Patent Publication Number: US-2020289827-A1

Title: Time multiplexed waveform for selective cell ablation

Description:
CROSS-REFERENCE TO RELATED PATENT DOCUMENTS 
     The present application claims the benefit of and priority to U.S. Provisional Patent Application 62/819,120, filed Mar. 15, 2019, titled TIME MULTIPLEXED WAVEFORM FOR SELECTIVE CELL ABLATION, the disclosure of which is incorporated herein by reference. The present application is also related to U.S. Provisional Patent Application 62/819,135, filed Mar. 15, 2019, and titled SPATIALLY MULTIPLEXED WAVEFORM FOR SELECTIVE CELL ABLATION, and U.S. Provisional Patent Application 62/819,101, filed Mar. 15, 2019, and titled WAVEFORM GENERATOR AND CONTROL FOR SELECTIVE CELL ABLATION, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Removal or destruction of diseased tissue is a goal of many cancer treatment methods. Tumors may be surgically removed, however, less invasive approaches garner much attention. Tissue ablation is a minimally invasive method of destroying undesirable tissue in the body. Ablation may be thermal or non-thermal. 
     Thermal ablation either adds or removes heat to destroy undesirable cells. For example, cryoablation kills cells by freezing of the extracellular compartment resulting in cell dehydration beginning at −15 C with membrane rupture occurring at colder temperatures. Cryoablation is known to (beneficially) stimulate an antitumor immune response in the patient. 
     Heat-based thermal ablation adds heat to destroy tissue. Radio-frequency (RF) thermal, microwave and high intensity focused ultrasound ablation can each be used to raise localized tissue temperatures well above the body&#39;s normal 37 degrees C. For example, RF thermal ablation uses a high frequency electric field to induce vibrations in the cell membrane that are converted to heat by friction. Cell death occurs in as little as 30 seconds once the cell temperature reaches 50 degrees C., while at higher temperatures cell death is instantaneous. Heat based ablation, however, may not prompt the desirable immune response associated with cryoablation. 
     Thermal ablation techniques using heat or cold each suffer from the drawback that they have little or no ability to spare normal structures in the treatment zone. Collateral injury to vascular, neural and other structures is undesirable. For this reason, various researchers have explored non-thermal ablation as well. 
     Non-thermal ablation techniques include electro-chemotherapy and irreversible electroporation. Electroporation refers to a phenomenon in which the plasma membrane of a cell exposed to high voltage pulsed electric fields becomes temporarily permeable due to destabilization of the lipid bilayer. Pores then form, at least temporarily. Electro-chemotherapy combines pore formation with the introduction of chemicals that cause cell death. Because the chemical molecules used are large, only cells subject to the electric fields will absorb the chemical material and subsequently die, making for useful selectivity in the treatment zone. Irreversible electroporation (IRE) omits the chemicals, and instead uses the electric fields, usually with increased amplitude, to expand pores in the cell membrane beyond the point of recovery, causing cell death for want of a patent cell membrane. The spatial characteristics of the applied field control which cells and tissue will be affected, allowing for better selectivity in the treatment zone than with thermal techniques. 
     One challenge with the electrical (whether thermal or not) ablation techniques is that of local muscle stimulation. A monophasic waveform is thought to provide better results for IRE in terms of causing certain cell death. However, monophasic waveforms tend to cause muscle stimulation, requiring the use of a paralytic to facilitate surgery, among other problems. A biphasic waveform avoids the muscle stimulation, but may not be as effective at the same energy level and/or amplitude as the monophasic waveform. Simply raising power to make the biphasic waveform more effective runs the risk of causing thermal ablation. Enhancements and alternatives to the state of the art are desired to allow a waveform to be used that is as effective as monophasic stimulus for IRE, while avoiding muscle stimulation and thus obtaining the benefits of both monophasic and biphasic therapy. 
     OVERVIEW 
     The present inventors have recognized, among other things, that a problem to be solved is the provision of ablation therapy that combines high efficacy and tissue selectivity while avoiding muscle stimulation. A number of examples shown below use a time multiplexing of therapy outputs to achieve such aims. 
     A first illustrative and non-limiting example takes the form of a signal generator adapted for use in delivery of tissue ablation energy comprising a therapy output block including to a voltage conversion circuit, an energy storage circuit and an output control circuit; an input/output circuit adapted to couple to a probe for delivery of tissue ablation energy, the input/output circuit defining a plurality of output channels such that a probe coupled thereto and having a plurality of electrodes can be used with separate activation of subsets of the plurality of electrodes; a user interface allowing a user to control the signal generator and adapted to display one or more parameters of tissue ablation energy to be delivered by the signal generator; a controller coupled to the therapy output block and the user interface; a memory coupled to the controller and having stored instructions for the delivery of a treatment cycle, the treatment cycle comprising: a first output of a first polarity at a first amplitude and having a first pulse width as a first phase; a second output of a second polarity, opposite the first polarity, at a second amplitude and having a second pulse width, the second pulse width being less than half the first pulse width; and a third output using the second polarity at a third amplitude less than the second amplitude, the third pulse width being greater than the second pulse width; wherein the sum of the first, second and third outputs yields a balanced charge to limit muscle stimulation associated with the multiphasic ablation waveform. 
     Additionally or alternatively to the first illustrative example, the stored instructions may define the first and second amplitudes to exceed an irreversible electroporation threshold, and the third amplitude to be less than an irreversible electroporation threshold. 
     Additionally or alternatively to the first illustrative example, the stored instructions may define at least one of the first and second amplitudes to exceed an irreversible electroporation threshold, and the third amplitude to be less than a reversible electroporation threshold. 
     Additionally or alternatively to the first illustrative example, the stored instructions may define the first, second and third pulse widths such that one of the first, second and third time periods exceeds the sum of the other two of the first, second and third time periods. 
     Additionally or alternatively to the first illustrative example, the stored instructions may define the first pulse width in the range of about 1 to 50 microseconds, and the second pulse width in the range of about 0.5 to 10 microseconds. 
     Additionally or alternatively to the first illustrative example, the stored instructions may define the first, second and third pulse widths as summing to a duration of less than one millisecond. 
     Additionally or alternatively to the first illustrative example, the signal generator may further comprise monitoring circuitry coupled to the input/output circuitry to monitor at least one of current or voltage in at least one therapy output channel, and the stored instructions may define a plurality of iterations in which: in a first iteration, the first, second and third outputs are generated while the control circuitry monitors one or more of impedance of current flow for each of the first, second and third outputs; in second iteration, the first, second and third outputs are again generated, except that at least one of the amplitude or pulse width of at least one of the first, second and third outputs is adjusted to reduce change imbalance, if any, resulting from the monitored impedances; and the first and second iterations are performed within a time period of less than 10 milliseconds. 
