in fig3 & amp ; 4 the minimal design is diagrammatically shown . no vulnerable to failure active semiconductor is used in the isolated patient circuit section shown which is passive in its regulation by the minimal component rf excitor circuit driving the primary side of the isolation transformer . the only active parts in the isolated power delivery section ( the patient circuit ) comprise the low power ( less than 100 mw ) switching and passive monitoring devices shown in fig1 . further , the monitoring function of other deployed electrodes will discern faulty switching mechanisms when in operation . the natural and uncompensated regulation curves , meaning regulation achieved without feedback control , are shown in fig6 ( a ),( b ),( c ) which are derived from the circuitry shown in fig7 where it can be seen that at short circuit the rf current is limited and over the range of tissue impedance power is delivered by virtue of the nature of the excitor circuit and resonant lc tank circuit phase inter - action . given a fixed phase and amplitude at the excitor output port , an excess current thru the transformer and series resonant tank circuitry reduces disproportionately the voltage amplitude available at the tissue impedance . a further graph 6 ( c ) is shown indicating the current regulation capacity of the circuit and includes short circuit . the degree of non - linearity of the power vs tissue impedance is largely determined by the separation of excitor frequency and the natural frequency of the resonating tank circuit . such boundary condition of limited power dissipated at short circuit impedances between the electrodes is highly desirable to avoid momentary tissue carbonisation ( when electrodes are inadvertently shorted ) such as might happen if overt microprocessor control were necessary with associated delays in regulation . it is also to be noted that at high tissue impedances , which represent desiccated tissue , the energy output is relatively constant . this is a unique and desirable property of the resonant lc output stage in this embodiment . it is further noted in these natural regulation curves 6 ( a ) that the maximum power output occurs between 200 and 450 ohms which is suited to the impedance of tissue in the early stages of cellular denaturation and excretion of cytoplasm conducive to convection and transport of heat between the electrodes . such a regulation curve commencing at lower energy and proceeding to maximum output power in this region of maximum liquid convection and conduction effects which are prevalent at the time of maximal cytoplasm excretion in the target tissue volume ensures better lesion integration and faster lesion creation . at the time of maximal convection the temperature distribution is also most uniform which is the optimum period to be depositing the maximum energy in the tissue . of collateral significance the rates of change of tissue impedances around the electrodes in this impedance range is maximal thus allowing greater sensitivity of the method herein and below described of determining the distribution of impedance throughout the said spatial network of electrodes . the application of feedback to “ flatten ” such natural regulation curves by adjustment of the excitor waveform as shown is held as not desirable and in the embodiment herein is applied minimally . such restriction of highly accurate constant power over the impedance range ( a flat curve ) is not efficacious given the advantageous characteristics of the output power versus load ( resistance ) of this embodiment of resonant lc tissue desiccator / oscillator as described herein . it is also emphasised that the reduction in the time taken for delivery of the therapy is a critical factor due to the usual discomfort of the patient undergoing such therapy and consequently such regulation curves are considered optimal in nature and a salient feature of this invention . turning now to fig5 , in accordance with an explanatory embodiment of the present invention , four needle like probes 20 a , 20 b , 20 c and 20 d are used . these needles have an insulated shaft , and a single electrode tip which is connected to a corresponding conductive lead 24 . in fig5 the four probes 20 are shown inserted into tissue lying immediately adjacent a nerve 25 which is generally transverse to the direction of insertion of the probes . it will be appreciated from fig5 that since each of the electrode tips 23 has an appreciable longitudinal extent , the four electrodes as placed define a thin , generally elongated volume 27 indicated by broken lines in fig5 which has a thickness which is comparable to , and determined by , the longitudinal extent of the electrode tips . the location of the probes 20 and their depth of insertion are arranged so that the volume 27 closely follows the course of the nerve 25 and may be on , or closely adjacent to , a bone surface . rather than energise the probes 20 in monopole fashion as indicated in fig1 , the probes 20 are energised in bipolar fashion so that each of the six pairs of equal area electrodes 23 a / 23 b , 23 b / 23 d , 23 d / 23 c , 23 c / 23 a , 23 a / 23 d and 23 b / 23 c are energised in turn . additionally the electrodes can have alternate management of current density surrounding each electrode by opposing one electrode with any other two , so electrode 20 b can oppose 20 c + 20 d where the current density on the electrodes c & amp ; d are halved in relation to the current density on 20 b at whatever