JP7496126B2 - Radiation Dosage Measurement System - Google Patents

Description

The present invention relates to a radiation dosimetry system that measures the capacitance of a capacitor based on a phase change or loss in power transmission caused by resonance of a resonant circuit , and measures the amount of radiation using the measured capacitor capacitance .

Conventionally, various sensors that detect changes in the capacitance of a capacitor are known. For example, many sensors that detect changes in capacitance are used in touch panels for inputting various types of data, human presence sensors, and the like.

Patent Document 1 shows an inexpensive radiation dosimeter that uses only a capacitor and a coil, taking advantage of the fact that the capacitance of a capacitor changes according to the radiation exposure dose. Here, an LC circuit in which a capacitor and a coil are connected in series has a frequency characteristic (resonance frequency) unique to the LC circuit, and the change in the capacitor capacitance is measured by analyzing the unique frequency using an external antenna and network analyzer. The radiation exposure dose is then measured based on this change in the capacitor capacitance. With this system, an LC circuit is embedded in the body, and an external antenna and network analyzer are used to measure the radiation dose inside the body.

Patent No. 6512623

Kurs A, Karalis A, Moffatt R et al. Wireless power transfer via strongly coupled magnetic resonances. Science (80- ) 2007;317:83-6

In Patent Document 1, the change in capacitance of a capacitor can be detected by position using an antenna or the like placed at a distance from the capacitor. However, if the distance between the antenna and the LC circuit increases, the detection accuracy deteriorates, and there is a possibility that sufficient measurement cannot be performed. On the other hand, considering the advantage of being able to detect the change in capacitance without contact, there is also a demand for detection from as far away as possible.

Non-Patent Document 1 describes a wireless power transmission method that uses a four-coil configuration with helical coils (resonators) installed on both the transmitting and receiving coils, and that uses Strongly Coupled Magnetic Resonance (SCMR) to maintain high transmission efficiency S21 even over long distances. Note that the S parameter represents the relationship between the power entering the circuit and the power leaving the circuit (the square root of the power), and S21 is the transmission coefficient (= transmission efficiency) from the input terminal to the output terminal.

The present invention includes a transmitting coil, a receiving coil arranged at a predetermined distance from the transmitting coil and sharing the magnetic field generated by the transmitting coil, a target coil arranged between the transmitting coil and the receiving coil, a capacitor whose capacitance changes depending on the dose of incident radiation and which determines the resonant frequency of a resonant circuit formed by being connected to the target coil, and a resonant frequency detection unit that analyzes the frequency dependence of power transmitted from the current flowing through the transmitting coil and the receiving coil and detects the resonant frequency of the resonant circuit from the phase change or loss of the transmitted power due to resonance of the resonant circuit, and detects the capacitance of the capacitor based on the detected resonant frequency , and detects the amount of incident radiation depending on the detected capacitance of the capacitor .

The transmitting coil is divided into a first coil and a second coil arranged close to each other, and the receiving coil is divided into a fourth coil and a fifth coil arranged close to each other, and the resonant frequency detection unit supplies a current to the first coil and detects the current in the fifth coil, thereby performing frequency analysis of the power transmitted from the transmitting coil to the receiving coil.

The first coil and the fifth coil may be single turn coils, and the second coil and the fourth coil may be multiple turn coils.

The resonant frequency detection unit may detect the resonant frequency from the differential value of the transmission efficiency of the power transmitted from the transmitting coil to the receiving coil.

According to the present invention, when power is transmitted from the transmitting coil to the receiving coil, the capacitance of the resonant circuit is detected by using the loss or phase change that occurs when the resonant circuit resonates. This makes it possible to detect the capacitance even if the transmitting coil and receiving coil are far from the resonant circuit and the size of the resonant circuit is small.

