WO2007107762A1

WO2007107762A1 - Indirect heating

Info

Publication number: WO2007107762A1
Application number: PCT/GB2007/001020
Authority: WO
Inventors: Jon Dobson; John Reed; Adrian Lovejoy; Neil Farrow
Original assignee: Keele University
Priority date: 2006-03-22
Filing date: 2007-03-21
Publication date: 2007-09-27
Also published as: GB0605672D0

Abstract

An apparatus and method for heating a target system are disclosed. The apparatus comprises at least one coil element locatable proximate to the target system, at least one capacitor element connected to the coil element, a direct current power supply connectable to the coil element via a switching element and a switch controller for selectively switching the switching element on at a desired frequency to thereby heat the target system via indirect heating.

Description

INDIRECT HEATING

The present invention relates to a method and apparatus for indirectly heating a target system. In particular, but not exclusively, the present invention relates to an apparatus and method in which one or more coils located close to a target system can be energised in a selectable manner to thereby indirectly heat the target system.

Various techniques and associated apparatus are known for indirectly heating. Many of these have distinct advantages and disadvantages associated with the specific nature of the indirect heating method and how this is achieved. For example, induction heating is a non-contact method that uses high frequency electricity to provide fast, consistent heat to electrically conducting materials. Since the process is non-contact, there is no contamination of the material being heated. It is also very efficient since the heat is actually generated inside the instrument. This can be contrasted with other heating methods where heat is generated in a flame or heating element, which is then applied to the instrument. There are many applications and uses for induction heaters where an electrically conductive material needs to be heated in a clean, efficient and controlled manner. A disadvantage of induction heating is that it is only applicable to metallic objects.

Other types of indirect heating include Neel heating in which a target system is made up of particles with single magnetic domains and heat is derived from the energy associated with rapid switching of the particles magnetisation vector. Another type of indirect heating is hysteresis heating which occurs when a system comprises particles with one or more magnetic domains. Heat is derived from the energy associated with domain wall movement driven by an alternating magnetic field close to the target system.

These and still further indirect heating techniques can be used for a wide range of purposes. For example, hyperthermia is a technique known for destroying cancer cells by raising the temperature of the cancer in the order of 5⁰C or above. The subject has been extensively researched over the past 20 years and numerous heating devices have been produced. Some hyperthermia devices are available commercially and are based on either focused ultrasound or electromagnetic radiation. However, none of these devices have been able to accurately deliver high heat loads to deeply situated cancers without also destroying the surrounding normal tissues. Thermoablation is another technique which can be used to treat cancer cells. During thermoablation burning of the cells occurs by raising the temperature of the cells via heating.

There are a number of ways that heat can be applied to the body, either by whole body heating techniques or by local applications of heat. The second method is more favourable and there are at least three techniques of doing this:

• Microwave: Heat produced by microwaves can be directed at tumours that are 1- 3 cm from the surface of the skin. Microwaves are rapidly absorbed as they penetrate deeper into the body. Thus, tumours located at depths greater than 3 cm from the surface of the body cannot be effectively heated with presently used microwave techniques.

• Interstitial RF: Interstitial treatments send RF energy through small needles placed into the tumour. After heating, interstitial radioactive therapy material can be introduced into the tumour site through the same probes used to introduce heat (This is called brachytherapy and has been used as a cancer treatment for many years.) Interstitial hyperthermia can also be used with external beam radiation. This technique allows greater control of heat application, but is an, invasive procedure (the placement of needles can be painful and restricts the movement of the patient). • Ultrasound: This technique uses ultra-high frequency sound waves to produce heat within the tumour. Ultrasound is more easily focused than other energy modalities and can be applied to tumours located from the skin to 8 cm within the body. This allows the treatment of tumours unreachable by other external modalities. Ultrasound doesn't require the use of radiowave shielding devices to protect medical personnel during treatment.

One of the most common applications of induction heating is for sealing the anti-tamper seals attached to the top of such items like medicine bottle caps, toothpaste tubes, and ointment cream containers. A foil seal coated with "hot-melt glue" is inserted into the plastic cap and screwed onto the top of each bottle during manufacture. These foil seals are then rapidly heated as the bottles pass under an induction heater on the production line. The heat generated melts the glue and seals the foil onto the top of the bottle. When the cap is removed, the foil remains providing an airtight seal and preventing any tampering or contamination of the bottle's contents until the customer pierces the foil. It is an aim of the present invention to at least partly mitigate the above-mentioned problems.

It is an aim of embodiments of the present invention to provide apparatus which can be used to heat a target system in a manner which can be carefully controlled and yet which is cost effective.