power level occurs on the regulation locus . all combinations of electrodes can be “ activated ” thus 6 of 2 or pairs , 4 of 2 to 1 , and 4 of 3 to 1 discharge “ sets ” are available . all patterns may be activated with different power levels . this assumes that the tissue is somewhat homogenous however if not subsequent adaptations of electrode pattern will be produced to create equally high impedance over the entire volume . in this way the entire volume 27 is inherently covered by a plethora of possible current pathways , however the strategies to achieve this homogeneous lesion needs be highly adaptive and optimised by additional control means to determine the end point of energy delivery . in the simplest example , rf current is made to pass between electrodes 23 a and 23 b in the first instance and similarly through each of the other five pairs in turn in a manner which may be termed sequential bi - polar excitation . for reason of reducing the time of power application it is also possible to activate two pairs , say 23 a - 23 b and 23 c - 23 a simultaneously , or also 23 a - 23 b / c or 23 b - 23 a / c / d . this may be consequent to the control algorithms below mentioned determining that current density of needles a or b need be in a region of × 2 current density or × 3 the minimum current density for some period of time . as coagulum builds up around an electrode it is necessary to monitor the changes occurring throughout the lesion and utilise algorithms dependent on sensory mechanisms provided by this device by way of momentary activation of electrodes as discussed previous . ( i ) each needle does not asymmetrically accumulate coagulum with respect to others in this network of electrodes to the point it is of high impedance and unusable for further energy delivery . ( ii ) each current pathway is monitored by a third inactive electrode which can detect the impedance changes between the active electrodes by momentarily sampling its own impedance path value to each of these active electrodes . in overview terms each electrode in the deployment can also act as a sensor of the distribution of tissue impedance which determines current distribution and attendant heat production . ( iii ) the degree of homogeneity in an adequately desiccated lesion is an indication that viable tissue may or may not remain between pathways . of significant benefit assisting lesion integration is the fact that as tissue desiccates its impedance increases — leaving lower impedance pathways attracting higher current density which facilitates lesion integration and end - point homogeneity . ( iv ) the end point of energy delivery will occur when such monitoring electrodes determine minimum change of distributed impedance for a number of activation sequences — such impedance will have reached a maximum and remain unchanged within programmed criteria . ( v ) according to heurism and mathematics in any medium such lesion integration , homogeneity and definition will only be made with a requisite density of electrodes . ( vi ) the monitoring electrodes perform a crucial part of the procedure after the energy delivery is considered complete according to ( iv ) above , by sampling and detecting impedance changes indicating instability of the thermal lesion . this procedure takes a short while , with no sensation felt by the patient , and such undesired changes in impedance post activation would indicate a limited anaesthetic period caused by reperfusion of the intended thermal lesion . extension of the activation process is then countenanced . turning now to fig7 , from the schematic circuit diagram of the radio frequency generator of approximately 30 w power , it will be seen that a microcontroller 30 is connected to , and controls the operation of , a multiplexer programmed into a programmable logic device 31 , a lagging phase full bridge controller 32 , and the variable voltage dc supply 34 also controlled by the microcontroller 30 . the variable control voltage supply 34 determines the dc voltage applied to the full bridge activated electrode set and is applied to a full bridge circuit formed by four mosfets q 1 - q 4 which are connected to the primary winding of a transformer t 1 which has a centre tapped secondary winding . the full bridge circuit has monitoring of the dc voltage applied across it and also the primary current . the patient circuit monitors the current in c 1 and c 2 by current transformer which indirectly is a measure of the output patient circuit voltage . the full bridge circuit acts as an excitor energy source for the resonating parallel lc tank circuit . each of the secondary windings passes through two windings of mutually coupled inductors l 1 and l 2 respectively to each of two capacitors c 1 and c 2 which are connected in series and equal in capacitance . the centre point of the capacitors is connected to the centre tapping of the secondary winding via a high isolation current transformer ct 1 , the output of which is connected to the phase controller 32 . the capacitors c 1 and c 2 together with the entire inductance constituted by the self inductance of transformer t 1 and the coupled inductors l 1 and l 2 constitute a resonant tank circuit of a specific resonant “ quality ” termed “ q ” which is damped by further parallel resistance represented by the tissue enclosed by the activated bipolar electrode pairs or sets . the frequency of oscillation of the excitor generator , preferably approximately 490 khz , is determined by