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technique

MECHANISM

Alternating electric current operated in the range of radio frequency can produce a focal thermal injury in living tissue.  Shielded needle electrodes are used to concentrate the energy in selected tissues. The tip of the electrode conducts the current, which causes local ionic agitation and subsequent frictional heat, which leads to localized coagulation necrosis.

Basically, the term radio frequency refers not to the emitted wave but rather to the alternating electric current that oscillates in the range of high frequency (200-1,200 kHz).  Schematically, a closed-loop circuit is created by placing a generator, a large dispersive electrode (ground pad), a patient, and a needle electrode in series.

With the bipolar technique, the dispersive electrode (ground pad) is replaced by a second electrode inserted parallel to the other electrode.


In this way, the closed-loop circuit is created between the generator, the patient, and both electrodes.

There is no loss of energy in the skin of the patient with deposition of the energy inside the lesion. The distance between the electrodes is 3 to 4 cm maximum.

 

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.

 

EQUIPMENT

Each radio-frequency device consists of an electrical generator, needle electrode, and ground pad. Each manufacturer has a different needle electrode design.

We are using a new commercially available device using a single probe system (18- to 16-gauge) with continuous infusion of saline without exceeding a 110°C maximum temperature threshold (60W, Berchtold®/ Tuttlingen, Germany).

The continuous infusion of saline at the tip of the needle allows increasing heat and electrical conductivity.  The Berchtold® radio frequency device relies on an electrical measurement of tissue impedance to determine that tissue boiling is taking place.  Impedance increases can be detected by the generator, which can then reduce the current output and increase the saline flow.  Injection of hypertonic saline during RF ablation can increase energy deposition, tissue heating, and induced coagulation.

 

Radio-frequency device with injection of continuous saline.  The same generator is used for bipolar technique.

1. Monitor of Energy and temperature.
2. Impedance control.
3. Injector for continuous injection of saline.
4. Power.
5. Timing.
6. Needle electrode input.

 

Needle electrode (16-gauge) with continuous infusion of saline around the tip.

 

 


Needle electrode with continuous infusion of saline.

 


Interventional CT room.

PROCEDURE

Radiofrequency technique is widely used in liver tumors.  In our department we are using an electrode with continuous saline infusion to increase the coagulation size (Berchtold®, Tuttlingen, Germany). The infused electrode of 18 to 16 gauges is inserted inside the tumor and a power of 40 to 50 Watts is used for 10 minutes with continuous infusion of hypertonic saline 5.85% (56 to 62 ml/hour).

The advantages of hypertonic saline are the increase of electrical conductivity and reduction of the fluid volume injected.  For large lesions (≥ 4cm), the procedure should be repeated after repositioning the needle electrode.

The guidance system is chosen based on operator preference and experience.  We are using CT guidance on a routine basis  for bone tumor ablations.

 

However MRI can be used for the needle insertion with thermal monitoring during ablation.

If a penetration of a thick bone is required, a coaxial technique should be applied for the insertion of the needle electrode.  However, the coaxial technique using a bone biopsy needle should be handled with care because the bone needles are not insulated and the active part of the electrode should not be in contact with these needles in order to avoid loss of energy and coagulation of the needle tract.

With conventional monopolar technique, dispersive grounding pad was typically attached to the patients as close as possible to the needle electrode.  In order to ensure that there is no interference with the heart rate, the heart should not be between the ground pad and the needle electrode.

For spinal metastases or lesions too close to sensitive organs (nerve root, cord, colon, etc.) a specific bipolar technique was used inserting two electrodes in tandem.  The major advantage of this bipolar system is that the coagulation is strictly limited between the electrodes.  In this way, the adjacent organs are protected.  Another advantage of bipolar technique is the fast coagulation of the area between the electrodes (approx. 5 minutes).

With bipolar technique the ground pad electrode is not necessary. The limitation of bipolar radiofrequency is the distance between the electrodes which is limited to a maximum of 3 to 4 cm. Furthermore, the bipolar procedure requires the use of two needle electrodes.

The number of ablations performed varied for each patient and depended on size, shape, origin, and location of the metastatic lesion.  The duration of a procedure depends on the number of ablations to be performed.

Curative ablation is limited to some specific indications.  Thyroid cancer metastases are a typical example.  RF ablation is performed in this indication to reduce tumor size and is followed by 131- iodine therapy to complete the ablation of residual tumor.

The RF ablation of bone is painful and the procedure should be performed under general anesthesia or regional anesthesia.  To avoid post procedural pain, injection of 10 ml of rupivacain 0.2 % in subperiosteal area is very useful with an analgesic effect of up to 12 hours.

 

Patient installation with ground pads inserted close to the lesion.

 

Large painful lytic metastasis.

 

RF ablation of the metastasis with monopolar technique.

 

MR imaging 10 days after the RF ablation.  Large but incomplete necrosis of the tumor.

 

Painful paravertebral metastasis.

 

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.

 

The electrodes are repositioned and another 5 minutes RF ablation is performed.

 

CT scan after the procedure.

 

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

 

View of the bipolar needle insertion with continuous infusion of saline.

 

Thyroid cancer metastasis treated by the association of RF ablation followed by radioisotope therapy.

 

Large necrosis of the tumor.  After tumor reduction, residual lesions are treated by radioisotope therapy.

 

Installation of the patient for RF ablation under general anesthesia.   Regional anesthesia can be used too.