AJR 2005; 184:391-397
© American Roentgen Ray Society
Bipolar Radiofrequency Ablation Using Wet-Cooled Electrodes: An In Vitro Experimental Study in Bovine Liver
Jeong Min Lee1,
Joon Koo Han,
Se Hyung Kim,
Seung Hong Choi,
Su Kyung An,
Chang Jin Han and
Byun Ihn Choi
1 All authors: Department of Radiology and Institute of Radiation Medicine,
Seoul National University College of Medicine and Clinical Research Institute,
Seoul National University, 28 Yeongon dong, Jongno-gu, Seoul 110-744, South
Korea.
Received October 8, 2003;
accepted after revision June 3, 2004.
Address correspondence to J.K. Han
(HANJK{at}RADCOM.SNU.AC.KR).
Abstract
OBJECTIVE. Our aim was to evaluate the performance of hypertonic
saline (HS)-enhanced bipolar radiofrequency ablation using wet-cooled
electrodes versus monopolar radiofrequency ablation to create coagulation
necrosis in explanted bovine liver.
CONCLUSION. HS-enhanced bipolar radiofrequency ablation using the
wet-cooled electrodes shows better performance in creating coagulation
necrosis than the monopolar mode.
Introduction
Radiofrequency ablation is a promising imaging-guided interventional
procedure for primary and secondary liver tumors and has become an established
alternative treatment to surgery in inoperable patients
[1–3].
Despite the development of several recent devices enabling the creation of
larger lesions, treating liver tumors larger than 3.5 cm in diameter often
requires multiple overlapping ablations
[4]. However, because this
approach is both time-consuming and technically challenging, especially under
sonographic guidance, there is a real need to increase the dimensions of
thermally mediated coagulation necrosis using a single probe application
[4,
5].
This limited dimension of coagulation necrosis produced by monopolar
radiofrequency ablation is related to the fact that in monopolar
radiofrequency ablation, the current density drops off as the inverse square
of the distance from the electrode and heating drops off as the inverse of the
fourth power of the distance
[5]. To increase
radiofrequency-induced coagulation necrosis, several investigators proposed
bipolar radiofrequency ablation
[6–10].
In the bipolar mode, a second electrode is used instead of the dispersive
plate, and there is a high and constant electric field gradient between the
two electrodes [6]. However,
previous studies [6,
8] showed that although bipolar
radiofrequency ablation could more quickly create round and regular lesions
than the monopolar mode, on several occasions rapid increases in impedance
occurred during the procedure because of a failure to avoid the rapid boiling
of the liver tissue adjacent to the electrode. Miao et al.
[11] reported that a
cooled-wet electrode, which allowed simultaneous internal cooling perfusion
and interstitial saline infusion, showed better efficiency at creating larger
ablation zones than other monopolar electrodes. On the basis of the results of
their study [11], we supposed
that during bipolar radiofrequency ablation, the simultaneous application of
internal electrode cooling and saline infusion could effectively avoid this
boiling and, therefore, allow higher currents to be delivered to tissue. To
enable saline interstitial infusion and intraelectrode cooling, we developed a
prototype wet-cooled electrode by modifying a 17-gauge internally cooled
electrode (Cool-tip, Radionics). In this context, we present the results of
our systematic evaluation of bipolar radiofrequency ablation using the
developed wet-cooled electrode versus the sequential monopolar mode, with
respect to the dimensions of ablation zones in liver tissue and the
temperature at the midline between the two electrodes.
Materials and Methods
Design of Wet-Cooled Electrode
Referencing a previous study by Miao et al.
[11], we developed a
wet-cooled electrode to allow simultaneous intraelectrode cooling perfusion
and interstitial saline infusion; we covered a 17-gauge cooled-tip electrode
having a 3-cm active tip with a15-gauge outer sheath, except for its 3.5-cm
distal portion (Fig. 1). The
insulated, metal outer sheath covered the shaft of the cooled-tip electrode. A
hole for saline infusion was positioned in the proximal portion of the sheath,
and the space between the 15-gauge sheath and the cool-tip electrode permitted
saline infusion along the electrode.
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Fig. 1. —Photograph showing wet-cooled electrode containing two
coaxial lumina that enable circulation of cooling water through electrode and
separate lumen for saline interstitial infusion. Hole for saline infusion was
positioned in proximal portion of sheath (arrow).
