WO2002079803A1

WO2002079803A1 - Method and apparatus for radiation dosimetry

Info

Publication number: WO2002079803A1
Application number: PCT/US2002/004646
Authority: WO
Inventors: Thomas C. Cetas; Eugene J. Gross; W. Timothy Holman; Frederick Ronald Thomas; John G. Williams
Original assignee: Cetas, Inc.
Priority date: 2001-04-02
Filing date: 2002-02-15
Publication date: 2002-10-10
Also published as: WO2002079803A8

Abstract

An ionizing radiation sensing element (12) of a dosimetry device (10) is adapted to receive operating power wirelessly from a remote power source (14) and to produce an electromagnetic signal that is representative of radiation dose. The radiation sensing element (12) is implanted in a body or test object (11). The electromagnetic signal produce by the radiation sensing element (12) is received and interpreted by a remote readout portion (16) of the device (10).

Description

METHOD AND APPARATUS FOR RADIATION DOSIMETRY

Background of the Invention

The present invention relates to a method and apparatus for radiation dosimetry, and more particularly to a method and apparatus for measuring the radiation dose at a specific location in vivo or within a test object, or phantom. It is particularly important for a patient undergoing radiation therapy to provide an accurate means for measuring radiation dose. In the prior art, radiation measurements are taken in a water tank used to simulate and thereby estimate the dosage received by a human being from the same radiation. This is based on the geometry and material properties of a patient's tissue, the geometry of the tank and the observation that the radiation absorption of tissues are close to those of water. However, since this does not provide a measured dosage at a specific location of interest within the body, it limits the precision with which a measured dose can be delivered.

Thomson, U.S. Patent No. 5,444,254 discloses a MOSFET radiation sensing device disposed at the end of a flexible probe, the probe carrying conductive tracks for connecting to suitable external circuitry. The device is inserted into the body through a catheter. Because of the hardwire connection to external circuitry, the device must remain in the catheter during the time that radiation dose is being monitored, with the disadvantage that the apparatus is limited to use at the bedside. Moreover, the device must be inserted and removed each time the method is employed.

The present inventors have previously suggested implanting a radiation dosimeter in the body and transponding a first signal from the dosimeter that reflects the cumulative exposure to ionizing radiation following implantation, and a second reference signal which is less sensitive to radiation, so that a reading proportional to accumulated radiation would be possible. Such a dosimeter would permit sensing radiation at all times, and one implantation would provide for on going uses. However, the requirements that, to be practical, such a device must be very small, must not require a hard wire connection outside the body and must be remotely interrogated from outside the body have heretofore presented daunting obstacles to the development of such a device. Accordingly, there has been an unfulfilled need for a method and apparatus for radiation dosimetry that provides a small radiation dosimeter that may be implanted in the body and remotely interrogated, and which does not require a hard wire connection outside the body.

Summary of the Invention

The method and apparatus for radiation dosimetry of the present invention solves the aforementioned problems and meets the aforementioned need by providing a radiation sensing dosimetry device for implantation into the body of a patient or test object and a means outside the body for energizing and interrogating that device wirelessly. A radiation sensing element is adapted to receive power from a field generated by a remote power source and to produce a radiated signal that is representative of radiation dose, preferably cumulative radiation dose. The radiated signal produced by the sensing element is received and interpreted by a remote reading portion of the device.

In a first embodiment the sensing element comprises a radio frequency ("RF") oscillator whose frequency of oscillation changes with cumulative radiation dose. A power source transfers power from outside the body of the patient or other test object to the oscillator wirelessly. The RF signal is received and inteipreted outside the patient or test object. A second oscillator, having the same temperature characteristics as the first oscillator but being essentially insensitive to cumulative radiation dose, may be included in the sensing element to produce a second RF signal that can be used to compensate for changes in temperature of the sensing element.

