we developed a stent - like electrode that consisted of four parts : an introductory sheath , a stent , a tube through which a guide wire is passed , and an adaptor ( fig .
the sheath was made from 8-f teflon tubes , and the tube through which the guide wire is passed was made from 3-f polyethylene tubes , with a tapered dilator attached to the tip of the guide wire tube .
the stent itself was knitted from a single thread of 0.22-mm nitinol wire in a tubular configuration and with an interlocking , diamond - shaped pattern .
one end of the stent was collapsed and fixed with a polyethylene tube , using nitinol wire .
the free end of the this wire , connected to the stent , was used as a connector , and insulated with a 3-f polyethylene tube ( figs . 1 , 2 ) .
in order to gauge the extent of rfa , we measured the depth of ablation in cow liver . using the stent - like electrode and an rf generator ( rf2000 , radiotherapeutics corporation , calif .
, radiofrequency energy was applied to excised cow liver through the parenchymal tract at 3-min - intervals ( 3 , 6 , 9 , 12 , and 15 mins ) and in 20-watt increments ( 10 , 30 , 50 , 70 , and 90 watts ) ( fig .
impedance , power output , and elapsed time were recorded at 15-sec - intervals until full system impedance was realized .
after the completion of rfa , the liver was sectioned serially across the ablation zone , either perpendicular to the electrode or parallel to the needle track , and the maximal transverse diameter of each lesion was measured .
correlation between the thickness of the thermal lesion and ( a ) elapsed time to full system impedance , and ( b ) power applied was determined using cox regression analysis ( spss for windows , version 10.01 ; chicago , ill .
on the basis of a protocol approved by the animal research committee at chosun university , 14 male white new zealand rabbits , weighing between 2.0 and 3.2 kg , were anesthetized via intramuscular injection using a mixture of rompun ( bayer , seoul , korea ) and ketamin ( yuhan , seoul , korea ) , both at a concentration of 1 mg per kg of body weight .
after the thighs and abdomen of each rabbit were shaved , the animals were placed in the supine position within a fixation device , and two 35-cm ground pads ( each with conductive gel applied to their surface ) were placed bilaterally on each thigh . under fluoroscopic guidance ,
the anterior wall of the stomach was sutured to the abdominal wall in a purse - string fashion using blue nylon 3.0 ( ailee , pusan , korea ) .
next , the stomach was punctured at the center of the purse string suture using an 18-gauge peripheral catheter ( angiocath , becton dickinson , seoul , korea ) and a guide - wire was advanced through the sheath of the catheter until it reached the duodenum .
the stent - like electrode was advanced over the guide - wire as far as the distal duodenum .
three different levels of rf energy were applied to three different sites of the duodenum in a retrograde sequence , moving from distal to proximal positions ( fig .
4 ) , and during each application , impedance ( in ohms ) was documented every 15 sec .
energy was applied under six sets of conditions : 10 watts for 1 min , 10 watts for 2 mins , 20 watts for 1 min , 20 watts for 2 mins , 30 watts for 1 min , and 30 watts for 2 mins .
higher levels of rf energy ( 40 , 50 , and 60 watts for 1 and 2 mins ) were applied to additional rabbits .
six rabbits were sacrificed immediately after the application of rf energy and the gross and microscopic pathological findings were correlated with the twelve different power and time settings used .
eight rabbits were monitored for 4 weeks following the procedure , during which time they were pathologically examined .
we developed a stent - like electrode that consisted of four parts : an introductory sheath , a stent , a tube through which a guide wire is passed , and an adaptor ( fig .
the sheath was made from 8-f teflon tubes , and the tube through which the guide wire is passed was made from 3-f polyethylene tubes , with a tapered dilator attached to the tip of the guide wire tube .
the stent itself was knitted from a single thread of 0.22-mm nitinol wire in a tubular configuration and with an interlocking , diamond - shaped pattern .
one end of the stent was collapsed and fixed with a polyethylene tube , using nitinol wire .
the free end of the this wire , connected to the stent , was used as a connector , and insulated with a 3-f polyethylene tube ( figs . 1 , 2 ) .
in order to gauge the extent of rfa , we measured the depth of ablation in cow liver . using the stent - like electrode and an rf generator ( rf2000 , radiotherapeutics corporation , calif .
