The development of radio wave technologies in recent years has led to their increasing use in a growing range of applications. Ubiquitous network technologies are expected to become widespread, with people anticipated to wear various data processing terminals on various parts of their body for communicating wirelessly. There is, however, a growing concern about the possible health hazards of the radio waves that are emitted from these wireless communications devices.
Japan has established safety guidelines on radio waves to prevent their adverse effects on the human body, and to allay people’s anxieties about them. These guidelines are based on the thermal effect that is created as the radio energy is absorbed by the human body. As one indicator for this, the amount of electromagnetic power absorbed per unit mass of a biological body, or Specific Absorption Rate (SAR), is used. Therefore, to study the safety of radio waves, it is necessary to precisely evaluate the SAR inside the body.
Absorption of radio waves strongly depends on the complex shape of the human body and the heterogeneous structure of the inner tissues. NICT has therefore developed whole-body voxel human models that simulate the complex structure of the human body and conducts numerical simulations to detailed dosimetry.
Development of whole-body voxel human models
To evaluate the safety of radio waves, it is necessary to make precise evaluations of SAR within the human body. SAR changes in a complex fashion, depending on the shape of the human body and the heterogeneous structure of the internal tissues. However, it is next to impossible to measure SAR directly within the human body, which was why assessments could thus far be made by using numerical human models or experimental phantoms with simple shapes and approximated structures.
Recent advances in medical diagnostic technologies have enabled the development of a precise whole-body voxel human model, based on accumulated anatomical image data such as MRI.
Development has been under way at NICT to create high-precision whole-body voxel human models that match the average body shapes of the Japanese. They represent a considerable step forward in this discipline, and are already in wide use in research in diverse fields both in Japan and other countries.
Japanese adult male and female whole-body voxel human models
Whole-body voxel human models depict the shape of the human body (tissues and organs) as an aggregate of minute elements. Each minute block, or voxel, is labeled with a tag representing the name of a tissue or an organ such as muscle or fat, and assigned with the electrical properties of the respective tissue/organ. By applying electrical constants that correspond to a specific tissue or organ, it is possible to simulate how radio waves are absorbed by the human body.
Under collaborative efforts undertaken with Kitasato University, Keio University, and Tokyo Metropolitan University, NICT has developed Japan’s first whole-body voxel human models that have the average body shape of Japanese adult men and women. Named Taro (male) and Hanako (female), these models possess 2 mm spatial resolution and 51 tissues and organs, based on the accumulated MRI images of adult Japanese volunteers of average build (height and weight). After the whole-body voxel human model were developed, they underwent medical supervisions and analyses of anatomical data. The results showed that these human-body model databases were superior to those that had been developed in the past in terms of spatial resolution and the number of tissues and organs identified, and that the dimensions of various parts of the body as well as organ weights were close to those of average Japanese people. Incidentally, Hanako, the female model, is the world’s first whole-body voxel model to have spatial resolution in millimeter units.
These models are not merely used to investigate the interaction between radio waves and the human body. By defining the elastic, radiation absorption, and other parameters specific to each tissue or organ, they can be adapted for numerical simulations in various fields of research. For example, the model may be used for simulation tests of damage suffered by passengers in crash tests, and for drawing up radiation treatment plans for cancer patients.
At present, these models are available to the public, both free of charge (for non-commercial and/or research purposes) and for a fee (for commercial purposes).
| Organization | Sex | Height (cm) | Weight (kg) | Num. of Tissues | Voxel Size (mm) |
|---|---|---|---|---|---|
| NICT (TARO) | M | 173.2 | 65 | 51 | 2×2×2 |
| NICT (HANAKO) | F | 160.8 | 53 | 51 | 2×2×2 |
| HPA (NORMAN) | M | 176.0 | 73 | 37 | 2×2×2 |
| HPA (NAOMI) | F | 163.0 | 60 | 41 | 2×2×2 |
| Utah Univ. | M | 176.4 | 71 | 29 | 2×2×3 |
| Victoria Univ. | M | 177.0 | 76 | 34 | 3.6×3.6×3.6 |
| Brooks AFB | M | 187.1 | 105 | 43 | 1×1×1 |
Child voxel models
The development of radio wave technologies in recent years has led to their increasing use in a growing range of applications. Dosimetry of children’s exposure to radio frequency radiation emitted from these devices has become an important research challenge in this field. However, for a variety of reasons, it has proved difficult to develop a child model with a similar level of quality as the adult whole-body voxel human models with internal structures.
To overcome this problem, NICT has developed a method of creating a whole-body child voxel model that precisely reconstructs a child’s average body, by three-dimensionally modifying the adult model. This modification method makes it possible to effectively develop not only the child models but also other human models, such as pregnant women.
Modification of the adult models
The proportion of various parts of the body vis-à-vis height differs significantly between adults and children.
NICT therefore took measurements of various parts (66 parts) of the bodies of children aged around 3, 5, and 7, and, based on these measurement values, modified the adult male voxel model, Taro, to match the measurement values corresponding to those of 3-, 5-, and 7-year-olds.
