fig1 a and 1b , which illustrate the problems inherent to state - of - the - art e - field applicators , have already been discussed in the background section above . fig2 shows a conductor in the form of a helix 20 traversed by rf current i24 . a helix radius in a catheter application is typically a = 1 mm and maximum desired radius of tissue heating for cardiac ablation is r = 10 mm . the resultant magnetic field typified by h21 , h22 , and h23 has primarily an axial ( z ) component and induces a transverse electric field e . sub . φ typified by e21 , e22 , and e23 ( and a proportional current density not shown ), primarily in the circumferential direction around the helix . the circumference of e22 corresponds to r . at an operating frequency of 915 mhz , and within a cylinder of a radius of 10 mm , the magnitude of induced electric field e . sub . φ ( and corresponding current density j = σe . sub . φ ) decreases approximately as a / r , where a is the radius of the helix . the z component of electric field e z , which follows field lines similar to h 21 , h 22 and h 23 , decreases with radius r even more slowly than a / r . hence the heating power dissipation decreases no faster than ( a / r ) 2 . for a typical catheter radius a = 1 mm and desirable depth of heat penetration in ablation r = 10 mm , the ratio of heat dissipation at the catheter wall to heat dissipation at r = 10 mm is approximately 100 : 1 . it should be noted that this is a major improvement over the ratio of heat dissipation for the e - field applicator which for the same conditions is 10 , 000 : 1 . with a solenoidal applicator , the effective heat dissipation radius r can be adjusted : r increases with decreasing frequency . for ablation of cardiac arrhythmias , ism ( industrial , scientific , and medical ) frequencies of 915 mhz and 2450 mhz are of interest . for hyperthermia treatment of cancer , a wider gamut of frequencies is needed depending on the size of a tumor . fig2 b shows an equivalent circuit of a helical heating applicator . within a volume inside a radius of 10 mm , a circular induced electric field e 100 multiplied by the length of its circumference gives a potential around each cylindrical shell which is approximately equal . the shell of lossy medium adjacent to the helix , energized by e21 , the shell at the intermediate distance energized by e22 , and the shell corresponding to r = 10 mm energized by e22 , appear in fig2 b as parallel resistances r21 , r22 , and r23 respectively , exposed to the same potential . current source i24 feeds the three resistances . now , if desiccation occurs adjacent to the helix , resistance r21 increases . this reduces power dissipation in r21 and increases power dissipation in resistances r22 and r23 . in general then , as power is increased to a point of desiccation at a catheter surface , the heat delivered to a desiccated volume decreases in a solenoidal applicator while it increases in an e - field applicator . thus , the solenoidal applicator is much less likely to cause excessive desiccation but even if desiccation occurs , it will not lead to a decrease in power dissipation in remote tissue at r = 10 mm . the helix in fig2 a is an example of a solenoidal applicator structure , characterized in general by current loop or loops and an electrical short as an end termination . solenoidal applicator generates a magnetic field in the surrounding tissue . this magnetic field by induction generates in turn , an electric field and current which heats the tissue . in contrast , the e - field applicator has an electrical open end termination and the primary , rather than induced , electric field heats the tissue . the preferred embodiment of the helical solenoidal applicator in an ablation catheter is shown in fig3 . coaxial line 43 consists of a center conductor 44 ( 0 . 16 mm diameter ), a dielectric 46 ( 1 . 35 mm outside diameter ), a metal braid 45 and insulating sleeve 57 ( 1 . 8 mm outside diameter ). small diameter and flexible construction makes the coaxial line 43 suitable for a biomedical catheter application . helical winding 50 is wound on a ceramic or ferrite core 51 . a heat - shrunk teflon ( tfe : tetrafluoroethylene sleeve 53 covers the helical winding 50 . distal end of the helical winding 50 is connected at distal peripheral terminal 58 to distal electrode 56 and to bypass capacitor 55 . bypass capacitor 55 is connected to braid 45 through metallized coating 52 on the inside of core 51 . the function of the bypass capacitor 55 is to ground the rf energy . thus during rf current flow through helical winding 50 , distal electrode 56 has no rf voltage thereby preventing e - field heating . distal electrode 56 in conjunction with a proximal ring electrode 47 picks up a cardiac electrogram voltage between them . the distance from the beginning of proximal ring electrode 47 to the end of the distal