, radiofrequency energy was applied to excised cow liver through the parenchymal tract at 3-min - intervals ( 3 , 6 , 9 , 12 , and 15 mins ) and in 20-watt increments ( 10 , 30 , 50 , 70 , and 90 watts ) ( fig .
3 ) . during the procedure , impedance , power output , and elapsed time were recorded at 15-sec - intervals until full system impedance was realized .
after the completion of rfa , the liver was sectioned serially across the ablation zone , either perpendicular to the electrode or parallel to the needle track , and the maximal transverse diameter of each lesion was measured .
correlation between the thickness of the thermal lesion and ( a ) elapsed time to full system impedance , and ( b ) power applied was determined using cox regression analysis ( spss for windows , version 10.01 ; chicago , ill .
on the basis of a protocol approved by the animal research committee at chosun university , 14 male white new zealand rabbits , weighing between 2.0 and 3.2 kg , were anesthetized via intramuscular injection using a mixture of rompun ( bayer , seoul , korea ) and ketamin ( yuhan , seoul , korea ) , both at a concentration of 1 mg per kg of body weight .
after the thighs and abdomen of each rabbit were shaved , the animals were placed in the supine position within a fixation device , and two 35-cm ground pads ( each with conductive gel applied to their surface ) were placed bilaterally on each thigh . under fluoroscopic guidance ,
the anterior wall of the stomach was sutured to the abdominal wall in a purse - string fashion using blue nylon 3.0 ( ailee , pusan , korea ) .
next , the stomach was punctured at the center of the purse string suture using an 18-gauge peripheral catheter ( angiocath , becton dickinson , seoul , korea ) and a guide - wire was advanced through the sheath of the catheter until it reached the duodenum .
the stent - like electrode was advanced over the guide - wire as far as the distal duodenum .
three different levels of rf energy were applied to three different sites of the duodenum in a retrograde sequence , moving from distal to proximal positions ( fig .
4 ) , and during each application , impedance ( in ohms ) was documented every 15 sec .
energy was applied under six sets of conditions : 10 watts for 1 min , 10 watts for 2 mins , 20 watts for 1 min , 20 watts for 2 mins , 30 watts for 1 min , and 30 watts for 2 mins .
higher levels of rf energy ( 40 , 50 , and 60 watts for 1 and 2 mins ) were applied to additional rabbits .
six rabbits were sacrificed immediately after the application of rf energy and the gross and microscopic pathological findings were correlated with the twelve different power and time settings used .
eight rabbits were monitored for 4 weeks following the procedure , during which time they were pathologically examined .
a total of 40 ablations were performed in the excised livers , showing that the dimensions of the thermal lesions increased both with elapsed time and increased rf energy levels .
cox regression analysis demonstrated that the thickness of the resultant lesion showed linear correlation with both elapsed time ( fig .
the regression equation used was as follows : diameter of the thermal lesion= -11.436 + 1.57 elapsedtime+0.315watts ( p value < .001 ; r2 , 82% ) .
rf ablation using the stent - like electrode was technically successful in all 14 rabbits . during ablation of the duodenum , impedance of between 60 and 120
impedance increased rapidly ( within 75 to 90 sec ) and both animals died within one day of the procedure .
one rabbit to which we applied rfa at power levels of 10 , 20 , and 30 watts for 2 min died 7 days after the procedure .
three animals ( two in the 1-min group and one in the 2-min group ) were sacrificed at the end of the 2nd week after ablation , and two more ( both in the 1-min group ) at the end of the 4th week ( table 1 ) . during the rf procedure ,
three sites were perforated and the duodenum was seen to adhere to adjacent organs such as the liver , abdominal wall , and colon ( fig .
the acute changes apparent in the rabbits ' small intestine following rf ablation included color change , cytoplasmic denaturation , fibrin deposition and hemorrhage , among which hemorrhage of the mucosal layer was the earliest finding .
sequential histologic findings correlated well with the six different power and elapsed time parameters used in this part of the study ( table 2 ) .
animals subjected to one minute of rf ablation manifested no histologic change either 2 or 4 weeks after the procedure , irrespective of the power applied .