1 is a block diagram showing a schematic configuration of a capacitor capacitance measuring system according to an embodiment of the present invention. FIG. 2 is a diagram showing an equivalent circuit of the capacitor capacitance measurement system of FIG. 1 . 1A and 1B are diagrams showing examples of coil arrangement, in which (a) shows step 1 (no target coil, small distance), (b) shows step 2 (large target coil, small distance), and (c) shows step 3 (small target coil, large distance). 11A and 11B are diagrams showing a comparison result between a simulation and an actual measurement of the transmission efficiency S21, where (a) shows step 1 and (b) shows step 2. FIG. 13A and 13B are diagrams showing a comparison result between a simulation and an actual measurement of a transmission efficiency S21 in Step 3, in which (a) shows S21 and (b) shows the derivative of S21. 13A to 13C are diagrams showing S21 and its derivative when the magnetic field is strengthened and the measurement distance is increased, where (a) is 150 mm, (b) is 200 mm, and (c) is 250 mm. 1A and 1B are diagrams showing examples of the configuration of a ferrite core, in which FIG. 1A shows only the ferrite core and FIG. 1B shows a state in which a coil is wound around the ferrite core. FIG. 1 is a diagram showing an example of the configuration of a radiotherapy device using the system of the present embodiment. FIG. 1 is a diagram showing an example of a configuration in which the capacitor capacitance measuring system is applied to a capacitive touch panel.

The following describes an embodiment of the present invention with reference to the drawings. Note that the present invention is not limited to the embodiment described here.

"Overview of the Configuration"
FIG. 1 is a block diagram showing a schematic configuration of a capacitor capacitance measuring system according to the present embodiment.

The transmitting coil 10 has two coils on the transmitting side, the first coil CL1 and the second coil CL2. The resonant circuit 12, which functions as a sensor, has a target coil CL3. The receiving coil 14 has two coils on the receiving side, the fourth coil CL4 and the fifth coil CL5. A network analyzer 20 is connected to the first coil CL1 and the fifth coil CL5 as a resonant frequency detection unit. The network analyzer 20 supplies a signal of a desired frequency to the first coil CL1 and analyzes the signal received by the fifth coil CL5. The transmitting coil 10 and the receiving coil 14 may each have one coil, or may have three or more coils. The first coil CL1 and the fifth coil CL5 are preferably single-turn coils, and the others are preferably multiple-turn coils. The resonant frequency detection unit may not perform frequency analysis like the network analyzer 20, but may detect the presence or absence of resonance at a predetermined frequency.

Figure 2 shows an equivalent circuit of the capacitor capacitance measurement system in Figure 1. Note that since the coil includes electrical resistance in addition to inductance, the inductance L and resistance R are shown separately in the equivalent circuit. In addition, the capacitor C may include stray capacitance, etc.

The first coil CL1 consists of a series connection of an inductance L1 and a resistance R1, to which a source resistor Rs and a capacitor C1 are further connected in series, and an AC signal is supplied from the network analyzer 20. The second coil CL2 consists of a series connection of an inductance L2 and a resistance R2, to which a capacitor C2 is further connected in series, and an induced current flows in response to the magnetic field generated by the first coil CL1.

The target coil CL3 consists of a series connection of an inductance L3 and a resistor R3, to which a capacitor C3 is further connected in series, and an induced current flows in response to the magnetic field generated by the second coil CL2. The fourth coil CL4 consists of a series connection of an inductance L4 and a resistor R4, to which a capacitor C4 is further connected in series, and an induced current flows in response to the magnetic field generated by the second coil CL2.

The fifth coil CL5 consists of an inductor L5 and resistor R5 connected in series, and a capacitor C5 and a load resistor RL are further connected in series. An induced current flows in response to the magnetic field generated by the fourth coil CL4, and this current (voltage) is measured by the network analyzer 20.

The resonant frequencies of the first, second, fourth and fifth coils CL1, CL2, CL4, CL5 are set to be the same, so that the first, second, fourth and fifth coils CL1, CL2, CL4, CL5 resonate at the resonant frequency, thereby achieving effective power transmission.

Here, the source resistor Rs and the load resistor RL are for impedance matching with the network analyzer 20, and the capacitors C1, C2, C4, and C5 are for setting the resonant frequencies of the transmitting and receiving coils (CL1, CL2, CL4, and CL5) to be the same, and it is preferable to use variable capacitors.

In such a system, when an AC signal of a predetermined frequency is supplied to the first coil CL1 by the oscillator 20a of the network analyzer 20, a current flows through the second coil CL2 due to the magnetic field generated in the first coil CL1, a current flows through the target coil CL3 and the fourth coil CL4 in response to the magnetic field generated by the second coil CL2, a current flows through the fifth coil CL5 in response to the magnetic field generated in the fourth coil CL4, and the current (voltage) in the fifth coil CL5 is detected by the network analyzer 20.

