called the Virtual Dental Patient.
MSI: That’s very exciting. When you say composites, you mean fillings?
RD: Yes, filling materials.
MSI: So, you’re using computer modeling to simulate the way light hits these materials. I’m guessing this is something that could not be done on a standard desktop computer?
JJ: The difficulty with this simulation is that it requires a lot of computational power, as well as a large amount of memory space. Unlike an analytical approach, Monte-Carlo simulation needs to sample large amount of data to compute an outgoing radiance on the surfaces. Parallel computing with multiple computing nodes is very useful in this kind of problem. In addition, we re-use a lot of intermediate results that we compute for the translucent objects. That’s why large amount of main memory is also helpful in this simulation. If we were running this kind of simulation in my laptop, it would take weeks, if not months. Without huge main memory space, the simulation would waste time on swapping data between main memory and secondary storage. Thanks to MSI, we can save a lot of time.
MSI: Did you use the supercomputers on this, or did you use the labs?
JJ: This is a large simulation that requires a lot of computing power. I used the Windows machines [through the labs] on Iron for the Monte Carlo simulation.
MSI: When you run your calculations, do you get a visualization, or do you get numbers that the manufacturer can use to create these false teeth?
JJ: The final results of this simulation are photo-realistic renderings of teeth and biomaterials. Through changing the optical properties of the restorative materials, we can arrive at the appropriate values for desired appearance as required by the materials scientists and dentists.
MSI: How close are we to the point where 3M will be able to use this?
JJ: It will take some time. One thing I have to overcome is the slow speed of the simulation. If we simulated the physically correct appearance of the teeth and the biomaterials fast enough, the material engineers and dentists would be happy to use this. Right now, we are modeling a real-time algorithm, with recent hardware technologies, based on the current mathematical model. We are expecting this research to be more helpful with better responsiveness. Once we have that, it’s going to be very useful, especially for the material designers, as well as dentists who can take advantage of these approaches.
MSI: So then, 3M will be able to make these materials so that a dentist will be able to build teeth based on the individual patient? Very individualized treatment?
RD: Once he’s got an equation, he needs the optical properties of the materials. 3M would measure those optical properties, and they would be plugged into some sort of a device, possibly a portable spectrometer. The device would capture an image of the tooth, and tell you what material to use. It makes it very easy to identify the right material and how to use it. The device would consider the multiple structures within the tooth, possible filling materials, and guide the dentist how to use them.
This optical information could lead manufacturers to changing colors of the restorative materials. Right now, the colors of the restorative materials are a shot in the dark (no pun intended). They provide a range, but they may not be the best colors to match natural teeth.
One other thing I should mention, Jin Woo gets most of his data from the MDRCBB microCT scans, which come out as shades of gray; it’s literally just a bunch of numbers. When he gets through, and you look at the image, it looks just like a natural tooth. There have been NIH SBIR [Small Business Innovation Research] grants given to do something like this, to render a tooth in a dental atlas. The way they had to do it was to take pictures of teeth as they slowly rotated them through different viewing angles, then they combined the pictures to get an apparent 3D image that could be rotated in space; a very tedious process. When you put those pictures up against Jin Woo’s, there’s no difference. And yet his technique is all mathematics with some physics.
Posted on November 13, 2013.
In the past few decades, researchers specializing in condensed matter physics have been intensely interested in studying the properties of strongly correlated quantum many-body systems, which are systems on the microscopic scale that include more than two particles that interact with each other. Researchers especially want to be able to explain how the macroscopic behavior of materials can be explained by the fundamental interactions of the material’s microscopic constituents.
Particular interest has focused on quantum spin-lattice systems, where the interactions can be simply described, but where different types of interaction can compete with one another. (The “spin” of an elementary particle is an inherent property it possesses in quantum mechanics that has no classical counterpart, although it can loosely be thought of as a form of internal angular momentum. In a standard computational model in condensed matter physics, these spins are arranged on a lattice and coupled through magnetic interactions.) The system can then find itself in a frustrated state in which different forms of ordering are trying to emerge in competition with one another. The often subtle interplay between this frustration and quantum fluctuations can lead to quantum spin-lattice models exhibiting ground-state (i.e., at zero temperature) phase diagrams that are very different from their classical counterparts. Of greatest theoretical interest are the so-called quantum critical points where phase transitions occur.
Computer models for quantum-mechanical wave functions of strongly interacting many-spin systems are extremely complex. This is especially so near the quantum critical points, where the ground-state phase has special properties and involves a very large set of fluctuating configurations. Methods for modeling these systems have included techniques from quantum field theory or large-scale numerical simulations such as Monte Carlo methods. Since the effects of quantum fluctuations, and hence the complexity of the wave functions, increase the closer one approaches the quantum critical points, very accurate quantum many-body techniques are necessary.
