Percutaneous ultrasound-guided radiofrequency electrocautery ablation of prostate tissue in dogs

Effects of Intraoperative Magnetic Resonance Thermal Imaging Signal Artifact During Laser Interstitial Thermal Therapy on Thermal Damage Estimate and Postoperative Magnetic Resonance Imaging Ablative Area Concordance

BACKGROUND

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has considered the impact of this artifact on TDE accuracy.

OBJECTIVE

To determine the effect of intraoperative MRTI signal artifact on postoperative magnetic resonance imaging (MRI)-predicted ablative area.

METHODS

All ablations were performed using the Visualase MRI-Guided Laser Ablation System (Medtronic). Patients were grouped based on whether the intraoperative MRTI contained signal artifact that distorted the ablative region. Cross-sectional area of the ablative lesion from the MRI image was measured, and the difference between intraoperative TDE and postoperative MRI cross-sectional area was calculated and compared between groups with and without intraoperative MRTI artifact.

RESULTS

A total of 91 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 43.9% of cases overall. The mean absolute difference between TDE and the postoperative MRI cross-sectional area was 94.8 mm2 (SEM = 11.6) in the group with intraoperative MRTI artifact and 54.4 mm2 (SEM = 5.5) in the nonartifact group.

CONCLUSION

MRTI signal artifact is common during LITT. The presence of signal artifact during intraoperative MRTI results in higher variation between intraoperative TDE and postoperative MRI cross-sectional ablative area. In cases in which intraoperative MRTI artifact is observed, there may be a larger degree of variation between observed intraoperative TDE and measured postoperative MRTI ablative area.

Understanding the Relationship Between Real-Time Thermal Imaging and Thermal Damage Estimate During Magnetic Resonance-Guided Laser Interstitial Thermal Therapy

OBJECTIVE

Magnetic resonance-guided laser interstitial thermal therapy is a minimally invasive procedure that uses intraoperative magnetic resonance thermometry (MRT) to generate a thermal damage estimate (TDE) of the ablative area. This study aimed to compare areas produced by the MRT heat map with the system-generated TDE produced by Visualase software.

METHODS

All ablations were performed using the Visualase laser ablation system. MRT heat map and TDE were quantified using MATLAB version R2014a. TDE was compared with the summed area of green, yellow, and red areas (heat map 63.9 [HM63.9]) and the summed area of light blue, green, yellow, and red areas (heat map 50.4 [HM50.4]) produced by the MRT heat map.

RESULTS

Fifty-six patients undergoing magnetic resonance-guided laser interstitial thermal therapy were examined. Mean TDE produced was 236 mm2 (SEM = 9.5). Mean HM63.9 was 231 mm2 (SEM = 8.7), and mean HM50.4 was 370 mm2 (SEM = 12.8). There was no significant difference between TDE and HM63.9 (P = 0.51). There was a significant difference between TDE and HM50.4 (P < 0.001) and between HM63.9 and HM50.4 (P < 0.001).

CONCLUSIONS

The system-generated TDE consistently remains contained within the boundaries of the MRT heat map. At standard factory settings, TDE and the area produced within the periphery of HM63.9 are similar in magnitude. The light blue portion of the MRT heat map may serve as an additional means of predicting when critical structures may be at risk during laser ablation if exposed to further thermal stress.

Ablation dynamics of subsequent thermal doses delivered to previously heat-damaged tissue during magnetic resonance-guided laser-induced thermal therapy

OBJECTIVE

Intraoperative dynamics of magnetic resonance-guided laser-induced thermal therapy (MRgLITT) have been previously characterized for ablations of naive tissue. However, most treatment sessions require the delivery of multiple doses, and little is known about the ablation dynamics when additional doses are applied to heat-damaged tissue. This study investigated the differences in ablation dynamics between naive versus damaged tissue.