     A second illustrative and non-limiting example takes the form of a signal generator adapted for use in delivery of tissue ablation energy comprising: a therapy output block including to a voltage conversion circuit, an energy storage circuit and an output control circuit; an input/output circuit adapted to couple to a probe for delivery of tissue ablation energy, the input/output circuit defining a plurality of output channels such that a probe coupled thereto and having a plurality of electrodes can be used with separate activation of subsets of the plurality of electrodes; a user interface allowing a user to control the signal generator and adapted to display one or more parameters of tissue ablation energy to be delivered by the signal generator; a controller coupled to the therapy output block and the user interface; a memory coupled to the controller and having stored instructions for the delivery of a treatment cycle, the treatment cycle comprising: a first pulse train comprising first pulses of a first polarity having a first amplitude and a first pulse width, alternating with second pulses of a second polarity opposite the first polarity, having a second amplitude and having a second pulse width less than the first pulse width; a second pulse train comprising third pulses of the first polarity having a third amplitude and a third pulse width, alternating with fourth pulses of the second polarity having a fourth amplitude and a fourth pulse width greater than the third pulse width; such that the first pulse train yields a first charge imbalance, and the second pulse train yields a second charge imbalance that offsets the first charge imbalance to prevent muscle stimulation. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first and second amplitudes to be equal, and the third and fourth amplitudes to be equal. 
     Additionally or alternatively to the second illustrative example, the stored instructions may require delivery of the first and second pulse trains in sequence such that a time from the start of the first pulse train to the end of the second pulse train is less than one millisecond. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first and fourth pulse widths as equal in duration, and the second and third pulse widths as equal in duration. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first pulse train to comprise a first quantity of first pulses and a second quantity of second pulses, and the second pulse train to comprise a third quantity of third pulses and a fourth quantity of fourth pulses, wherein the first, second, third and fourth quantities are all equal. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first pulse widths as twice the second pulse widths, and the fourth pulse widths as twice the third pulse widths. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first, second, third and fourth amplitudes to each exceed an irreversible electroporation threshold. 
     Additionally or alternatively to the second illustrative example, the stored instructions may define the first, second, third and fourth pulse widths each in a range of about 0.1 to 500 microseconds. 
     Another illustrative and non-limiting example takes the form of a system comprising a signal generator as in any of the first or second illustrative, non-limiting examples, and alternatives or additions thereto, along with a probe adapted for use with the signal generator, the probe having a plurality of ablation therapy delivery electrodes thereon. 
     A third illustrative and non-limiting example takes the form of a method of delivering a multiphasic ablation waveform comprising: generating a first output of a first polarity at a first amplitude for a first time period as a first phase; generating a second output of a second polarity, opposite the first polarity, at a second amplitude for a second time period, the second time period being less than half the first time period; and generating a third output using the second polarity at a third amplitude less than the second amplitude for a third time period, the third time period being greater than the first time period; wherein the sum of the first, second and third outputs yields a balanced charge to limit muscle stimulation associated with the multiphasic ablation waveform. 
     Additionally or alternatively to the third illustrative example, at least one of the first and second amplitudes may exceed an irreversible electroporation threshold, and the third amplitude may be less than an irreversible electroporation threshold. 
     Additionally or alternatively to the third illustrative example, at least one of the first and second amplitudes may exceed an irreversible electroporation threshold, and the third amplitude may be less than a reversible electroporation threshold. 
     Additionally or alternatively to the third illustrative example, one of the first, second and third time periods may exceed the sum of the other two of the first, second and third time periods. 
     Additionally or alternatively to the third illustrative example, the first time period may be in the range of about 1 to 50 microseconds, and the second time period may be in the range of about 0.5 to 10 microseconds. 
     Additionally or alternatively to the third illustrative example, the first, second and third time periods may add up to a duration of less than one millisecond. 
     Another illustrative and non-limiting example may comprise in a first iteration, performing the method of the third illustrative and non-limiting example while monitoring one or more of impedance of current flow for each of the first, second and third outputs; and in second iteration, again performing the method of the third illustrative and non-limiting example, and adjusting at least one of the amplitude or pulse width of at least one of the first, second and third outputs to reduce change imbalance, if any, resulting from the monitored impedances; wherein the first and second iterations are performed within a time period of less than 10 milliseconds. 
     A fourth illustrative and non-limiting example takes the form of a method of delivering a multiphasic ablation waveform comprising: generating a first pulse train comprising first pulses of a first polarity having a first amplitude and a first pulse width, alternating with second pulses of a second polarity opposite the first polarity, having a second amplitude and having a second pulse width less than the first pulse width; generating a second pulse train comprising third pulses of the first polarity having a third amplitude and a third pulse width, alternating with fourth pulses of the second polarity having a fourth amplitude and a fourth pulse width greater than the third pulse width; such that the first pulse train yields a first charge imbalance, and the second pulse train yields a second charge imbalance that offsets the first charge imbalance to prevent muscle stimulation. 
     Additionally or alternatively to the fourth illustrative example, the first and second amplitudes may be equal, and the third and fourth amplitudes may be equal. 
     Additionally or alternatively to the fourth illustrative example, the first and second pulse trains may be delivered in sequence such that a time from the start of the first pulse train to the end of the second pulse train is less than one millisecond. 
     Additionally or alternatively to the fourth illustrative example, the first and fourth pulse widths may be equal in duration, and the second and third pulse widths may be equal in duration. 
     Additionally or alternatively to the fourth illustrative example, the first pulse train may comprise a first quantity of first pulses and a second quantity of second pulses, and the second pulse train may comprise a third quantity of third pulses and a fourth quantity of fourth pulses, wherein the first, second, third and fourth quantities are all equal. 
     Additionally or alternatively to the fourth illustrative example, the first pulse widths may be twice the second pulse widths, and the fourth pulse widths may be twice the third pulse widths. 
     Additionally or alternatively to the fourth illustrative example, the first, second, third and fourth amplitudes may each exceed an irreversible electroporation threshold. 
     Additionally or alternatively to the fourth illustrative example, the first, second, third and fourth pulse widths may each be in a range of about 0.1 to 500 microseconds. 
     A fifth illustrative and non-limiting example takes the form of a method of delivering a multiphasic ablation waveform comprising: generating a first pulse of a first polarity having a first amplitude and a first pulse width; generating a first pulse train having a plurality of second pulses of a second polarity opposite the first polarity, the second pulses having second amplitudes and second pulse widths, the second pulse widths being less than ½ of the first pulse width; such that the first pulse yields a first charge imbalance, and the second pulse train yields a second charge imbalance that offsets the first charge imbalance to prevent muscle stimulation. 
     Additionally or alternatively to the fifth illustrative example, the plurality of second pulses may each be separated by an interpulse interval, the interpulse interval being between one-half and twice the second pulse width. 
     Additionally or alternatively to the fifth illustrative example, the first pulse width may be equal to a sum of the second pulse widths. 
     Additionally or alternatively to the fifth illustrative example, the first amplitude and the second amplitude may each exceed an irreversible electroporation threshold. 
     Additionally or alternatively to the fifth illustrative example, the first amplitude may be less than the second amplitude; and the sum of the second pulse widths may be less than the first pulse width. 
