Abstract:
Device for delivering radio frequency (RF) energy, for example during tissue ablation procedures, comprising an electrode having a distal end associated with tissue puncturing means and a proximal end connectable to a radio frequency energy source, wetting means for wetting the proximity of the distal end of the electrode with a non-toxic (RF) conductive solution and cooling means for cooling at least the distal end of the electrode. The invention further relates to a process for cooling and wetting a radio frequency energy delivering device and to a guidance element therefor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to novel devices for delivering radio frequency energy (RF), for example during tissue ablation procedures. 
     The invention relates in particular to a novel concept of an electrode for the optimization of radio frequency ablation. This concept will hereunder be nominated as the cooled-wet electrode. 
     2. Description of the Prior Art 
     Although surgical resection is still considered as a primary option for the treatment of malignant tumors, minimally invasive alternatives including intraoperative cryosurgery, local injection of ethanol, microwaves, interstitial laser therapy focused ultrasound and radio frequency (RF) tissue ablation have been developed in order to ablate the tumor less invasively for the safety of the patient and reduction of the costs and/or to broaden our capability in treatment of the patient. 
     Among these approaches, RF ablation has shown the greatest impact on recent experimental and clinical research because of its low invasiveness, simplicity and favorable cost-effectiveness. 
     In RF ablation the radio-frequency waves are emitted from a generator through an uninsulated part of the electrode which is inserted into a target tissue. The tissue destruction in a form of coagulation necrosis is caused primarily by resistive heating in the surrounding tissue and secondarily by the peripheral passive heat conduction. 
     Resistive heating is proportional to the square of the distance between the central electrode and adjacent tissue. Therefore, significant resistive heating only occurs within a rim of tissue in direct contact with the electrode. Beyond this rim, the tissue is further heated as a result of passive conduction of increased temperature. However, the RF emission is readily terminated as a result of impedance rise at the electrode-tissue interface, which is secondary to tissue desiccation and carbonization. Due to such non-optimal RF energy delivery and dissipation, the lesion size induced by known prototypes electrodes is smaller than 2 cm, which is obviously insufficient for tumor ablation. Similar to the principle in surgical resection, the ideal range of RF tissue destruction should involve the entire tumor and a layer of adjacent normal tissue as a safety margin to avoid incomplete ablation. 
     Many known technical innovations have been made to increase the lesion size in RF ablation. These include the introductions of: 
     1) bipolar electrodes; 
     2) a cooled electrode and cooled-clustered electrodes; 
     3) a “wet” electrode with hypertonic saline infusion; and 
     4) an expandable electrode. 
     According to the principle of minimal invasiveness, a monopolar is preferred to multipolar electrode. 
     As shown in table 1, although markedly increased, the lesion sizes induced by these modified devices are still limited, normally less than 4 cm in diameter. If a tumor larger than 2 cm, there is little chance to achieve complete ablation by a single session. Therefore there is still a demand to further optimize these devices and techniques. 
     Table 1 shows the lesion sizes induced by different known designs of electrode in RF Ablation. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Electrode Type 
                 Lesion size (cm) 
                 No. Reference 
               
               
                   
               
             
             
               
                 Prototype Electrode 
                 0.8-1.5 
                 1 
               
               
                 Bipolar Electrode 
                 5 (the width between poles) 
                 2 
               
               
                 Cooled Electrode 
                 1.4-3.6 
                 3 
               
               
                 Wet Electrode 
                 4.5 ± 0.75 
                 4 
               
               
                 Expandable electrode 
                 4.5 
                   5,6 
               
               
                 Cooled-clustered 
                 4.7 ± 0.1 
                 7 
               
               
                   
               
               
                 References cited are:  
               