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Experimental Preparation
Radiofrequency ablation was performed in 22 freshly excised bovine livers
weighing, on average, 7.5 kg. The livers were cut into 10 x 10 x 7
cm blocks, which were immersed into a 50 x 20 x 25 cm
saline-filled bath. The radiofrequency ablation system comprised 15-gauge
wet-cooled electrodes in the form of needles with tip exposures of 3 cm and a
480-kHz generator (CC-3, Radionics). Two wet-cooled electrodes were then
placed in a liver at a distance of 3 cm through an acrylic plate containing
several holes at 5-mm intervals. The tips of two wet-cooled electrodes were
advanced at least 4 cm into the liver block. One radiofrequency-induced
ablated region was created in each block.
To monitor the local tissue temperature during the procedure, we inserted a
thermocouple midway between the two electrodes. A peristaltic pump was used to
infuse normal 0°C saline solution into the lumina of the electrodes at a
rate sufficient to maintain a tip temperature of 10–25°C.
Radiofrequency Energy Delivery
In monopolar radiofrequency delivery mode, radiofrequency was applied to
one of both electrodes at an initial generator output of 200 W and was flowed
from one of the electrodes to a metallic dispersive pad. Then, sequentially,
radiofrequency energy was delivered to the other electrode
(Fig. 2A). Initial impedance
was controlled at 110 
at the place of the two electrode insertions by
controlling the distance between the electrodes and the dispersive pad. The
radiofrequency power was manually increased to the maximum (
200 W) in 1
min and held for a total of 20 min (10 + 10 min).
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Fig. 2A. —General setting in simultaneous monopolar and bipolar
radiofrequency ablation in in vitro bovine liver model. Illustration of
sequential monopolar mode shows wet-cooled electrode and injector used to
continuously inject hypertonic saline. Thermocouple (T1) is inserted 15 mm
from electrodes (E1 and E2). Sequential monopolar mode applies current to one
of two probes; current flows from probe to dispersive pad.
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In bipolar radiofrequency delivery mode, one electrode tip was connected to
the generator radiofrequency output, and the other connected to the generator
ground output (Fig. 2B). In
this mode, current was flowed from one electrode to the other at 150 W for 20
min; therefore, the dispersive pad was not necessary.
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Fig. 2B. —General setting in simultaneous monopolar and bipolar
radiofrequency ablation in in vitro bovine liver model. Illustration of
bipolar mode shows two wet-cooled electrodes used as active electrode (E1) and
dispersive electrode (E2), respectively. Current then flows from one electrode
to other. T1 = thermocouple.
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Ablation Protocol
Two sets of experiments were performed. The first set of experiments was
performed to find the appropriate concentration and injection speed of the
hypertonic saline (HS) that prevented a rapid impedance rise during bipolar
radiofrequency ablation. On the basis of previous study results regarding
saline-enhanced radiofrequency ablation
[8–12],
we tested three concentrations of HS (6%, 20%, and 36%) to increase the
electric conductivity of liver tissue. With bipolar radiofrequency energy
application mode, the three concentrations of HS were infused at rates of 1,
2, and 3 mL/min through the 15-gauge wet-cooled electrode using two infusion
pumps, and five thermal ablation zones were created at each condition. On the
basis of a previous study [8],
for bipolar radiofrequency ablation, we placed two wet-cooled electrodes 3 cm
apart in liver tissue, without a dispersive pad and attached to a generator
(Fig. 2A,
2B). Radiofrequency power was
increased manually to 150 W, and radiofrequency energy was applied for 20 min
in the bipolar mode, in which the current flows from one electrode to the
other.
In a second set of experiments, 10 ablation zones were created in the
sequential monopolar mode (group A, 10 + 10 min), and an additional 10
ablation zones were created in bipolar mode (group B, 20 min). For monopolar
radiofrequency ablation, the initial impedance was controlled at 80
with two electrode insertions. To set the impedance at 80 , we immersed
the liver blocks in the saline-filled bath (50 x 20 cm), and the
distance between the electrodes and the dispersive metallic pad was altered.
Two electrodes were placed in the liver at a distance of 3 cm through an
acrylic plate containing multiple holes at 5-mm intervals. Radiofrequency was
applied to one of the two electrodes for 10 min at an initial generator output
of 200 W, and sequentially it was applied to the other electrode for 10 min in
the monopolar mode, in which current flows from one of the two electrodes to
the dispersive metallic pad. In the bipolar mode, radiofrequency power was
increased manually to 150 W; then, it was automatically changed depending on
impedance changes. During radiofrequency energy application, 6% HS was infused
at a rate of 2 mL/min on the basis of the findings of the first set of
experiments.
The applied current, power output, and impedance were continuously
monitored by the generator during radiofrequency ablation and were recorded
automatically using a computer program (Real Time Graphics Software version
2.0, Radionics). The technical aspects of the radiofrequency ablation
including impedance and wattage changes, tissue temperature at the midpoint
between the two electrode tips, and the dimensions of the
radiofrequency-coagulated area were compared for each technique.
