U.S. patent application number 12/136683 was filed with the patent office on 2009-12-10 for method for low temperature electrosugery and rf generator.
Invention is credited to Alexander B. VANKOV.
Application Number | 20090306642 12/136683 |
Document ID | / |
Family ID | 41400980 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090306642 |
Kind Code |
A1 |
VANKOV; Alexander B. |
December 10, 2009 |
METHOD FOR LOW TEMPERATURE ELECTROSUGERY AND RF GENERATOR
Abstract
Method and apparatus for electrosurgery including tissue
coagulation using very high voltage pulses of electrical energy
applied to the electrosurgical probe. This minimizes heating of the
surrounding tissue in the probe and is especially suitable for
precise and limited coagulation and fulguration without excessive
tissue charring or other damage. The power at rated load of the
applied pulses to the probe is typically over 300 W and the
duration of the on time is very short, so each group of pulse
bursts is of relatively low duty cycle. An RF generator is also
provided for delivering electrical energy to an electrosurgical
probe with the proper characteristics, including fast switching
times.
Inventors: |
VANKOV; Alexander B.; (Menlo
Park, CA) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
755 PAGE MILL RD
PALO ALTO
CA
94304-1018
US
|
Family ID: |
41400980 |
Appl. No.: |
12/136683 |
Filed: |
June 10, 2008 |
Current U.S.
Class: |
606/33 ;
327/171 |
Current CPC
Class: |
A61B 18/1206 20130101;
H03K 17/0822 20130101; A61B 2017/00154 20130101; A61B 18/18
20130101; H03K 17/691 20130101; H03K 3/02 20130101 |
Class at
Publication: |
606/33 ;
327/171 |
International
Class: |
A61B 18/18 20060101
A61B018/18; H03K 3/00 20060101 H03K003/00 |
Claims
1. A method of coagulating tissue, comprising the acts of: applying
groups of bursts of pulsed electrical energy to a probe in contact
with the tissue; each group of bursts having a power of at least
300 W at a rated load during an on time and zero during an off
time, the RF groups of bursts being of frequency in a range of 100
kHz to 5000 kHz and each group of bursts having a duty cycle in a
range of 1% to 50%; applying a plurality of the bursts in
succession; and providing an interval between successive
pluralities of the bursts of at least 1 msec.
2. The method of claim 1, wherein a temperature of the probe
remains below 100.degree. C.
3. The method of claim 1, wherein a tip of the probe is of metal
with an insulation layer thereover, the insulation defining on a
side of the probe a plurality of openings to expose the metal.
4. The method of claim 3, wherein each opening has a diameter in
the range of 0.02 mm to 0.010 mm.
5. The method of claim 3, wherein at an edge of the probe the
insulation defines an opening to expose the metal.
6. Electrosurgery apparatus, comprising: a probe adapted to be
applied to tissue; and a source of electrical energy electrically
coupled to the probe, the source applying groups of bursts of
pulsed electrical energy to the probe; each group of bursts having
a power of at least 300 W at rated load during an on time and zero
during an off time, the RF groups of bursts being of frequency in a
range of 100 kHz to 5000 kHz and each group of bursts having a duty
cycle in a range of 1% to 50%; the source applying a plurality of
the bursts in succession; and the source providing an interval
between successive pluralities of the bursts of at least 1
msec.
7. A circuit to generate high frequency signals, comprising: a
first channel and a second channel, each including a switching
transistor, an output terminal of each transistor being coupled to
a common output node; a control terminal of each transistor being
coupled to an output terminal of a driver; and an input terminal of
each driver being coupled to a source of a negative voltage bias;
wherein the first channel provides positive going signals at the
common output node and the second channel provides negative going
signals at the common output node.
8. The circuit of claim 7, further comprising coupling the input
terminal of each driver to ground.
9. The circuit of claim 8, further comprising: a current driver
circuit having an output terminal coupled to the input terminal of
each driver.
10. The circuit of claim 8, further comprising a current protection
circuit coupled to each channel.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrosurgery for biological
tissue.
BACKGROUND OF THE INVENTION
[0002] The field of electrosurgery is well known, see for instance,
Palanker U.S. Pat. No. 7,238,185 and Palanker, et al. U.S. Pat. No.