               
                 1 Goldberg, S. N. et al. (Academic Radiology 1995;2:399-404)  
               
               
                 2 Goldberg, S. N. et al. (Acad. Radiol. 3/929, 1996)  
               
               
                 3 Lorentzen, T. A. (Acad. Radiol. 3:556, 1996)  
               
               
                 4 Miao, Y. et al. (J. Surg. Res. 71:19, 1997)  
               
               
                 5 Rossi, S. et al. (AJR. Am. J. Roentgenol., 170:1015-1022, 1998)  
               
               
                 6 Patterson E J, et al. (Ann Surg, 227:559-565, 1998)  
               
               
                 7 Goldberg S. N. et al. (Radiology 209:371-379; 1998)  
               
             
          
         
       
     
     SUMMARY OF THE INVENTION 
     The main object of the invention is to provide new device and methods yielding good RF ablation results and providing larger lesion size. In particular whereby the lesion size is larger than 5 and preferably more than 6 cm. 
     According to the invention this is realized by a combination of separately known features, which in combination surprisingly results in a more effective RF ablation. This is realized by an increased conductivity of the target tissue as well as at the electrode tissue interface in relation to a decreased tip temperature. 
     The invention therefore provides a device for delivering radio-frequency energy combining the characteristics of a “wet” electrode and of a cooled electrode. 
     A main object is a minimal invasiveness of the radio-frequency ablation technique. A minimal invasiveness is obtained by a precise puncturing and guidance towards the tissue to be treated. It is therefore a further object of the invention to improve the efficiency of the puncturing and guidance of the radio-frequency electrode and more broadly of all instruments used in RF ablation. The puncturing is presently performed by the sharpened distal end of the electrode. As this distal end is often open introduction sometimes causes obstruction and once introduced blocks off these openings at the distal tip. It will be understood that the use of guidance means is not necessary for the use of the cooled wet electrode. The puncturing can be performed by the sharpened distal end of the cooled wet electrode as sole puncturing mean. 
     As a solution to this disadvantage the invention provides further a separate guidance element for the guidance of an instrument, in particular a radio-frequency electrode. The guidance element according to the invention is substantially formed by a open hollow shaft having a cylindrical central bore which is adapted in dimensions for the temporarily housing and axial displacement of an instrument during radio-frequency ablation procedures. Said instrument-can be for example a puncturing needle for a smooth introduction towards the tissue to be treated, a radio-frequency electrode for the radio-frequency ablation step and further a biopsy needle or biopsy clamp for providing proof of the efficiency of the radio-frequency ablation procedure by the collection of a tissue sample. 
     It will be understood that several cooled wet electrodes, for example two, three, four or more can be used as a clustered cooled wet electrode device when the tumor to be treated is of an excessive dimension. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention shall be elucidated hereunder with reference to the drawing wherein schematically is shown in: 
     FIGS. 1 to  7  partially broken away cross-sectional and perspective views of five preferred embodiments of the cooled-wet electrode according to the invention; 
     FIG. 8 a schematic illustration of radio-frequency ablation system using a cooled-wet electrode according to the invention; 
     FIG. 9 a partial broken away perspective view of a guidance element (FIG. a), a puncturing needle (FIG. b) and a biopsy needle (FIG. c). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the Figures the thin printed arrows define the flow pattern of a cooling medium and a wetting medium and the bold printed arrows the direction of the movement of parts of the electrode. The cooling and wetting medium are preferable solutions and in a preferred embodiment the wetting solution is a saline solution and more preferably a hypertonic (for example 0.9% saturated) saline solution. The cooling solution is preferably water or cooled media such as 0° C. saline. 
     A rigid hollow needle electrode  1  comprises a proximal end  2 , a distal end  3  and there between a longitudinal part  4 . The electrode  1  comprises a number of cylindrical wall elements  5  forming three concentric channels, i.e. an inner concentric channel  6  and two outer concentric channels  7 ,  8 , which outer concentric channels  7 ,  8  are connected at the distal end  3  of the electrode  1  forming a closed loop  9 . The outer concentric channels  7 ,  8  define a flow path for a cooling solution such that at least the distal end  3  of the electrode  1  can be sufficiently cooled. 