the set frequency of the excitor controller of the full bridge circuit . the natural resonant frequency of the resonant tank circuit is designed to be close to , but not equal to , this excitor energy feed frequency . at specific settings of the frequency difference between excitor and resonator the changing tissue impedance during desiccation reflects greater or lesser current through the coupled inductors which represent a complex impedance such that for example , greater current drawn by the load provides less output voltage and lesser current provides greater output voltage given the voltage over the full bridge has been set to a constant value known to be commensurate with an output regulation of power locus such as shown in 6 ( a ). as is expected at other settings close to resonance , as the load impedance increases the resonant voltage also increases which also results in a form of current regulation shown in graph 6 ( b ). an example of the resonant voltage curves is shown in 6 ( c ). in simpler terms the manner this circuit regulates output power ( it being proportional to vrms multiplied by irms ) is where , increasing current across the coupled inductors is reflected to the primary of the transformer t 1 as a phase lag which then draws more current from the fixed frequency excitor oscillation . by design the phase relationship can ensure limited current at zero impedance 6 ( b ) and such other desirable power output regulation characteristics as described herein . such a circuit provides a family of curves and characteristics depending principally on the relation between the excitor frequency and the resonant output frequency . the circuit is able to dynamically alter the frequency of the excitor stage or to increase or decrease energy supplied to the resonating tank by pulse shaping options which could be supplied by transformer ct 1 which determines the “ flatness ” of curves of fig6 ( a ) if required . such manipulations are able to be performed by the microcontroller in concert with the pld device in the preferred embodiment but it seems not to be advantageous . the sole control of output power is set by the output voltage from the variable output voltage ac to dc power factor controller ( 34 ) which is set a priori with the voltage commensurate with the desired output power level , or regulation curve for the therapy . at high impedances large circulating currents occur in the excitor bridge as the resonant current needed to activate the capacitors in the resonant tank circuit are alternately charged and discharged of voltage at high frequency . such currents are largely conducive to zero voltage switching of the excitor bridge switching fets — a situation highly desirable for reliability and efficiency . such high currents as are circulating are reactive and returned to the dc driving voltage output . this circuit has an efficiency only limited to a practical degree by the magnetisation power loss in the inductors and transformer , consequently achieving such high efficiency as 95 %. hitherto efficiencies of devices using “ class ab ” amplifier output stages for rf ablation are generally 50 %. for the output stages alone . the dissipation of this energy loss needs large heat sinking and packaging and less than optimal reliability due to the necessary use of power semiconductors in linear mode . this subject embodiment circuit of fig7 can be thus seen to provide the natural regulation that allows fast switching between electrode bipolar pairs or sets that have different impedances , it not being necessary to switch off the continuous wave output of the resonating tank circuit provided a limiting damping resistance is applied in parallel with the resonant tank circuit which otherwise theoretically approaches infinity at open circuit or other damping dissipative load , but which in practice does not due to natural circuit element parasitic losses . any loading by tissue causes immediate reversion to the regulation curve locus at a “ power out ” position dependent on the load impedance value . the features combined of high efficiency , minimal parts count , minimum active parts , no linear active parts , high isolation to patient circuit and attributable in part to the natural self regulation properties of the circuitry allow the device to be embodied in a small sterilisable package and used conveniently as a small medical instrument . in further explanation of the circuit in fig7 , each of the capacitors c 2 and c 1 is connected via a corresponding switch ea 1 and eb 1 respectively each of which is joined to a corresponding electrode e 1 shown in more detail in fig8 . the system can have any number of electrodes but is typically sixteen . as a consequence of this arrangement , each pair of electrodes ( eg e 1 - e 2 ; e 1 - e 3 ; . . . , e 1 - en ; etc ) is able to be energised in two ways . for example , the pair of electrodes e 1 and e 2 can be energised by turning on the switches ea 1 and eb 2 or by turning on switches ea 2 and eb 1 . in general the microcontroller will sequentially energise all pairs or sets of electrodes in a deployment in a time period referred to herein as a “ frame period ”. each electrode pair or set is available for activation for an equal period inside this frame period such that each electrode bipolar pair or set has a maximum activation time equal to the frame time divided by the number of electrode pairs or sets to be activated each frame period . this activation period available to each electrode