Early small-bowel ischemia: dual-energy CT improves conspicuity compared with conventional CT in a swine model

PURPOSE

To compare dual-energy computed tomography (CT) with conventional CT for the detection of small-bowel ischemia in an experimental animal model.

MATERIALS AND METHODS

The study was approved by the animal care and use committee and was performed in accordance with the Guide for Care and Use of Laboratory Animals issued by the National Research Council. Ischemic bowel segments (n = 8) were created in swine (n = 4) by means of surgical occlusion of distal mesenteric arteries and veins. Contrast material-enhanced dual-energy CT and conventional single-energy CT (120 kVp) sequences were performed during the portal venous phase with a single-source fast-switching dual-energy CT scanner. Attenuation values and contrast-to-noise ratios of ischemic and perfused segments on iodine material-density, monospectral dual-energy CT (51 keV, 65 keV, and 70 keV), and conventional 120-kVp CT images were compared. Linear mixed-effects models were used for comparisons.

RESULTS

The attenuation difference between ischemic and perfused segments was significantly greater on dual-energy 51-keV CT images than on conventional 120-kVp CT images (mean difference, 91.7 HU vs 47.6 HU; P < .0001). Conspicuity of ischemic segments was significantly greater on dual-energy iodine material-density and 51-keV CT images than on 120-kVp CT images (mean contrast-to-noise ratios, 4.9, 4.3, and 2.1, respectively; P < .0001). Although attenuation differences on dual-energy 65- and 70-keV CT images were not significantly different from those on 120-kVp images (55.0 HU, 45.8 HU, and 47.6 HU, respectively; 65 keV vs 120 kVp, P = .15; 70 keV vs 120 kVp, P = .46), the contrast-to-noise ratio was greater for the 65- and 70-keV images than for the 120-kVp images (4.4, 4.1, and 2.1 respectively; P < .0005).

CONCLUSION

Dual-energy CT significantly improved the conspicuity of the ischemic bowel compared with conventional CT by increasing attenuation differences between ischemic and perfused segments on low-kiloelectron volt and iodine material density images.

Creation of short microwave ablation zones: in vivo characterization of single and paired modified triaxial antennas

PURPOSE

To characterize modified triaxial microwave antennas configured to produce short ablation zones.

MATERIALS AND METHODS

Fifty single-antenna and 27 paired-antenna hepatic ablations were performed in domestic swine (N = 11) with 17-gauge gas-cooled modified triaxial antennas powered at 65 W from a 2.45-GHz generator. Single-antenna ablations were performed at 2 (n = 16), 5 (n = 21), and 10 (n = 13) minutes. Paired-antenna ablations were performed at 1-cm and 2-cm spacing for 5 (n = 7 and n = 8, respectively) and 10 minutes (n = 7 and n = 5, respectively). Mean transverse width, length, and aspect ratio of sectioned ablation zones were measured and compared.

RESULTS

For single antennas, mean ablation zone lengths were 2.9 cm ± 0.45, 3.5 cm ± 0.55, and 4.2 cm ± 0.40 at 2, 5, and 10 minutes, respectively. Mean widths were 1.8 cm ± 0.3, 2.0 cm ± 0.32, and 2.5 cm ± 0.25 at 2, 5, and 10 minutes, respectively. For paired antennas, mean length at 5 minutes with 1-cm and 2-cm spacing and 10 minutes with 1-cm and 2-cm spacing was 4.2 cm ± 0.9, 4.9 cm ± 1.0, 4.8 cm ± 0.5, and 4.8 cm ± 1.3, respectively. Mean width was 3.1 cm ± 1.0, 4.4 cm ± 0.7, 3.8 cm ± 0.4, and 4.5 cm ± 0.7, respectively. Paired-antenna ablations were more spherical (aspect ratios, 0.72-0.79 for 5-10 min) than single-antenna ablations (aspect ratios, 0.57-0.59). For paired-antenna ablations, 1-cm spacing appeared optimal, with improved circularity and decreased clefting compared with 2-cm spacing (circularity, 0.85 at 1 cm, 0.78 at 2 cm).

CONCLUSIONS

Modified triaxial antennas can generate relatively short, spherical ablation zones. Paired-antenna ablations were rounder and larger in transverse dimension than single antenna ablations, with 1-cm spacing optimal for confluence of the ablation zone.

Temperature isotherms during pulmonary cryoablation and their correlation with the zone of ablation

PURPOSE

To determine the expected ablation zone size and associated isotherms when using clinically available percutaneous cryoprobes for pulmonary cryoablation in a porcine lung model.

MATERIALS AND METHODS

Seven ablations were performed in the lungs of three adult pigs using clinically available 2.4-mm cryoprobes (Endocare, Inc, Irvine, California) and a 10-minute double-freeze protocol. Five 18-gauge thermocouples were positioned at 5-mm increments (ie, 5, 10, 15, 20, and 25 mm) from the cryoprobe. Real-time tissue temperatures were recorded during the cryoablation. The isotherms obtained during the ablation and the pathological ablation zones were measured.