     Additionally or alternatively to the fifth illustrative example, the second pulses may be each separated by an interpulse interval, wherein the sum of the second pulse widths and the interpulse intervals is equal to the first pulse widths. 
     Additionally or alternatively to the fifth illustrative example, a duration from the start of the first pulse to the end of the first pulse train may be less than one millisecond. 
     A sixth illustrative and non-limiting example takes the form of a method of delivering a multiphasic ablation waveform comprising: delivering a first pulse train comprising a plurality of first pulses each having a pulse width and an amplitude, wherein a first in time of the first pulses has a first amplitude, and each successive pulse of the first pulses has a larger amplitude than an immediately preceding pulse, each of the first pulses having a first polarity; delivering a second pulse train comprising a plurality of second pulses each having a pulse width and an amplitude, wherein a first in time of the second pulses has the first amplitude, and each successive pulse of the second pulses has a larger amplitude than an immediately preceding pulse, each of the second pulses having a second polarity opposite of the first polarity; wherein the first pulse train and second pulse train are delivered within a time window of less than about one millisecond, such that charge balance is achieved upon conclusion of the second pulse train. 
     Additionally or alternatively to the sixth illustrative example, within the first pulse train, the first in time pulse may have an amplitude that is less than an irreversible electroporation threshold, and the last in time pulse may have an amplitude that is greater than an irreversible electroporation threshold; and within the second pulse train, the first in time pulse may have an amplitude that is less than an irreversible electroporation threshold, and the last in time pulse may have an amplitude that is greater than an irreversible electroporation threshold. 
     Another illustrative and non-limiting example takes the form of a pulse generator configured for use with a probe for delivering ablation therapy to a patient, the pulse generator comprising output circuitry for delivering voltage-based therapy, monitoring circuitry for monitoring characteristics of delivered therapy pulses, and control circuitry comprising a non-volatile memory containing an executable instruction set adapted to deliver therapy as in any of the third to sixth illustrative examples and additions and alternatives thereto. Another example may be a system comprising such a pulse generator and a probe adapted for use with the pulse generator and comprising a plurality of therapy delivery electrodes. 
     This overview is intended to provide an introduction to the subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  shows an approximation of different therapy modalities associated with a combination of electrical field strength and pulse duration; 
         FIGS. 2-4  show various impacts of application of electrical field to a cell; 
         FIG. 5  shows a prior art “Leveen” needle; 
         FIGS. 6-8  show various waveform features; 
         FIG. 9  shows a signal generator in block form; and 
         FIGS. 10-15  show various time multiplexed therapy patterns. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an approximation of different biophysical responses dependent on the amplitude-time relationship of delivered electrical pulses. The thresholds between cellular responses ( 10 ,  20 ,  30 ) operate generally as a function of the applied field strength and pulse duration. Below a first threshold  10 , no effect occurs; between the first threshold  10  and a second threshold  20 , reversible electroporation occurs. Above the second threshold  20 , and below a third threshold  30 , primarily irreversible electroporation (IRE) occurs. Above a third threshold  30 , the effects begin to be primarily thermal, driven by tissue heating. Thus, for example, at a given field strength and duration there may be no effect (location  12 ), and extending the duration of the field application can yield reversible electroporation (location  22 ), irreversible electroporation (location  32 ), and thermal ablation (location  40 ). 
     As described in U.S. Pat. No. 6,010,613, a transmembrane potential in the range of about one volt is needed to cause reversible electroporation, however the relationship between pulse parameters, such as timing and duration, and the transmembrane potential required for reversible electroporation remains an actively investigated subject. The required field may vary depending on characteristics of the cells to be treated. At a macro level, reversible electroporation requires a voltage in the level of hundreds of volts per centimeter, with irreversible electroporation requiring a still higher voltage. As an example, when considering in vivo electroporation of liver tissue, the reversible electroporation threshold field strength may be about 360 V/cm, and the irreversible electroporation threshold field strength may be about 680 V/cm, as described in U.S. Pat. No. 8,048,067. Generally speaking, a plurality of individual pulses are delivered to obtain such effects across the majority of treated tissue; for example, 2, 4, 8, 16, or more pulses may be delivered. Some embodiments may deliver hundreds of pulses. 
     The electrical field for electroporation has typically been applied by delivering a series of individual pulses each having a duration in the range of one to hundreds of microseconds. For example, U.S. Pat. No. 8,048,067 describes analysis and experiments performed to illustrate that the area between lines  20  and  30  in  FIG. 1  actually exists, and that a non-thermal IRE therapy can be achieved, using in several experiments a series of eight 100 microsecond pulses delivered at 1 second intervals. 
     The tissue membrane does not return instantaneously from a porated state to rest. As a result, the application of pulses close together in time can have a cumulative effect as described, for example, in U.S. Pat. No. 8,926,606. In addition, a series of pulses can be used to first create pores in a cell membrane and then move large molecules through generated, reversible pores, with electric field serving both to maintain the pores and to move the molecules as described in US PG Patent App. Pub No. 2007/0025919. 
       FIGS. 2-4  show various impacts of application of electrical field to a cell. At electric field strengths below the threshold for reversible electroporation, as shown in  FIG. 2 , the cell membrane  62  of cell  60  remains intact and no pores occur. As shown in  FIG. 3 , at a higher electric field strength, above the threshold for reversible electroporation and below the threshold for irreversible electroporation, the membrane  72  of cell  70  develops pores  74 . Depending on the characteristics of the applied field and pulse shapes, larger or smaller pores  74  may occur, and the pores developed may last for longer or shorter durations. 
     As shown in  FIG. 4 , at a still higher electric field strength, above the threshold for irreversible electroporation, the cell  80  now has a membrane  82  with a number of pores  84 ,  86 . At this higher amplitude or power level, pores  84 ,  86  may become so large and/or numerous that the cell cannot recover. It may be noted as well that the pores are spatially concentrated on the left and right side of the cell  80  as depicted in  FIG. 4 , with few or no pores in the region  88  where the cell membrane is parallel to the applied field (assuming here that the field is applied between electrodes disposed to the right and left sides of the cell shown in  FIG. 4 ). This is because the transmembrane potential in region  88  remains low where the field is closer to parallel, rather than orthogonal, to the cell membrane. 
       FIG. 5  shows a prior art “Leveen” needle, which may be used as a probe to deliver therapy. As described in U.S. Pat. No. 5,855,576, the device comprises an insertable portion  100  having a shaft  104  that extends to a plurality of tissue piercing electrodes  102  that can be extended or retracted once a target tissue  112  of a patient  110  is accessed. The proximal end of the apparatus is coupled by an electrical connection  106  to a power supply  108 , which can be used to supply RF energy. 
     Conventionally, the Leveen needle would be used to deliver thermal ablation to the target tissue. For example, as described in the &#39;576 patent, a return electrode in the form of a plate or plates may be provided on the patient&#39;s skin, a return electrode could be provided as another tissue piercing electrode, or a return electrode may be provided on the shaft  104  near its distal end, proximal of the tissue piercing electrodes  102 . 