The adult male voxel model Taro was modified to create a child’s physique by using a Free-form Deformation algorithm.
Whole-body voxel human models with a child’s physique
Pregnant woman voxel models
To simulate how the radio waves emitted from devices, etc., behave inside a pregnant woman’s body and fetus and affect them, we have jointly developed with Chiba University a whole-body voxel human model of a Japanese woman in the 26th week of pregnancy. At present, this model is being distributed free of charge for non-commercial and/or research purposes.
A fetus voxel model (including tissues specific to pregnant women)
A fetus model created from a woman’s abdominal MRI image
Resolution: 0.742 x 0.742 x 0.742 mm
Tissues: 6 types (fetus, fetal head, fetal eyeballs, amniotic fluid, placenta, uterine wall)
Abdomen of an adult female voxel model modified to match her physique during pregnancy
Free-Form Deformation is used to modify the abdomen of the adult female voxel model Hanako.
Pregnant woman voxel models
Pregnant woman voxel models are being developed by combining a fetus model and a female model whose abdominal area has been modified. Efforts are currently under way to develop whole-body voxel models, not only of women in the 26th week of pregnancy but also of various other stages of pregnancy. As with the other models, we plan to make them available to researchers.
Week of pregnancy: 26th week
No. of tissues: 56 types
Resolution: 2 x 2 x 2 mm
Abdominal shape: Matches the average value
Posture-independent voxel models
The whole-body human voxel models developed by NICT simulate humans in a standing position.
This standing-position voxel human-body models can be modified to resemble the more realistic positions that people take, such as while actually using wireless communications devices. A variety of postures can be created by taking the 19 parts, divided according to the human-body model’s joints, and moving them in any directions using the Free-form Deformation feature.
Examples of posture-independent voxel models
Sitting on a chair
Walking
Numerical simulation using whole-body voxel human models
High-resolution whole-body voxel human models with anatomical structures, and models that simulate the body tissues in a simple fashion, are employed to estimate the behaviors of radio waves inside the human body and SAR. At NICT, our studies focus on the frequency bands that are mainly used in wireless communications, from several ten kHz to several hundred GHz. Particular emphasis is being placed on studies either in the VHF/UHF bands used at broadcasting stations and mobile phones, or in the intermediate frequency range used in RF-ID devices and IH cooking equipment.
Analysis of the VHF/UHF bands (30 MHz – 3 GHz)
Analysis of SAR distribution during plane-wave exposure
The finite-difference time-domain (FDTD) method is used for analyzing SAR distribution in human bodies exposed to plane waves in the VHF band (30 MHz ~ 300 MHz) and the UHF band (300 MHz ~ 3 GHz). The example here shows the SAR distribution (surface) when the adult male and female whole-body voxel models are exposed from the front to vertical polarized plane waves (1 mW/cm²). In this Figure, areas with extensive radio wave absorption are indicated in red, and those with little or no absorption in blue.
This analysis requires a long calculation time and huge amounts of memory, so a supercomputer is employed.
Higher-precision analysis using even higher-definition models (with 2 mm resolution or more) is recently becoming increasingly feasible, thanks to enhanced computer processing capabilities and larger-scale memory.
Analysis of the SAR distribution in the human head during mobile phone use
A detailed characterization of SAR distribution in the human head during mobile phone use is carried out, using numerical simulations employing the head model. We make use of the advantages of the Method of Moments (MoM) technique suitable for establishing a model of an electromagnetic wave source, and that of the FDTD method suitable for establishing a model of the body tissue, to develop the FDTD/MoM-hybrid method that analyzes interactions between the source and the body, and carry out detailed computation of the electric field inside the human head during mobile phone use.
Analysis of SAR during mobile phone use inside an elevator
The SAR distribution inside a human body when a mobile phone is used inside an elevator is evaluated, using an anatomical model (modeling a mobile phone with a dipole antenna). Although this computation uses a supercomputer, it takes the size of the actual elevator into consideration. Therefore, if the overall elevator is divided into 2-mm blocks, like in the human body, computation becomes difficult, even with a supercomputer, requiring a large-scale memory. We therefore used the FDTD method using a non-uniform mesh to save memory, enabling computation by a supercomputer.
Analysis of the intermediate frequency band (300 Hz – 10 MHz)
Use of radio waves in the intermediate frequency range (from 300 Hz to 10 MHz) has been increasing in recent years, such as with RF-ID, EAS (Electronic Article Surveillance) system, and IH (induction heating) cooking hobs. However, not much is known about the effects of intermediate-frequency band radio waves on living organisms, and there is a need to accumulate scientific evidence to assess their impact. Regarding the relevant frequency range, the International Commission on Non-Ionizing Radiation Protection, or ICNIRP, has established basic restrictions on induced current density. At NICT, we use a variety of human-body voxel models, including those of children, to analyze induced current density when exposed to magnetic field in the intermediate frequency band.
