Computational human phantoms are models of the human body used in computerized analysis. Since the 1960s, the radiological science community has developed and applied these models for ionizing radiation dosimetry studies. These models have become increasingly accurate with respect to the internal structure of the human body.
As computing evolved, so did the phantoms. Graduating from phantoms based on simple quadratic equations to voxelized phantoms, which were based on actual medical images of the human body, was a major step. The newest models are based on more advanced mathematics, such as non-uniform rational B-spline (NURBS) and polygon meshes, which allow for 4-D phantoms where simulations can take place not only 3-dimensional space but in time as well.
Phantoms have been developed for a wide variety of humans, from children to adolescents to adults, male and female, as well as pregnant women. With such a variety of phantoms, many kinds of simulations can be run, from dose received from medical imaging procedures to nuclear medicine. Over the years, the results of these simulations have created an assortment of standards that have been adopted in the International Commission on Radiological Protection (ICRP) recommendations.Xu, X.G.; Eckerman, K.F. Handbook of Anatomical Models for Radiation Dosimetry. Taylor & Francis, 2010. .
Stylized (first-generation) computational phantoms
The very first generation computational phantoms were developed to address the need to better assess organ doses from internally deposited radioactive materials in workers and patients. Until the late 1950s, the ICRP still used very simple models.ICRP. Report of Committee II on Permissible Dose for Internal Radiation International Commission on Radiological Protection (Oxford: Pergamon Press), 1959. In these calculations, each organ of the body was assumed to be represented as a sphere with an "effective radius". The radionuclide of interest was assumed to be located at the center of the sphere and the "effective absorbed energy" was calculated for each organ. Phantoms such as the Shepp-Logan Phantom were used as models of a human head in the development and testing of image reconstruction algorithms. However, scientists attempted to model individual organs of the body and ultimately the entire human body in a realistic manner, the efforts of which led to stylized anthropomorphic phantoms that resemble the human anatomy.
In general, stylized computational phantom is a mathematical representation of the human body which, when coupled with a Monte Carlo radiation transport computer code, can be used to track the radiation interactions and energy deposition in the body. The feature of stylized computational phantom is finely tuned by adjusting individual parameters of the mathematical equations, which describes the volume, position, and shape of individual organs. Stylized computational phantom has a long history of development through the 1960s to 1980s.
MIRD phantom
The MIRD phantomReport of the Task Group on Reference Man: ICRP Publication 23. was developed by Fisher and Snyder at Oak Ridge National Laboratory in the 1960s with 22 internal organs and more than 100 sub-regions.Fisher, H. L. J. and Snyder, W. S. "Variation of dose delivered by 137Cs as a function of body size from infancy to adulthood." ORNL-4007 (Oak Ridge, TN: Oak Ridge National Laboratory), P. 221, 1966.Fisher, H. L. J. and Snyder, W. S. "Distribution of dose delivered in the body size from a source of gamma rays distributed uniformly in an organ", ORNL-4168 (Oak Ridge, TN: Oak Ridge National Laboratory), p. 245, 1967. It is the first anthropomorphic phantom representing a hermaphrodite adult for internal dosimetry.
Phantoms derived from MIRD
Based on MIRD phantom, many derivations of phantoms were developed for the following decades. The major types of phantom include: stylized "Family" phantom series developed in the 1980s by Cristy and Eckerman; "ADAM and EVA" developed by GSF, Germany; CAM (Computerized Anatomical Man) phantom developed by NASA unknown by the mainstream radiation protection dosimetry community, etc.
Limitation on stylized phantom
Although many efforts were undertaken to diversify and extend its applications in radiation protection, radiation therapy, and medical imaging, one cannot overcome its inborn limitation. The representation of internal organs in this mathematical phantom was crude, by capturing only the most general description of the position and geometry of each organ. With the powerful computer and tomographic imaging technologies became available in the late 1980s, the history launched a new era of voxel phantoms.
Voxel (second-generation) phantoms
The stylized phantoms provided only basic information with a large degree of error. More accurate methods of simulating a human body were necessary to advance. To allow further research, the computer technology had to become more powerful and more readily available. This did not occur until the 1980s. The real breakthrough occurred when computed tomography (CT) and magnetic resonance imaging (MRI) devices could generate highly accurate images of internal organs in three dimensions and in digital format. Researchers discovered that they could take that diagnostic data and transform it into a voxel (volumetric pixel) format, essentially re-creating the human body in digital form in 3D. Today there are over 38 human phantoms in voxel format, for many different uses.Zaidi, H. and Xu, X.G. (2007). "Computational anthropomorphic models of the human anatomy: The path to realistic Monte Carlo modeling in radiological sciences", Annu Rev Biomed Eng. 9, p. 471.
Challenges for implementation
Two major issues with development of the reference phantoms are difficulty in obtaining useful images and handling the large amount of data created from these images. CT scans give the human body a large dose of ionizing radiation – something the computational phantom was designed to circumvent in the first place. MRI images take a long time to process. Furthermore, most scans of a single subject cover only a small portion of the body, whereas a full scan series is needed for useful data. Handling this data is also difficult. While the newer computers had hard drives large enough to store the data, the memory requirements for processing the images to the desired voxel size were often too steep.
