SPECIFICATIONElectromagnetic applicatorThe present invention relates to an electromagnetic applicator intended for heating material, especially body tissue, and comprising a resonant circuit for generating radiation in the range of from below 27 MHz to above 915 MHz.
An applicator of this type may be used for heating tissue in the treatment of e.g. cancer. It may also be used in industrial heating, drying and food processing.
In clinical use, such applicators should be easy to use. That is they should be compact, lightweight, well matched to the source of electromagnetic energy, and it should be possible to make the size of the heated area equal to the treatment area. Further, to obtain reasonable penetration of the radiation into high water content tissue, the frequency should be reasonably low. For example, plane wave penetration into muscle at 2450 MHz is only about 1 cm whereas at 27 MHz it is about 15 cm.
UK patent application No. 8402813, filed on 2nd February 1984, describes methods of making efficient compact applicators for operation at frequencies as low as 100 MHz. Such applicators have now been in clinical use for over two years (Johnson et al, IEEE BME 31, 1984, pp. 28-37). These applicators are designed to be matched to the tissue to be treated, usually spaced from it by a temperature-controlled, water-filled cushion or bolus. The applicator also incorporates an overlying layer of appropriate thickness of high-permittivity low-loss dielectric which makes the frequency of operation almost independent of the load and contains the near field of the radiation which would otherwise cause local hot spots.If it is desired to operate at frequencies of the order of from 100 to 200 MHz to obtain improved heat penetration, the cost and weight of the required high-permittivity low-loss dielectric are disadvantages, even though they represent a large saving over a comparable dielectric loaded waveguide.
Inductive applicators consisting of solenoids or pancake coils tuned by capacitors to operate at frequencies below or about 27 MHz do not provide reasonably uniform heating patterns and can produce hot spots due to the electric field that exists between adjacent turns (Hand, Proc. IEE,128 PtA, pp. 593-601).
An inductive applicator consisting of current-carrying conductors above a ground plane, tuned by capacitors for resonance at 150 MHz has been described and shown to overcome the excessive surface heating associated with capacitor or coil systems (Anderson et al, IEEE BME 31, 1984, pp. 21-27), and the idea has been extended to 27 MHz in a linear induction applicator. (Franconi et al, IV International Symposium on Hyperthermic Oncology, Aarhus, Denmark, July 1984).
It is an object of the present invention to provide an electromagnetic applicator which is compact, lightweight and well matched to the source of electromagnetic energy for varying load or temperature conditions.
According to the invention, this object is achieved by the provision of an electromagnetic applicator which is characterized in that the resonant circuit comprises a single turn coil and a capacitor which is arranged between the ends of the coil, a screening box accommodating said resonant circuit and having a radiating aperture adjacent which a part of the single turn coil is located, electromagnetic energy being radiated from the part of the single turn coil located adjacent said aperture and radiation from the remainder of the resonant circuit being prevented by the screening box.
The invention will now be described in greater detail hereinbelow in some embodiments and with reference to the accompanying drawings, in which:Fig. 1 shows an inductively coupled applicator suitable for operation at about 27 MHz.
Fig. 2 shows a capacitively coupled applicator suitable for operation above about 100 MHz.
Fig. 3 shows a variant of the applicator in Fig. 1; andFig. 4 shows a variant of the applicator in Fig. 2.
In Fig. 1, there is shown an applicator comprising a resonant circuit having a single turn coil forming a radiating source, and a capacitor connected between the ends of the coil. The coil may consist of a flat high-conductivity plate 1 which is folded back to form a U, as seen in Fig.
1. The capacitor consists of conducting plates 2 which are arranged between the arms 2a of theU and are substantially parallel to the base 2b of the U. Alternatively, the capacitor may be a commercially available R.F. capacitor.
The resonant circuit is accommodated in a compact metal screening box 7, one side of which is apertured and contains the central part of the U. This side of the screening box 7 may be covered with a suitable dielectric 8 (e.g. perspex or P.T.F.E.). Electromagnetic energy is radiated from that part of the coil which is located adjacent the aperture due to R.F. current carried in the coil. Radiation from the remainder of the resonant circuit is however prevented by the screening box.
The radiator may be compared in principle to a dipole source, the ends of which are screened so that energy is only radiated from the central region of current maximum, thus preventing the high electric fields normally associated with the dipole ends from producing local hot spots in the adjacent tissue.
The resonant circuit may be excited by variable inductive or capacitive coupling to match an energy source.
Fig. 1 shows an example of an inductive coupling in which the resonant circuit is excited by a single turn coupling loop 4 which is rotatable and connected by a coaxial input 4a and a coaxial cable (not shown) to an energy source (not shown) which supplies electromagnetic energy to the applicator. The matching of the applicator to the energy source is brought about by rotating the single turn coupling loop 4. Alternatively, the loop 4 may have more than one turn.
Coarse control of the resonant frequency is obtained by moving the position of attachment 5 of the capacitor plates to the arms of the U, either nearer to or further from the base of the U.
Fine tuning is provided by moving a conducting plate 6 to vary the capacitance between the arms of the U. This construction is preferred for operation below about 100 MHz, such as 27MHz. If so desired, ferrite rods 3 (125 mmx8 mm dia. Neosid F 29) may be suitably secured to the inside of the arms of the U to reduce the resonant frequency by about 25%.
At frequencies above about 100 MHz, the resonant circuit is more conveniently capacitively coupled to the energy source. In Fig. 2, there is shown an example of how the applicator may be designed in such a case. A conducting plate 9 is connected to the coaxial input 4a. The plate 9, together with an arm 2a of the U, forms a capacitor the capacitance of which may be varied and, hence, the amount of energy transmitted to the resonant circuit from the energy source connected to the input, in that the plate 9 is moved closer to or further from the arm 2a of the U.