     The inner concentric channel  6  is open  10  at the distal end  3 . The inner concentric channel  6  defines the flow path for the wetting solution and a housing for puncturing means which is formed by an axial (arrow  55 ) retractable and protruding pith organ  54 . The pith organ  54  closes the open end  10  when being inserted into the target tumor  11  in order to avoid obstruction in the channel  10 . An accesory biopsy needle of the same size can be replaced before ablation for sampling tumor tissue for histopathologic examination. After insertion of the electrode  1  the pith organ  54  is retracted upwards making free the flowing path of the wetting solution in the channel  6  (FIG.  2 ). When the electrode  1  is introduced towards a tumor  11  on a target organ  12  radio-frequency energy will be delivered via a non-insulated part of the electrode  1 , being at least the distal end  3  of the electrode  1  while simultaneously the distal end  3  is cooled by a cooling solution and the proximity of the distal end  3  is being wetted  13  by a wetting solution. The distal end  3  of the electrode  1  is preferably sharpened such that is has a further puncturing function. The separate flow control of the cooling and wetting for example in concentration, temperature, etc. results in a superior lesion size. 
     The electrode  1  has in general a substantially rigid structure in order to be able to be aimed precisely in the tumor. 
     The axial slidable pith organ  54  is used in order not to obstruct the channel for the wetting solution  6 . Once the electrode  1  is positioned in the centre of the tumor  11  the pith organ  54  is upwardly retracted and removed. A RF-energy delivery can start when the pith  54  is retracted and the wetting solution  13  is delivered simultaneously with the RF-energy. 
     The embodiment disclosed in FIG. 3 comprises two concentric channels  27  and  28  forming a closed loop at the closed end  29  of the distal end  30 . This closed loop channel ( 27 , 28 ) defines the flow channel for the cooling solution as arrows  31  (down) and  32  (up) indicate (comparable to FIG.  2 ). At the distal end  30  an open lateral channel  33  is provided as flow path for the wetting solution, which is preferably a hypertonic saline solution. At the distal end  30  the channel  33  is provided with multiple openings  35  for the outflow of the wetting solution  13  in order to create sufficient spreading of the wetting solution  13  at the proximity of the distal end  30 . Arrows  36  (down) and  35  (out) define the flow path for the wetting solution. 
     The diameter of these needle electrodes should preferably be as small as possible and is preferably smaller than 3 mm. 
     The embodiment of the electrode  37  depicted in FIG. 4 comprises three concentric channels: an outer channel  38  provided with multiple holes  41  at the distal end  42  of the electrode  37  and two concentric channels  27  and  28  forming a closed loop  29  at the distal end  42  of the electrode  37  and defines the flow path for the cooling solution. 
     The electrode of FIG. 5 discloses another preferred embodiment wherein a separate lateral channel  44  for the wetting solution  13  is provided having at the distal end  45  of the electrode a helical formed part  46  around the distal end  45  of the electrode and is provided with multiple openings  48  in order to create a flow path for the wetting solution through and out the electrode. 
     In the embodiment of FIG. 6 the cooling and wetting solution is one and the same thus. This has the advantages of a more compact and simpler structure of the electrode  49 . However, in the other embodiments the separate flow rate can be adjusted for their purposes i.e. the cooling solution normally has a higher flow rate than the wetting solution. 
     The embodiment of FIG. 7 discloses a further preferred embodiment comprising an axial (arrow  56 ) slidable temperature measurement organ  44  comprising multiple thermosensors  50  on a determined distance of each other. Normally radiofrequency radiation and energy will spread radially in relation with the distal end of the electrode. The retractable thermosensor will provide in an objective manner a measurement of the efficiency of the radiofrequency ablation method. 
     The use of these concentric channels does not only provide the advantage of a sufficient and controlled flow rate but also the synergetic effect that the wetting solution is simultaneously cooled off. 
     It is obvious for a skilled man that any combination of the form or the position of the channels, the described central pith and the retractable thermosensor can be varied, for example the central thermosensor can be positioned laterally, also the pith can be positioned in a different manner without departing from the scope of the invention. 
     FIG. 8 depicts a schematic illustration of the radio-frequency ablation of a target tissue  11 , for example the liver with a cooled-wet electrode of the invention. The cooling means  57  comprise in general a reservoir  58  for a cooling solution connected to an opening  59  at the proximal end  60  of an electrode  61  and further comprising circulation means  62  in order to circulate the cooling solution. 
     The wetting means generally contain an infusion pump  63  connected to a hypertonic solution  64  and connected to the opening  59  at the proximal end  60  of the electrode  61 . The proximal end  60  of the electrode  61  is connected to a radio-frequency energy source  65  and in order to close the electric circuit a ground path  66  is provided under the organ  11 . The lesion size is substantially enlarged by using a cooled/wet electrode of the invention up to 6-10 cm. 