METHODS

The authors examined 168 ablations from 60 patients across various surgical indications. All ablations were performed using the Visualase MRI-guided laser ablation system (Medtronic), which employs a 980-nm diffusing tip diode laser. Cases with multiple topographically overlapping doses with constant power were selected for this study. Single-dose intraoperative thermal damage was used to calculate ablation rate based on the thermal damage estimate (TDE) of the maximum area of ablation achieved (TDEmax) and the total duration of ablation (tmax). We compared ablation rates of naive undamaged tissue and damaged tissue exposed to subsequent thermal doses following an initial ablation.

RESULTS

TDEmax was significantly decreased in subsequent ablations compared to the preceding ablation (initial ablation 227.8 ± 17.7 mm2, second ablation 164.1 ± 21.5 mm2, third ablation 124.3 ± 11.2 mm2; p = < 0.001). The ablation rate of subsequent thermal doses delivered to previously damaged tissue was significantly decreased compared to the ablation rate of naive tissue (initial ablation 2.703 mm2/sec; second ablation 1.559 mm2/sec; third ablation 1.237 mm2/sec; fourth ablation 1.076 mm/sec; p = < 0.001). A negative correlation was found between TDEmax and percentage of overlap in a subsequent ablation with previously damaged tissue (r = -0.164; p < 0.02).

CONCLUSIONS

Ablation of previously ablated tissue results in a reduced ablation rate and reduced TDEmax. Additionally, each successive thermal dose in a series of sequential ablations results in a decreased ablation rate relative to that of the preceding ablation. In the absence of a change in power, operators should anticipate a possible reduction in TDE when ablating partially damaged tissue for a similar amount of time compared to the preceding ablation.

Lower Urinary Tract Neoplasia

Lower urinary tract neoplasia in companion animals is a debilitating and often life-threatening disease. Tumors of the bladder, urethra, and prostate often occur independently, although extension of these tumors into adjacent regions of the lower urinary tract is documented frequently. The most common lower urinary tract tumor in dogs and cats is transitional cell carcinoma (TCC). In both dogs and cats, TCC affecting the urinary bladder is generally considered to be highly aggressive with both local and metastatic disease potential, and this disease poses unique treatment challenges. Whereas much literature exists regarding the TCC disease process, treatment options, and prognosis in dogs, relatively few studies on feline TCC have been published due to the lower incidence of TCC in this species. Prostate tumors, most commonly adenocarcinomas, occur less commonly in dogs and cats but serve an important role as a comparative model for prostate neoplasia in humans. This article serves as a review of the current information regarding canine and feline lower urinary tract neoplasia as well as the relevance of these diseases with respect to their human counterparts.

Low-frequency and low-intensity ultrasound-mediated microvessel disruption enhance the effects of radiofrequency ablation on prostate cancer xenografts in nude mice

The aim of the present study was to examine the impact of low-frequency, low-intensity ultrasound (US)-stimulated microbubbles (USMB) on radiofrequency ablation (RFA) in the treatment of nude mice with human prostate cancer xenografts. The tumor‑bearing nude mice were divided into three groups: The USMB+RFA group was treated with USMB immediately followed by RFA, the RFA group was treated with RFA alone, and the control group remained untreated. The animals underwent enhanced US to calculate the tumor volumes, ablation volumes and ablation rates. Subsequently, the tumors were excised for hematoxylin and eosin staining, to identify necrosis in the tumors following the treatments, and immunohistochemical staining, to analyze the apoptotic index (AI), proliferative index (PI) and microvessel density (MVD) at 1, 4 and 7 days post-treatment. Each group contained five mice at each time‑point. Necrosis was apparent in the center of the tumors in the treatment groups. Ablation lesion volumes of the USMB+RFA group were larger than those in the RFA group at 1 and 4 days post‑treatment (P=0.002 and P=0.022, respectively), and the ablation rates of the USMB+RFA group were significantly higher, compared with the RFA group at the three time‑points (all P<0.001). There were fewer apoptotic cells and more proliferative cells in the RFA group, compared with the control group 1,4 and 7 days post‑treatment (all P<0.05). The AI of the USMB+RFA group was higher than that of the control group and lower than that of the RFA group 1 day post-treatment (P=0.034 and P=0.016, respectively). The PI of the USMB+RFA group was lower than that of the control group and higher than that of the RFA group 4 and 7 days post-treatment (all P<0.05). No significant differences were observed in MVD among the three groups throughout the experiment. In conclusion, exposure to USMB prior to RFA produced larger volumes of ablation, compared with treatment with RFA alone, and increased AI and reduced PI in the residual carcinoma cells induced by RFA.