It is an aim of embodiments of the present invention to provide a method for heating a target system which can be carried out very conveniently and in a cost effective manner.

It is an aim of embodiments of the present invention to provide a technique for destroying cancer cells by raising the temperature of cancer cells by a sufficient amount to cause damage to the cells.

According to a first aspect of the present invention there is provided an apparatus for heating a target system, comprising: at least one coil element locatable proximate to the target system; at least one capacitor element connected to said coil element; a direct current (DC) power supply connectable to said coil element via a switching element; and a switch controller for selectively switching said switching element on at a desired frequency to thereby heat said target system via indirect heating.

According to a second aspect of the present invention there is provided a method for heating a target system, comprising: locating at least one coil element proximate to the target system, at least one capacitor element being connected in parallel with said coil element; and selectively supplying power to said coil element by selectively connecting a direct current (DC) power supply to the coil element at a desired frequency.

According to a third aspect of the present invention there is provided a method for destroying cancer cells, comprising the steps of: indirectly heating a target system proximate to at least one cancer cell by the steps of: locating at least one coil element proximate to the target system, at least one capacitor element being connected in parallel with said coil element; and selectively supplying power to said coil element by selectively connecting a direct current (DC) power supply to the coil element at a desired frequency.

Embodiments of the present invention provide an instrument for the indirect heating of magnetic or magnetisable particles or solids by an alternating magnetic field in the radio range of frequency (about 2kHz to 3MHz). The power, frequency and duration of the heating effects can be varied to suit the system to be heated. The system is defined as the magnetic or magnetisable particles or solids or electrically conductive or electrically conductible particles or solids plus any other substances such as water, biological tissue or solids in contact with them. The instrument can thus be used to heat magnetic or magnetisable particles or solids to test their inherent ability to be heated or to indirectly heat substances in direct contact with the magnetic or magnetisable particles or solids. The instrument can also be used to heat electrically conductive particles or solids to test their inherent ability to be heated or to indirectly heat substances in direct contact with the electrically conductive particles or solids.

Embodiments of the present invention provide an instrument which provides flexibility for a number of operational parameters. Notably, the frequency of an alternating magnetic field can be varied simply by replacing banks of capacitors. Also, the waveform of the alternating magnetic field can be varied to provide, for example, a square wave, sinusoid or saw tooth. This has a number of advantages, as will be appreciated by those skilled in the art. Still furthermore, the size, shape and material of the inductor coil which the target system to be heated is placed in or is placed next to, can be varied. This provides a very versatile system. In addition, impedance matching does not need to be carried out with a specific coil to be used.

Embodiments of the present invention provide the advantage that the power output of an indirect heating apparatus can be varied but that this power output which is responsive to an alternating magnetic field strength can be held relatively constant over a particular frequency range.

Embodiments of the present invention will now be described hereinafter, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates a heating coil and circuitry for energising the coil; Figure 2 illustrates use of the coil to test heating in a test sample; and

Figure 3 illustrates use of a coil to destroy cancer cells.

In the drawings like reference numerals refer to like parts.

Figure 1 illustrates an indirect heating system 10 which includes a winding 11. The geometric design of the coil 11 (number of turns, radius and length) has an effect on a magnetic field strength generated by the coil. It will be appreciated that embodiments of the present invention are not restricted to any specific design of coil and that indeed one or more coils of various designs may be employed in accordance with embodiments of the present invention. The coil is formed from coiled tubing formed of a metal, such as copper. The tubing enables a cooling fluid, such as water, to be pumped through the coil to reduce direct heating effects. It will be understood that embodiments of the present invention are not restricted to use of a tubular coil structure nor to a coil structure including any cooling elements.

A node 12 at an end of the coil 11 is connected to a high voltage rail 13 of a high voltage direct current (DC) power supply 14. The low voltage rail 14 is grounded and is connectable to a coil tap 16 via a MOSFET 17. The MOSFET 17 provides a current path between its drain and source and this current path can be controlled by selectively providing a control voltage to the gate of the MOSFET. This control voltage V_CTRL is provided by a function generator 18 which is connected to the gate via a first low resistance element R₁ and higher resistance element R₂. For example the resistor R-i has a value of 50 ohms whilst the resistor R₂ has a value of 1000 ohms. These values may be selected as appropriate as will be appreciated by those skilled in the art.