pair or set is commonly referred to as the pwm period and such is the control that within this pwm period the electrodes are activated with a “ duty cycle ” such that relative energy delivery between electrode pairs or sets is controlled as determined in assessment by the control algorithms as to the need or not of energy to create an integrated lesion of the desired form . as pairs or sets enter desiccation and are dropped out of the sequence or sets get larger for simultaneous activation , the frame period will not be constant . in this particular embodiment , each probe ( for example probe 20 a in fig5 ) has a single common sized electrode and thus the number of electrodes and probes are equal . in the general case and for other desired thermal lesions electrodes need not be common sized nor need there be equal numbers of electrodes on probes . the equivalent circuit in this example formed between each pair of electrodes is indicated schematically in fig5 and appears to the system to be a resistance connected in parallel to the output port of the resonator circuit . irrespective of small parasitics of leads , selector switches etc and connections the inter - electrode impedance is predominantly resistive and not reactive . as seen in fig8 , each of the probe switches ea ( n ), eb ( n ) is formed from two series connected fets — qa and qb . a signal transformer tn has its primary winding supplied by the selector 31 and its secondary winding is connected to a signal diode bridge . the output of the diode bridge is connected to the gate of each of the fets qa and qb such that a resistor dumps gate charge to quickly terminate gate on charge and the remaining resistor avoids gate circuit coupling at switch on threshold between fets qa and qb . other means of effecting the power switching are available to those skilled in the art and such function is recognised as that of a solid state relay and may involve photovoltaic isolation . in order to provide an indication of the current flowing through the switch qa - qb , a corresponding current transformer ctn seen in fig9 is provided which has its secondary winding output rectified by diode bridge dbn and passed through a low pass filter in the form of a capacitor and resistor c , r . the output of the low pass filter is in turn provided to the microcontroller analog to digital signal converter for quantitative measurement and monitoring of the current . thus all electrodes have their rf current monitored , a necessary parameter for the measurement and control of the distributed currents in the desired lesion . it follows from the above circuits that each of the electrodes e 1 . . . en can be used in either one of two ways . the first way is in an energising mode in which pairs of electrodes are energised by the rf current . this energy discharge causes tissue to coagulate preferentially around certain electrodes in preference to others . this distribution of coagulum on the electrodes is not uniform because the tissue and its perfusion is not uniform as stated . this leads to the second function of the similar electrodes which is to monitor the progress of the desiccation and , in particular , the build up of coagulum . this will be explained in particular with reference to fig9 where only three electrodes e 1 , e 2 and em are illustrated . the impedance between the electrodes e 1 and e 2 varies as the energy is dissipated by the tissue between them . eventually , due to non - uniform perfusion and other factors , one electrode , say e 1 in this example will build up a volume of coagulum as represented by the shaded area . if the tissue between electrodes e 1 and e 2 is energised , then impedance between e 1 - e 2 increases over time as the desiccation proceeds ( typically from 100 ohms to 2000 ohms but dependent on perfusion , type of tissue , hydration and electrode size ). concurrent with the energisation of e 1 - e 2 , one of the two switches connecting em to either rf power supply rail as shown previous is momentarily activated . this means that em will be at equipotential with say , e 2 , and thus the current to e 1 will be shared between e 1 and em . the difference in current drawn by em when compared to its reading when activated to the second rf rail , say e 1 , will be reflective of the impedance around each electrode . the manner such an indirect impedance measurement changes is a sensitive monitor of the build up of coagulum and a demonstration that three electrodes can achieve better lesion integration than two electrodes and four will improve it further . it is implied that the initially referred to electrode em will also be used to discharge energy and in a symmetrical fashion e 1 & amp ; e 2 will in turn be monitoring electrodes . as a consequence , in order to provide a more uniformly desiccated lesion in an electrode deployment of say 4 , the electrodes pairs including electrode e 1 would be energised less and electrode pairs including electrode e 2 would be energised for a longer period of time in the manner pulse width modulation can regulate energy delivery or by dropping pairs or sets from the activation sequence . the general intention of this method being to form coagulum and later desiccated tissue approximately equally on all the electrodes being energised and the spaces between so as to form a relatively uniform lesion which extends over an entire volume defined by the spatial arrangement , density and number of electrodes , each as arbitrarily determined for specific