Lesion-Size Measurement
The liver blocks containing lesions were dissected along the axis of the
probe insertion and then cut transversely into slices. Because the white
central area of the radiofrequency-induced ablation zone has been shown to
correspond to the zone of coagulation necrosis by macroscopic examination
[13], two observers measured
the vertical diameter (Dv) of the ablation region in the electrode direction
on the plane along the electrode insertion axis (longitudinal plane) and the
ablation distance perpendicular to this, called the long-axis diameter (Dl) by
consensus. In addition, the short-axis diameter (Ds) of the ablation regions
was measured on the other perpendicular plane (transverse plane) with calipers
(Fig. 3A,
3B,
3C,
3D). The volume of the ablation
zone obtained with radiofrequency was evaluated by approximating the lesion to
a sphere using the following formula:
 |
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Fig. 3A. —Comparison of radiofrequency-induced coagulation created by
applying radiofrequency in two groups. Photograph shows cut section of
specimen along electrode insertion axis from group A (monopolar mode).
Arrowheads indicate electrode insertion sites. Vertical solid arrow indicates
vertical diameter, and horizontal dotted arrow indicates long-axis
diameter.
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Fig. 3B. —Comparison of radiofrequency-induced coagulation created by
applying radiofrequency in two groups. In photograph of cut section of same
specimen (A) along perpendicular plane to A, arrow indicates
short-axis diameter.
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Fig. 3C. —Comparison of radiofrequency-induced coagulation created by
applying radiofrequency in two groups. Photograph of specimen from group B
(bipolar mode) shows that shortest vertical diameter of coagulation necrosis
at midpoint between two electrodes (arrowheads) is larger in bipolar
mode than in monopolar mode.
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The shape of the radiofrequency-induced ablation zone was characterized by
determining the ratio between the long-axis and vertical diameters (Dl /
Dv).
Statistical Analysis
In the first set of experiments, the impedance and current changes during
bipolar radiofrequency ablation using different HS infusion rates were
compared using one-way analysis of variance followed by Bonferroni's post hoc
test for intergroup comparisons. In the second set of the experiments, the
dimensions of the thermal ablation areas and the technical parameters of the
two groups (monopolar vs bipolar modes) were compared using the unpaired
Student's t test. To compare the temperature at the midpoint between
the two electrodes, we performed the Mann-Whitney U test. For all
statistical analyses, a p value of less than 0.05 was considered
statistically significant. Statistical calculations were performed using SPSS
version 9.0 (Statistical Package for the Social Sciences) for Windows
(Microsoft).
Results
First Set of Experiments
In the first set of experiment with 6% HS infusion at a rate of 1 mL/min,
impedance values showed a marked increase to more than 250 during
radiofrequency ablation. However, with 6% HS infusion at a rate of 2 mL/min,
impedance was controlled to less than 100 in four (80%) of five cases
but, in the fifth case, increased to more than 250 14 min after the
inputting of radiofrequency energy. In addition, with 6% HS infusion at a rate
of 3 mL/min, impedance was well controlled in all cases. With 20% HS infusion
at a rate of 1 mL/min, the impedance was controlled to less than 100
in all cases but showed a gradual decrease of less than 40 10 min
after starting radiofrequency energy application. With higher infusion rates
of 20% HS or with infusion of 36% HS, impedance was gradually decreased less
than 40 within 5 min from starting radiofrequency ablation. Because
impedance decreased less than 50 , the current was saturated up to the
generator-limited level (2,000 mA), and then, wattage was gradually decreased
(Table 1). Furthermore,
infusion of 36% HS was frequently blocked in seven of 15 trials because of
crystallization of salts related to vaporization of water during
radiofrequency ablation.
The mean current values with infusion of 6% HS at rates of 1, 2, and 3
mL/min were 1,223 ± 644 mA (SD), 1,813 ± 438 mA, and 1,966
± 75 mA, respectively (p < 0.05)
(Table 1). The difference in
the mean current values at 1 and 2 mL/min and between mean current values of 1
and 3 mL/min was significant (p < 0.05), but the mean current
values at 2 and 3 mL/min were similar (p > 0.05). With infusion of
20% and 36% HS, the current was increased at the level of generator current
limitation (2,000 mA) and wattage was gradually decreased. Therefore, infusion
of 6% HS at a rate of 2 mL/min was used for bipolar radiofrequency ablation in
the main experiment.