In a second embodiment the sensing element comprises a paramagnetic material encapsulated in a bio-compatible capsule. The magnetic characteristics of the paramagnetic material vary with cumulative radiation dose. A magnetic resonance imaging ("MRI") system provides power to the sensing element by a radio frequency excitation signal in the presence of a strong magnetic field. The paramagnetic material emits a relaxation RF signal whose spectral content is dependent in cumulative radiation dose. The relaxation signal is received and inteipreted by the

MRI system. Therefore, it is a principal object of the present invention to provide a novel and improved method and apparatus for radiation dosimetry.

It is another object of the present invention to provide a method and apparatus for implanting a radiation dosimeter for sensing radiation dose at a specific location inside a body.

It is yet another object of the present invention to provide a method and apparatus for remotely interrogating a radiation dosimeter implanted within the body.

It is still another object of the present invention to provide a method and apparatus for wireless operation of a radiation dosimeter implanted within a body. The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.

Brief Description of the Drawings

Figure 1 A is a diagram of a first embodiment of a radiation dosimeter according to the present invention.

Figure IB is an enlarged diagram of a radiation-sensing element of a dosimeter according to the present invention. Figure 2 is a schematic diagram of an exemplary ring oscillator for use in the dosimeter of Figure 1 according to the present invention.

Figure 3 is a schematic diagram of a wireless power source for use in the dosimeter of Figure 1 according to the present invention.

Figure 4 is a schematic diagram of a radiation-sensing element of the dosimeter together with an exemplary power-receiving inductor for use therewith according to the present invention.

Figure 5 is a schematic diagram of a rectifier, voltage regulator and buffer circuit for use as a power circuit in the radiation-sensing element of the dosimeter of Figure 1. Figure 6 is a schematic diagram of an exemplary bandgap voltage regulator for use in the power circuit of Figure 5 according to the present invention. Figure 7 is a cross section of a second embodiment of a radiation-sensing element according to the present invention.

Figure 8 is a diagram of the use of the second embodiment of Figure 7 with an MRI system.

Detailed Description of the Invention

Referring to Figures 1 A and IB, a diagram of a first radiation dosimeter 10 according to the present invention is shown. The dosimeter 10 is for measuring an amount of ionizing radiation (hereinafter "radiation") encountered by the device. It is particularly adapted for accurately and precisely measuring radiation at a specific location inside the body 11 of a human patient, for example, inside the body of a cancer patient undergoing radiation therapy, or a test object. However, the dosimeter 10 may be used to accurately and precisely measure radiation dose at a specific location in any animal or material, for any purpose. The dosimeter 10 includes one or more implantable ionizing-radiation sensing elements 12, a power source 14, and a readout device 16. The sensing elements are implanted inside the body, while the power source, and readout device are disposed outside the body or test object in wireless communication therewith. This enables the size of the device that must be implanted in the body to be minimized, and avoids complications that could result from interconnecting the sensing element with a readout device by metal wires or other conductors. It also permits a patient to remain ambulatory despite implantation of one or more sensing devices.

In that regard, the present inventors have determined that the size of the sensing device 12 may be made 2.25 millimeters square, and have a maximum length of 10 millimeters if not smaller, though larger devices may be used without departing from the principles of the invention. The aforementioned dimensions are such that, in use, several radiation-sensing elements 12 may be implanted around a region in the body 11 of the patient to be treated by radiation, as shown in Figure 1A.

In the first embodiment, the radiation-sensing element 12 includes a circuit 18 that is sensitive to radiation. The circuit 18 employs MOSFET transistors as radiation sensors. As explained in Buehler et al., U.S. Patent No. 5,332,903, incorporated by reference herein in its entirety, FET dosimeters are advantageous because they are small and provide an RF electrical signal representative of cumulative radiation dose. They operate on the principle that ionizing radiation causes a shift in threshold voltage due to the accumulation of trapped charge in the gate oxide.