6,780,178 both incorporated herein in their entireties. Briefly,
application of a voltage to an electrode is useful for cutting,
ablating and fulgurating biological tissue. This is generally known
as electrosurgery. Typically the voltage is applied as a train of
high frequency pulses in the radio frequency (RF) range to a probe
in contact with the tissue.
[0003] A problem with electrosurgery is preventing excessive
application of heat to the tissue being cut, fulgurated,
dessicated, etc. since this tends to produce undesirable affects
such as charring and collateral tissue damage. This is typically
caused by high temperatures induced by the application of the
electrical energy.
[0004] Some highly localized high temperature is required during,
for instance, tissue coagulation (sealing) for denaturation of
blood and vascular tissue (veins and arteries) followed by
occlusions of the blood vessels. Typically dessication occurs below
or close to 100.degree. C. and fulguration at higher temperatures
above 100.degree. C. A high temperature during fulguration outside
the immediate area being treated results in undesirable tissue
charring and buildup of debris on the electrosurgical probe, which
decreases its efficiency of coagulation. This may also result in
adhesion of charred tissue to the probe and damage to the areas of
the probe with low melting temperatures such as plastic components.
Typically this might require cleaning of the probe after each
session of coagulation. Also, high temperature may result in smoke
obscuring the surgical field, especially for laparoscopic
procedures.
SUMMARY
[0005] In accordance with this invention, a method and apparatus
for pulsed applications of heat in electrosurgery provide
sufficient peak temperature for tissue coagulation (and "blend"
cutting) and allow for cooling of tissue between the application of
electrical energy pulses, so avoiding excessive heating. Typically
groups of pulse bursts are separated by a time interval sufficient
for cooling both the probe and immediately neighboring tissue to
close to ambient temperature.
[0006] In one embodiment this is achieved by using RF high power
groups of pulse bursts, such as power levels of 300 W or higher
during on time and zero during off time, the groups of bursts being
of high frequency such as 100 kHz to 5000 kHz and each group of
bursts having a duty cycle in the range of 1% to 50%. Duty cycle
refers to the ratio of time when RF power is applied to the rated
load to the full duration of the group of bursts. According to this
definition a sine wave has a 100% duty cycle. For coagulation and
blend cutting electrosurgery, the sine wave cycles (on time) occupy
a short time in each burst, with a substantial part of each burst
having no RF energy present (off time). A number of such bursts are
grouped together, with an interval of at least 1 millisecond
between each group of bursts, to allow for tissue cooling. Each
burst of pulses has enough electrical power to rapidly heat the
tissue to temperatures adequate for coagulation.
[0007] The active portion of the electrosurgery probe itself is
typically of relatively small size to provide a short cooling time.
Moreover the probe is bare metal or metal covered with a layer of
insulation, with the layer of insulation defining an opening at the
edge where the electrical pulses are actually applied to the tissue
for coagulation and cutting and further defining a number of spaced
apart small openings on its side surfaces (flat portions), each
having a diameter for instance of 0.02 mm to 0.10 mm for extensive
coagulation.
[0008] Also provided in accordance with the invention is an RF
pulse generator for low temperature electrosurgery tissue cutting.
This pulse generator provides square wave alternating positive and
negative pulses with a fast switching time and a pulse amplitude of
up to 1000 Volts peak to peak. The particular circuit disclosed
here, also referred to as a pulse generator or radio frequency
generator in the field, is based on a conventional half bridge
inverter with high power transistors serving as high and low side
switches. In order to overcome the well known problem of Miller
capacitance, each channel of the inverter (there being a positive
pulse voltage channel and a negative pulse voltage channel) is
provided with a gate driver circuit driving the gate of each
switching transistor. Moreover an input terminal of the gate
driving circuit is negative biased and also coupled to ground via a
resistance. Further, each channel also includes a current driver
(booster) with a disable function to provide protection of the
circuit in short circuit conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIGS. 1a, 1b, 1c show a set of high power groups of bursts
indicating the nature of the electrical energy applied to the probe
in accordance with the invention for 3 types of coagulation
respectively spray, pinpoint, and blend.
[0010] FIG. 2 shows a graph of thermal relaxation time vs. probe
size.