Second Set of Experiments
Technical parameters.—In group A, the impedance values
during radiofrequency application showed a mild variation within the range
50–100 (mean, 55 ± 9 ). In group B (bipolar mode),
the impedance was controlled to less than 100 in eight (80%) of 10
cases but rose intermittently to greater than 300 after 12 min in the
other two cases (mean, 80 ± 46 ). However, there was no
significant difference in mean impedance value during radiofrequency ablation
between the two groups (p > 0.05). The mean current values of
groups A and B were 1,830 ± 124 mA and 1,809 ± 146 mA,
respectively (p > 0.05) (Table
2). In the two cases that showed impedance rises during bipolar
radiofrequency application, a large hepatic vessel (> 5 mm) was included in
the blocks and some of injected HS leaked out
(Fig. 4).
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TABLE 2 The Measured Values of Radiofrequency-Induced Coagulation Necrosis,
Tissue Temperature, and Ablation Data According to Radiofrequency Power
Application Mode
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Effects of tissue ablation.—A graph showing the mean
temperatures at the midpoint between the two electrodes is presented in
Figure 5. In groups A and B,
mean final temperature values were 78 ± 13°C and 105 ±
13°C, respectively. The difference in the temperatures measured at the
midpoint between the two electrodes in the two groups was significant
(p < 0.05).
In the experiments designed to compare the efficacy of the monopolar and
bipolar modes in terms of the dimensions of the thermal ablation zones
produced, no significant differences were found between the impedances and
mean current values of the monopolar mode (group A) and the bipolar mode
(group B). The maximal long-axis diameter (Dl) of the radiofrequency-induced
central white zone measured in the gross specimens of the two groups was as
7.2 ± 0.3 cm in group A and 6.7 ± 0.7 cm in group B (p
> 0.05) (Table 2). In
addition, the mean shortest vertical diameters (Dv) at the midpoint between
the two electrodes were 4.1 ± 0.3 cm in group A and 5.4 ± 0.4 cm
in group B (p < 0.05) (Fig.
3A,
3B,
3C,
3D). In groups A and B, the
mean short-axis diameters (Ds) of the ablated spheres were 4.2 ± 0.6 cm
and 5.8 ± 0.9 cm, respectively (p < 0.05). Compared with
monopolar radiofrequency ablation, bipolar radiofrequency ablation tended to
produce a round coagulation with less prominent waist formation at the
midpoint between the two electrodes (i.e., the ratio of Dl /
Dv was 1.75 ± 0.2 in group A and 1.25 ± 0.1 in group B
[p < 0.05]). Furthermore, the volumes of ablation zones obtained
with monopolar and bipolar modes were 65.7 ± 12.7 cm3 and
111.6 ± 30 cm3, respectively, and the difference in volumes
of the ablation zones in both groups was statistically significant (p
< 0.05).
Discussion
The most restrictive factor concerning the broader applicability of
radiofrequency ablation is the limited size of the radiofrequency-induced
ablation zone due to the uncontrollable impedance rise caused by the delivered
current in the tissue near the electrode
[5,
6]. Recently, a few studies
[6,
8] reported that bipolar
radiofrequency ablation using dual electrodes could increase power deposition
in the region between the electrodes, but it had a problem of impedance rise
related to too high current condensation near the electrodes. Problems of
tissue desiccation and charring at the interface between the electrode and the
target tissue can be resolved by saline infusion, which increases the electric
conductance and thermal conductivity
[11,
12,
14]. In our study, the bipolar
HS-enhanced radiofrequency ablation using the wet-cooled electrodes created
larger lesions than the sequential monopolar radiofrequency ablation (65.7
± 12.7 cm3 vs 111.6 ± 30 cm3).
Furthermore, the ratio of Dl / Dv of the ablation zones
generated by the bipolar technique was larger than that of the sequential
monopolar technique (i.e., bipolar radiofrequency ablation created an oval
ablation zone without waist formation midway between the electrodes, which is
much more ideal for tumor ablation than overlapping monopolar radiofrequency
ablation showing waist formation midway between the electrodes [Fig.
3A,
3B,
3C,
3D]). This finding could be
valuable in clinical radiofrequency application of liver tumors because focal
liver lesions are usually round or oval.
The larger lesions created using the wet-cooled electrode in the bipolar
mode could be attributed to the greater energy efficiency of the bipolar mode
[10,
15]. In monopolar modes, heat
is diverted from the ablation site in all directions and a precipitous drop of
current density occurs with distance from the energy source (1 /
r4) [5]. In
contrast, in the bipolar mode, one electrode is thermally shielded by the
opposing second electrode, which also actively heats tissue in its proximity;
because heat is trapped between the two electrodes, higher temperatures are
achieved because less cooling occurs in the direction of the collateral
electrode than is the case with monopolar ablation
[8,
15]. Furthermore, current is
deposited in the region between the electrodes
[6].
