Patent Application
20060114003
1. Field of the Invention
The present invention relates to a biological tissue equivalent phantom unit (phantom unit) used by a specific absorption rate measuring system for evaluating absorption of electromagnetic wave energy; a specific absorption rate measuring system using the phantom unit; and a method thereof.
2. Description of the Related Art
In recent years and continuing, requirements are increasing for quantitatively evaluating a thermal effect caused by an electromagnetic wave emitted by a wireless radio transmitter; and a system that is capable of accurately and swiftly measuring a specific absorption rate (SAR), which is an index of the reaction of the electromagnetic wave on a living body, such as a human, is desired.
A SAR value is proportional to an electric field (|E|2), and is often used for evaluating the energy absorbed by a human body when a cellular phone is used near the human body, SAR being defined by the following Equation 1.
SAR=σ|E|2/ρ [Equation 1]
Here, σ and ρ represent conductivity [S/m] and density [kg/m3], respectively, of the biological tissue equivalent phantom.
Usually, when measuring SAR, an electric-field measuring method is used, wherein a short dipole detects an electric field generated in a medium (live body), which electric field is converted into SAR using Equation 1.
Here, the electric field generated in the phantom is measured by arranging a measuring target instrument 108, such as a cellular phone, near the specific absorption rate measuring system 100 as shown in
The electric field generated in the phantom is measured by arranging a measuring target instrument 127, such as a cellular phone, near the specific absorption rate measuring system 200 as shown in
In either of the conventional examples, the probe 103 or 122, as applicable, for detecting the electric field is required. Each of the probes 103 and 122 for detecting the electric field includes a detecting element 110 as shown in detail on the right-hand side of
However, since the short dipole antenna and the high resistance wires, both being conductive, are present in the electric field to be measured, the electromagnetic field distribution near the detecting element 110 is disturbed. This is a problem of the electric-field measuring method. Further, since it is difficult to reduce the length of the dipole elements 111, 112, it is expected that the disturbance will become greater as the frequency becomes higher.
Then, in an attempt to reduce the disturbance to the electromagnetic field generated by the measuring target 108, 127 (e.g. a cellular phone), the disturbance being due to the probe 104, 122 for detecting the electric field, an electric-field sensor 300 using an optical waveguide type modulator and a laser beam has been developed as shown in
The electric-field sensor 300 includes a laser luminous source 131, an electric-field probe 132, an optical waveguide type modulator 133, a minute dipole 134 that consists of metal, and an optical receiving unit 135.
Since the electric-field sensor 300 is configured only by dielectric materials, except for the minute dipole 134, it is capable of measuring the electric field with a precision that is higher than the electric-field detecting methods that use the high resistance wires.
[Problem(s) to be Solved by the Invention]
Nevertheless, since the short dipole is used according to the electric-field measuring method using the electric-field sensor 300, wherein the optical waveguide type modulator and the laser beam are used, the disturbance remains, although the disturbance becomes smaller than in the case of the electric-field measuring methods using the high resistance wires. Further, since the probe for detecting the electric field, or a 3-dimensional electric-field sensor, is moved in the liquid phantom for measuring SAR, the liquid (a phantom solvent) is agitated, and noise is generated by vibration of the probe or sensor. If a time until the solvent settles into a steady state is waited for in order to avoid the noise, measurement will take a long time. If two or more electric-field sensors are arranged in two dimensions or three dimensions in the phantom in order to shorten the measuring time, the aggregate of the sensors (short dipoles) will behave as a conductor, and will generate a great disturbance to the electromagnetic field to be measured. Consequently, a SAR distribution that is different from actual may be measured, which is a problem.
The present invention provides a specific absorption rate measuring system, a biological tissue equivalent phantom unit, and a method thereof that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.
Features of the present invention are set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Problem solutions provided by the present invention will be realized and attained by a specific absorption rate measuring system, a biological tissue equivalent phantom unit, and a method thereof particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these solutions and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a specific absorption rate measuring system, a biological tissue equivalent phantom unit, and a method thereof as follows.
[Means for Solving the Problem]
An aspect of the present invention provides a biological tissue equivalent phantom unit that is to be used by a specific absorption rate measuring system for evaluating absorption of electromagnetic wave energy. The biological tissue equivalent phantom unit includes
According to another aspect of the present invention, a high dielectric constant material is applied to the surface of the optical fibers of the biological tissue equivalent phantom unit.
