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Advantages of the bipolar technique

 

The bipolar radiofrequency method uses two electrodes that pass a current through a section of tissue. The current heats and destroys a cylindrical shaped volume of tissue clamped between the electrodes. First, we have tested the technique on five ex-vivo beef livers. We propose a bipolar method for creating larger, faster and more controllable lesions.

  • The lesions are less dependent on local heterogeneity of the tissue such as blood perfusion and are strictly limited between the probes. The major contribution toward larger lesion size of bipolar ablation is based on a thermodynamic effect. Heat is trapped between the two probes and higher temperatures are reached. This results in a lesion of larger size than that of two lesions produced sequentially by monopolar ablation with probes placed at the same position.

 


Bipolar RF.  The bipolar RF technique creates a cylindrical shaped lesion (red) around the tumor.

  • The bipolar RF technique creates a cylindrical shaped lesion.  Dependence of lesion size on local differences in cooling mediated by perfusion is reduced. This is supported by the fact that the standard deviation of bipolar ablation lesion size relative to its mean is smaller than for the monopolar case. Furthermore when a tumor is localized near sensible structures (Bowel, nerve root), the necrosis is well limited between the probes and accidental coagulation of the nearby organs can easily be avoided.
    The proposed bipolar method reduces treatment time. The use of bipolar technique with a distance of 3 cm produces a well-limited coagulation between the electrodes with a maximum diameter of 4.3 cm plus/minus 0.5 cm after 5 to 10 minutes ex vivo.
     

 

Drawbacks of the bipolar technique

 

The bipolar technique also introduces new drawbacks and difficulties:

  • The probes must ideally be parallel, the best distance between probe is 3 cm, probe temperature can be different and there is risk of hot saline diffusion.
    The best results are obtained when the probes are parallel together and the tumor is limited in between. However Insertion of two parallel probes can be difficult in some cases. The ideal gap between the two probes depends upon the local properties of the ablation site and is different for each ablation. We performed preliminary ex vivo experiments with probe distances of 4, 3.5, 3, and 2.5 cm. When the distance was 4 and 3.5 cm, in some cases we found a gap of viable tissue between two lesions created by the two probes after performing ablation. We chose a distance of 3 cm for subsequent experiments. A different distance might be appropriate if probes with other geometries are used.
  • In RF ablation, most of the active heating occurs within a range of a few millimeters from the electrodes. Similar resistivity and current density are present in the vicinity of both probes. Therefore, a comparable amount of energy is converted into heat next to each of the two probes. If one probe is cooled more by blood perfusion than the other, more heat energy is carried away. One probe can therefore reach a higher temperature than the other. This can lead to boiling and vaporization. Impedance will rise, and the RF generator shuts down. In 5 patients, during bipolar ablation the impedance showed a sudden rise resulting in the shutdown of the generator. In these cases, we used the temperature-controlled method. If the temperature of the hotter probe is controlled to be kept at 95 °C, the other probe will not reach this temperature and heating near this probe will be kept lower.
  • Another drawback of the wet electrode is the risk of diffusion of hot saline. This risk can be decreased with the reduction of the volume of fluid injected by using hypertonic saline (5.85%). Using this method reduces the amount of saline injected.

 

Principle of Monopolar RF.

 


Principle of Bipolar RF.

 


1. RF generator. 2. Continuous saline infusion. 3. Dispersive electrode (ground pad). 4. Closed loop. 5. Heating. 6. Saline infusion.

 


1. RF generator. 2. Continuous saline infusion. 3. Bipolar needle electrodes inserted parallel together.

 


1. Heating. 2. Closed loop between the electrodes. 3. Continuous infusion of saline.

 


Bipolar technique to avoid neurological complications.

 


CT scan during RF ablation.

 


Ablation between the electrodes. Coagulation outsides the electrodes limited to 5 mm maximum. RF ablation duration 5 minutes perposition.

 


CT scan after the procedure.

 


MR imaging 2 days after the procedure (T1-weighted + gadolinium). Large necrosis of the tumor. No complications. Excellent pain relief.