Histotripsy of the Prostate in a Canine Model: Characterization of Post-Therapy Inflammation and Fibrosis

INTRODUCTION

Histotripsy is a nonthermal, noninvasive, pulsed ultrasound technology that homogenizes tissue within the targeted volume. From previous experiments, it appeared that the resultant fibrotic response from histotripsy was limited compared with the typical tissue response seen after thermoablation. The objective of this study was to characterize the inflammatory response and quantify patterns of collagen deposition 6 weeks after in vivo canine prostate histotripsy.

METHODS

Histotripsy was applied to the left half of eight canine prostates to produce an intraparenchymal zone of tissue homogenization. Six weeks after treatment, prostates were harvested, sectioned, and stained with hematoxylin and eosin for histologic evaluation, CD3, CD20, and Mac387 immunohistochemistry to characterize the inflammatory components, and picrosirius red staining to identify collagen.

RESULTS

Seven of eight treated prostates exhibited only minimal residual inflammation. Visual microscopic analysis of picrosirius red slides revealed a band of dense collagen (0.5 mm wide) immediately adjacent to the cavity produced by histotripsy. This was surrounded by a second band (1 mm wide) of less dense collagen interspersed among glandular architecture. A lobar distribution of epithelial atrophy and basal cell hyperplasia reminiscent of periurethral glands and ducts was apparent surrounding the margin of the treatment cavities. Tissue loss (-31%) was apparent on the treated side of all prostates while four demonstrated a net decrease in collagen content.

CONCLUSIONS

In vivo histotripsy of canine prostate produced a decrease in prostate volume coupled with a limited inflammatory and fibrotic response. A narrow (1.5 mm) band of fibrosis around the empty, reepithelialized treatment cavity was observed 6 weeks after treatment. In four cases, an overall reduction in collagen content was measured. Further studies are planned to correlate these histologic findings with alteration in mechanical tissue properties and to explore histotripsy strategies for treatment of benign prostatic hyperplasia that optimize tissue volume removal with minimization of fibrosis.

Radiofrequency ablation during continuous saline infusion can extend ablation margins

AIM: To determine whether fluid injection during radiofrequency ablation (RFA) can increase the coagulation area.

METHODS: Bovine liver (1-2 kg) was placed on an aluminum tray with a return electrode affixed to the base, and the liver was punctured by an expandable electrode. During RFA, 5% glucose; 50% glucose; or saline fluid was infused continuously at a rate of 1.0 mL/min through the infusion line connected to the infusion port. The area and volume of the thermocoagulated region of bovine liver were determined after RFA. The Joule heat generated was determined from the temporal change in output during the RFA experiment.

RESULTS: No liquid infusion was 17.3 ± 1.6 mL, similar to the volume of a 3-cm diameter sphere (14.1 mL). Mean thermocoagulated volume was significantly larger with continuous infusion of saline (29.3 ± 3.3 mL) than with 5% glucose (21.4 ± 2.2 mL), 50% glucose (16.5 ± 0.9 mL) or no liquid infusion (17.3 ± 1.6 mL). The ablated volume for RFA with saline was approximately 1.7-times greater than for RFA with no liquid infusion, representing a significant difference between these two conditions. Total Joule heat generated during RFA was highest with saline, and lowest with 50% glucose.