In our study, which was conducted with a 200-W generator and wet-cooled
electrodes, the shortest Dv of the ablation zones formed at the midpoint
between the two electrodes was larger than those found in a previous study
[8] using HS-enhanced bipolar
radiofrequency ablation, a 60-W generator, and two perfused electrodes (5.4
± 0.4 cm vs 3.1 ± 0.8 cm). This finding could be explained by
two factors: First, the more effective control of tissue impedance during
radiofrequency energy application could have been achieved by the use of a
wet-cooled electrode, which permits simultaneous intraelectrode cooling and
saline perfusion to the tissue. Both internal cooling perfusion and
interstitial saline infusion are indispensable for adequate radiofrequency
energy delivery in bipolar mode. Second, the more effective control could have
been caused by the use of a higher wattage (150 vs 60 W), which would have
resulted in a higher electric current density in target tissue.
In several previous studies
[11,
12,
14,
16,
17], HS infusion before or
during radiofrequency energy application caused substantial improvement the
efficacy of radiofrequency ablation to create coagulation necrosis compared
with radiofrequency alone. In a previous study by Burdio et al.
[9], physiologic saline was
injected at a rate of 600 mL/hr for bipolar radiofrequency ablation, and
impedance was gradually lowered during radiofrequency application. Given that
more saline was infused, the risk of unexpected thermal injuries to adjacent
vital structures and the loss of control of the radiofrequency-induced
ablation area and shape could be increased
[18]. Thus, the infusion of
saline at a rate of 600 mL/hr is probably too much. On the basis of previous
study results regarding HS-enhanced radiofrequency ablation
[8–12],
we tested three concentrations of HS to increase the electric conductivity of
liver tissue: 6%, 20%, and 36%. With 6% saline infusion at a rate of 2 mL/min
during bipolar radiofrequency ablation, an uncontrolled impedance rise was
successfully prevented, and the delivery of a high current to the tissue was
permitted. Indeed, adjuvant NaCl injection leads to substantial increases in
radiofrequency-induced heating compared with radiofrequency alone. However,
increasing electric conductivity induced by saline infusion has competing
effects on radiofrequency ablation: It enables increased energy deposition and
greater heating, but it also increases the energy required to heat a given
volume of tissue. If this amount of energy cannot be delivered beyond the
maximal generator out-put, then less actual heating and less coagulation
results [16]. In our
experiments with infusion of concentrations of HS higher than 6%,
radiofrequency energy delivery reached the maximal generator current output of
2,000 mA during the procedure; then, progressive decrease of wattage occurred
with increasing concentrations of HS. In our study, therefore, 6% HS at an
infusion rate of 2 mL/min was used, but still there is a room to further
optimize the concentration of HS and the infusion rate, because infusion of 40
mL of HS into the target tumor in clinical application may not be possible
without leakage around the injection needle because of heterogeneous tissue
texture.
Our experimental study has certain limitations. First, the extent to which
the results of the present in vitro study can be transposed to the human liver
is limited, as are those of other in vitro experimental studies. In living
tissue, heat is lost because of blood flow
[5], which limits the dimension
of radiofrequency-induced ablation zones. Moreover, all ablations involved the
normal liver parenchyma, not tumor tissue. Despite these short-comings, our
model provides a reliable basis for a comparative study of the efficiencies of
the different radiofrequency modes. Second, we tested relatively few infusion
rates of the 6% HS, and further optimization of the HS concentration for
bipolar radiofrequency ablation is warranted. Finally, in our study, the
bipolar radiofrequency mode using the wet-cooled electrode was shown to have
the greater efficiency, but further comparative studies are needed to show
whether this remains the case for other monopolar radiofrequency systems like
the Radiotherapeutics and RITA systems, which have higher maximal power
outputs [19].
In conclusion, the wet-cooled electrode allows bipolar radiofrequency
ablation of liver tissue to be undertaken without a marked impedance rise.
Moreover, HS-enhanced bipolar radiofrequency ablation using the electrodes
showed better efficacy than sequential monopolar ablation for creating
coagulation necrosis. Therefore, we believe that this design could be applied
to the radiofrequency ablation of larger tumors.
Acknowledgments
We thank Bonnie Hami from the department of radiology, University Hospitals
of Cleveland, for her editorial assistance and manuscript preparation.
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Abstract
Introduction
)
than in simultaneous monopolar mode (•).