electrode 56 is 20 mm . a number of turns on the helical winding 50 is chosen so that at an operating frequency of 915 mhz , the helix is somewhat short of being a quarter wavelength resonator . the proximal end of the helical winding 50 is connected to a variable tuning capacitor 48 at proximal peripheral terminal 49 . variable tuning capacitor 48 is moved with respect to neutral electrode 47 during manufacture for tuning to a precise quarter wavelength resonance . details of the tuning capacitor 48 are shown magnified in fig3 a . rf power is coupled into the helical resonator by connecting the center conductor 44 to the helical winding 50 at feed terminal 54 . the connection at feed terminal 54 is shown magnified in fig3 b . the position of feed terminal 54 on the helix is selected for good match between the characteristic impedance of the line and the impedance of the resonator . the choice of an axial quarter wavelength resonator is by no means unique . one could just as well select any multiplicity of quarter wavelengths e . g ., half wavelength or full wavelength resonators . in some applications , it may be desirable to distort the axisymetrical form of the induced e - field . this can be accomplished by partially covering a dielectric sleeve 53 with metal foil ( not shown ). currents induced in such foil modify the shape of a heating pattern and so serve as an aperture antenna . an asymmetrical field pattern can also be accomplished by a loop antenna . in cardiac ablation , it is highly desirable to be able to monitor intracardiac electrogram just before and after the application of heat . fig4 shows a block diagram of a system which combines dosimetry control of the solenoidal heat delivery with monitoring of intracardiac electrograms . the rf power is generated in an rf power source 41 . the rf power is controlled and monitored in controller 42 which couples the rf power to the coaxial line 43 through capacitor 62 , which for rf represents substantially a short - circuit . the center conductor 44 is attached at feed terminal 54 to the helical winding 50 wound on a core 51 . quarter wavelength resonance tuning is accomplished by adjustment of variable tuning capacitor 48 connected to the helical winding 50 at proximal peripheral terminal 49 . the rf ground is maintained by the bypass capacitor 55 connected to the distal electrode 56 and then to helical winding 50 at distal peripheral terminal 58 . distal electrode 56 in conjunction with the proximal ring electrode 47 picks up the local intracardiac electrograms and feeds this electrogram signal through the coaxial line 43 to capacitor 62 . capacitor 62 represents a short circuit for the rf power and an open circuit for the much lower frequency band ( typically 0 . 1 hz to 100 hz ) associated with intracardiac electrogram activity . the electrogram signal appears therefore on lines 63 and 64 at the input to the low - pass filter 61 . filter 61 has a high input impedance to the rf and hence has no effect on transmission of rf power between controller 42 and coaxial line 43 . filter 61 blocks the transmission of the rf power to switch 60 while allowing passage of the electrogram signal . switch 60 is closed simultaneously with application of rf power , thus providing additional protection for monitor 59 . electrogram signal processing , display , and recording is accomplished by monitor 59 . standard existing equipment is suitable for application as monitor 59 . solenoidal catheter applicator for hyperthermia treatment of tumors follows largely the same design as the one represented in fig3 except that in this case , there is no need for the distal electrode 56 and the proximal ring electrode 47 . since the depth of heat penetration depends inversely on the square root of frequency , the frequency of operation and the helical winding design can be tailored to the required depth of penetration depending on tumor size . while certain specific embodiments of improved rf heating applicator and systems have been disclosed in the foregoing description , it will be understood that various modifications within the scope of the invention may occur to those skilled in the art . therefore it is intended that adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments .
radiofrequency heating applicator , located at the distal end of a coaxial line catheter , produces deeper and more uniform heat dissipation . the active applicator element is a conductor helix fed via the coaxial line . the applicator has provisions for interception of intracardiac electrogram signal . a cardiac ablation system using the above catheter , ablates cardiac tissue responsible for ventricular tachycardia . the ablation system provides means to monitor intracardiac electrograms and to control the rf power . a variation of the helical applicator can be used in a hyperthermia system for treatment of malignant tumors .