CONCLUSION: RFA with continuous saline infusion achieves a large ablation zone, and may help inhibit local recurrence by obtaining sufficient ablation margins. RFA during continuous saline infusion can extend ablation margins, and may be prevent local recurrence.

Contrast Enhanced Ultrasound for Radio Frequency Ablation of Canine Prostates: Initial Results

PURPOSE

We determined the feasibility of contrast enhanced ultrasound for radio frequency ablation of the entire prostate as a method of minimally invasive treatment for prostate cancer in a canine model.

MATERIALS AND METHODS

Approval of the Institutional Animal Use and Care committee was obtained. Initially 5 dogs (group 1) were tested using variable power (5 to 30 W), time (4 to 12 minutes), bolus (0.01 to 0.04 ml/kg) and infusion (3 to 11 ml per minute at 0.015 microl/kg) injections of an ultrasound contrast agent with conventional grayscale power Doppler and pulse inversion harmonic imaging to establish optimal parameters. Subsequently 4 dogs (group 2) underwent entire prostate ablation using parameters based on group 1. The size of the thermal lesions and residual viable tissue was measured with ImageJ software (National Institutes of Health, Bethesda, Maryland) on ultrasound and pathological study. Linear regression and Student's t test were used for statistical analysis.

RESULTS

A bolus of 0.04 ml/kg, an infusion of 11 ml per minute at 0.015 microl/kg and the contrast enhanced pulse inversion harmonic imaging mode were ranked best for guiding ablation. Thermal lesion volume was proportional to ablation power and time. There was no significant difference in measured thermal lesion size in group 1 between ultrasound and pathological findings (mean +/- SD 1.51 +/- 0.74 and 1.46 +/- 0.74 cm3, p = 0.56) or in residual viable tissue in group 2 (0.43 +/- 0.043 and 0.41 +/- 0.291 cm3, p = 0.21). The average volume of prostate ablation achieved in group 2 was 96.3%.

CONCLUSIONS

Contrast enhanced pulse inversion harmonic imaging is able to guide, monitor and control radio frequency ablation of the entire prostate.

Radio-frequency tissue ablation of the liver: in vivo and ex vivo experiments with four different systems

The aim of this study was to test the efficacy of four different radio-frequency ablation (RFA) systems in normal hepatic parenchyma in large animals. The RFA was applied to pig livers in vivo and to calf livers ex vivo using the Radionics cluster needle, RITA starburst XL needle, Radiotherapeutics Le Veen 4.0 needle, and the Berchtold 14-G saline-perfused 15-mm active-tip needle based on constructor specifications. The volume of tissue coagulation from RF was calculated from measurements of the vertical diameter (Dv) and transverse diameter (Dt). Lesion shape was characterized using the ratio between Dt/Dv. Radiotherapeutics and RITA produced in vivo lesion volume of 42+/-10, 39+/-4 cm3 with a reproducible spherical shape (Dt/Dv of 1.01+/-0.16 and 0.97+/-0.1, respectively). Radionics produced in vivo RF lesions volume of 29+/-11 cm3) with an ovoid shape (Dt/Dv 0.88+/-0.09). The RF lesions with the Berchtold device could not be assessed in vivo as 5 of 8 animals died during treatment. Ex vivo RF lesions had similar volumes with each system; however, the Radiotherapeutics device produced more reproducible shaped lesions than the other systems. In our experimental study, we found no difference between expandable needle systems in vivo. Cooled needles produced slightly smaller and ovoid shape in vivo lesions.

Percutaneous radiofrequency ablation of liver metastases in potential candidates for resection: the "test-of-time approach"

BACKGROUND

Some surgeons have advocated delaying resection of liver metastases to allow additional metastases which may be present, but are undetected, to be identified. This "test-of-time" approach can limit the number of resections performed on patients who ultimately will develop additional metastases. The current study evaluated the potential role and possible advantages of performing radiofrequency (RF) ablation during the interval between diagnosis and hepatic metastasectomy as part of a test-of-time management approach.