     Enhancements on the original design can be found, for example, in U.S. Pat. No. 6,638,277, which discusses independent actuation of the tissue piercing electrodes  102 , both in terms of movement of the electrodes as well as separately electrically activating individual ones of the electrodes. The U.S. Pat. Nos. 5,855,576 and 6,638,277 patents are incorporated herein by reference for showing various therapy delivery probes. U.S. Provisional Patent Application Ser. No. 62/620,873, the disclosure of which is incorporated herein by reference as showing various therapy delivery probes, discloses updates and enhancements on the Leveen needle concept, allowing flexibility in the spacing, size and selection of electrodes. 
       FIGS. 6-8  show various waveform features. Referring to  FIG. 6 , a monophasic waveform is shown at  150 . The waveform  150  is shown relative to a baseline or equipotential  152 . An idealized square wave is shown having an amplitude  154 , a pulse width  156 , and a cycle length  158 . The waveform  150  is shown as an ideal square wave, with a vertical upswing from baseline  152  to the designated amplitude  154 . When describing such a waveform, the frequency typically refers to the inverse of the cycle length  158 . So, for example, if a waveform having a one microsecond pulse width  156  is delivered at two microsecond intervals  158 , the “frequency” of the waveform may be described as 500 kHz (the inverse of two microseconds). The waveform  150  may be a current controlled or voltage controlled waveform. Either approach may be used in various examples, as further described below. 
     In any real application the edges of the generated waveform will be rounded and the upswing from baseline  152  will be more as shown in  FIG. 7 , where the upward divergence from the baseline, shown at  162 , is characterized by a rise time  160 . At the end of the output, there is also a non-ideal fall 164 characterized by fall time  166 . Real application of the waveform will also include some variation in the peak amplitude, as shown, which may include for example overshoot of the amplitude if the signal output is underdamped, or rounding off of the edges for a critically damped or overdamped signal. 
     In some examples, one or more of the rise or fall time  160 ,  166  can be manipulated. In an illustrative example, the output circuitry of a system may include selectable elements, such as resistors, inductors or the like, that can slow the rise time if switched into the circuit. For example, the current through an inductor cannot be instantaneously changed, so switching an inductive element into an output circuit can slow the rise time as the inductor begins to allow current to flow. 
     Rise and fall time may be manipulated in several different ways. For example, the process settings may be selected to modify the peak voltage target; a higher target can yield a faster rise time as various components respond in exponential fashion to being turned on or switched into an output circuit. By monitoring the output, the system can artificially increase a peak voltage target to reduce rise time, and once the true peak voltage is met, the system may switch voltage sources or use an output regulation (such as by using a rectifier or by redirecting output current through a separate discharge path) to cap the voltage output. In another example, component selection may be used, such as by having a plurality of different HV switches available and selectable to the system, with different HV switch types having different rise and fall times. For example, if three output switches are available, each with a different rise/fall characteristic, the system may respond to a user input requesting longer or shorter rise/fall time by selecting an appropriate output switch for use during a particular therapy output session. High pass or low pass filtering may be switched into the output circuit as well to control slew rate, or may be switched into the control signal circuit; a slow turn-on of an output transistor for example can cause slower rise time for the transistor itself and conversely fast turn-on of the output transistor can speed the rise time. In another example, a digital to analog converter may be used as an output circuit, allowing digitized control of rise or fall time. In still a further example, control signals to the output switches can be generated by a digital to analog converter, thus manipulating the on/off signal to the output circuitry itself. In still a further example, using a capacitor stack output as shown in U.S. Provisional Patent Application 62/819,101, filed Mar. 15, 2019 and titled WAVEFORM GENERATOR AND CONTROL FOR SELECTIVE CELL ABLATION (the disclosure of which is incorporated herein by reference), a fast rise time may be effected by using a single switched output from the top (or desired target level) of the capacitor stack, while a slow rise time may be effected by sequentially turning on an output using less than all of the capacitor stack and then subsequently adding more of the capacitor stack to the output; appropriately placed diodes in the output circuitry will prevent back-current or shorting of the newly added portions of the capacitor stack during such a maneuver. 
       FIG. 8  shows further details, this time for a biphasic signal. Here, the waveform is shown at  180 , with a first, positive pulse at  182  quickly followed by a negative pulse at  190 . The positive pulse  182  has an amplitude  184 , and the negative pulse  190  has an amplitude  192  which is usually equal in voltage to, but of opposite polarity than, the positive pulse. The positive pulse  182  has a pulse width  186 , and the negative pulse  190  has a pulse width  194 ; again, typically the two pulse widths  186 ,  194  would be equal to one another. For a signal as shown, the cycle length can be determined as shown at  196 , from the start of the positive pulse  182  to the initiation of a subsequent cycle; again, frequency is the inverse of the cycle length. 
     In a typical application or use of biphasic signals, the aim is, in part, to achieve charge balancing at the end of each cycle. For that reason, the pulse widths of the two phases are kept equal, and the amplitudes are also equal though of opposite polarity. Whether using a voltage controlled or current controlled system, charge balance can be reasonably maintained by controlling just the pulse width and amplitude. For example, in a voltage controlled system, the current flow will be more or less constant within a cycle, assuming the cycle length  196  is in the millisecond range or less. That is, while it is known that during ablation procedures the tissue impedance changes as cells are destroyed, expelling cellular media which generally reduces impedance, the impedance does not change so quickly that a simple biphasic waveform would fail to provide charge balance. 
     An interphase period  188  represents a time period spent at baseline between the positive and negative pulses, and is ordinarily minimized in accordance with the physical constraints of the underlying circuitry. Thus, for example, if a first switch must turn off to end the positive pulse  182 , and a second switch is used to initiate the negative pulse  190 , assuming digital control, the system may allow a few digital clock cycles to expire after turning off the first switch before turning on the second switch, to avoid any possible internal shorting. Faster switches can reduce the interphase time, and much engineering effort has gone into reducing this time period  188 . 
     For example, a very short interphase period  188  can be achieved using a design as shown in U.S. Pat. No. 10,154,869. In the 10,154,869 patent, an inductor is placed in parallel with the output load. A power source is applied to the load and inductor during an initial phase of therapy delivery. Opening a switch between the power source and the load/inductor causes a near immediate reversal of current through the load as the inductor draws current from the load after the power source is disconnected. 
     The background to be gathered from  FIGS. 6-8  is that of typical usage. In several embodiments described further below, monophasic pulses are used to achieve biphasic results with respect to charge balancing that prevents muscle stimulation. It should be noted that within all the examples herein, the term “without causing muscle stimulation” allows for some muscle stimulation, but only an amount tolerable within the relevant intervention and/or surgical domain. For example, the stimulation that occurs is not so much that the patient is made uncomfortable. In another example, the stimulation that occurs is small enough that surgery to ablate tissue is not subject to interference due to stimulated patient movement. In another example, the muscle stimulation that occurs is insignificant to the surgery and allows surgery to be performed without requiring administration of a paralytic. In some examples, the stimulation that occurs does not affect probe placement and securement, or is small enough that migration of the probe does not occur. 