[0011] FIG. 3 is a planar view of a portion of an electrosurgical
probe showing the openings defined on the side surfaces of the
probe through the insulation layer.
[0012] FIG. 4a is a schematic circuit diagram of a RF generator in
accordance with the invention.
[0013] FIG. 4b shows an output waveform of the RF generator.
[0014] FIG. 5a shows the potential problem of noise in the present
RF generator.
[0015] FIG. 5b shows how the problem of noise is overcome in
accordance with the present RF generator.
[0016] FIG. 6 shows a schematic diagram of a protection circuit
used with the RF generator of FIG. 4a.
DETAILED DESCRIPTION
High Voltage Electrosurgery
[0017] The present description is directed to high voltage RF
electrical energy applied to an electrosurgical probe for tissue
coagulation and cutting. It is understood that the electrosurgery
probe itself may be of the types disclosed here or in the above
described patent applications or other types as known in the field.
Typically the probe has a relatively small surface area at its
active electrode portion (tip) to minimize heating of the tissue
being treated. The probe may be uninsulated (bare metal) or partly
covered with a high dielectric insulating layer. The probe may be
mono or bi-polar. In some applications the probe is immersed in the
tissue being operated on, which has naturally occurring fluid
present or some type of liquid is provided immediately around the
probe in the surgical field. In other uses for, e.g., fulguration
no liquid is present.
[0018] The present method is intended primarily for use with
electrosurgical coagulation, but can be used for simultaneous
tissue cutting and coagulation. For tissue coagulation purposes
some amount of charring is in fact desirable since that is the
intent of coagulation (to seal tissue). However, tissue charring is
undesirable beyond the immediate area being coagulated. The goal is
to maintain a relatively low probe temperature and hence minimize
heat transfer to the surrounding tissue while still accomplishing
coagulation or dessication or fulguration. Hence the present method
is directed to what is sometimes called "cold coagulation."
[0019] The edge of the probe is intended for both cutting and
coagulation, and the flat (side) portion thereof with "dimples"
(the openings) serves for coagulation only. As the probe edge cuts
through the tissue, the flat portion of the probe sends an
electrical arc to the walls of the wound to heat and close the
blood vessels. The dimples help the electrical arc to reach all
blood vessels, as in an uninsulated probe, but the small diameter
of the dimples advantageously provides a short thermal relaxation
time and, as a result, low temperature during pulsed
coagulation.
[0020] This is accomplished here by applying high power RF groups
of bursts with relatively long off times and thus relatively low
duty cycles compared to conventional electrosurgical coagulation.
While equipment limitations may prevent use of RF power levels
above 300 W given current materials and electrical components,
ultimately this is not limiting. Hence generally, the present
invention is directed to use of high rated power RF (above 300 W)
during the pulse on time. Since power depends on the load, the
rated load is by definition the load where the maximum (rated)
power can be achieved. A typical voltage here (both positive and
negative) is up to 12,000 Volts peak to peak under open circuit
conditions. A typical waveform for this condition is a damped sine
wave.
[0021] In one embodiment the RF power has a carrier frequency of
approximately 460 KHz, so the duration of each period (pulse) is
approximately 2.2 microseconds. The on time RF pulses can be sine
waves, but usually a sine wave is good only for pure cutting. For
blended cut and coagulation purposes, periods of pulses are
clustered in each burst with no RF energy between them. In one
embodiment, there is only one period per burst but this is not
limiting; there may be 2, 3, or more pulses per burst as shown
respectively in FIGS. 1a, 1b, 1c for different types of
coagulation. The repetition rate for the bursts is e.g. 30 KHz. A
typical frequency for the groups of burst is 25 Hz. A group of
these bursts defines the on time, followed by the off time. Hence
the duty cycle of each group (during on time) is in the range of 1%
to 50%. That is, only 1% to 50% of the total time during each group
of bursts is actually occupied by RF energy and the remainder is of
zero voltage applied to the load, as shown in FIGS. 1, 1a, 1b, 1c.
A number of such pulse bursts may also be grouped together.
Typically the off time between each group of bursts is about 1
millisecond or more to allow further cooling of the probe and
associated tissue. The on time can be from 100 microseconds to 10
ms, followed by the off time off interval, at least 1 ms in
duration. This modulation further reduces the duty cycle by a
factor of 0.01 to 0.9.