Here, part of the induced current generated by the magnetic field of the second coil CL2 flows through the target coil CL3, and the induced current flowing through this target coil CL3 reduces the induced current flowing through the fourth coil CL4. The resonant circuit 12 is composed of an LC circuit consisting of the target coil CL3 and capacitor C3 connected in series. Therefore, the resonant circuit 12 resonates at its natural resonant frequency, and a large current flows at this time.

In this embodiment, the network analyzer 20 detects the power transmission efficiency S21 of the first coil CL1 to the fifth coil CL5. Since S21 decreases significantly when the resonant circuit 12 resonates, the resonant frequency of the resonant circuit 12 (target coil CL3) can be detected by detecting the frequency of this decrease peak (peak of the change in transmission efficiency), and therefore the capacitance of the capacitor C3 can be measured.

In this embodiment, a capacitor whose capacitance changes is used as capacitor C3, and the capacitance of capacitor C3 is measured by network analyzer 20. By using a capacitor whose capacitance changes depending on the amount of incident radiation, as described in Patent Document 1, for example, as capacitor C3, it is possible to measure the radiation dose.

The resonant frequency f of resonant circuit 12 is f = 1/2π√(L3·C3), and when the frequency of the transmitted signal matches this resonant frequency, a resonant current flows in resonant circuit 12, and the amount of current flowing therein increases. In this case, the current flowing through the fourth coil CL4 and the fifth coil CL5 decreases, and by detecting this decrease with the network analyzer 20, the resonant frequency of resonant circuit 12 can be detected, from which the capacitance of capacitor C3 can be determined. The amount of incident radiation can then be determined from the capacitance of capacitor C3.

In this embodiment, the network analyzer 20 sweeps the supplied AC signal at a frequency close to the resonant frequency of each coil to detect the resonant frequency of the resonant circuit 12.

"Calculation of mutually induced current"
When two coils are electromagnetically coupled, a phenomenon occurs in which the currents flowing through the two coils affect each other (mutual induction). When a current flows through one coil due to mutual induction, the magnetic field generated from that coil generates an induced electromotive force V2 in the other nearby coil.

Here, _I1 is the current in one coil, t is time, and M is the mutual inductance. If the distance between the coils is _d12 , the radii of the two coils are _r1 and _r2 , and the numbers of turns are _N1 and _N2 , the mutual inductance _M12 between one coil and the other coil can be calculated as follows using the first kind elliptic integral K(k) and the second kind complete elliptic integral E(k).

Furthermore, since mutual induction affects not only adjacent coils but all coils, the relationship between the voltage V, current I, and impedance Z in each coil is expressed as follows:

Here, M is the mutual inductance, Vs is the voltage applied to the first coil CL1, _VL is the voltage applied to the fifth coil CL5, and Vs is the voltage applied to the source resistor Rs. Note that Vs may be the voltage applied to the source resistor Rs, and VL may be the voltage applied to the resistor _RL .

The transmission efficiency S21 can be calculated by the following formula, since the respective current values (I ₁ to I ₅ ) can be calculated by solving the inverse matrix of the above formula.

Here, in the network analyzer 20, the transmission efficiency S21 is expressed in decibels, so the obtained S21 is substituted into the following formula to obtain S21 _{Nwrwork Analyzer} . The following offset is an attenuation correction term due to the internal attenuator of the network analyzer, and is obtained by actual measurement.

"Measuring resonance frequency"
According to Faraday's law, when a coil is placed in a space where a magnetic field exists, an induced electromotive force is generated due to the change in magnetic flux penetrating the loop surface. In this case, if the number of turns of the coil is N, the magnetic flux penetrating the coil is Φ, and the time is t, the induced electromotive force V is expressed by the following formula.

In addition, since the coil is equivalent to an RLC series circuit, the impedance is expressed by the following formula, where the impedance Z is at its minimum value R at the resonant frequency, and the induced current due to the induced electromotive force is at its maximum. In other words, when the frequency of the electromagnetic waves penetrating the loop surface of the coil matches the natural resonant frequency of the coil, the induced current is at its maximum.