a total of 40 ablations were performed in the excised livers , showing that the dimensions of the thermal lesions increased both with elapsed time and increased rf energy levels .
cox regression analysis demonstrated that the thickness of the resultant lesion showed linear correlation with both elapsed time ( fig .
the regression equation used was as follows : diameter of the thermal lesion= -11.436 + 1.57 elapsedtime+0.315watts ( p value < .001 ; r2 , 82% ) .
rf ablation using the stent - like electrode was technically successful in all 14 rabbits . during ablation of the duodenum , impedance of between 60 and 120
ohms was maintained . during the 4-week follow - up period , three of eight rabbits in the 2-min group died . in two rabbits ,
impedance increased rapidly ( within 75 to 90 sec ) and both animals died within one day of the procedure .
one rabbit to which we applied rfa at power levels of 10 , 20 , and 30 watts for 2 min died 7 days after the procedure .
three animals ( two in the 1-min group and one in the 2-min group ) were sacrificed at the end of the 2nd week after ablation , and two more ( both in the 1-min group ) at the end of the 4th week ( table 1 ) . during the rf procedure ,
three sites were perforated and the duodenum was seen to adhere to adjacent organs such as the liver , abdominal wall , and colon ( fig .
the acute changes apparent in the rabbits ' small intestine following rf ablation included color change , cytoplasmic denaturation , fibrin deposition and hemorrhage , among which hemorrhage of the mucosal layer was the earliest finding .
sequential histologic findings correlated well with the six different power and elapsed time parameters used in this part of the study ( table 2 ) .
animals subjected to one minute of rf ablation manifested no histologic change either 2 or 4 weeks after the procedure , irrespective of the power applied .
radiofrequency ablation systems are comprised of three components : a radiofrequency generator , an active electrode , and dispersive electrodes .
rf energy ( alternating current ) is introduced into tissue via the active electrode , and as it moves from there to the dispersive electrode ( i.e. the electrosurgical return pad ) , and back again , ions within the tissue oscillate as they attempt to follow this change in direction .
this movement results in frictional heating of the tissue , and as its rises above 60 , cells begin to die .
rf thermal ablation using high - frequency electrical current is now an accepted procedure for the control of certain tumors . for both sexes , carcinoma of the gastrointestinal tract was the leading cause of cancer death in the united states in the year 2000 , with a 5-year relative survival rate of only 5% . in about 58% of patients with esophageal cancer and 40% of those with gastric cancer ,
curative resection is not feasible and palliative anastomotic surgery is the usual option . in patients with inoperable cholangiocarcinoma ,
several authors have reported that intracavitary radiation therapy can be an effective local treatment and has a positive effect on patient survival ( 13 - 15 ) .
using currently available rf ablation technology , zones of necrosis of up to 5 cm in diameter can be created in a single application , and tumors of larger diameter can be addressed with multiple applications of rf energy from different deployment sites ( 16 - 19 ) .
there are several possible differences between radiofrequency ablation of a gastrointestinal tract lesion and other solid organs such as the liver .
first , we found it impossible to apply maximal power for an extended period , so estimation of the ablation depth required was essential prior to rfa .
the gastrointestinal tract is adjacent to the common bile duct and other structures , and to avoid possible injury , care thus needs to be taken if the tract undergoes rfa for the maximum possible time and at maximal power . secondly ,
if the center of the target lesion contains a fluid- or air - filled space , the dimensions of the resultant thermal lesion may be affected due to heat conduction and convection within that tissue . in our study ,
two rabbits in the 2-min group experienced a rapid increase in the level of impedance during rf ablation and both died the next day .
we surmise that the duodenum of these animals was seriously injured by rf ablation , indicating that bowel circumstances could affect the procedure .
thirdly , it is very hard to monitor ablation while it is on - going , and the plan for rfa must thus be decided in advance , according to the size of the target lesion , after determining this by means of ct , mri , or ultrasound , for example . although the regression equation for depth of a thermal lesion due to elapsed time and applied power derived from the ex - vivo study had a high correlation value , correlation was low when the equation was used in our in - vivo study . prior to its use in a clinical situation , the equation should be refined to a higher probability through additional ex - vivo studies .
two factors present in our ex - vivo study may affect the extent of a thermal lesion : tissue composition and the lack of perfusion .