According to the invention, one embodiment of the circuit 18 is preferably a ring oscillator. A ring oscillator employing MOS transistors oscillates at a radio frequency with a period that depends on the threshold voltage of the transistors. Depending on the type of transistors and the parameters of the circuit, the threshold voltage increases or decreases as radiation dose is accumulated in a way that will be understood by persons of ordinary skill in the art. Accordingly, for the radiation sensitive circuit 18, the frequency of oscillation will increase or decrease in response to accumulated radiation.

Figure 2 shows a three-stage ring oscillator 19 using the Bi-CMOS process that illustrates a preferred architecture for the circuit 18. For example, one design uses a 2 millimeter square chip having 9 CMOS devices or stages with a line-width of 0.35 μm. Although three CMOS stages 21 are shown in this example, it is to be recognized that the ring oscillator may have any odd number of stages of three or greater.

The threshold voltage of MOS transistors is a function of temperature, so that the circuit 18 may require temperature compensation. Alternatively, however, it is advantageous to provide an additional radiation sensing circuit 20, as shown in Figure

IB, that retains the temperature dependence of the circuit 18 yet is relatively insensitive to radiation, so that the circuit 20 may function as a reference. To this end, the circuit 20 is preferably constructed to be substantially the same as the circuit 18, except that the oxide volume is made smaller. This decreases the aforementioned accumulation of charge and, therefore, radiation sensitivity. For example, a two-to- one ratio of oxide thicknesses has been found satisfactory, and this is conveniently accommodated in manufacturing by using two different oxide processes in the fabrication of transistors of the circuit 18 and 20. However, it is to be recognized that other ratios for making different oxide volumes may work just as well, or better, without departing from the principles of the invention.

Turning to Figure 3, the power source 14 for the first embodiment of the dosimeter is illustrated. The power source is preferably located remotely from the sensing element 12, outside the body of the patient or test object, helping to minimize the size of the implantable sensing element. The power source is adapted to produce an alternating electromagnetic power signal at a selected frequency, preferably the 13.56 megahertz allotted by the FCC for medical use. A sinusoidal voltage source 23 is employed to produce a 13.56 megahertz signal, which is amplified with an RF amplifier 25. The output of the RF amplifier is applied through a loading resistor R_s to an impedance matching transformer Z_matc)] for matching the source resistance to a resonant output tank circuit. A coil L in the tank circuit tuned with capacitor C produces an electromagnetic power signal that varies sinusoidally at 13.56 megahertz. Returning to Figure IB, to receive the power signal, the sensing portion 12 of the dosimeter includes a small power-receiving inductor 29. While a variety of different inductor configurations may be designed by a person of skill in the art, one design employs a planar, square coil formed of deposited aluminum as is compatible with standard monolithic semiconductor manufacturing processes, as shown at 30 in Figure 4. In this case the sensitivity of the inductor is proportional to the number of turns of the coil and the area the turns cumulatively encompass. The width "w" of the aluminum strip forming the coil is preferably approximately 2.4 μm, and the inductor occupies about 30% of the chip area. It is to be appreciated, however, that an effective power-receiving inductor design is within the ordinary skill of a person working in this field and that the most effective design may vary depending on the rest of the components of the dosimeter and the particular application and environment. It is also to be understood that, while RF magnetic induction coupling of power to the ionizing-radiation sensing element is preferred, power might be coupled by electric field inductive coupling, ultrasonic power coupling, or other wireless means without departing from the principles of the invention.

Turning to Figure 5, a bridge rectifier circuit 32 is coupled to the inductor 29 at input port 31 and produces full wave rectification of the output of the inductor. The output of the bridge rectifier is smoothed with a low pass RC filter 34 and provided to a voltage regulator 39, since the oscillation frequencies of the oscillating circuits 18 and 20 depend on the voltage supplied to them.

Referring to Figure 6, the voltage regulator 39 is preferably a bandgap voltage regulator as is known in the art. The regulator has an input 47 and an output 49. The bandgap voltage regulator provides a number of advantages. These are primarily that it operates independently of threshold voltage and, therefore, of radiation dose, and that it is relatively temperature insensitive.