       FIG. 9  shows a signal generator in block form. A signal generator  200  may be a self-contained unit, or it may comprise several discrete components coupled together with wires and/or wireless connections. A control block is shown at  202  and may comprise a plurality of logic circuits in the form of a state machine, a microcontroller and associated digital logic, or a microprocessor, or even an off the shelf computing unit such as a laptop or desktop computer, as desired. A memory  204 , which may or may not be separate from the control block  202 , is included to store executable instruction sets for operation as well as keeping a log of activity of the system and any sensor outputs received during therapy. The memory  204  may be a volatile or non-volatile memory, and may include optical or digital media, a Flash drive, a hard drive, ROM, RAM, etc. A UI or user interface  206 , which may also be integrated with the control block (such as when using a laptop for control  202 , which would include each of memory  204  and a UI  206 ). The UI  206  may include a mouse, keyboard, screen touchscreen, microphone, speakers, etc. as desired. 
     Power in  208  may include a battery or batteries, and will typically include an electrical coupling to plug into a wall socket to receive line power. A therapy block is shown at  210  and includes several stages. An isolation and voltage conversion circuit is shown at  212  and may include, for example, one or more transformers or other step-up converters (such as a capacitive step-up conversion circuit) to take a battery or line voltage and increase to a high voltage output that is stored in HV storage  214 . The HV storage  214  may include batteries, inductors or other circuit elements, but will typically be a capacitive storage block such as a stack of capacitors. HV storage  214  may be helpful to take the HV signal from block  212  and smooth it out over time to provide a more stable high voltage output that is then delivered by an HV output circuit  216 . Also, the HV storage  214  may enable a lower power voltage input to generate very high power outputs by storing energy over a longer period of time to be delivered in short bursts. 
     The HV output circuit  216  may serve as an output control circuit. The HV output circuit may include a number of switches and other elements, including for example, high voltage switches such as silicon controlled rectifiers, high power Mosfets, and other elements, allowing selective outputting of the high voltage signal to an IO block shown at  218 . The IO block  218  may provide a number of sockets to receive plugs from one or more delivery probes  220 , as well as one or more outputs for one or more indifferent electrodes to be placed on the body of a patient, serving as return electrodes or simply grounding the patient and system. 
     In some alternative approaches to the therapy block  210 , rather than HV Out  216  using sets of switches to directly output a signal from HV storage, a resonant circuit may be powered by the HV signal, with outputs of the resonant circuit used for therapy delivery by selectively switching the output of the resonant circuit. A topology that uses a set of four switches in an “H-bridge” to drive an RF circuit is shown, for example, in U.S. Pat. No. 10,105,172. In some embodiments, control over the individual pulses is achieved in the present invention by omitting the driven RF circuit and simply relying on a form of extended H-bridge circuit, as shown in U.S. Provisional Patent Application 62/819,101, filed Mar. 15, 2019 and titled WAVEFORM GENERATOR AND CONTROL FOR SELECTIVE CELL ABLATION, the disclosure of which is incorporated herein by reference. 
     One or more sensing circuits  224  may be included to provide feedback to the control block  202 . For example, the sensing circuits may measure voltage at the output nodes to the probe  220 , or may measure current going to the output nodes that couple to the probe  220 , allowing tissue characteristics to be monitored. For example, voltage measuring circuits are well known in the art, including, for example, direct-conversion, successive approximation, ramp-compare, Wilkinson, integrating, Delta-encoded, pipelined, sigma-delta, and/or time-interleaved ADC, any of which may be used as suited to the application. Current measuring circuitry may use, for example, trace resistance sensing, a current sensor based on Faraday&#39;s Law such as a current transformer or Rogowski coil, or the use of magnetic field sensors (Hall effect, Flux gate, and/or a magneto-resistive current sensor) electrically or magnetically coupled to one or more transmission lines. Current sensing of the output circuitry may be used for safety purposes to prevent or limit shorting or overcurrent conditions, for example. 
     In another example, the probe  220  may include a sensor, such as a temperature sensor, a force sensor, or a chemical or pH sensor, any of which can be used to monitor tissue characteristics during therapy delivery. For example, a temperature sensor may be used to manage a non-thermal therapy such as electroporation by observing whether the temperature in a region is raising above a threshold temperature or showing an increasing trend, in which case one or more elements of power output may be reduced to ensure that the desired therapy type is dominant. If the probe contains such items, the sensing circuits  224  may include any suitable amplifier, filter or the like to allow the sensed signal to be conditioned for use by the control block  202 . 
     Sensing circuits  224  may include a cardiac rhythm sensor that is adapted for use with one or more electrodes (such as surface electrodes placed on the patient&#39;s chest) to capture cardiac rhythms and identify physiological windows for therapy deliver, as discussed below. A cardiac signal for purposes of identifying a physiological window for therapy may be received instead from an in-clinic ECG monitor, an implantable medical device such as a cardiac monitor, pacemaker or defibrillator, or from a variety of wearable products that sense cardiac rhythms. 
     Optionally, “other therapy” block  222  may be included. “Other” therapy may include, for example, the delivery of a chemical or biological agent to provide additional therapy, to enhance therapy being delivered, or to trigger immune response to facilitate the body healing itself after ablation. Such other therapy  222  may comprise a reservoir (which may be refillable) of material to be delivered to a patient via, for example, a syringe or catheter or through a probe. An “other therapy”  222  may include introducing a substance that enhances, augments, is synergistic with, or independently adds to the ablation effects of therapy delivered electrically. For example, a substance may be injected to modify or enhance electric field effects, as disclosed in U.S. patent application Ser. No. 16/188,343, titled IRREVERSIBLE ELECTROPORATION THROUGH A COMBINATION OF SUBSTANCE INJECTION AND ELECTRICAL FIELD APPLICATION, the disclosure of which is incorporated herein by reference. 
     In some examples, a cryotherapy may be integrated into the system to allow tissue cooling before, during or after electrical ablation, prompting immune response if desired. Cryotherapy may be delivered using, for example, a balloon on a therapy probe  220  or provided separately with a nozzle in the balloon coupled to a pressurized fluid source, such as nitrous oxide; the pressurized fluid when expelled through the nozzle will expand or go through a phase change from liquid to gas, which causes localized cooling, as disclosed for example in U.S. Pat. No. 6,428,534. In another example, a fluid (gas or liquid) may be externally cooled and introduced via a catheter for cryogenic purposes, or, in the alternative, externally heated and introduced via a catheter for heat ablation purposes. 
     In still other examples, other therapy  222  may include delivery of energy such as mechanical energy (ultrasound, for example) or optical energy using, for example, a laser source (such as a vertical cavity surface emitting laser) coupled to an optical fiber that extends through a probe to allow laser energy to be delivered to targeted tissue. In some examples, a secondary or “other” therapy may be used, as noted, to trigger the immune response even if it is not used as a primary approach for destroying targeted tissue. 