Professor Charles Campbell (Physics and Astronomy) and his colleagues Professor Raymond Bishop and Dr. Peggy Li at the University of Manchester (UK), and their collaborators elsewhere, have developed and adapted one such many-body method, the so-called coupled cluster method (CCM), to study a large and diverse array of two-dimensional (2D) quantum spin systems of theoretical and experimental interest. The CCM is now widely accepted as being one of the most successful and most widely applicable of all modern methods of microscopic quantum many-body theory. The CCM techniques pioneered by Professor Bishop and his collaborators are probably now the best available for these strongly frustrated 2D quantum spin-lattice systems, and their results are now setting benchmarks in the field. The group runs their CCM codes on MSI’s supercomputers. The interesting magnetic phenomena displayed by such systems make them suitable candidates for a large number of technological applications, many of which are already in widespread use. This research is also providing insights into exciting new systems, such as exotic superconducting systems and non-superconducting systems that have unusual magnetic properties.
Professor Bishop’s contributions to the development and applications of the CCM resulted in his sharing the Eugene Feenberg Memorial Medal in 2005. (The Feenberg Medal is awarded for work that significantly advances the field of many-body physics.) His co-recipient, the late Hermann Kuemmel, is generally acknowledged as the inventor of the CCM. Among other contributions, Professor Bishop has adapted the CCM to several important quantum many-body systems, including the quantum magnetism problems described above. Professor Campbell, who is a long-time researcher at MSI in the field of quantum fluid research, has been working with Professor Bishop and his former student and now post-doctoral associate Dr. Li, adding his own area of expertise to the CCM. Bishop, Li, and Campbell have used MSI resources for several years in their work to advance this technique.
Approximately 10 papers describing this research using MSI have been published since the start of 2012. These have appeared in the journals Physical Review B, Journal of Physics: Condensed Matter, and the European Physical Journal B. Two of the articles were chosen by the editors for special highlighting. The group has especially concentrated on the spin-1/2 J1-J2-J3 model on the honeycomb lattice and the spin-1/2 J1-J2 model on the checkerboard lattice (otherwise known as the anisotropic planar pyrochlore), which have recently become very hot topics in the field.
Image Description: Phase diagram of the spin-1/2 J1−J2 model on the honeycomb lattice (with J1 > 0 and x ≡ J2/J1 > 0), as obtained by a CCM analysis. The four phases shown are Néel, plaquette valence-bond crystalline (PVBC), staggered dimer valence-bond crystalline (SDVBC), and Néel-II. The quantum critical points (phase transitions) are at xc1 ≈ 0.207(3), xc2 ≈ 0.385(10), and xc3 ≈ 0.65(5), as shown in the diagram. From “Valence-bond crystalline order in the s = 1/2 J1−J2 model on the honeycomb lattice,” R.F. Bishop, P.H.Y. Li and C.E. Campbell, Journal of Physics: Condensed Matter 25:306002, DOI=10.1088/0953-8984/25/30/306002 (2013) ©2013 IOP Publishing Ltd.
Posted on October 30, 2013.
As the earth’s climate changes, scientists are concerned about the effects these changes will have on the earth’s ecosystems. Regents Professor Peter B. Reich (Forest Resources, Institute on the Environment) and post-doctoral researcher Emily Peters (Institute on the Environment) specialize in discovering the impacts of these changes. The Reich group’s main area of focus is the part of central North America that includes Minnesota. In this part of the continent, several types of ecosystems converge, including boreal forests (forest consisting mostly of coniferous trees), temperate hardwood forests, oak woodlands/savannas, and grasslands.
MSI has been working with Professor Reich and Dr. Peters to develop a distributed computing framework for the parallel photosynthesis and evapotranspiration model (PPnET). PPnET allows researchers to efficiently use PnET-CN (a widely used and well-tested ecosystem model) to simulate the effects of many simultaneously changing environmental factors on forests over large geographic areas. MSI is providing hardware, software, and consulting support to this project. Dr. Shuxia Zhang, in the HPC Operations group, developed an MPI-based program that allowed parallel jobs to start and restart flexibly. This made allowances for the availability of software licenses at any given time, as well as the availability of compute nodes on the supercomputers. Dr. Zhang also developed script tools that verified data integrity and the success of hundreds of thousands of inputs.
The Reich group used PPnET to simulate ecosystem responses to changes in climate and atmospheric CO2 concentrations in the Great Lakes region of North America. This simulation had 1 km spatial resolution, consisting of 200,000 forest grid cells. The computing time, which would take 25 days for serial runs – and would therefore be impractical – was reduced to six hours using 96 cores on a Linux cluster. This research has been published in the Canadian Journal of Forest Research (“Potential Climate Change Impacts on Temperature Forest Ecosystem Processes,” EB Peters, K Wythers, S Zhang, JB Bradford, PB Reich, Canadian Journal of Forest Research, DOI:10.1139/cjfr-2013-0013, published online July 17, 2013.)