[0022] The open circuit waveform is, e.g., a damped sine wave at
the carrier frequency (such as 460 kHz in FIG. 1c) which shrinks to
a sine wave cycle as shown in FIG. 1a at a low impedance load. The
amplitude (voltage) of these pulses decreases with the resistance
of the load in such a way that the average RF power achieves a
maximum at a so called "rated load", typically 100 to 1000 Ohm. The
time T between pulse centers corresponds to the inverse of the
carrier frequency (for example T=1/460 kHz). The number of pulses
in a pulse burst determines the rated load, roll-off points on the
load curve, the type of the surgical mode (called in the field for
instance blend, desiccation, fulguration), and the length of the
spark. The repetition rate of the pulse bursts is typically 20 to
60 kHz (indicated as fburst of 30 kHz in FIG. 1b). The purpose of
these bursts is not a cooling of the tissue during the burst off
time, since tissue temperature cannot decrease during mere tens of
microseconds. Instead the off time allows for collapse of
undesirable vapor bubbles formed on or near the probe, arising
during the pulse on time. Otherwise due to the vapor bubbles the
tissue experiences problematic "micro explosions" of the bubbles,
repulsing tissue from the probe, and precluding effective
coagulation.
[0023] In the present method therefore for coagulation the pulse
bursts are grouped together, with a time interval between them
(determined by the burst duty cycle) longer than the thermal
relaxation time for a particular probe. The thermal relaxation time
t can be assessed as t=r.sup.2c.rho./.pi..sup.2k=r.sup.20.7
.mu.s/.mu.m.sup.2, where r is the characteristic size of the
electrode in .mu.m, c is heat capacity, .rho. is density, and k is
thermal conductivity of liquid as plotted in FIG. 2. For instance,
for a 10 .mu.m effective probe electrode size the thermal
relaxation time is 70 .mu.s, but for 1 mm probe this value is 0.7
seconds. So, a small probe electrode is generally required
here.
[0024] The small probe can coagulate only a small area adjacent to
the probe electrode. With a spark (arc) length of 1 mm and a point
electrode that is 0.1 mm in diameter, one can coagulate a spot of
tissue 2.1 mm in diameter. Multiple small electrodes, representing
small openings in the probe insulation are introduced on the flat
portion of the blade and spaced apart to coagulate a large solid
area of tissue. The spark circles should overlap to cover the whole
tissue surface. The size of the individual electrodes (the
openings) is small enough to provide fast cooling. At pulsed mode
as described above, low average tissue temperature can be achieved.
As a result, the probe provides shallow strong coagulation with a
safe temperature of the probe.
[0025] FIG. 3 shows a partial view of a side surface of the
associated probe 30 in which the overlying insulating layer 34
defines a pattern of openings (dimples) 36a, 36b, 36c, etc. to
expose the underlying metal 40 of the electrode of probe 30. In
this case the size of each opening is shown as about 0.05 mm by
0.05 mm (each being approximately a square), however, this size and
shape are not limiting. The spacing between openings (center to
center) is in the range of 0.2 to 1.0 mm, not limiting. Also
conventionally the metal edge 42 of probe 30 is also exposed
through the overlying insulating layer 34. The actual materials of
the probe metal 40 and insulation 34 are conventional as explained
in the above referenced patents and as well known in the field.
[0026] Typically the associated equipment (RF generator) has
variable outputs and can be adjusted by the operator to provide
pulses of various frequencies and timing durations so that the
present pulse regime is thereby accomplished. This pulse regime may
depend on probe size, the nature of the surgical procedure being
undertaken such as fulguration, dessication, coagulation, and other
factors as determined by the operator (surgeon). A typical range of
correct frequencies for the pulses is 100 KHz to 5 MHz, of which
the above described 460 KHz is merely illustrative. In accordance
with this approach, the probe and the associated tissue may be kept
below or above 100.degree. C., depending on what is required for
the particular surgical procedure being undertaken. Advantageously
the relatively low temperature of the probe-tissue interface
results in reduced adhesion of the charred tissue to the probe,
decreasing smoke and providing better performance for
coagulation.