     In several examples, a biphasic effect reducing or minimizing muscle stimulation is achieved while separating positive and negative phases of a signal in time to provide for monophasic therapeutic effects. Therapy may be delivered using one or more pulse trains that meet each of two rules:
         Charge balance rule: the pulse train is completed thereby providing charge balance or an approximation of charge balance within:
           A time period that is less than the time constant of surrounding tissue, which can depend on factors such as tissue type and water content. The time constant of surrounding tissue reflects the complex impedance of the tissue and cells in the electrical field. For example, the time constant of the tissue between two electrodes would be determined by the complex impedance thereof; in a simplified model the time constant would be the capacitance multiply the resistance of the tissue, including cells, within the electrical field that would be generated between two electrodes. Cells or tissue which is already polarized may have a greater or lesser effective time constant.   A time period of less than about one millisecond   A maximum time period tolerable for the patient, as determined by testing the patient. For example, to test a patient, a therapy output may include first and second portions separated by a period of time, and the period separating the first and second portions can be extended until muscle contraction is observed, until the patient reports feeling a contraction or tension, or until discomfort is indicated by the patient, wherein the first portion of the therapy is a first monophasic pulse or pulses that impart a charge imbalance, and the second portion of the therapy is configured to remove the charge imbalance. For example, a biphasic output may be separated into two portions by controlling and expanding the interphase period ( FIG. 8, 188 ) to a multiple of the pulse width—such as using 5 microsecond pulses separated by tens or hundreds of microseconds, or even more, out to several milliseconds, as tolerated by the patient and while still staying within the therapy completion rule noted below.   
           Therapy completion rule: the pulse train is to be delivered within a physiological window determined by observation of a non-therapy factor, such as the cardiac rhythm of the patient.       

     Regarding the therapy completion rule, using the heart as the driver, the cardiac rhythm contains various components known by convention as the R-wave, QRS complex, P-wave, and T-wave. Stimulus of non-cardiac tissue for ablation purposes ought not interfere with the cardiac rhythm, and the heart may be less susceptible to electrical signal interference in an interval between the R-wave peak (or end of the QRS complex) and the T-wave. Sometimes this interval can be called the S-T interval (the S-wave ends the QRS complex); the S-T interval for a given patient is likely to last tens of milliseconds and may range from 5 to 100 milliseconds. Approximately 60 milliseconds is typical for a healthy individual, though it is noted that the therapies discussed herein are not necessarily for healthy or typical people and, therefore, the S-T interval may not be “typical”. In an example, R-waves are sensed and therapy bursts are delivered after a delay of about 50 milliseconds from the R-wave detection or R-wave peak. In any event, in some examples, therapy is started and completed within the S-T interval window. A cardiac signal useful for identifying the S-T interval, or other physiologically useful window, may be obtained from a separate device (external or implantable) or may be sensed by a therapy generator having inputs for receiving cardiac signals from electrodes placed in or on the patient. Other sources may be the drivers; for example, detecting diaphragm movements may be useful as well, to time delivery of therapy for when the patient has inhaled, or exhaled. 
     In other examples, one, the other, or both of these timing rules may be omitted. In some examples, the windows may be approximated, such as by setting a rule that a pulse train must return to a balanced charge state in less than one millisecond, or 800 microseconds, or 500 microseconds. 
       FIG. 10  shows an illustrative therapy waveform  300 . This example illustrates a method of delivering a multiphasic ablation waveform comprising: generating a first output  310  of a first polarity (in the example show, pulse  310  has a negative polarity) at a first amplitude  314  for a first time period using pulse width  312 , as a first phase. In the method, the next step is generating a second output  320  of a second polarity (here, positive polarity as shown), opposite the first polarity, at a second amplitude  324  for a second time period using pulse width  322 , the second time period  322  being less than half the first time period  312 . In an example, the second time period may be, for example, 1-5 microseconds, while the second time period is 10 microseconds, though other periods may be used. In some examples, the first time period or pulse width  312  may be in the range of about 1 to 50 microseconds, and the second time period or pulse width  322  may be in the range of about 0.5 to 10 microseconds. The method further includes generating a third output  330  using the second polarity (again a positive polarity is shown) at a third amplitude  334  less than the second amplitude for a third time period, shown as pulse width  332 , the third time period being greater than the first time period. In the example the third time period may be, for example, in the range of about 10 to about 500 microseconds, as desired. Further in the example, the sum of the first, second and third outputs yields a balanced charge to limit muscle stimulation associated with the multiphasic ablation waveform. So, as indicated, the integral of the output current—which may be in this case approximately the integral of the voltage, during the entire time period  340 , is approximately zero. In a numeric, and non-limiting, example, the first pulse width  312  may be about 10 microseconds, with a first amplitude  314  of 800 volts (8 millisecond-volts), the second pulse width  322  may be about 4 microseconds, with a second amplitude  324  of 800 volts (3.2 millisecond-volts), and the third pulse width  332  may be about 100 microseconds with a voltage of about 48 volts (4.8 millisecond-volts), a sequence which, assuming generally constant impedance during the total period  340 , would yield a balanced charge outputs. 
     In some examples, the first and second amplitudes  314 ,  324  each exceed an irreversible electroporation threshold, and the third amplitude  334  is less than an irreversible electroporation threshold. For example, the first and second amplitudes may be in the range of about 700 to 5000 volts, or higher or lower, depending on the distance between the electrodes in use, recognizing that an IRE threshold may be defined in volts per unit distance, such as 670 volts per centimeter, while the third amplitude is half, or less than half, the amplitude of the first and second amplitudes, for example, or otherwise below an IRE threshold. 
     In some examples, the first and second amplitudes  314 ,  324  each exceed an irreversible electroporation threshold, and the third amplitude  334  is less than a reversible electroporation threshold. For example, when configuring therapy outputs, the distance between therapy electrodes may be estimated or known, using methods noted above. Then, the first and second amplitudes  314 ,  324  can be calculated to exceed an IRE threshold such as exceeding 670 volts/cm, while the third amplitude is calculated to be below a reversible threshold such as below 330 volts/cm. In other examples, the third amplitude may be more than a reversible electroporation threshold. Keeping the third amplitude higher may be useful to encourage pores that have opened during the first and/or second phases of therapy delivery  320 ,  330 , to remain open for a longer period of time, or even grow and become irreversible. 
     Curves such as those shown in  FIG. 1  may be useful to aid in the planning of therapy. It may be noted that the longer—or shorter—durations may determine what field strength is needed to achieve reversible, or irreversible, electroporation. While the examples may note that the amplitude is set above or below an electroporation threshold (whether reversible or not), it is to be understand that the meaning is that the amplitude, taken in light of pulse width, is above or below such a threshold. Thus reference to amplitude is not meant as an exclusive statement, and the relevant threshold for any of these examples is the threshold for field strength at a given pulse width. 
     In some examples, the third time period is greater than the sum of the first and second time periods. Such examples may help in providing the desired charge balance while mimicking the desired monopolar therapy approach. In some examples, the second time period may be well shorter than noted above, such as by using a second time period that is one-fourth, or one-tenth, the first time period. 