Therefore, the energy U stored in the coil due to the induced current can be expressed by the following equation using the current I flowing through the coil.

From the above, it is believed that when the desired target coil is placed in the space where power transmission is taking place, the impedance becomes small near the resonant frequency of the target coil, a portion of the transmitted power is stored in the target coil, and a phase change occurs in the transmission efficiency S21.

Furthermore, since the impedance of the coil is minimum at the resonant frequency, when the transmission efficiency S21 is observed with the network analyzer 20, a phase change that is maximum at the resonant frequency of the target coil appears, and power loss occurs due to resistance in the circuit. Therefore, the phase change can also be detected as a change in transmission efficiency (power loss), and can also be detected in the same way by measuring, for example, the standing wave ratio (SWR value).

In this way, the transmission efficiency S21 varies with frequency, and by utilizing the fact that the transmission efficiency decreases significantly when the frequency of the transmitted signal matches the resonant frequency of the target coil CL3, which is the target coil, it is possible to detect the resonant frequency of the resonant circuit 12 (target coil CL3) from the outside. That is, the network analyzer 20 sweeps the transmission frequency, determines the transmission efficiency S21 at each frequency, and identifies the frequency at which the transmission efficiency S21 drops as the resonant frequency of the resonant circuit 12 (target coil CL3).

"Experiment: Simulation, Actual Measurement"
Regarding the measurement of the resonant frequency in this embodiment using the transmission efficiency S21, verification was carried out in stages (Step 1 → Step 2 → Step 3) starting with a simple measurement system, and the validity of the measurement of the resonant frequency using the transmission efficiency S21 was confirmed.

For this measurement, a network analyzer manufactured by Keysight (product name: E5071C) was used.

<Step 1>
As shown in FIG. 3A, the transmission efficiency S21 is measured in a situation where the resonant circuit 12 including the target coil CL3 as the target is not present and the distance between the second and fourth coils is relatively short (100 mm).

<Step 2>
In the same measurement system as in Figure 3(a), as shown in Figure 3(b), a target coil CL3 (diameter: 18 mm, number of turns: 4, length: 8 mm) is placed at the midpoint between the second coil CL2 and the fourth coil CL4 (communication distance 50 mm), and it is verified whether a phase change occurs at the resonant frequency of the resonant circuit 12 including the target coil CL3 in the transmission efficiency S21.

<Step 3>
In Step 3, the target coil CL3 was changed to a small coil (diameter 2.4 mm, number of turns 8, length 10 mm). Furthermore, a ferrite core (diameter 2 mm, length 15 mm) was inserted as the core of the target coil CL3. The communication distance was targeted to be 100 mm, and frequency measurement was performed in the system shown in FIG. 3(c).

In the resonant circuit 12, the phase change of the transmitted power depends on the induced electromotive force, so it is believed that the larger the mutual inductance M, the larger the phase change peak. Mutual inductance is a parameter that depends on the distance and radius between the coils, and can be calculated by simulation using the above formula. Therefore, the radius that increases the mutual inductance M at a communication distance of 100 mm was determined to be 90 mm. Furthermore, since the phase change peak due to the target coil CL3 is expected to become smaller as the desired communication distance becomes longer, there is a possibility that the phase change peak will not appear clearly. Therefore, we devised a way to make it possible to detect it as a clear signal by differentiating the obtained S21 [dB] with respect to frequency (horizontal axis).

The resonant frequencies of the resonant circuit 12 used in Step 2 and Step 3 were measured in advance using the RFID principle, and were 21.52 MHz and 17.68 MHz, respectively.

"Experimental result"
<Step 1, Step 2>
4(a) and 4(b) show the comparison results of the simulation and the actual measurement of the transmission efficiency S21. As can be seen from FIG. 3(a), the simulation and the actual measurement results are almost the same. It is presumed that the slight deviation between the simulation and the actual measurement is due to the fact that each coil is handmade and the coil placement in the actual measurement is slightly misaligned.

In addition, comparing FIG. 3(a) with FIG. 3(b), it can be seen that a phase change peak occurs in the transmission efficiency S21 at the resonant frequency of the resonant circuit 12 including the target coil CL3. From the above, it can be seen that it is possible to wirelessly measure the frequency by utilizing the peak of the phase change portion.