     If appropiate temperature control means are further provided at the distal end of the electrode to monitor and to control the temperature. All the depicted configurations of channels and elements in or on the electrode are, as is obvious adjustable and combinable or interchangable. 
     Guidance element  100  is substantially formed by an open elongated shaft  101  provided with a central cylindrical bore  102  and a open blunt distal end  103 . The diameter of the cylindrical bore  102  is thus adjusted that instruments to be guided by the guidance element  100  can be introduced and be displaced in the axial direction of the bore with a minimal radial tolerance but still providing smooth axial guidance. The puncturing can preferably be performed by a puncturing needle  104  which is introduced in the guidance element  100  and provided with a sharpened distal end  106  being used as a puncturing mean for introducing of the combination guidance element  100  and needle  104  towards to the tissue to be treated. A smooth introduction can be obtained due to the sharpness and to the form and dimensions of the needle  104 . Once introduced the needle  104  is retracted out of the cylindrical bore  102  of the guidance element  100  while maintaining the introduced position of the guidance element  100 . A radio-frequency electrode can then be entered through the cylindrical bore  102  of the guidance element  100  until protruding at the distal end of the guidance element  100 . When the radio-frequency ablation procedure is terminated, the electrode is retracted out of the guidance element  100 , while this element is maintained in the previous obtained position. 
     For providing proof of the efficiency of the radio-frequency ablation a biopsy needle  109  can be introduced through the same cylindrical bore of the guidance element  100  towards the treated tissue. The distal end of the biopsy needle  109  is provided with a clamp  108  for collecting treated tissue samples for further investigation. 
     The advantages and the specific characteristics of the cooled-wet electrode are founded on the following experiments. 
     Materials and Methods of the Experiments 
     The subjects of RF ablation are: 
     1. Commercial beef liver: 4 pieces of beef livers of about 10 kilograms each were purchased from a local butcher. The temperature of the liver was warmed up from 4° C. to room temperature before RF ablation. 
     2. Swine liver: fifteen livers were excised from the pigs and immediately brought to the site of RF ablation. 
     3. Twelve domestic pigs of 40-60 kg body weight. 
     The used equipment comprised a demo RF generator (RFG-3E, Radionics, USA); a cooling pump: Watson-Marlow 31.3 (Watson-Marlow Limt. England); a wetting saline infusion pump (Ismatic, Switzerland); cool-wet electrodes according to the invention and a MRI scanner: 1,5 Tesla Mangetom Vision (Siemens, Erlangen, Germany). 
     Experimental Groups ex vivo Tests 
     1. Group A: Conventional RF mode, 22 sites of ablation (without cooling perfusion and saline infusion) 
     2. Group B: Cooled only mode: 27 sites of ablation (RF at 50 W for 10 min with cooling perfusion at 40 ml/min) 
     3. Group C: Wet only mode: 20 sites of ablation (RF at 50 W for 10 min with 5% saline infusion at 1 ml/min) 
     4. Group D: Continuing cooled-wet mode, 20 sites of ablation (RF at 50 W for 10 min with 5% saline infusion at 1 ml/min and cooling perfusion at 40 ml/min) 
     5. Group E: Cooled-wet mode with disconintuing saline infusion, 20 sites of ablation (RF at 50 W for 10 min with cooling perfusion at 40 ml/min and 5% saline infusion at 1 ml/min for only first 5 min) 
     6. Group F: Cooled-wet mode with discontinuing cooling perfusion, 13 sites of ablation (RF at 50 W for 10 min with 5% saline infusion at 1 ml/min and cooling perfusion at 40 ml/min and for only first 5 min) 
     7. Group G: RF of cooled-wet mode by manual control at 70-90 W during 10-30 min:10 sites. 
     In vivo Liver Ablation in the Swine 
     Under general anesthesia and intubated ventilation, 12 swines were laparotamized with left and right liver lobes exposed for RF ablation. Under laparotomy, 72 RFA lesions were created in 12 pigs using a novel “cooled-wet” elecrode that combines internal cooling perfusion and hypertonic saline interstitial infusion. Both power control mode (Group A. cooled only, B. wet-only and C. cooled-wet) at 90 W and manual control mode (Group D. cooled-only, E. wet-only and F. cooled-wet) were compared for impedance, current and lesion size. MRI was performed for measurement of lesion size. T 1  and T 2  weighted MR 1  were performed immediately after RF ablation. 
     The results in ex vivo tests with excised pork and beef livers are summarized in table 2. RF ablation at 50 W for 10 min created the largest lesion size with cooled-wet mode (group D) right than with any other modes left. Continuity of RF energy delivery was ensured only in group D, so that the lesion size reached close to 10 cm if ablation duration was prolonged to 30 min and the power was set to 70-90 W. Due to a sudden increase of impedance in other groups sooner or later after ablation started, the delivery of RF energy almost ceased and the lesion size did not further increase. 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Tip 
                   
