System for controlling tissue ablation using temperature sensors
Body tissue ablation is carried out by inserting a probe into a body of a living subject, urging the probe into contact with a tissue in the body, generating energy at a power output level, and transmitting the generated energy into the tissue via the probe. While transmitting the generated energy the ablation is further carried out by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level. Related apparatus for carrying out the ablation is also described.
1. A method for ablating body tissue, comprising:
inserting a probe into a body of a subject;
urging the probe into contact with a tissue in the body;
generating ablative energy at a power output level at a level of current;
transmitting the generated ablative energy into the tissue via the probe;
determining a measured temperature of the tissue by assessing a thickness of the tissue concurrently with the transmitting of the generated ablative energy;
determining a measured power level of the generated ablative energy during the transmitting of the generated ablative energy into the tissue;
determining a power deviation by comparing a difference between the measured power level and a predetermined target power level;
determining a temperature deviation by comparing a difference between the measured temperature of the tissue and a predetermined target temperature;
calculating a target current value from a control function, wherein the control function is:
I
new
=
I
present
+
k
M
in
{
(
P
targ
-
P
meas
P
targ
)
,
(
T
targ
-
T
meas
T
targ
)
}
,
or
I
new
=
I
present
+
kC
(
P
targ
-
P
meas
P
targ
)
(
T
targ
-
T
meas
T
targ
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in which:
I new is the target current value;
I present is the target current value in a previous iteration;
P meas is the measured power level;
P targ is the target power level;
T meas is the measured temperature;
T targ is the target temperature;
k is a damping constant; and
C is a constant having a value of −1 if both P meas is greater than P targ and T meas is greater than T targ , and +1 otherwise; and
controlling the power output level responsively to the calculated target current value by incrementally adjusting the level of current to the target current value gradually over time to generate the ablative energy at a new power output level until the measured temperature of the tissue reaches the predetermined target temperature and the measured power level reaches the predetermined target power level.
2. The method according to claim 1 , wherein the generated ablative energy is radiofrequency energy, ultrasound energy or laser-produced light energy.
3. The method according to claim 1 , wherein the determining a measured temperature of the tissue by assessing the thickness of the tissue comprises acquiring signals from one or more ultrasound transducers using an ultrasound processor, and analyzing the signals in a temperature analyzer to determine the measured temperature.
4. The method according to claim 1 , wherein the determining the power deviation, determining the temperature deviation, calculating the target current value, and controlling the power output level are performed iteratively.
5. The method according to claim 1 , wherein the controlling the power output level is performed by limiting an increment or decrement thereof so as not to exceed a predetermined limiting condition, wherein the limiting condition is selected from the group consisting of a maximum current, a minimum electrode temperature, a maximum electrode temperature, a maximum temperature of the tissue, and a maximum power demand.
6. The method according to claim 4 , wherein in subsequent iterations after a first iteration, changes in power demand (ΔD) are calculated using Equation 4:
Δ D=D 0*Min(( P targ− P meas)/ P targ,( T targ− T meas)/( T targ)) Equation 4
wherein D0 is a constant predefined change in the demand or power.
7. The method according to claim 6 , wherein at each iteration, the current value (I) corresponding to the power calculated by Equation 5, is output onto the electrode:
Di+ 1= Di+ΔD. Equation 5
8. The method according to claim 6 , wherein when Min((Ptarg−Pmeas)/Ptarg, (Ttarg−Tmeas)/(Ttarg))>1, then Equation 6 is used to limit the increment in the power level:
Δ D=D 0. Equation 6
9. The method according to claim 6 , wherein when Min((Ptarg−Pmeas)/Ptarg, (Ttarg−Tmeas)/(Ttarg))<−1.1, then the power output is set at 0 in order to allow the tissue to cool.
10. The method according to claim 6 , wherein when Ptarg<Pmeas or Ttarg<Tmeas, then the value ΔD is negative and the power output will be decreased.
11. The method according to claim 3 , wherein the one or more ultrasound transducers are also configured to generate the ablative energy.
12. The method according to claim 1 , further comprising determining a location of the probe using a positioning sub-system.
13. The method according to claim 12 , wherein the positioning sub-system comprises a positioning processor that measures location and orientation coordinates of the probe.
14. The method according to claim 12 , wherein the positioning sub-system comprises a magnetic tracking arrangement that determines the position and orientation of the probe by generating magnetic fields in a predefined working volume and sensing the magnetic fields at the probe using field generating coils.
15. An ablation system, comprising:
a catheter having a distal portion for insertion into a body and configured to bring the distal portion into contact with a tissue in the body;
a power generator for generating ablative energy at a power output level having a level of current;
an ablation element disposed on the distal portion of the catheter, configured to accept the ablative energy from the power generator via the catheter and to conduct the ablative energy to the tissue for ablation of the tissue;
one or more ultrasound transducers configured to generate signals;
a processor operative for determining a measured temperature of the tissue from the signals from the one or more ultrasound transducers, and to determine a measured power level of the ablative energy conducted through the ablation element, the processor configured to:
determine a power deviation by comparing a difference between the measured power level and a predetermined target power level;
determine a temperature deviation by comparing a difference between the measured temperature of the tissue and a predetermined target temperature;
calculate a target current value from a control function, wherein the control function is:
I
new
=
I
present
+
k
M
in
{
(
P
targ
-
P
meas
P
targ
)
,
(
T
targ
-
T
meas
T
targ
)
}
,
or
I
new
=
I
present
+
kC
(
P
targ
-
P
meas
P
targ
)
(
T
targ
-
T
meas
T
targ
)
in which:
I new is the target current value;
I present is the target current value in a previous iteration;
P meas is measured power;
P targ is a target power level;
T meas is measured temperature;
T targ is a target temperature;
k is a damping constant; and
C is a constant having a value of −1 if both P meas is greater than P targ and T meas is greater than T targ , and +1 otherwise; and
control the power output level responsively to the calculated target current value by incrementally adjusting the level of current to the target current value gradually over time to generate the ablative energy at a new power output level until the measured temperature of the tissue reaches the predetermined target temperature and the measured power level reaches the predetermined target power level.
16. The ablation system according to claim 15 , wherein the measured temperature is a temperature of the ablation element; or wherein the ablation energy is radiofrequency energy, ultrasound energy or laser-produced light energy.
17. The ablation system according to claim 15 , further comprising a positioning sub-system configured to determine a location of the catheter.
18. The ablation system according to claim 17 , wherein the positioning sub-system comprises:
a positioning processor configured to measure location and orientation coordinates of the catheter; and/or
a magnetic position tracking arrangement that determines the position and orientation of the catheter by generating magnetic fields in a predefined working volume and sensing the magnetic fields at the catheter using field generating coils.
19. The ablation system according to claim 15 , wherein the determining the power deviation, determining the temperature deviation, calculating the target current value, and controlling the power output level are performed iteratively.
20. The ablation system according to claim 19 , wherein in subsequent iterations after a first iteration, changes in power demand (ΔD) are calculated using Equation 4:
Δ D=D 0*Min(( P targ− P meas)/ P targ,( T targ− T meas)/( T targ)) Equation 4
wherein D0 is a constant predefined change in the demand or power.