RF Generator
[0027] Also disclosed here is an RF generator 50 for e.g. pulsed
cutting of tissue (see circuit diagram FIG. 4a), compatible with a
pulsed coagulation method and probe described above. Such an RF
generator is believed to be novel for electrosurgery, where out
coupling typically represents a transformer, although generally RF
generators are well known in the electronics field for generating
high frequency electrical signals. Such RF generators typically are
half bridge inverters. The present RF generator has only capacitors
in series with the load as required by regulatory rules (for a
capacitance <5 nF), and the usual RF transformer is omitted.
Associated waveforms are described in Palanker U.S. Pat. No.
7,238,185, incorporated herein by reference in its entirety, and
represent a true bipolar square wave. The present RF generator for
tissue cutting may be a part of a system producing also coagulation
waveforms according to FIG. 1 to combine cutting and coagulation
ability for a single probe.
[0028] In accordance with similar circuits, the present RF
generator apparatus or circuit 50 conventionally includes a half
bridge inverter with high power Field Effect Transistors (FET) Q2,
Q1 used as respective low and high side switches. In such an RF
switching generator or power supply the amount of time for each
switch (transistors Q1, Q2) to turn on or off is important. For
proper performance the switching transistors should be capable of
switching in less than approximately 10% of the period of the
output pulse. For a 4 MHz frequency pulse this requires that each
transistor's Q1, Q2 gate terminal be charged/discharged in less
than 25 nanoseconds.
[0029] As well known in the field, the effective gate capacitance
or input capacitance of such field effect transistors Q1, Q2
includes the gate-source capacitance and the gate-drain
capacitance, also referred to as Miller capacitance. The total gate
charge required to charge the gate of a typical field effect
transistor from 0V to 3V (enough to switch the transistor) is 80
nC. This total charge includes the Miller charge required to
discharge the gate-drain capacitance when the transistor switches
from the off state with a drain-source voltage of 450V, to the on
state. If the entire charge is to be delivered in a 25 nanosecond
period as indicated above, then the gate driver circuit which
provides the signal to the gate must apply an average current of 80
nC/25 ns=3.2 amps with a peak current as high as 12 amps. To meet
this, the gate driver circuits 52, 54 in this generator are
selected to provide a 20 amp maximum current, but this is merely
illustrative.
[0030] For typical electrosurgery applications, the electrical
charge injected in the tissue from the probe must be close to zero
to minimize undesirable muscle stimulation. Thus it is important to
have balanced positive and negative portions of the pulsed current
provided to the probe. In the present RF generator therefore two
channels 58, 60 are used, e.g., connected to two Direct Current
(DC) power supplies (not shown), one providing +500V and the second
providing -500V output signals at respectively nodes 115, 117. The
output terminal 112 (to the probe) for generator 50 therefore is
connected at a midpoint node 66 between the two channels 58, 60.
Voltage at this terminal 66 therefore swings between positive and
negative voltages as described further below.
[0031] The current driving for the gate of each switching
transistor Q1, Q2 is provided here with a radio frequency isolated
independent driving direct current power supply. The RF isolation
is required because the gate of each transistor Q1, Q2 is
referenced to the source of the transistor, which switches with
slew rates of more than 30 Volt/nanosecond. The high side driving
reference point 66 has to have a minimum coupling capacitance and
leakage inductance to the ground of the drivers 52, 54.
[0032] Transformers 78, 80 are provided as is conventional in each
channel 58, 60 for galvanic isolation and level shifting required
for each switching transistor Q1, Q2. An advantage of this is at
the high-side gate driver circuit 52 does not require a floating
power supply since the power to transistor Q1 is coupled through
the transformer 78. The leakage inductance of the windings of each
transformer 78, 80 makes it difficult to obtain the rapid rise of
the current required and causes excessive ringing which must be
suppressed. Improved operation is obtained here by using a large
sinusoidal drive current since the leakage inductance of the
transformers 78, 80 along with the input capacitance of each
transistor Q1, Q2 can be included in a resonant circuit.