METHODS

Eighty-eight consecutive patients with 134 colorectal carcinoma liver metastases were potential candidates for hepatic metastasectomy. They were treated with percutaneous RF ablation using single (101 treatments) or triple-probe cluster (22 treatments) 18-gauge internally cooled electrodes. Treatment was performed under conscious sedation (22 of 119 treatments), anesthesia (14 of 119 treatments), or general anesthesia (83 of 119 treatments). At the time of the initial RF ablation procedure, 49 of 88 patients (56%) were found to have 1 metastasis, 32 of 88 patients (36%) had 2 metastases, and 7 of 88 patients (8%) had 3 metastases. Metastases ranged from 0.6 to 4.0 cm in greatest dimension (mean, 2.1 cm). Follow-up with serial computed tomography scans scans ranged from 18 to 75 months (median, 33 months) after the initial RF ablation.

RESULTS

A total of 119 RF ablations were performed. Complete necrosis was obtained in 53 of 88 patients (60%) and in 85 of 134 lesions (63%). During follow-up of these 53 patients, 16 (30%) remained free of disease and 37 (70%) developed new lesions. New lesions were intrahepatic in 26 of 37 patients (70%), extrahepatic in 4 patients (11%), and both intrahepatic and extrahepatic in 7 patients (19%). Of 26 patients whose new lesions were intrahepatic only, 15 (58%) were retreated with RF and 7 were free of disease at the time of last follow-up (median follow-up, 28 months). Ten additional patients with only intrahepatic new lesions were deemed untreatable and 1 patient underwent resection. Overall, among the 53 patients in whom complete tumor necrosis was achieved after RF ablation therapy, 52 (98%) were spared surgical resection: 23 (44%) because they have remained free of disease and 29 (56%) because they developed disease progression. Among all 88 patients, 21 (24%) underwent resection after RF ablation (8 were free of disease at the time of last follow-up), 23 (26%) remained free of disease after successful RF ablation, and 56 (64%) developed untreatable disease progression (44 after RF alone, 12 after RF and surgery). Lesions in 35 of 88 patients (40%) demonstrated local tumor recurrence on follow-up imaging studies. Twenty of these 35 patients (57%) underwent surgical resection, whereas the remaining 15 patients (43%) developed additional, untreatable metastases. New lesions were intrahepatic in 9 of 15 patients (60%), extrahepatic in 1 of 15 patients (7%), and both intrahepatic and extrahepatic in 5 of 15 patients (33%). No patient who had been treated with RF ablation became unresectable due to the growth of metastases and there was no evidence of needle track seeding in any patient after RF ablation. Overall, among the 35 patients in whom complete tumor necrosis was not achieved after RF ablation therapy, 15 (43%) were spared surgical resection.

CONCLUSIONS

The results of the current study suggest that current RF ablation techniques, when used as part of a test-of-time management approach, can decrease the number of resections performed. The approach results in complete tumor necrosis in some patients and provide an interval for others who ultimately will develop new intrahepatic and/or extrahepatic metastases to do so.

Clinical short-term results of radiofrequency ablation in liver cancers

AIM: To study local therapeutic efficacy, side effects, and complications of radiofrequency ablation (RFA), which is emerging as a new method for the treatment of patients with hepatocellular carcinoma (HCC) with cirrhosis or chronic hepatitis and metastatic liver cancer.

METHODS: Thirty-six patients with primary and secondary liver cancers (21 with primary hepatocellular carcinoma, 12 with colorectal cancer liver metastases and 3 with other malignant liver metastases), which were considered not suitable for curative resection, were include in this study. They were treated either with RFA (RITA2000, Mountain View, California, USA) percutaneously (n = 20) or intraoperatively (n = 16). The procedures were performed using the ultrasound guidance. The quality of RFA were based on monitoring of equipments and subject feeling of the practitioners. Patients treated with RFA was followed according to clinical findings, radiographic images, and tumor markers.