A capacitor bank 19 is connected in parallel with the coil 11 between node 12 and a further end of the coil 11. The capacitor element 19 is a removable array of silvered mica tuning capacitors. A set of such capacitor elements is provided so that an array of the capacitors can be selected from the set which has a value which can be used to select when resonance occurs in the coil 11. It will be understood that rather than using an array of capacitors single capacitors having various capacitances could be used or indeed a variable capacitor. The coil 11 and capacitors 19 form a resonant circuit due to the inductive nature of the coil and the capacitive nature of the capacitors being connected in parallel. When supplied with current at a correct selected frequency the circuit resonates. The resonant frequency of the circuit is set by the values of the coil inductance and the capacitors. The frequency can be changed by changing the coil or the capacitors or both. Using banks of capacitors means that a bank can readily be exchanged for another bank to change the resonant frequency. When the frequency is changed the alternating current (AC) magnetic field strength inside the coil changes. At high frequencies the AC magnetic field decreases and at low frequencies the AC magnetic field increases. To maintain a constant AC magnetic field strength inside the coil when changing the frequency, the power output of the power supply is adjusted to compensate.

The waveform of the AC magnetic field can also be adjusted using the control of the function generator 18. This provides an additional level of control but is not required when this level of control is not required.

Two further capacitors C1 , C2 are connected in parallel between the power supply rails 13, 15. The first capacitive element C1 is a 1 μF 100V polyester capacitor. The second capacitive element C2 is a 10 μF 100V electrolytic capacitor. These are included to help keep the power supply impedance low at radio frequencies. It will be appreciated by those skilled in the art that these capacitors may be replaced by other types or indeed omitted entirely.

Embodiments of the present invention make use of a direct current (DC) power source. Rather than use an alternating current (AC) power source which would require complex impedance matching in the circuitry, use of a DC power supply overcomes such problems.

As illustrated in Figure 1 , the coil 11 provides an inductor element connected to the bank of capacitors 19. The resultant oscillating circuit, sometimes referred to as a tank circuit, stores energy in the form of oscillating voltage and current. In this way the capacitor and inductor will exchange energy between them back and forth creating their own AC voltage and current cycles. In this way current through the coil 11 is alternating with the

MOSFET used to pump more power into the circuit at predetermined intervals in phase with the alternating current. Figure 2 illustrates how an embodiment of the present invention can be used to test whether a target system 20 can be heated via indirect heating. The coil 11 is provided with power via the circuitry illustrated in Figure 1 and is located close to a target system. In this sense the target system is defined as the magnetic or magnetisable particle or particles or solids or electrically conductive or electrically conductable particle or particles or solids plus any other substance such as water, biological tissue or solid in contact with them. Such material can be introduced into an environment proximate to a location where heating is to be achieved. The proximate location may be achieved either by locating the sample within a solenoid type coil or surface coil configurations may be utilised where the sample need not be placed in the coil but only adjacent to it. If a target system is selected which can be readily heated then this heating can be advantageously utilised. By locating the coil 11 close to the target system and then energising the coil indirect heating of the target 20 can be achieved by one or more of the methods noted above. It is to be noted that the samples may be of magnetic non- metallic metal oxide compounds which do not heat by all the same mechanisms as nonmagnetic metals but do use the same AC magnetic field for heating. Non-magnetic metals are heated via induction by eddy currents induced by an AC magnetic field which gives rise to dual-resistance heating. If the metals are also magnetic then additional heating via magnetic hysteresis heating is also involved. Plastics may also be heated if they are doped with a necessary magnetic or metallic compound. The type of heating induced by an AC magnetic field is hysteresis, Neel and Brownian motion heating. One, two or more of these mechanisms may be in operation in a target sample 20. Whether or not a target sample is heated by the coil 11 or to what degree is measured by a temperature sensor 21 located close to the test sample.

Figure 3 illustrates how embodiments of the present invention may be used to help destroy cancer cells in a human body. A patient 30 with a cancerous cell 31 is identified and the location of the cancerous cell determined. Next target systems which can be heated in an indirect manner as noted above are introduced into the patient. These may be injected into a patient and include known techniques for locating themselves proximate to a cancer cell. Alternatively the target system to be heated may be surgically implanted. It is to be understood that according to embodiments of the present invention particles to be heated, and/or cells and formulations etc containing or having particles attached to them, can locate at the target site (e.g. tumor) by passive techniques (random or characteristic properties of the target), direct location to the target site (e.g. by injection) or by a targeting technique of the particle and/or cells and formulations etc. containing or having particles attached to them. The coil 11 is then brought into close proximity with the cell 31 and power supplied so as to heat the target system. Heating the target system which is located close to the cancer cell heats the cancer cell. Sufficient heat is provided so as to harm or destroy the cancerous cell.