the first of these is the principal factor that can alter the extent of coagulation following rf ablation .
since heat is conducted through different tissues at various rates , the power settings for rf ablation vary according to the tissue involved .
in addition , heat transfer through excised cow liver is undoubtedly different from that through the bowel tissue of a living rabbit .
equally as important to the development of a thermal lesion is the degree of perfusion within tissue .
the movement of blood or other fluids through tissue has been reported to reduce heat generation during thermal ablation , a phenomenon labeled the heat - sink effect ( 20 ) . in this way
, ablation near major blood vessels can require either longer periods of time or higher levels of power , or both , if the resultant lesions are to encompass the areas of tissue to the desired degree .
since perfusion - mediated tissue cooling reduces the extent of coagulation necrosis produced by thermal ablation , tissue that is not perfused ( such as excised cow liver ) will exhibit larger zones of ablation than are normally seen in perfused tissue . as a result
, the correlation noted in the ex - vivo study is less clinically useful than in an in - vivo or tumor study ; however , it certainly provides the basis for developing a clinical model for use in perfused tissue .
the stent - like electrode developed for this work has some advantages over currently configured needle electrodes used in soft tissue coagulation , providing easy control of the extent of the thermal lesion induced .
its shape , furthermore , permits its use in any tubular structure , including the esophagus , gastric outlet , bile duct , and vessels .
more importantly , in this study the stent - like electrode did not directly perforate the gastrointestinal tract , and because it has no sharp end or point , perforation during future applications is unlikely . in addition ,
when applied to the duodenum , it was easily expanded and retracted within the tissue .
because the bowel changes its shape and location during rfa with this electrode , real - time ultrasonographic guidance could well be useful .
the changes noted in the duodenum following rf ablation in this study correspond closely with these reported in other studies involving soft tissues ( 1 , 10 , 17 ) . the resultant thermal lesions were comprised mainly of coagulation necrosis and fibrosis , and any changes in the mucosal layer had healed completely within two weeks . however , long application times and/or high power applications resulted in submucosal and serosal injuries that lead to peritonitis or ulcer formation and eventual bowel perforation .
we believe that rfa procedures that combine lower power with shorter elapsed time will , in future , be the preferred manner of using rfa to address gastrointestinal malignancies , including biliary caner .
while the results of this experimental study are encouraging , determination of a more accurate algorithm for applying radiofrequency energy to tissue within the gastrointestinal tract will require further investigation and long - term follow - up in animals .
in addition , the effect of bowel circumstances on rf ablation should be investigated before its application in cases involving unresectable gastrointestinal lesions .
 objectiveto assess the technical feasibility of a newly designed stent - like electrode in rabbits.materials and methodsa stent - like electrode was knitted from a single thread of nitinol wire and interconnected to a generator using similar wire . in order to gauge the extent of radiofrequency ablation ( rfa ) , we measured the depth of the ablated area in cow liver using a combination of 180-sec time intervals and 20-watt power increments . for data processing , 
 cox regression analysis was used . 
 rfa was also applied to the small intestine of rabbits using this stent - like electrode under six different sets of conditions : 10 watts for 1 min , 10 watts for 2 mins , 20 watts for 1 min , 20 watts for 2 mins , 30 watts for 1 min , and 30 watts for 2 mins . to determine the gross and microscopic findings , 
 six animals were sacrificed immediately after the procedure and the results obtained under the different sets of conditions were correlated . 
 eight rabbits were monitored for 4 weeks prior to sacrifice.resultsfor both ex - vivo and in - vivo ablations , the depth of the thermal lesion showed linear correlation with both the duration of rfa and the power applied . 
 rfa of the duodenum was technically successful in all 14 rabbits . 
 the acute changes occurring in the rabbits ' small intestine included color change , cytoplasmic denaturation , fibrin deposition and hemorrhage , among which hemorrhage of the mucosal layer was the earliest finding . 
 rf ablation for 2 mins at 30 watts caused serosal hemorrhage . 
 the gross and histologic changes occurring showed close correlation under all six sets of conditions.conclusionuse of the stent - like electrode proves technically feasible but to determine the nature of the chronic change occurring in the gastrointestinal tract after rf ablation , further investigation and long - term follow - up in animals are required . 