Returning to Figure 5, the circuits 18 and 20 are coupled in parallel to the output of the low pass filter. However, to avoid interaction between the two circuits, each is coupled to the low pass filter through a buffer 42.

As mentioned above, the oscillation frequencies of the circuits 18 and 20 are preferably arranged to be substantially equally temperature dependent, while the frequency of oscillation of the circuit 18 is sensitive to radiation dose. The sensing element 12 broadcasts respective RF output signals of circuits 18 and 20 tlirough a inductor or broadcast antenna 36, shown in Figure 1, that is about the same size as, and may be, for example, constructed in substantially the same manner as, the inductor 29. Each circuit 18 and 20 drives the broadcast inductor or antenna through respective buffers similar to the buffers 42. As with the power-receiving inductor, the design of an appropriate inductor or broadcast antenna is within the ordinaiy skill of a person working in the field and a variety of configurations therefor may be used.

Like the power source 14, the readout device 16 is also remote from the ionizing-radiation sensing element 12, outside the body of the patient or test object. Returning to Figure 1, the readout device therefore preferably includes an antenna 38 for receiving each of the RF signals produced by the oscillator circuits 18 and 20 broadcast by the antenna 36. The readout device further interprets the received electromagnetic signals as being indicative of radiation dose.

As the electromagnetic signals preferably indicate radiation dose by their frequency, the readout device includes one or more frequency counters 40 for measuring the frequency of each of the signals. Preferably, a common spectrum analyzer is used as the readout instrument.

The difference in the frequencies produced by the circuits 18 and 20 is attributed to radiation dose, since the frequencies are assumed to change equally with temperature. This difference in frequency may be related to actual dose by calibrating the response of the circuits to a known radiation source. Measured frequencies may be provided to a computer (not shown) for analysis and reporting. Turning now to Figures 7 and 8, in a second embodiment 40 of the dosimeter the ionizing-radiation sensing elements 12 comprise a paramagnetic or other magnetic material 42 encapsulated in a bio-compatible material 44.

The paramagnetic material is sensitive to radiation such that its magnetic characteristics vary with cumulative radiation dose. By way of example, but not of limitation, the paramagnetic material may be ferrous sulphate or ferrous ammonium sulphate, which are known to exhibit such behavior. Specifically, the ferrous ions convert to ferric ions when irradiated. However, other materials whose magnetic characteristics change with radiation dose may be used without departing from the principles of the invention.

Preferably, the paramagnetic material 42 is encapsulated in a capsule similar to a gel-cap that does not dissolve, though other structures may be employed as appropriate. The capsule material 44 may be, by way of example, but not of limitation, borosilicate glass. What is important is that the material be bio-compatible and that it can form a structure that will provide a heraietic seal for the paramagnetic material. Preferably, it should also not perturb an MRI system.

As is well understood, an MRI system 46, as shown in Figure 8, generally comprises a magnet 48 that produces a strong magnetic field and MRI electronics 50 that produces an RF exciting signal and receives an RF relaxation signal. In this case, the exciting signal, in the presence of the strong magnetic field, supplies power to the paramagnetic material within the ionizing-radiation sensing devices 12, and receives relaxation signals from that material by which the location and spectral characteristics of the sensing device may be determined in accordance with MRI principles that are generally known in the art. It is to be recognized that, while particular methods and apparatuses for measuring radiation dose have been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention of the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims

Claims:

1. An ionizing radiation dosimeter, comprising:

an ionizing radiation sensing element encased in a housing for implantation into an object, said ionizing radiation sensing element having a radiation dose output for producing a dosage- indicating signal responsive to the amount of ionizing radiation received by said sensing element; and

a power source adapted to transfer power from outside said object to said ionizing radiation sensing element wirelessly so as to generate said dosage-indicating signal.

2. The ionizing radiation dosimeter of claim 1, wherein said dosage- indicating signal is an electromagnetic signal.

3. The ionizing radiation dosimeter of claim 1, wherein said ionizing radiation sensing element includes a first oscillator circuit for producing said dosage- indicating signal, said first oscillator circuit having a frequency of oscillation that is sensitive to said ionizing radiation.