     In some examples, the first, second and third time periods add up to a duration of less than one millisecond. By ensuring that the total duration  340  is less than a millisecond, charge balancing can be achieved within a time frame that will avoid or reduce the likelihood of muscle stimulation. 
     In some examples, the first and second outputs  310 ,  320  are separated by an interphase period  316  of at least 10 nanoseconds, and the second and third outputs are not separated by an interphase period. In the drawing of  FIG. 10 , no interphase period is shown. Such an approach may be achieved by using a topology as shown in U.S. Pat. No. 10,154,869. In other examples, however, an interphase period  316 , in which no output voltage is being generated, may have a duration in the range of 1 nanosecond out to tens or even hundreds of microseconds, keeping in mind the aim to achieve charge balancing within a time period of less than a few milliseconds. During interphase, the output electrodes may be subject to an open circuit, or may be grounded or tied to a reference voltage, if desired. Most often, however, the electrodes will be open-circuited and placed in a high impedance state. 
     In some examples, the waveform  300  may be delivered repeatedly in first and second iterations; third or more iterations may be used as well. In some such examples, a first iteration is performed using a set of parameters that are preset, with the outputs delivered while monitoring one or more of impedance or current flow for each of the first, second and third outputs  310 ,  320 ,  330 . Then, in the second (and possibly subsequent) iterations, the method includes adjusting at least one of the amplitude or pulse width of at least one of the first, second and third outputs  310 ,  320 ,  330  to reduce change imbalance, if any, resulting from the monitored impedances. In some such examples, the first and second iterations are performed within a time period of less than 10 milliseconds, or a time period of less than 1 millisecond. For example, returning to the above numeric example, a total period  340  for therapy delivery was discussed using about 115 microseconds. If iterations start at 200 microsecond intervals, up to five iterations could be performed within a 1 millisecond window. As noted above, the window for repeated iterations can be longer, for example, to fit within an S-T cardiac cycle window, such as up to about 50 milliseconds or longer. Whether each iteration is adjusted using impedance or not, it is noted that the waveform as shown in  FIG. 10  can be delivered repeatedly. 
       FIG. 11  shows an illustrative therapy waveform. This example shows a method of delivering a multiphasic ablation waveform comprising generating a first pulse train  430  comprising first pulses  410  of a first polarity (negative, in the illustration) having a first amplitude  414  and a first pulse width  412 , alternating with second pulses  420  of a second polarity opposite the first polarity, having a second amplitude  424  and having a second pulse width  422  less than the first pulse width  412 . The example further includes generating a second pulse train  460  comprising third pulses  440  of the first polarity having a third amplitude  444  and a third pulse width  442 , alternating with fourth pulses  450  of the second polarity having a fourth amplitude  454  and a fourth pulse width  452  greater than the third pulse width  442 . The example method may be performed such that the first pulse train  430  yields a first charge imbalance, and the second pulse train  460  yields a second charge imbalance that offsets the first charge imbalance to prevent muscle stimulation. The charge imbalance of the first pulse train  430  would be proportional to the difference between the product of amplitude  414 , pulse width  412  and the quantity of first pulses  410  of the first pulse train  430 , and the product of amplitude  424 , pulse width  422 , and the quantity of second pulses  420  of the first pulse train  430 . 
     In some examples, the first and second amplitudes  414 ,  424  are the same, and the third and fourth amplitudes  444 ,  454  are the same. Further, the method may be performed such that a time  470  from the start of the first pulse train  430  to the end of the second pulse train  460  is short enough to avoid muscle stimulation due to the charge imbalance of the first pulse train  430 . For example, time  470  may be shorter than one millisecond, or shorter than two milliseconds, or some other duration, as desired. In some examples, the first and fourth pulse widths  412 ,  452  are equal in duration, and the second and third pulse widths  422 ,  442  are equal in duration. For example, the first and fourth pulse widths  412 ,  452  may be in the range of about 1 to about 20 microseconds, and the second and third pulse widths  422 ,  442  may be in the range of about 0.1 to about 10 microseconds. In some examples, the first pulse width  412  is about double the second pulse width  422 , and the fourth pulse width  452  is about double the third pulse width  442 . In other examples, the first, second, third and fourth pulse widths are each in a range of about 0.1 to 50 microseconds and may have other suitable ratios. In general, the concept is to provide two pulse trains, each of which would be imbalanced if delivered alone, with delivery taking place in a short enough period of time to achieve charge balance without muscle stimulation. 
     In some examples, the first pulse train  430  comprises a first quantity of first pulses  410  and a second quantity of second pulses  420 , and the second pulse train  460  comprises a third quantity of third pulses  440  and a fourth quantity of fourth pulses  450 , wherein the first, second, third and fourth quantities are all equal. 
     In some examples, the first, second, third and fourth amplitudes each exceed an irreversible electroporation threshold. As noted, the “threshold” may be in part dependent on pulse width as well as the distances between electrodes. In other examples, the first, second, third and fourth pulse widths are each in a range of about 0.1 to 500 microseconds. 
     In an alternative formulation, a pulsetrain  430  may comprise an odd number of pulses, such as pulses p 1  to p 5 , each having the same amplitude, in which pulses p 1 , p 3  and p 5  are of the same polarity and each have a pulse width PW, while pulses p 2  and p 4  are of opposite polarity and each have pulse width 1.5×PW, which would yield a charge balanced output even though pulses delivered in each polarity are unequal in charge content. In another example, a pulsetrain  430  may comprise an odd number of pulses each having the same pulse width, such as pulses p 1  to p 5 , in which pulses p 1 , p 3 , and p 5  are of the same polarity and each have an amplitude V, while pulses p 2  and p 4  are of opposite polarity and each have an amplitude 1.5×V, again providing an asymmetric output that, upon conclusion of the pulse train, provides charge balance. 
       FIG. 12  shows an illustrative therapy waveform. In this example, the method of delivering a multiphasic ablation waveform  500  comprises generating a first pulse  510  of a first polarity having a first amplitude  514  and a first pulse width  512 , and generating a first pulse train  520  having a plurality of second pulses  522  of a second polarity opposite the first polarity, the second pulses  522  having second amplitudes  526  and second pulse widths  524 , the second pulse widths  524  is a fraction of the first pulse width, such as being less than half, or less than ¼ of the first pulse width  514 . The ablation waveform  500  may be delivered such that the first pulse  510  yields a first charge imbalance, and the first pulse train  520  yields a second charge imbalance that offsets the first charge imbalance to prevent muscle stimulation. In this example, the wider pulse width  512  of the first pulse may be relatively long, in fact long enough to start to cause heating of the tissue if delivered repeatedly. However, the offsetting pulse train  520  that follows uses shorter pulse widths and should therefore reduce or even negate the heating effect. 