RESULTS: Thirty-six patients underwent RFA for 48 nodules. RFA was used to treat an average 1.3 lesions per patient, and the median size of treated lesions was 2.5 cm (range, 0.5-9 cm). The average hospital stay was 5.6 d overall (2.8 d for percutaneous cases and 7.9 d for open operations). Seven patients underwent a second RFA procedure (sequential ablations). Sixteen HCC patients with a high level of alpha fetoprotein (AFP) and 9 colorectal cancer liver metastases patients with a high level of serum carcinoembryonic antigen (CEA) have a great reduction benefited from RFA. Four (11.1%) patients had complications: one skin burn; one postoperative hemorrhage; one cholecystitis and one hepatic abscess associated with percutaneous ablations of a large lesion. There were 4 deaths: 3 patients died from local and system diseases (1 at 7 month, 1 at 9 month, and 1 at 12 month), 1 patients died from cardiovascular shock, but no RFA-related death. At a median follow-up of 10 mo (range, 1-24 mo), 6 patients (16.7%) had recurrences at an RFA site, and 20 patients (56.7%) remained clinically free of disease.

CONCLUSION: RF ablation appears to be an effective, safe, and relatively simple procedure for the treatment of unresectable liver cancers. The rate and severity of complications appear acceptable. However, further study is necessary to assess combination with other therapies, long-term recurrence rates and effect on overall survival.

Radiofrequence ablation of liver cancers

Primary and secondary malignant liver cancers are some of most common malignant tumors in the world. Chemotherapy and radiotherapy are not very effective against them. Surgical resection has been considered the only potentially curtive option, but the majority of patients are not candidates for resection because of tumor size, location near major intrahepatic blood vessels and bile ducts, precluding a margin-negative resection, cirrhotic, hepatitis virus infection or multifocial. Radiofrequence ablation (RFA), which is a new evolving effective and minimally invasive technique, can produce coagulative necrosis of malignant tumors. RFA should be used percutaneously, laparscopically, or during the open laparotomy under the guidance of ultrasound, CT scan and MRI. RFA has lots of advantages superior to other local therapies including lower complications, reduced costs and hospital stays, and the possibility of repeated treatment. In general, RFA is a safe, effective treatment for unresectable malignant liver tumors less than 7.0 cm in diameter. We review the principle, mechanism, procedures and experience with RFA for treating malignant liver tumors.

Percutaneous radio-frequency ablation of hepatic metastases from colorectal cancer: long-term results in 117 patients

PURPOSE

To describe the results of an ongoing radio-frequency (RF) ablation study in patients with hepatic metastases from colorectal carcinoma.

MATERIALS AND METHODS

In 117 patients, 179 metachronous colorectal carcinoma hepatic metastases (0.9-9.6 cm in diameter) were treated with RF ablation by using 17-gauge internally cooled electrodes. Computed tomographic follow-up was performed every 4-6 months. Recurrent tumors were retreated when feasible. Time to new metastases and death for each patient and time to local recurrence for individual lesions were modeled with Kaplan-Meier analysis. Modeling determined the effect of number of metastases on the time to new metastases and death and effect of tumor size on local recurrence.

RESULTS

Estimated median survival was 36 months (95% CI; 28, 52 months). Estimated 1, 2, and 3-year survival rates were 93%, 69%, and 46%, respectively. Survival was not significantly related to number of metastases treated. In 77 (66%) of 117 patients, new metastases were observed at follow-up. Estimated median time until new metastases was 12 months (95% CI; 10, 18 months). Percentages of patients with no new metastases after initial treatment at 1 and 2 years were 49% and 35%, respectively. Time to new metastases was not significantly related to number of metastases. Seventy (39%) of 179 lesions developed local recurrence after treatment. Of these, 54 were observed by 6 months and 67 by 1 year. No local recurrence was observed after 18 months. Frequency and time to local recurrence were related to lesion size (P < or =.001).