Another aspect of the present invention provides a specific absorption rate measuring system for evaluating the absorption of the electromagnetic wave energy using the biological tissue equivalent phantom unit. The specific absorption rate measuring system includes
Another aspect of the present invention provides a specific absorption rate measuring method of evaluating absorption of the electromagnetic wave energy using the biological tissue equivalent phantom that receives irradiation of the electromagnetic wave. The specific absorption rate measuring method includes
According to another aspect of the present invention, the step of reflecting the light that is provided to the electro-optical crystals of the specific absorption rate measuring method is a step of reflecting the light by a dielectric reflective film prepared on a surface countering a surface, through which the light is provided, of the electro-optical crystal.
According to another aspect of the present invention, the step of sequentially providing the light to each of the electro-optical crystals through the optical-path switcher of the specific absorption rate measuring method is a step of sequentially providing the light to each of the electro-optical crystals by selecting an optical fiber by the optical-path switcher, wherein the optical-path switcher is connected to each of the electro-optical crystals.
According to another aspect of the present invention, as for the specific absorption rate measuring method, a high dielectric constant material is applied to the surface of the optical fiber such that the equivalent dielectric constant of the optical fiber becomes substantially equal to the dielectric constant of the biological tissue equivalent phantom.
As described above, according to the present invention, since the electric-field detecting element is constituted by nonmetallic materials, it is possible to measure the SAR distribution without the disturbance that is generated in the case of the conventional technology. Further, since the electro-optical crystals having a dielectric constant approximately equal to that of the phantom are used as a sensor head, reflection due to difference of the dielectric constants is reduced, and the SAR distribution can be more accurately measured. Furthermore, since spatial resolution of the measurement is proportional to a diameter of a beam of the light that penetrates the electro-optical crystal, the spatial resolution can be raised, theoretically, to as small as the wavelength of the light (several μm). Furthermore, since a change in a refractive index of the electro-optical crystal at the measuring point depends on deviation of a dipole that follows the electromagnetic wave, the SAR measurement is available in a wide band range, from the MHz band to the THz band.
[Effect of the Invention]
According to the present invention, disturbance of the electric field in the electro-optical crystal by interface reflection is reduced, and the influence of the interface reflection on the electromagnetic field near the electro-optical crystals is reduced by using the electro-optical crystal that has a dielectric constant approximately equal to that of the phantom. Therefore, the specific absorption rate measuring system capable of obtaining an accurate SAR distribution is realized.
In the following, embodiments of the present invention are described with reference to the accompanying drawings, wherein common reference numbers are assigned to items that have the same functions, and the descriptions are not repeated.
The specific absorption rate measuring system 40 further includes
The specific absorption rate measuring unit 44 includes an analyzer 11, a photodetector 12, an electrical signal line 13, a signal processing unit 14, and a SAR distribution image display 15.
The specific absorption rate measuring system 40 is for measuring an electric field in the phantom 1 using the electro-optical crystals 3, the electric field being generated by the electromagnetic wave generator 2 (such as a cellular phone) arranged near the phantom 1, as shown in
The linearly polarized light irradiated by the luminous source 4 is provided to the polarization regulator 7 via the circulator 6 and the polarization maintenance fiber (PMF) 5. The polarization regulator 7 changes the polarization of the linearly polarized light into a predetermined polarization state, and outputs the light.
The polarization state is determined by a crystallographic axis of the electro-optical crystal 3 arranged in the phantom 1 and a vibrating direction of the electric field generated by the electromagnetic wave generator 2. For example, when detecting an electric field that vibrates in parallel to the y-axis using CdTe, which is a lead marcasite type crystal, crystallographic faces (001) (100), and (010) of CdTe are perpendicularly arranged to the y, x, and the z axes, respectively; or to the y, z, and x axes, respectively; and the polarization regulator 7 is adjusted so that the polarization axis of the linear polarization or an elliptical polarization may become parallel to the x axis or the z axis.
The light, the polarization of which has been adjusted, is transmitted via the single mode fiber (SMF) 8 to the optical-path switcher 9, then to the electro-optical crystals 3.
The light is reflected by a dielectric reflective film prepared on a surface of the electro-optical crystal 3, the surface countering a surface through which the light is provided, and the light goes back along the incidence path. When the light goes back along the incidence path within the electro-optical crystal 3, a phase difference arises between components of the predetermined polarization due to a refractive-index change (Pockels effect) that is proportional to the field strength to the first power that is applied, and the polarization state is changed, i.e., polarization modulation occurs.