In both the device of Fig. 1 and the device of Fig. 2, the operating frequency is almost independent of the load, but may be fine-tuned by the plate 6 during operation, and both the plate and the tuned circuit may be insulated from the energy source. As opposed to Fig. 1, the device of Fig. 2 has the plate 6 located on the opposite side of the U in relation to the coaxial input 4a and movable in a direction towards and away from the arm of the U.
The above-described method of frequency control requires adjustment of the tuning capacitance between the arms of the U. The frequency adjustment available by this method at frequencies below about 100 MHz, such as 27 MHz, is thus limited and at frequencies above about 100 MHz, variation of the coupling capacitance to the coaxial input requires a corresponding adjustment of the tuning capacitance.
Figs. 3 and 4 show further embodiments of the present invention. The first of these embodiments provides a substantial tuning adjustment, particularly at lower frequencies, such as 27MHz, and the second a means of varying the coupling capacitance independently of tuning effects.
Fig. 3 shows a medical applicator similar in principle to that shown in Fig. 1 but incorporating a more effective tuning method. As in the applicator of Fig. 1, a flat high-conductivity plate 1 is folded back to form a U, and conducting plates 2 attached to the arms of the U form a capacitor, The position of attachment 5 provides coarse setting of the resonant frequency of the assembly which is excited by the coupling loop 4, the rotation of which matches the assembly to an energy source. As opposed to the device in Fig. 1, fine tuning is provided by rotation of a high-conductivity strip 10, induced currents in which effectively vary the inductance of the resonant circuit.In the position shown where the plane of the strip 10 is perpendicular to the plane of the radiating plate 1 and parallel to the direction of current flow, the effective inductance is at a minimum corresponding to the highest resonant frequency. When the plane of the strip 10 is rotated through 90 to be parallel to the plane of the plate 1, its effect on the circuit inductance is a minimum and the resonant frequency is at its minimum. As in Fig. 1, the assembly is enclosed in a metal screening box 7 with the radiating metal plate 1 covered by a suitable dielectric 8 which forms the radiating aperture. The construction shown in Fig. 3 is suitable for operation below about 100 MHz, such as 27 MHz where a fine frequency adjustment of at least 1 MHz is obtained.
This method of fine frequency adjustment can also be applied at frequencies above 100 MHz where it offers an alternative to the capacitor plate 6 shown in Fig. 2.
At frequencies above about 100 MHz, the preferred method of coupling to the coaxial input is capacitive. The effect of varying such coupling capacitance on the resonant frequency becomes noticeable as the desired operating frequency band is increased because it then becomes a significant fraction of the capacitance required to tune the resonant circuit.
Fig. 4 shows an applicator similar in principle to that shown in Fig. 2, but having a different method of coupling adjustment and fine tuning. In this construction, the resonant circuit formed by the U and the capacitor plates 2 is modified by extending the arms of the U so that additional capacitor plates 13 are provided symmetrically in a plane parallel to and suitably spaced from the nearest capacitor plate 2. Above this plane and parallel to it is located a split circular conducting disc, the two halves 11, 12 of which are connected, respectively, to the centre and outer conductors of the coaxial input. The plane of the split disc 11, 12 can be moved nearer to or further from that of the plates 13 to provide fine frequency adjustment.
Additionally, the split disc 11, 12 can be independently rotated. In the position shown in Fig. 4, coupling to the coaxial input is a maximum when the two half discs 11, 12 are respectively aligned with the plates 13. Rotation of the split disc through 90 couples the half disc 11 equally to each of the two plates 13 so providing minimum excitation to the tuned circuit.
Rotation and spacing of the split disc 11, 12 can be achieved by suitable screw adjustments.
Alternatively, a coaxial input fixed to the screening box 7 can be provided, using either a sliding contact or a flying lead to the half disc 11. In the latter case, rotation of the disc should be limited to about a quarter turn.
The following is a list of a representative sample of applicators constructed according to this invention.
Frequency,MHz 22 27 168 200 400 900Radiator,LxW, cm 18x14 18x20 8.5x9.5 8x9 8x9 3x4.5Aperture,LxW, cm 22x20 22x26 11x13 11x13 11x13 5x7Height, cm 7 24 8.5 8 6.5 4Weight, kg 8.6 5.5 1.0 0.6 0.5 0. 2 The applicators operated satisfactorily at the maximum test power available which was 600 W below 200 MHz and 50 W above 400 MHz. If required, a cooling fluid may be circulated for continuous high power operation. Air dielectric was used for all except the 22 MHz applicator for which it was liquid dielectric. Either liquid or solid dielectric, such as paraffin or P.T.F.E., may be used to reduce the capacitor dimensions. Alternatively, a commercial R.F. capacitor may be employed.A particular feature of the invention is that the operating frequency can be reduced almost independently of the aperture size, although it cannot be increased to the extent that the aperture becomes a significant fraction (e.g. > 1/5) of the free space wavelength. The applicators may be operated in arrays, either to improve field penetration or to control heating profile.
Alternatively, the radiating current sheet or flat plate may be curved or shaped, or may be divided into two or more segments of equal or differing widths in the direction of current flow to improve or control heating profile. The aperture shape can be square or rectangular, circular or elliptical with the longer dimension either parallel to or perpendicular to the current direction (which is the direction of polarisation of the radiated electric field). The applicator can be placed in contact with the tissue or spaced from it with or without a bolus.