[0033] In this case the sine wave output of the resonant circuit
has higher amplitude than the transistor switching threshold
voltage, to minimize switching time. However, fast switching
results in shorter high voltage swings on the source/drain
terminals of the switching transistors Q1, Q2. Short intense
transients therefore travel back from the mid-point 66 of the half
bridge to the gate terminals of transistors Q1, Q2 due to the
Miller capacitance into the output of each gate driver circuit 52,
54 from each of the transformers 78, 80. Each gate driver circuit
52, 54 has a 0.6 Ohm output resistance in both high and low output
voltage regimes. Therefore the energy of the transient goes mostly
to the low voltage ground as indicated in FIG. 5a and causes
ringing. This undesirable ringing may affect the input of the gate
driver circuits 52, 54 causing simultaneously opening and closure
of the switching transistors Q1, Q2 which, of course, must be
avoided. In order to increase signal to noise discrimination level
and avoid ringing, a negative bias voltage as shown of -1V is
applied to the input ("In") terminal of each gate driver circuit
52, 54. Additionally in this case the same input terminal of each
gate driver circuit 52, 54 is also coupled to ground via a low
resistance (22 Ohm) resistor 80, 82. Also, 22 Ohm resistors 86, 88
are coupled across the primary and/or secondary windings of each
transformer 78, 80 to damp ringing. Also, to decrease quality
factor of the resonant circuit, inductances of the primary and
secondary windings of each transformer 78, 80 are chosen to be
minimal e.g. 1.6 microH. With the input capacitance of the MOSFET
Q1, Q2 (1 nF) resonant frequency of the contour
f=1/(2.pi.(LC).sup.0.5)=4 MHz is equal to the operation frequency.
The inductance of the ground path to the input terminal of each
gate driver circuit 52, 54 is minimized with short and wide leads.
Also, a DC/DC converter 90, 92 is coupled to the input terminal of
each gate driver circuit 52, 54 to create the above mentioned
negative direct current bias of -1 Volt to that input terminal and
effectively discriminate noise at the input terminal of each gate
driver circuit 52, 54.
[0034] As shown in FIG. 4a, effectively the negative input bias at
the input terminal "In" of each gate driver circuit 52, 54 is -1
Volt in this example. In the left hand portion of FIG. 4a are shown
(as waveforms) the input control signals 100, 102 applied to each
input terminal 106, 108 of the two channels of the RF generator and
shown as a set of square waves which determines timing for the
pulse bursts and pulses as explained above. The input control
signals are generated conventionally. Conventionally in the far
right hand portion of FIG. 4a is the RF generator output terminal
112 labeled "pulse out" which is connected to the probe. FIG. 4b
shows an output waveform (at node 112) of the RF generator 50. Also
provided is a high voltage ground terminal 116 connected to the
probe ground terminal or to a return line connected to the patient.
The remaining circuit elements in FIG. 4a are conventional; in some
cases component numbers or values are shown, but these are only
exemplary.
[0035] FIG. 5a shows via waveforms how the circuit of FIG. 4 would
have an accumulation of noise on a low voltage ground resulting in
an uncontrollable wave form, e.g., due to ringing. The horizontal
axis here refers to time and the vertical axis is the voltage at
the input terminal of each gate driver. The horizontal broken line
at 3 Volt is the threshold voltage at the input terminal of the
gate drivers 52, 54. FIG. 4b shows how the above described negative
bias of -1 Volt applied to that same input terminal (and also shown
as the horizontal broken line in FIG. 5b) reduces the amount of
noise compared to FIG. 4a at the In terminal to the gate driver
circuits 52, 54.
[0036] Also provided in one embodiment in the circuit of FIG. 4a is
over current protection to prevent damage to the switching
transistors and/or the other components. Typically failure of such
a RF generator is caused by excessive currents flowing either
through the switching transistors or into the output terminal.
Various conventional protection circuits are known and an example
is shown in FIG. 6 which would be coupled conventionally to
generator 50. These protection circuits typically include current
transformer sensors connected either to the return patient cable
(ground to the patient, e.g., at node 116) or to the high voltage
lines at node 112. The circuit of FIG. 4a since it has two channels
58, 60 would typically have two such protection circuits, one
coupled to each channel 58, 60.
[0037] This description is illustrative and not limiting. Further
modifications and improvements will be apparent to those skilled in
the art in light of this disclosure and are intended to fall within
the scope of the pending claims.
* * * * *