When CdTe is arranged, e.g., as described above, a phase difference Γ arises between polarization components that are parallel to the x axis and the z axis, the phase difference Γ being expressed by the following Equation 2.
Γ=(2π/λ)n03r41Ed [Equation 2]
Here, λ, n0, r41, E, and d represent the wavelength [m] of the incident light, the refractive index of the electro-optical crystal 3, the Pockels constant [m/V], field strength [V/m], and the length [m] of the electro-optical crystal 3 in a direction of the oscillation of the electric field, respectively.
The light that is reflected and polarization-modulated reaches the circulator 6 through the optical-path switcher 9 and the polarization regulator 7; then, the light is branched to the analyzer 11 by the circulator 6. The modulation component of the branched light is taken out by the analyzer 11, and is converted into an electrical signal by the photodetector 12. The amplitude of the electrical signal is proportional to the field strength of the electromagnetic wave that is measured. The amplitude of the electrical signal is converted into a SAR value by the signal processing unit 14. Such values, with position information attached, constitute a SAR distribution that can be displayed by the SAR distribution image display 15.
According to the electric-field measuring method, the specific absorption rate (SAR) is defined by Equation 1. According to the present embodiment, by using the specific absorption rate measuring system 40, SAR can be defined by the following Equation 3, based on Equations 1 and 2.
SAR=σK|Γ2|/ρ [Equation 3]
Here, K is a constant determined by the crystallographic axis of the electro-optical crystal 3 and the vibrating direction of the electric field irradiated by the electromagnetic wave generator 2. The constant K can be expressed by the following Equation 4 when CdTe is arranged as described above.
K=λ/(2πn03r41d) [Equation 4]
Further, when detecting the electric field that vibrates parallel to the x (or z) axis by using CdTe, the crystallographic faces (110), (1(1 bar) 0), and (001) of CdTe are arranged perpendicularly to the x(z), y, and z(x) axes, respectively; then the polarization regulator 7 is adjusted so that the polarization axis of the linear polarization or the elliptical polarization may become parallel to the x(z) axis. In this case, a phase difference Γ expressed by the following Equation 5 arises between the polarized components that incline 45° to the x axis and the z axis.
Γ=(2π/λ)ln03r41Ed [Equation 5]
Here, “l” represents the length [m] of the electro-optical crystal 3 in the direction of the light passage. Further, when SAR is defined by Equation 3, the constant K can be expressed by the following Equation 6.
K=λ/(2πln03r41) [Equation 6]
As described above, according to the specific absorption rate measuring system 40 of the embodiment, since the electric-field detecting element is constituted from dielectric materials, the disturbance to the electromagnetic field to be measured is removed, the disturbance conventionally being due to the aggregate of the short dipoles, and the disturbance being the problem with the conventional electric-field measuring method. The dielectric constants of the phantom 1 are prescribed by ARIB. Although reflection (Fresnel reflection) of the electromagnetic wave can arise at the interface between the phantom 1 and the electro-optical crystal 3 depending on the kind of the electro-optical crystal 3 according to the difference in the dielectric constants, such reflection is very small as compared with the disturbance due to the aggregate of short dipoles.
For the same reason, about 28% of reflection occurs at the bare fiber 10 that connects the optical-path switcher 9 and the electro-optical crystal 3, which reflection may cause a disturbance to the electromagnetic field to be measured. The diameter of a common bare fiber is 250 μm including a covering layer, and the reflection factor per mm2 of the cross section is 1/16 (about 1.8%). The covering layer is provided in consideration of a micro bend property at low temperatures. However, since the bare fiber 10 of the specific absorption rate measuring system 40 according to the embodiment is covered by the phantom 1, a clad fiber having a diameter of 80 μm without a covering layer can be used. By using the clad fiber, the reflection factor per mm2 can be lowered to 0.2% or less.
With reference to
On the other hand, it is possible to prevent the reflection of the electromagnetic wave from occurring by applying a material having a great dielectric constant to the surface of the bare fibers 10, making the equivalent dielectric constant equal to the phantom. Since the direction of the main axis and sintering temperature can adjust the specific inductive capacity to a range between 40 and 120, TiO2 and BaTiO3 that have a sintering temperature lower than a softening temperature (about 1500° C.) of glass are suitable as the material to be applied.
Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.
The present application is based on Japanese Priority Application No. 2004-319387 filed on Nov. 2, 2004 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-319387 | Nov 2004 | JP | national |