CONCLUSION

RF ablation is an effective method to treat hepatic metastases from colorectal carcinoma.

Radio-frequency electrocautery ablation of mammary tissue in swine

PURPOSE

To establish the size, configuration, and histopathologic features of acute, subacute, and chronic radio-frequency (RF) electrocautery of mammary tissue in swine.

MATERIALS AND METHODS

Eighteen RF treatments were performed in the mammary tissue of three domestic swine under ultrasonographic (US) guidance. Histopathologic examination was performed immediately after (acute animal); 2 weeks after (subacute animal); and 4 weeks after (chronic animal) treatment.

RESULTS

In the acute animal, lesions were firm nodules on palpation and had a distinct line of demarcation between necrotic and viable mammary tissue (mean lesion volume, 14.24 cm(3); largest volume, 29.06 cm(3)). In the subacute animal, there was diffuse coagulation necrosis with neutrophilic infiltrates at the periphery (mean lesion volume, 6.46 cm(3); largest volume, 9.47 cm(3)), and two treatment areas had a secondary bacterial infection. In the chronic animal, lesions were still palpable and firm (mean lesion volume, 11.67 cm(3); largest volume, 25.5 cm(3)), and five of six treatment sites had an area of gray to white fibrotic tissue that blended with the surrounding tissue. However, one site had a pale yellow area of central necrosis surrounded by a fibrotic area. In both the subacute and chronic animals, two and one treatment site, respectively, had minimal areas of skin necrosis.

CONCLUSION

RF ablation of breast tissue is feasible in this animal model. Problems included minimal skin erythema, residual firm treatment regions at 4 weeks, slightly variable margins of coagulation necrosis, and occasional bacterial infection.

Open-configuration MR imaging, intervention, and surgery of the urinary tract

The open-configuration MR imaging system provides new applications both in diagnosis and in therapy of conditions in the urinary tract. In addition to conventional imaging, the open configuration permits MR imaging of patients in many positions. This has already been shown to be useful in imaging the pelvis during voiding, where a sitting position allows urodynamic evaluation. The lithotomy position can be used for imaging the prostate, which also permits procedural access. The ultimate purpose of the interventional MR imaging suite is to integrate therapeutic tools and techniques with MR imaging. From surgical planning through specialized imaging systems with minimally invasive surgical applications, new methods are being developed and implemented. This new field of image-guided therapy will require extensive clinical development and evaluation for applications in the urinary tract. This will require a large concentrated interdisciplinary effort of surgeons, radiologists, computer scientists, engineers, and physicists. Successful integration of basic research and clinical work will result in a number of cutting-edge technologies with direct clinical application in the urinary tract. Initial projects have included biopsies, endoscopies, and real-time procedural control of high-temperature and cryogenic ablations. It is anticipated that the current surge in image-guided interventions will motivate even more research activity in this field, and will ultimately define the role of MR imaging guidance in urologic intervention and surgery.

A cooled needle electrode for radiofrequency tissue ablation: thermodynamic aspects of improved performance compared with conventional needle design

RATIONALE AND OBJECTIVES

I developed and tested a cooled needle electrode (CNE) for radiofrequency (RF) tissue ablation in vitro.

METHODS

A 2-mm needle electrode with two lumina for internal water perfusion and irrigation of the needle tip and a conventional needle electrode were tested in ex vivo calf liver during different levels of output power (wattage). RF lesions produced by the two needle types were further evaluated with a thermal camera.

RESULTS

When the CNE was used, a significant increase in the duration of ablation was observed, which caused a significant increase in delivered energy and lesion size when compared with the conventional needle electrode. The largest lesion produced with the CNE was almost spherical and measured 41 x 37 mm (longitudinal x transverse). The cooling effect was reflected in the image obtained with the thermal camera.