     In some further examples, the plurality of second pulses  522  are each separated by an interpulse interval  528 , the interpulse interval  528  being between one-half and twice the second pulse width  524 . In some examples, the interpulse interval  528  may be even longer than twice the second pulse width  524 . For example the interpulse interval may be tens or hundreds of times the pulse widths, such as up to 1000 times the second pulse width. An interphase interval is shown at  516 . The interphase interval  516  may be omitted, if desired. If included, an interphase interval may be any suitable length, from a few nanoseconds out to hundreds of microseconds. By spacing the pulse train  520  from the initial pulse  510 , a more monophasic result may be achieved. As with other examples, the total period  530  may be selected to be short enough to achieve charge balance without causing muscle stimulation, such as by being shorter than one millisecond. 
     In some examples, the first pulse width is equal to a sum of the second pulse widths. In still other examples, the first amplitude and the second amplitude each exceed an irreversible electroporation threshold. As noted previously, the relevant IRE threshold may be different depending on the tissue and pulse widths in use. 
     The example shown indicates that the same amplitude is used for both the first pulse  510  and the pulses  522  of the pulse train  520 . In other examples, the first amplitude  514  is less than the second amplitude  526 , and the sum of the second pulse widths  524  is less than the first pulse width  512 . In some examples, the sum of the second pulse widths  524  and the interpulse intervals  528  (that is, the total duration of the pulsetrain  520 ) is equal to the first pulse width. The order may be reversed, if desired. 
       FIG. 13  shows an illustrative therapy waveform. In this example, two complex monopolar outputs are combined to yield a charge balance. A first output  560  comprises a first pulse  562  of relatively lower amplitude, followed by several pulses  564  of higher amplitude and shorter pulse width. The order of  562  and  564  may be reversed, if desired. In another example, the higher amplitude pulses may be superimposed on the first pulse  562 . 
     A second output  570  is delivered some duration of time  552  later. Duration  552  may be any suitable duration but is preferably relatively long, such as being equal to or greater than the duration of the first output  560 , while still allowing the total therapy  550  to be delivered in a period  580  of less than one millisecond. The second output  570  may again include a longer pulse width and lower amplitude portion  572  and short pulse width, higher amplitude train  574 . In the example, amplitudes for pulse  562  and  572  are different, as are the pulse widths, but that need not be the case and in other pulses  562 ,  572  are equal in terms of amplitude and pulse width, but of opposite polarity. In an example, the amplitudes for the short pulses  564 ,  574  are above an IRE threshold (again, in view of the distance between electrodes and the applicable pulse widths), while the other pulses  562 ,  572  are below the IRE threshold. 
     In some examples, pulses  562  and  572  are priming pulses that are delivered to prepare the tissue for IRE caused by subsequent pulses. In a further example, the “priming” pulses  562 ,  572  are of opposite polarity relative to the short pulses  564 ,  574 , respectively. 
       FIG. 14  shows an illustrative therapy waveform. In this example, a first pulse pair  610  is delivered and is itself charge balanced. The first pulse pair  610  includes a first pulse  612  of a first polarity with first pulse width and first amplitude, and a second pulse  614  delivered after an interphase delay  616 , the second pulse  614  having an amplitude which is more than twice, and in some examples more than three times the amplitude of the first pulse  612 . To achieve charge balancing, the second pulse  614  is much shorter in pulse width than the first pulse  612 , wherein a product of the amplitude and pulse width of the first pulse  612  is equal to the product of the amplitude and pulse width of the second pulse  614 . A second pulse pair  620  may also be delivered and would be a repeat of the first pulse pair  610 , with an interpulse period  618  between the two  610 ,  620 . The time needed to deliver the first pulse pair  610  is preferably less than a time constant of surrounding tissue—that is, pulses  612  and  614  would both be delivered in time to achieve charge balancing without muscle stimulation. The two pulse pairs  610 ,  620  would be delivered in a physiological window, such as the ST Interval, so that duration  630  is less than the window will allow. 
       FIG. 15  shows an illustrative therapy waveform  650 . This example shows two stepped waveforms  660 ,  670 . More particularly, the figure shows a method of delivering a multiphasic ablation waveform  650  comprising delivering a first pulse train  660  comprising a plurality of first pulses each having a pulse width and an amplitude, wherein a first in time of the first pulses  662  has a first amplitude, and each successive pulse of the first pulses has a larger amplitude than an immediately preceding pulse, each of the first pulses having a first polarity. In some examples, the amplitude of the first pulse  662  in train  660  is less than an IRE threshold, while the last pulse  664  has an amplitude that exceeds the IRE threshold for the tissue, taking into account pulse width and electrode distance. The example method also includes delivering a second pulse train  670  comprising a plurality of second pulses each having a pulse width and an amplitude, wherein a first in time of the second pulses  672  has the first amplitude, and each successive pulse of the second pulses has a larger amplitude than an immediately preceding pulse, each of the second pulses having a second polarity opposite of the first polarity. In some examples, the amplitude of the first pulse  672  in train  670  is less than an IRE threshold, while the last pulse  674  has an amplitude that exceeds the IRE threshold for the tissue, taking into account pulse width and electrode distance. In some examples, the two pulse trains  660  and  670  are equal and opposite, such that, for example, first pulses  662  and  672  each have the same amplitude (but opposite polarity) and pulse width, and the same is true for the last pulses  664 ,  674 . The two pulse trains may be separated by an interval  652  that is at least greater than any of the individual pulse widths, and is preferably at least 4, 8 or 16 times the pulse widths. 
     In the example, the first pulse train and second pulse train are delivered within a time window  680  of less than about one millisecond, such that charge balance is achieved upon conclusion of the second pulse train  670  without causing muscle stimulation. 
     In another example, a series of monophasic pulses can be provided which do not add up to a charge balanced pulse train. Within the series of monophasic pulses, some may exceed an IRE threshold, with others exceeding only the reversible electroporation threshold are also applied. A long duty cycle pulse at a lower amplitude can be provided to balance out the series of monophasic pulses, as desired and for charge balancing purposes. 
     Several embodiments of the present invention take the form of a pulse generator configured for use with a probe for delivering ablation therapy to a patient, the pulse generator comprising output circuitry for delivering voltage-based therapy, monitoring circuitry for monitoring characteristics of delivered therapy pulses, and control circuitry comprising a non-volatile memory containing an executable instruction set adapted to deliver therapy as in any of the above methods. 
     Several embodiments of the present invention take the form of a system comprising a probe for insertion into a patient having a plurality of electrodes for ablation therapy delivery, and a pulse generator configured for use with the probe for delivering ablation therapy to a patient, the pulse generator comprising output circuitry for delivering voltage-based therapy, monitoring circuitry for monitoring characteristics of delivered therapy pulses, and control circuitry comprising a non-volatile memory containing an executable instruction set adapted to deliver therapy as in any of the above methods. 
     It should be noted that in the above examples, the terms first, second, third, fourth, etc. when applicable to pulses that are to be delivered do not necessarily indicate an order of delivery of the pulses. A pulse labeled as a “third” pulse may be the first in time to be delivered, relative to pulses labeled as “first,” “second,” or “fourth”, and other orders may be used as well. 
     Each of these non-limiting examples can stand on its own, or can be combined in various permutations or combinations with one or more of the other examples. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic or optical disks, magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. 
     The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
     Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.