<Step 3>
5A shows a comparison between the simulation and actual measurement of the transmission efficiency S21 at step 3. As shown above, as the distance increases, the peak of the phase change portion becomes less visible. Therefore, FIG. 5B shows the transmission efficiency S21 differentiated with respect to frequency.

As can be seen from Figures 5(a) and 5(b), the simulation and actual measurements matched well even when the target distance was extended, and Figure 5(b) confirmed that a phase change occurred at the resonant frequency of target coil CL3 even when the communication distance was longer.

It was also confirmed that the resonant frequency of the target coil CL3, which had been measured separately in advance, matched the frequency at which the phase change peak occurred. Therefore, it was found that by combining power transmission by SCMR with the phase change that occurs in the transmission efficiency S21, it is possible to measure the resonant frequency using a small resonant circuit 12, even when the measurement distance is long.

"Extending the measurement distance"
6(a) to 6(c) show the results (S21 and the derivative of S21 with respect to frequency) for the first, second, fourth, and fifth coils CL1 to CL5, when the magnetic field generated is strengthened by using a large coil and the measurement distance is 150 mm, 200 mm, and 250 mm. According to this, the phase change peak becomes smaller as the distance increases due to the differential value of S21 in the lower part, but it can be seen that at this measurement distance, it is possible to sufficiently distinguish by differentiating the transmission efficiency S21.

In addition, in this embodiment, a ferrite core is used for the target coil CL3 to extend the communication distance. However, ferrite cores can also be used for the other coils, such as CL2 and CL4. By using a ferrite core, the magnitude of the magnetic field between the coils increases, so the communication distance can be further extended.

<Ferrite core shape>
In the above step 3, a cylindrical ferrite core is used, and a coil is wound around the ferrite core. Here, as shown in Fig. 7(a) and Fig. 7(b), both ends in the axial direction of the cylindrical ferrite core 30 can be expanded in a conical shape to form a tapered portion 32 whose diameter gradually increases. Then, by winding the coil 34 around the cylindrical portion, the movement of the coil 34 in the ferrite core 30 in the axial direction can be suppressed. When the target coil CL3 is embedded in the body, the movement of the coil is suppressed, and more reliable detection can be performed.

<System Configuration>
8 shows an example of the configuration of a radiotherapy device using the system of this embodiment. A patient 42 is placed on an examination table 40, and therapeutic radiation 44 is irradiated to the affected area of the patient 42. A resonant circuit 12 (target coil CL3) is embedded in a predetermined position of the patient 42. This makes it possible to detect the amount of incident radiation at the location where the resonant circuit 12 is placed by the change in capacitance of a capacitor C3 of the resonant circuit 12.

The target coil CL3 in this embodiment is a cylindrical helical coil that is sensitive to the magnetic field coming from its central axis (generating a large induced current), but the closer it is to the perpendicular direction, the less sensitive it becomes.

Therefore, as shown by the solid line in Figure 11, when the orientation of the target coil CL3 is aligned with the orientation of the transmitter coil 10 and receiver coil 14, the sensitivity is good, but when they differ, as shown by the dashed line, the sensitivity is poor.

It is therefore advisable to make both or either of the examination table 40 on which the patient 42 rests and the transmitting and receiving coils 10, 14 movable. This allows the sensitivity to be maintained at all times, enabling highly sensitive measurements. When irradiating a patient with therapeutic radiation, it is often necessary to set the position of the examination table 40 in order to determine the irradiation position of the therapeutic radiation, and it is therefore advisable to make the orientations of the transmitting coil 10 and receiving coil 14 adjustable. The orientation of the target coil CL3 may be detected from sensitivity, or it may be detected from X-ray or ultrasound images.

"Other configuration examples"
9 is a diagram showing a configuration example in which the capacitor capacitance measurement system is applied to a capacitive touch panel. In this way, a metal plate 36 is connected to the resonant circuit 12, and the capacitance of the resonant circuit 12 changes when a hand or the like approaches the metal plate 36. That is, in addition to the capacitor C3, a stray capacitance of the metal plate 36 is added, and this changes when a hand or the like approaches. Note that if the capacitance of the resonant circuit 12 changes, it is not necessarily necessary to provide an independent metal plate 36.