CONCLUSION

This technique produced very large lesions compared with conventional methods and may have a role in many different kinds of RF needle ablation.

The loop electrode: in vitro evaluation of a device for ultrasound-guided interstitial tissue ablation using radiofrequency electrosurgery

RATIONALE AND OBJECTIVES

I developed and tested in vitro a device for ultrasound-guided monopolar radiofrequency (RF) electrosurgical interstitial tissue ablation.

METHODS

The current was applied to an electrode with a distal loop form (radius = 1 cm). The superelastic properties of the electrode allowed cannula introduction in the experimental medium (calf liver) before subsequent rotation of the electrosurgical cutting electrode, resulting in cutting off and isolation of a spherical lesion interstitially. The optimal setting of the RF unit and the optimal cutting speed were evaluated. Under ultrasonographic guidance and monitoring, approximately 150 lesions were produced with different loop designs and sizes. The gross appearance was evaluated and correlated to ultrasonography.

RESULTS

An output effect of 200 W was optimal for resection and cutting with the loop electrode. An ellipsoid loop configuration could produce an almost spherical lesion with a diameter of 2.0 cm interstitially. Ultrasonography could guide the introduction of the loop electrode into tissue and visualize the upper part of the lesion.

CONCLUSION

The loop electrode is a technique for percutaneous ultrasonographically guided tissue ablation. It was proved to be efficient in vitro by producing 2-cm liver lesions.

Radiofrequency tissue ablation: importance of local temperature along the electrode tip exposure in determining lesion shape and size

RATIONALE AND OBJECTIVES

We determined whether heat distribution along a radiofrequency (RF) electrode would be uniform when longer tip exposures are used and whether local temperature effects would influence the shape of induced tissue coagulation.

METHODS

Thermistors were embedded within 18-gauge RF electrodes at both ends and in the middle of the exposed tip. The length of tip exposure varied from 1 to 7 cm. RF was applied in vitro to pig liver for 6 min using a constant tip temperature, which was varied in 10 degrees C increments from 60 degrees C to 110 degrees C. Experiments were performed in triplicate. The 3- and 5-cm probes were used at a 90 degrees C tip temperature to create lesions in live pig liver and muscle using similar parameters. Temperature was measured throughout the procedure. Observable coagulation necrosis was measured at the end of the treatment. Regression analysis was used to evaluate the local temperature-lesion diameter relationship.

RESULTS

Temperatures were not uniform along the tip exposure for any given trial. Temperature variation increased with higher tip temperatures and longer tip exposures. The diameter of local coagulation necrosis was a function of the local mean temperature. For in vitro trials, no coagulation was seen when the local temperature was less than 50 degrees C. Temperatures above this threshold resulted in progressively greater lesion diameter, with a minimum of 1 cm of necrosis occurring at 71 degrees C. Additional increases in lesion diameter (1.4-1.6 cm) were observed at approximately 90 degrees C. Mathematical modeling demonstrated a best-fit curve: lesion diameter (in cm) = ¿1.4 + 0.03 (tip exposure)¿ ¿1 - e [-0.067(local temp - 49.5 degrees C)]¿, r2 = .986, SD = 0.14 cm for each curve. In living tissue, less uniformity in the shape of coagulation necrosis was seen around the electrodes. Local temperature-lesion diameter data fit the same logarithmic relation, but the threshold for coagulation necrosis was 8.5 degrees C higher than for in vitro specimens.

CONCLUSION

Using a single-probe technique for RF-induced tissue necrosis, the diameter of tissue coagulation may be predicted by the local temperature along the exposed electrode. The uniformity of temperature decreases with increased tip exposures. This effect may be partially corrected by creating lesions at higher tip temperatures, where necrosis diameter is increased. Because effects are more pronounced in vivo, uniform volumes of tissue necrosis are limited to tip exposures of 3 cm or less.