                   
                   
                   
                   
               
               
                   
                   
                 Saline 
                 Cooling 
                 Tip 
                 Power 
               
               
                   
                 No. 
                 Infusion 
                 Perfusion 
                 Temp. 
                 Output 
                 Impedance 
                 Current 
                 Lesion Size 
               
               
                 Group 
                 Sites 
                 (ml/min) 
                 (ml/min) 
                 (° C.) 
                 (W) 
                 (Ò) 
                 (A) 
                 (cm) 
               
               
                   
               
             
             
               
                 A 
                 22 
                 0 
                  0 
                 93.6 ± 3.9 
                 13.7 ± 1.5 
                 &gt;900 
                 0.13 ± 0.1 
                 0.86 ± 0.3 
               
               
                 B 
                 24 
                 0 
                 40 
                 31.5 ± 4.8 
                 16.1 ± 3.3 
                 81.2 ± 16.5 
                 0.85 ± 0.1 
                 2.43 ± 0.5 
               
               
                 C 
                 18 
                 1 
                  0 
                 99.6 ± 0.9 
                  45.2 ± 10.8 
                  99.8 ± 113.4 
                 0.94 ± 0.4 
                 3.80 ± 0.5 
               
               
                 D 
                 20 
                 1 
                 40 
                 35.9 ± 6.8 
                 49.5 ± 2.4 
                 55.8 ± 50.7 
                 1.14 ± 0.2 
                 4.90 ± 0.6 
               
               
                 E 
                 20 
                 1 × 5 min 
                 40 
                 42.9 ± 4.4 
                 17.8 ± 2.7 
                 725.6 ± 229.3 
                 0.15 ± 0.0 
                 3.89 ± 0.6 
               
               
                 F 
                 13 
                 1 
                 40 × 5 min 
                 99.5 ± 0.9 
                   38 ± 12.2 
                 412.5 ± 138.3 
                 0.46 ± 0.4 
                 4.27 ± 0.5 
               
               
                   
               
               
                 Notes:  
               
               
                 1. Group A: Conventional RF mode, (without cooling perfusion and saline infusion)  
               
               
                 2. Group B: Cooled only mode (RF at 50 W for 10 min with cooling perfusion at 40 ml/min)  
               
               
                 3. Group C: Wet only mode (RF at 50 W for 10 min with 5% saline infusion at 1 ml/min)  
               
               
                 4. Group D: Continuing cooled-wet mode according to the invention (RF at 50 W for 10 min with 5% saline infusion at 1 ml/min and cooling perfusion at 40 ml/min)  
               