4. The ionizing radiation dosimeter of claim 3, wherein said oscillator circuit includes a ring oscillator.

5. The ionizing radiation dosimeter of claim 3, wherein the oscillating frequency of said first oscillator is temperature dependent, and said sensing element includes a second oscillator circuit for producing a second dosage-indicating signal, said second oscillator circuit having a frequency of oscillation that is substantially equally temperature dependent and substantially less sensitive to said ionizing radiation as compared to said first oscillator circuit.

6. The ionizing radiation dosimeter of claim 1, further comprising a readout portion for receiving said dosage-indicating signal wirelessly and being adapted to interpret said dosage-indicating signal for determining the amount of said ionizing radiation.

7. The ionizing radiation dosimeter of claim 1, wherein said ionizing radiation sensing element comprises an ionizing radiation-sensitive electronic circuit, and said power source includes a power field pick up element connected to said ionizing radiation sensing element for supplying power thereto, and a power field generating element for producing a power field in the location of said power field pick up element.

8. The ionizing radiation dosimeter of claim 7, wherein said power field pick up element comprises an inductor, and said power field generating element comprises a source of an alternating magnetic field.

9. The ionizing radiation dosimeter of claim 7, wherein said power field pick up element comprises a conductor, and said power field generating element comprises a source of an alternating electric field.

10. The ionizing radiation dosimeter of claim 7, further comprising a power regulator circuit for receiving power from said pick up element and providing regulated power to said electronic circuit.

11. The ionizing radiation dosimeter of claim 10, wherein said power regulator is a bandgap voltage regulator.

12. The ionizing radiation dosimeter of claim 1, wherein said ionizing radiation sensing element comprises an encapsulated paramagnetic material, and said power source comprises an excitation signal source within a magnetic resonance imaging system.

13. The ionizing radiation dosimeter of claim 12, wherein said encapsulated paramagnetic material comprises a paramagnetic material encapsulated within a bio-compatible capsule.

14. The ionizing radiation dosimeter of claim 13, wherein said paramagnetic material comprises a ferrous compound.

15. The ionizing radiation dosimeter of claim 14, wherein said capsule comprises borosilicate glass.

16. The ionizing radiation dosimeter of claim 12, wherein said capsule comprises borosilicate glass.

17. A method for measuring ionizing radiation dose, comprising the steps of:

providing a device that, when supplied with power, produces a primary signal representative of the cumulative ionizing radiation dose to which said device has been exposed;

implanting said device within an object; and

supplying power to said device wirelessly for producing said signal representative of dose.

18. The method of claim 17, wherein said primary signal comprises an electromagnetic signal.

19. The method of claim 17 wherein the object is a human body.

20. The method of claim 17 wherein the object is a test object.

21. The method of claim 17, wherein said primary signal representative of cumulative dose indicates said dosage by undergoing a change in frequency.

22. The method of claim 17, wherein said primary signal is characterized by a frequency having a temperature dependence, the method further comprising producing by said device a secondaiy signal representative of cumulative dose that is relatively insensitive to the amount of said ionizing radiation as compared to said primary signal, wherein said secondary signal is characterized by a frequency having substantially the same temperature dependence as said primary signal, and comparing the frequency of said primary signal with the frequency of said secondary signal to determine the amount of said ionizing radiation.

23. The method of claim 17, wherein said supplying power step comprises producing an alternating magnetic field at the location of said device.

24. The method of claim 17, wherein said supplying power step comprises producing an alternating electric field at the location of said device.

25. The method of claim 17, further comprising receiving said primary signal representative of dose outside said object and interpreting said signal to determine cumulative dose to which said device has been exposed.

26. The method of claim 25, wherein said supplying power step comprises producing an excitation signal with a resonance imaging system, and said receiving step comprises receiving a relaxation signal from said device with a resonance- imaging system.