In the network analyzer 20, a predetermined signal is transmitted from the transmitting coil 10, and the received signal of the receiving coil 14 is analyzed, and the change in the resonant frequency in the resonant circuit 12 can be detected from the phase change peak position (frequency) of the transmission efficiency S21. Note that by shifting the resonant frequency of each resonant circuit 12 by a method such as changing the capacitance of the capacitor C3 of each resonant circuit 12, it is possible to identify the resonant circuit 12 whose resonant frequency has changed.

The system of this embodiment can also be used when there is a need to detect changes in capacitance remotely. The system of this embodiment can also be used when detecting changes in capacitance in elements (capacitors) that use piezoelectric, pyroelectric, or ferroelectric materials.

Also, by connecting a capacitor, coil, and switch in series, it is possible to generate an antenna that has a resonant frequency only when the switch is pressed. By preparing a large number of similar capacitors, coils, and switches and setting each of their resonant frequencies to different values, it is possible to detect the on/off state of the switch due to the presence of a specific resonant frequency. Therefore, it is also possible to make a switch that contains a coil and a capacitor function as a computer keyboard simply by placing it in space without power supply. In this case, the on/off state can be detected by detecting whether a resonant signal exists at a predetermined frequency for each switch, so there is no need to perform frequency analysis using a network analyzer or the like.

"Effects of the embodiment"
Currently, wireless communication technology is used in various fields, and the resonant frequency of a target coil is usually measured by S11 using electromagnetic induction. However, in frequency measurement using S11, the communication distance is short, and even the longest is about a few centimeters.

In this embodiment, the resonant frequency of a target coil 10 cm away was measured by detecting the phase change in the induced electromotive force in the target coil based on power transmission by SCMR. It was also found that it is possible to measure the resonant frequency over a long distance even with a small target coil.

In addition, SCMR is a transmission method that utilizes magnetic resonance, and it has been confirmed that it is less affected by dielectric materials such as living bodies and that the transmission efficiency remains almost unchanged. Therefore, it is believed that the resonant circuit 12 can be used as an internal sensor as is, and there is a possibility that problems associated with existing implantable devices can be improved.

In Step 3 above, the coil radius etc. was verified to perform measurements suitable for a measurement distance of 100 mm, but it is believed that the distance can be extended further by rigorous optimization (number of coil turns, coil pitch, resonant frequency band, etc.).

REFERENCE SIGNS LIST 10 Transmitting coil, 12 Target coil, 14 Receiving coil, 20 Network analyzer, 30 Ferrite core, 32 Tapered portion, 34 Coil, 36 Metal plate, 40 Examination table, 42 Patient, 44 Therapeutic radiation.

Claims (4)

A transmitting coil;
A receiving coil that is disposed at a predetermined distance from the transmitting coil and shares a magnetic field generated by the transmitting coil;
a target coil disposed between the transmitting coil and the receiving coil;
a capacitor whose capacitance changes depending on the dose of incident radiation , the capacitor being connected to the target coil to determine a resonant frequency of a resonant circuit;
a resonance frequency detection unit that analyzes the frequency dependency of power transmitted from the current flowing through the transmitting coil and the receiving coil, and detects a resonance frequency of the resonance circuit from a phase change or loss of the transmitted power due to resonance of the resonance circuit;
Including,
detecting a capacitance of the capacitor based on the detected resonant frequency , and detecting an amount of incident radiation according to the detected capacitance of the capacitor ;
Radiation dosimetry system . 2. The radiation dosimetry system according to claim 1,
the transmitting coil is divided into a first coil and a second coil arranged close to each other, and the receiving coil is divided into a fourth coil and a fifth coil arranged close to each other, and the resonant frequency detection unit supplies a current to the first coil and detects a current in the fifth coil, thereby performing frequency analysis of the power transmitted from the transmitting coil to the receiving coil;
Radiation dosimetry system. The radiation dosimetry system according to claim 2,
The first coil and the fifth coil are single turn coils, and the second coil and the fourth coil are multi-turn coils.
Radiation dosimetry system. A radiation dosimetry system according to any one of claims 1 to 3,
The resonance frequency detection unit is
detecting the resonant frequency from a differential value of a transmission efficiency of the power transmitted from the transmitting coil to the receiving coil;
Radiation dosimetry system.