The heating properties of the target system, and in particular any particles therein, are dependent upon many factors including particle size and shape, chemical composition and location. For example, whether the particles are free in solution or attached/ embedded in a solid. In addition, the rate of heating is dependent upon the AC magnetic field strength and the frequency of the AC magnetic field. For this reason, it is important to be able to change the frequency of the instrument in order to fine the most appropriate frequency for heating the sample. The alternating magnetic field strength is adjusted by selecting the power input from the DC power supply.

In addition when considering heating of samples in living tissue, in order to prevent intrinsic tissue heating via eddy currents, the product of the AC magnetic field strength and the frequency of the alternating magnetic field should be less than 4.85 x 10⁸ AIvT¹S^'1. This value has been determined from experimental data as is known to those skilled in the art. To be within this limit and maintain the flexibility of choosing both alternating magnetic field strength and frequency, it is helpful to be able to adjust both the alternating magnetic field strength and the frequency.

Embodiments of the present invention provide a number of advantageous features. Firstly, power deposition in the biological systems is limited by the frequency and power of the RF radiation. By varying the power and frequency the system can be tuned to remain inside biologically acceptable limits. Secondly, embodiments of the present invention provide a system which is relatively simple and inexpensive in design which allows it to be purchased and easily serviced. Furthermore, the design circuitry does not require excessive cooling of electronic component parts which makes the circuitry relatively cheap and efficient to manufacture and maintain. Likewise, it is not always necessary to cool the coil.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

1. Apparatus for heating a target system, comprising: at least one coil element locatable proximate to the target system; at least one capacitor element connected to said coil element; a direct current (DC) power supply connectable to said coil element via a switching element; and a switch controller for selectively switching said switching element on at a desired frequency to thereby heat said target system via indirect heating.

2. The apparatus as claimed in claim 1 , further comprising: said switching element comprises a MOSFET and said switch controller comprises a function generator arranged to selectively supply a waveform having a voltage arranged to turn a MOSFET drain-source current on.

3. The apparatus as claimed in claim 1 or claim 2, further comprising: said at least one capacitor element comprises a removable array of tuning capacitors.

4. The apparatus as claimed in claim 3, further comprising: said removable array comprises one array from a set of removable arrays each associated with a respective reactance value.

5. The apparatus as claimed in any preceding claim, further comprising: said at least one coil comprises a coiled metal tube through which cooling fluid can be pumped.

6. The apparatus as claimed in any preceding claim, further comprising: a coil tap connected in a current path of said switching element, a selectable position of said coil tap determining a magnetic field strength of the coil element when said coil element is energised.

7. The apparatus as claimed in any preceding claim, further comprising: said target system comprises one or more magnetic particles or a magnetic solid material.

8. The apparatus as claimed in claim 7 wherein said apparatus is arranged to heat said target system via hysteresis heating effects.

9. The apparatus as claimed in any preceding claim wherein said target system comprises one or more electrically conductive particles or an electrically conductive solid.

10. The apparatus as claimed in claim 9 wherein said apparatus heats via inducing eddy currents in said target systems.

11. A method for heating a target system, comprising: locating at least one coil element proximate to the target system, at least one capacitor element being connected in parallel with said coil element; and selectively supplying power to said coil element by selectively connecting a direct current (DC) power supply to the coil element at a desired frequency.

12. The method as claimed in claim 11 , further comprising the steps of: for each target system, selecting a desired frequency at which said coil element is supplied with power.

13. The method as claimed in claim 11 or claim 12, further comprising the steps of: heating the target system via indirect heating.

14. The method as claimed in claim 13 wherein said target system comprises one or more magnetic particles or a magnetic solid material and said indirect heating comprises heating via a hysteresis heating method.

15. The method as claimed in claim 13 or claim 14 wherein said target system comprises one or more electrically conductive particles or an electrically conductive solid and said indirect heating method comprises heating via inducing eddy currents in the target system.

16. A method for destroying cancer cells, comprising the steps of: indirectly heating a target system proximate to at least one cancer cell by the steps of: locating at least one coil element proximate to the target system, at least one capacitor element being connected in parallel with said coil element; and selectively supplying power to said coil element by selectively connecting a direct current (DC) power supply to the coil element at a desired frequency.

17. The method as claimed in claim 16, further comprising the steps of: introducing the target system into a human or animal body comprising said at least one cancer cell.

18. The method as claimed in claim 16 or claim 17, further comprising: delivering one or more magnetic particles inside or adjacent to said at least one cancer cell and heating said magnetic particles via hysteresis heating.

19. The method as claimed in any one of claims 16 to 18, further comprising the steps of: delivering one or more electrically conductive particles inside or adjacent to said at least one cancer cell and heating said electrically conductive particles via induction heating.

20. Apparatus constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

21. A method substantially as hereinbefore described with reference to the accompanying drawings.