               
                 5. Group E: Cooled-wet mode according to the invention with discontinuing saline infusion (RF at 50 W for 10 min with cooling perfusion at 40 ml/min and 5% saline infusion at 1 ml/min for only first 5 min)  
               
               
                 6. Group F: Cooled-wet mode according to the invention with discontinuing cooling perfusion at 40 ml/min for only first 5 min).  
               
             
          
         
       
     
     The swine tolerated in the in vivo tests the RF ablation well and life signs were kept normal during and after ablation. The lesion size appeared smaller than that in ex vivo tests probably due to the cooling effect from hepatic inflow. In vivo results: together with lower impedance and higher power output, the lesion sizes in group C (4.8±0.6 cm) and F (6.5±0.8 cm) were significantly larger (P&lt;0.01) than that in group A (2.4±0.5 cm), B (3.1±1.0 cm), D (3.3±0.6 cm) and E (3.5±0.9 cm). 
     In RF ablation with a cooled electrode, the inner cavity channel of the electrode is preferably irrigated with cold or tap water. By the cooling effect, the distal end tip of the electrode is maintained at low temperature and free of charring thereby facilitating the conductivity of electrode-tissue interface and preventing an impedance rise. However, to a certain extent, the lesion size can no longer be further increased, because 
     1) the dimension of the electrode and hence the adjacent areas ablated with resistive and conductive heating are limited; 
     2) the conductivity of the tissue itself is relatively low if no external conductive agent is added; 
     3) steaming and tissue desiccation always occur next to the electrode-tissue interface which causes a rise of impedance. 
     The technique of a wet electrode and of a cooled electrode were separately known and eventuate in several drawbacks. 
     In RF ablation with a wet electrode, a hyper-conductive saline as an example of a wetting solution is prior and continuously infused via a the electrode into the target tissue while RF energy is delivered. The conductivity of 0,9% normal saline is 3-5 times higher than that of the blood and 12-15 times higher than that of tissues. With more than 5 times of increased concentration, further improvement of conductivity is expected. Infused saline functions as a “liquid electrode” within the tissue to be ablated and spreads applied RF energy away from the metal electrode to the surrounding tissue. Therefore, both the central resistive heating rim and peripheral passive heating area are increased, hence a larger lesion can be obtained. When saline is infused, some convective cooling also occurs at the tip. Besides, steaming is retarded by the increased boiling temperature of tissue fluid in which hypertonic saline is added. The effect of RF ablation with saline infusion appears already superior to that with cooling perfusion. However, this was still not optimal. The tip temperature still frequently raises above the boiling temperature at the electrode-tissue interface. Furthermore, infusion of a large amount of saline into the tumor may increase the static interstitial pressure which in turn may force individual tumor cells to migrate into adjacent or remote areas. 
     As demonstrated in our experiments, the current invention of cooled-wet electrode combines the advantages and meanwhile overcomes the disadvantages of each separate technique, yielding an optimal result of RF ablation with lesion sizes larger than 6 cm. This is realized by an increased conductivity of the target tissue as well as at electrode-tissue interface and a decreased tip temperature. The amount of infused saline can be reduced in comparison to that in “wet” alone mode. Unlike other more invasive approaches such as RF with bipolar, clustered and expanded electrodes and multiple applications of a single electrode, the present cooled-wet embodiment only use a single needle, through a single puncture but cause a large lesion ideal for tumor ablation or eradication. The proposed cooled-wet electrode and the described procedure allows to obtain by a single needle and in one session, a lesion of sufficient size. This is in contrast with the currently existing devices which necessitate either multiple deliveries of expanded electrodes or multiple applications of a single electrode to obtain similar results. Obviously, the application of a single electrode in one session is easier to perform and to control.