Basic development process of a voxel phantom
While there have been many voxel phantoms developed, they have all followed a similar path to completion. First, they must obtain the raw data, from CT scans, MRI imaging, or direct imaging through photography. Second, the components of the body must be segmented, or identified and separated from the rest. Third, the density of each component must be identified, along with the composition of each. Lastly, the data must be unified into a single 3D structure so it may be used for analysis.
Early developments
The earliest work on voxelized phantoms occurred independently at about the same time by Dr. Gibbs, of Vanderbilt University, and Dr. Zankl at the National Research Center for Environment and Health (GSF) in Germany.Gibbs, S. and Pujol, J. (1982). "A Monte Carlo method for patient dosimetry from diagnostic x-ray." Dentomaxillofac Radiol. 11, p. 25.Zankl, M. et al. (1988). "The construction of computer tomographic phantoms and their application in radiology and radiation protection." Radiat Environ Biophys, 27, p. 153. This occurred about 1982. Dr. Gibb's work started with X-ray images, not CT or MRI images, for the reconstruction of a human phantom which was used for medical dose simulations. M. Zankl and team did use CT imaging to create 12 phantoms, ranging from BABY to VISIBLE HUMAN.
Advancements in voxel phantom design by country
United States Dr. Zubal and team at Yale University developed the VoxelMan phantom in 1994.Zubal, I.G. et al. (1994). "Computerized three-dimensional segmented human anatomy." Med Phys, 21, p. 299. This original phantom was complete only from head-to-torso, and was designed specifically for improving nuclear medicine. Since its initial development it has been improved to include arms and legs to represent a complete human body, and a dedicated head has been completed which delineates the small internal sub-structures of the brain. In 2000, Dr. George Xu and two students at Rensselaer Polytechnic Institute (RPI) created the VIP-Man phantom from data retrieved from the National Library of Medicine's (NLM) Visible Human Project (VHP).Xu, X.G., Chao, T.C., and Bozkurt, A. (2000) "VIP-Man: An image-based whole-body adult male model constructed from color photographs of the Visible Human Project for multi-particle Monte Carlo calculations." Health Phys, 78, p. 476. This phantom was the most complex model to date, with over 3.7 billion voxels. This model was used in many studies concerning health physics and medical physics. Dr. Bolch and team at the University of Florida created a set of pediatric phantoms from 2002 to 2006.Lee, C. et al. (2006). "Whole-body voxel phantoms of paediatric patients—UF Series B." Phys Med Biol, 51, p. 4649. Child computational phantoms had been severely underrepresented until this point. The team developed models ranging from newborn to mid-teens. The U.S. Food and Drug Administration (FDA), in collaboration with the Foundation for Research on Information Technologies in Society (IT'IS) and other partners, developed the surface-based Virtual Family (VF) body phantoms for electromagnetic exposure and thermal evaluations. The four VF models are based on high-resolution magnetic resonance imaging (MRI) data of healthy volunteers (female and male adults and children). Switzerland The four VF full body phantoms developed by Prof. Niels Kuster and his team at the IT'IS Foundation in Zurich in collaboration with the U.S. FDA correspond to a 34-year-old adult male, 26-year-old female, an 11-year-old female, and a 6-year-old male – were based on full-body magnetic resonance imaging (MRI) scans of healthy volunteers. The original VF was later expanded with four more juvenile models of the Virtual Classroom. More models – including an elderly male and an obese male – were later added to form the Virtual Population (ViP). Brazil Dr. Kramer in Brazil improved on the data from the Yale Zubal team in an attempt to create a phantom similar to International Commission on Radiological Protection requirements, and created the MAX phantom.Kramer, R. et al. (2003). "All about MAX: A male adult voxel phantom for Monte Carlo calculations in radiation protection dosimetry." Phys Med Biol, 48, p. 1239. United Kingdom The NORMAN phantom was developed by a team led by Dr. Dimbylow.Dimbylow, P.J. (1996). "The development of realistic voxel phantoms for electromagnetic field dosimetry," in Proceedings of the Workshop on Voxel Phantom Development, Chilton, U.K. This was created by analyzing magnetic resonance images of a human male in 1996. In 2005, the team created a female phantom. Australia At Flinders University, Dr. Caon and team created a torso phantom to simulate a teenage girl in 1999.Caon, M., Bibbo, G., and Pattison, J. (1999). "An EGS4-ready tomographic computational model of a 14-year-old female torso for calculating organ doses from CT examinations." Phys Med Biol, 44, p. 2213. The name of the phantom was ADELAIDE. This was the only teenage female phantom for a number of years. Japan The first Asian phantom was developed by Dr. Saito and team at the Japan Atomic Energy Research Institute (JAERI) in 2001.Saito, K. et al. (2001). "Construction of a computed tomographic phantom for a Japanese male adult and dose calculation system." Radiat Environ Biophys, 40, p. 69. This was primarily used for radiation dosimetry studies.