therapy by the clinician or according to configure files for recognised or routine therapies utilising standardised numbers of electrodes , types of electrodes and associated probes and standardised approximate placements of same electrodes and probes in tissue . turning now to fig1 , a variation on the situation illustrated in fig3 is illustrated . here an additional probe 20 e is provided . this additional probe 20 e is inserted at some critical area where no hyperthermia effects are desired . this “ boundary ” electrode 20 e can allow close causation of a thermal lesion but is never itself part of an activated pair ( other than momentarily sampling surrounding currents , or if containing a thermometry device only providing such monitoring as required for the method ). this insertion of probe 20 e can be carefully performed under the abovementioned x - ray and image intensifier control and is specifically intended to preserve tissue close to critical anatomy such as efferent and other afferent fibres or ligature and bone or cartilage structures while ensuring the desired lesion boundary to be as close as possible depending on the density of the arbitrary placement of electrodes . this also provides safety and the remaining probes 20 a - 20 d which are deemed needed to be inserted may require less precision and caution as to their placement . it is also be generally common knowledge that high energy activations of electrodes produce smaller lesions than lower energy longer term activations . this is also a controlled factor applied to the other pairs or sets when determining the control strategy when using such a boundary control electrode as 20 e . since no lesion is required at the position of electrode 20 e , this electrode is never activated to create hyperthermia in its immediately surrounding tissue . it is however energised momentarily in monitoring mode with selected other electrodes and hence discharges such little energy that no coagulum forms on the electrode of probe 20 e . instead , the probe 20 e is used , in particular , to monitor the build up of coagulum on electrodes 20 b and 20 d . in this way , the energisation can proceed so that the electrode pairs 20 a - 20 c and 20 a - 20 b and 20 c - 20 d are energised preferentially and that the electrode pair 20 b - 20 d is energised only to the extent that electrode 20 e detects no current density indicative of hyperthermia creation considering its resistive isolation to either monitored and intended active pair or set when sensed current is assessed to both polarities of excitation which in effect measures the impedance of a parallel path between activated pairs . electrode 20 e is additionally activated only momentarily for the purpose of sampling current distribution and impedance as appropriate for pairs such as 20 a - 20 c and in this while discharging so little energy in the tissue that no hyperthermia is caused . alternately or concurrently , a temperature probe can be used at position 20 e . the result is that the resulting lesion 127 does not extend towards probe 20 e but an environment of viable and unaffected tissue around probe 20 e and beyond is maintained . in this way not only can a lesion of approximately uniform consistency be created , but the shape of the lesion can be better determined . the complexity of the shape of the volume 127 is dependent on the density of the electrode placement according to spatial sampling principles . a communication pathway exists between the rf generator control 30 and a laptop or other computer such that configuration files and other configuration and therapy relevant information can be displayed , stored and downloaded into the controller 30 . this is not a critical function in terms of the therapy delivery and default config files can be loaded to non - volatile memory for semi permanent storage inside controller 30 such that it can be used for a limited number of pre - configured therapies without the communication interface being active . however such monitoring as allowed by the connected pc if desired collects treatment records and has numerous safety warning and imaging functionalities . the foregoing describes only one embodiment of the present invention and modifications obvious to those used skilled in the biomedical or electronic arts , can be made thereto without departing from the scope of the present invention . for example , the probes can each be provided with a thermal detector such as a thermocouple or thermistor so as to monitor the temperature of the region being desiccated . similarly , the probes can be provided with more than one electrode so that greater lesion thickness and a deeper or more longitudinal dimensional aspect in the direction of probe penetration to the lesion can be achieved . further the electrodes need not be of uniform size while subject to control of coagulum build up as herein . for example a larger electrode can be included in a bigger set of opposition polarity smaller electrodes to decrease the period required for desiccation . in the setup of the control such parameters as electrode sizes are significant factors of the subject control process and the complexity of the desired thermal lesion shape will determine the electrode surface areas and surface current density and plurality of same . furthermore , the impedance between any , and all , pairs of electrodes is measured at some time interval after the initial desiccation process is terminated . if the impedance drops during