(Left) Forest types with a 1-km grid resolution over the northern Great Lakes region of the United States, also known as the Laurentian Mixed Forest Province. This region includes six major forest types.
(Right) Map of changes in above-ground net primary production predicted from 1970 to 2100, under a high-emissions climate change scenario.
©Canadian Journal of Forest Research, NRC Research Press (2013)
posted on October 9, 2013.
Update, October 28, 2013: The Minneapolis Star-Tribune published an article about climate change's effects on the north woods of Minnesota. Professor Reich and his project, B4WARMED, are discussed at the end of the article.
In 2004, the U.S. Department of Homeland Security established the National Center for Food Protection and Defense (NCFPD), headquartered at the University of Minnesota. This research consortium, which includes researchers from over 30 other universities, addresses the vulnerability of the nation’s food supply to attack. The NCFPD’s research and education mission is to reduce the potential for contamination of the food supply and to mitigate the potentially catastrophic public-health and economic effects of such an event. Dr. Amy Kircher, who has been working with MSI on the projects described below, was recently appointed Director of the NCFPD.
One threat to the food supply is called “economically motivated adulteration” or EMA. This type of food adulteration, in which less-expensive products are added to or substituted for higher-priced food in order to increase profits, has occurred for centuries. Recent examples from the news include the selling of horsemeat as beef in Europe, and, in China, adding melamine as an extender to milk and wheat gluten. The NCFPD is working to better understand the food supply, identify high-risk supplies, and do real-time analysis using a variety of data.
One of the agencies with which NCFDP works is U.S. Customs and Border Patrol, which monitors shipments of food into the U.S. MSI was contacted by NCFPD to develop tools that they could use to store and analyze this data. This project, which was completed earlier this year, included:
A database to store the CBP data and a tool to automatically import new data.
A private web interface that allows NCFPD researchers to select and perform statistical analyses on selected datasets using their own algorithms. The interface generates plots and other summaries.
Visualizations of imported food using the Google Maps API. Researchers can select countries and food products to see what is entering the country. They can view entry to specific ports or the view can show a single connection to the country exporting the food with a line thickness proportional to the weight of the imported food. The graphic above shows importation information for apples.
MSI is also providing hosting services for the application. MSI is also currently working with the NCFPD on a further project to extend these tools and capabilities.
These projects are part of an MSI program that provides long-term, at-cost, individualized support for projects meeting certain criteria. Information can be found on the MSI website.
Posted on September 25, 2013.
Polymers, which many people think refer only to plastic, are actually a large group of natural and man-made materials. They have many uses in industry and as consumer products. Some of these include the super-absorbant polymers used in disposable diapers, the heat-stable materials used for non-stick cookware, and the fiber spandex, used for stretchy clothing like athletic wear and foundation garments. The research group of Associate Professor Kevin Dorfman (Chemical Engineering and Materials Science) is using MSI for research into the structures and dynamics of polymers, especially DNA. On the engineering side, understanding the dynamics of DNA has many important applications in genomics. On the scientific side, DNA is a model system for investigating the basic physical properties of semiflexible polymers. The Dorfman group is using several computer-simulation techniques, including Metropolis and chain growth Monte Carlo methods.
Professor Dorfman and graduate student Douglas Tree, along with their colleague Yanwei Wang (Soochow University, China), recently published research in Physical Review Letters concerning DNA confinement in nanochannels. DNA confinement is becoming an important tool for genomics research. It also provides researchers a platform for testing theories concerning confined wormlike polymers. The classical theories for polymer chains in confinement only work in cases where the nanochannels are very small or very large compared to the polymer. The Dorfman group has investigated an intermediate case between the two models. In the graphic above, the Odijk theory applies to DNA in small channels and the Flory-de Gennes theory works for large channels. The newly proposed regime of behavior, called the “Gauss-de Gennes” regime by the researchers, works for the intermediate channel sizes that have been typically used in genomic devices. The researchers propose that this regime applies to the general class of semiflexible polymers, which includes DNA as a special case.
The article can be read on the American Physical Society website: “Extension of DNA in a Nanochannel as a Rod-to-Coil Transition,” DR Tree, Y Wang, DK Dorfman, Physical Review Letters, 110:208103, DOI:10.1103/PhysRevLett.110.208103 (2013).
Image description: Illustration of the analogy between free solution and confined configurations of a wormlike chain. The classical theories renormalize the chain into a series of subchains, where these subchains are either rodlike (Odijk) or excluded-volume blobs (de Gennes). (For clarity, the authors refer to the classic de Gennes regime as the “Flory-de Gennes” regime.) The middle drawing illustrates a universal Gauss-de Gennes regime in confinement that is an intermediate step between the two classical ones. © 2013 American Physical Society
Posted on September 11, 2013.