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From Schematics to Real People
With a nod to its early ancestors, including a tank of water and Karl, the phantom torso, Xu created VIP-Man using a comprehensive set of images collected from a cadaver. The image collection, known as the Visible Human Project, was an ambitious project undertaken by the National Library of Medicine in the late 1980s and was made available to the public in the mid-’90s. “Here was this huge dataset out there,” says Xu, “and not too many people had clear ideas about the potential engineering applications of this invaluable set of anatomical data.”

Xu did. As someone who did his doctoral research using simple geometric shapes to represent the human body in the early 1990s, he knew exactly how to take advantage of a realistic and descriptive dataset. Picture the difference between a preschooler’s stick drawing and Leonardo DaVinci’s Vitruvian Man. Xu rendered his “A-ha” moment into a prestigious Faculty Early Career Development Award (CAREER) from the National Science Foundation in 1999 and successfully used the four-year funding to make his virtual patient, painstakingly combining precise organ anatomy with computer codes simulating the movement of radioactive particles through the body. Although Xu has subsequently received various grants from NIH, the Department of Energy (DOE), and the nuclear industry, he says “it was the CAREER award that really validated my original ideas and gave me the freedom and confidence to pursue innovative research.”

Xu and his two doctoral students had to be innovative because they were the first to try to simulate radiation in a whole-body image set with so many voxels in it. In addition to a complex geometry, the human body has tissues that differ in density and atomic composition that affect the travel of radioactive particles. A tank of water was once used to approximate living tissue — humans are more than 50 percent water. Next came high-tech plastic, such that Karl is composed of three “tissue-equivalent” materials to model the density and composition of bone, soft tissue, and lung.

VIP-Man trumps both methods by accounting for dozens of tissues with regard to density and composition, not to mention precise scale and anatomical shape. In addition, VIP-Man provides critical new insight into such tissues as skin, gastrointestinal track mucosa, eye lenses, and red bone marrow, which are particularly sensitive to radiation but were too small to be modeled by physical phantoms or previous computer simulations. Xu is one of the few in the world who have successfully combined the Monte Carlo codes (those used to simulate nuclear weapons) to whole-body human models like the VIP-Man. VIP-Man still holds the record for having the largest number of voxels ever used for radiation simulations.

After publishing a series of papers on radiation protection of workers using VIP-Man from 2000 to 2002, Xu turned to medical applications. Radiation is employed in a variety of diagnostic and therapeutic procedures including computed tomography (CT) scanning, nuclear medicine, and radiation treatment. Calculating doses to different parts of the patient body in each of these procedures accurately has long been difficult, risking either doses that are too high, which cause side effects, or doses that are too low to effectively treat a tumor. Modeling specific medical modalities and how the radiation interacts with VIP-Man will provide never-before-seen scientific data to optimize the benefit-to-risk ratios of these procedures to patients.

Virtual Patients
Since his development of VIP-Man, Xu has nurtured collaborations with medical researchers at Vanderbilt University, the University of Florida, and Massachusetts General Hospital to use virtual patients to further medical diagnostics and therapeutics. NIH’s study section that reviewed his proposal has been so impressed by the ideas and the multi-center collaboration that NIH has awarded Xu and his collaborators $2.1 million of funding over three years to study clinical applications and to expand the virtual subject population. Xu has been invited to serve on a study section for the Biomedical Information Science and Technology Initiative (BISTI) at NIH.

Brian Wang, a doctoral student who graduated in May from Rensselaer, lends some insight into how Xu manages to attract such ambitious collaborations. When attending a national conference, students in Xu’s lab were instructed to meet
20 new people. “Otherwise, it’s a failure in attending such a national conference,” says Wang, who has accepted an offer as a clinical medical physics faculty member at Cooper University Hospital in New Jersey. For a research career, interacting with the right people can be more important than learning the results of the presentations at a conference, Xu believes. Wang says Xu led by example, illustrating for his students the value of networking. “It’s his style,” says Wang, and it’s why he has such “tremendous connections within the scientific community.”

Wesley Bolch, professor of nuclear and biomedical engineering at the University of Florida, describes a scientific session at the recent international conference called Monte Carlo 2005 that Xu organized last April in Chattanooga, Tenn., giving him credit for bringing “the world community” together. Xu planned the meeting with Dr. Keith Eckerman of Oak Ridge National Laboratory, a world authority on radiation dosimetry, by sending personal invitations to researchers as far away as Japan, Korea, China, Australia, and Europe. Although they have read each other’s papers on radiation human modeling, “this group of researchers — doing this kind of work — had never really met in one room before,” says Bolch. The meeting was a scientific success as well, a tribute to Xu’s “personal character, his initiative, and his organization skills,” Bolch says.


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Rensselaer (ISSN 0898-1442) is published in March, June, September, and December by the Office of Communications.

 
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