this examination period , say below 1000 ohms , this indicates reperfusion is effective and that the lesion has access to electrolytes and nutrients which may indicate a faster recovery of the nerve fibre and its immediate surrounding tissue . if so , the network of electrodes can be re - activated to discharge energy in the same manner as before to create a more stable lesion . this process is continued until a suitably low reperfusion rate is achieved . it is known that for some applications the probes are hollow cannulas , and chemicals such as anaesthetic or other therapeutic or ablative fluids are used without need for any change in the functionality of the embodiment herein described where such cannulas have one or two or more electrode surfaces associated with such probes . use of such a probe as 20 e above for both anaesthetic chemical delivery and thermal or current monitoring is common and advantageous . in such routine placements as mentioned herein when a configuration file is well developed the probes needed for the specific therapy can each be mechanically attached to a manifold or formwork construction such that simultaneous placement is done with needles in a fixed spatial arrangement suitable for a specific anatomical presentation thus further automating the therapy to achieve increased safety and lower cost . further , alternate probe construction may allow the placement in tissue of detachable electrodes which have flexible electrical leads withdrawing from the parent cannula making connection between the rf generator with its component parts as herein and said electrodes which are placed with the same spatial density paradigm for lesion design as otherwise described . such construction allows electrode placement in mobile musculature and skeletal parts indicated for anaesthesia . 1 . improving quality and predictability of anaesthetised period by reducing indeterminacy caused by making successive manual placements in relation to the anatomy and memorised previous placements . 2 . reducing the period of time required to perform such therapies which is of benefit to fluoroscopy theatres and clinicians managing an existing excessive case load . 3 . reducing the discomfort for the chronic pain suffering patient by reducing the period required for the therapy 4 . reducing the instance of physical damage to the fibre by desiccating a larger volume of tissue of a defined shape with fewer punctures in its immediate surround than current practice 5 . reducing radiation exposure to healthcare workers , as most electro - anaesthesia is performed under fluoroscopy limiting exposure is advantageous . 6 . in limiting the activation time the patient discomfort is limited commensurately . 7 . increasing safety by virtue of faster placement needing less accuracy providing bigger lesions to account for individual enervation with fewer needles and consequently better infection control . 8 . increasing useability and thus allows lesser skilled clinicians to perform spinal electro - anaesthesia . this both improves patient access to such therapy and increases competition which over time again suppresses costs to patients and healthcare institutions . 9 . reducing the learning time required by clinicians to perform the procedure with safety reducing costs of teaching and increasing safety for patients . 10 . allowing new procedures to be executed such as therapy for the sacroiliac joint for which there is presently no therapy and where analgesia is of little effect . 11 . providing an rf generator and associated electrodes and probe equipment small enough and sterilisable allowing location entirely within the sterile boundary . the clinician thus has complete control of surgical procedures with the flexibility to attach or detach probe connectivity according to anatomical presentation . orthopaedic surgery and prosthetic placement with erp stimulation to guide denervation is one such instance . while chemical or gaseous sterilisation of this embodiment is implied presently , no theoretical limits should prohibit 120 deg c . autoclaving of future embodiments . 12 . providing such device functionality as herein described may allow electro - anaesthesia procedures to be developed for much if not all muscular - skeletal pain . 13 . improving reproducibility and consequent expansion of therapies by providing procedure configuration files which provide the clinicians with setup and placements for recognised and repetitive procedures , particularly with the manifold or formwork described herein . with the repetitive manual placements presently practiced such automation is impractical . the term “ comprising ” ( and its grammatical variations ) as used herein is used in the inclusive sense of “ including ” or “ having ” and not in the exclusive sense of “ consisting only of ”.
a method and device for inducing anaesthesia in mammals by the application of rf energy to create hyperthermia derived neural anaesthesia . an rf generator drives a plurality of electrodes placed in tissue surrounding the target nerve fibre to desiccate the desired length of nerve fibre to be desiccated in a single deployment . the device allows high - speed selection / de - selection of bipolar electrode pairs or sets under continuous rf excitation . activation of electrode pairs is adapted in response to sensed current density and temperature in order to create lesions of complex and well defined shape necessary for the production of hyperthermia derived neural anaesthesia .