RESULTS

The pathologic zone of complete necrosis had a mean diameter of 2.4 + or - 0.2 cm, with a mean area of 4.6 + or - 0.6 cm(2) and a circularity of 0.95 + or - 0.04. In comparison, the mean diameter (+ or - standard deviation) of the 0 degrees C, -20 degrees C, and -40 degrees C isotherms were 3.1 + or - 0.2 cm, 2.3 + or - 0.3 cm, and 1.8 + or - 0.4 cm, respectively. The -20 degrees C isotherm was most closely related to the pathologic zone of ablation.

CONCLUSIONS

This study establishes the temperature isotherms and associated ablation zone size that can be expected with modern percutaneous cryoprobes in an in vivo porcine lung model.

Pulmonary thermal ablation: comparison of radiofrequency and microwave devices by using gross pathologic and CT findings in a swine model

PURPOSE

To compare the performance of equivalently sized radiofrequency and microwave ablation applicators in a normal porcine lung model.

MATERIALS AND METHODS

All experiments were approved by an institutional animal care and use committee. A total of 18 ablations were performed in vivo in normal porcine lungs. By using computed tomographic (CT) fluoroscopic guidance, a 17-gauge cooled triaxial microwave antenna (n = 9) and a 17-gauge cooled radiofrequency (RF) electrode (n = 9) were placed percutaneously. Ablations were performed for 10 minutes by using either 125 W of microwave power or 200 W of RF power delivered with an impedance-based pulsing algorithm. CT images were acquired every minute during ablation to monitor growth. Animals were sacrificed after the procedure. Ablation zones were then excised and sectioned transverse to the applicator in 5-mm increments. Minimum and maximum diameter, cross-sectional area, length, and circularity were measured from gross specimens and CT images. Comparisons of each measurement were performed by using a mixed-effects model; P < .05 was considered to indicate a significant difference.

RESULTS

Mean diameter (3.32 cm +/- 0.19 [standard deviation] vs 2.70 cm +/- 0.23, P < .001) was 25% larger with microwave ablation and mean cross-sectional area (8.25 cm(2) +/- 0.92 vs 5.45 cm(2) +/- 1.14, P < .001) was 50% larger with microwave ablation, compared with RF ablation. With microwave ablation, the zones of ablation were also significantly more circular in cross section (mean circularity, 0.90 +/- 0.06 vs 0.82 +/- 0.09; P < .05). One small pneumothorax was noted during RF ablation but stabilized without intervention.

CONCLUSION

Microwave ablation with a 17-gauge high-power triaxial antenna creates larger and more circular zones of ablation than does a similarly sized RF applicator in a preclinical animal model. Microwave ablation may be a more effective treatment of lung tumors.

Radiofrequency ablation: simultaneous application of multiple electrodes via switching creates larger, more confluent ablations than sequential application in a large animal model

PURPOSE

To compare radiofrequency (RF) ablations created by using a sequential technique to those created simultaneously by using a switching algorithm in ex vivo and in vivo liver models.

MATERIALS AND METHODS

RF ablation was performed by using either sequential or switched application of three cooled electrodes in a 2-cm triangular array in ex vivo bovine liver (28 total ablations) and in vivo swine liver (12 total ablations) models. For sequential ablations, electrodes were powered for 12 minutes each with a 5-minute rest interval between activations to simulate electrode repositioning. Switched ablations were created by using a multiple-electrode switching system for 12 minutes. Temperatures were measured during ex vivo experiments at four points in the ablation zone. Ablation zones were measured for minimum and maximum diameter, cross-sectional area, and isoperimetric ratio. Mann-Whitney and Wilcoxon matched pairs tests were used to identify differences between groups.

RESULTS

The switched application created larger and more circular zones of ablation than did the sequential application, with mean (+/-standard deviation) ex vivo cross-sectional areas of 25.4 cm(2) +/- 5 .3 and 18.8 cm(2) +/- 6.6 (P = .001), respectively, and mean in vivo areas of 17.1 cm(2) +/- 5.1 and 13.2 cm(2) +/- 4.2 (P < .05). Higher temperatures and more rapid heating occurred with the switched application; switched treatments were 74% faster than sequential treatments (12 vs 46 minutes). In the sequential group, subsequent ablations grew progressively larger due to local ischemia.

CONCLUSIONS

Switched application of three electrodes creates larger, more confluent ablations in less time than sequential application. Thermal synergy and ablation-induced ischemia both substantially influence multiple-electrode ablations.

Multiple-electrode radiofrequency ablation: comparison with a conventional cluster electrode in an in vivo porcine kidney model

PURPOSE

To compare multiple-electrode radiofrequency (RF) ablation versus RF ablation with a cluster electrode in an in vivo porcine kidney model.

MATERIALS AND METHODS

Thirteen female pigs (mean weight, 45 kg) were used for the study. In each animal, RF ablations were performed for 12 minutes with a conventional cluster electrode in one kidney (controls, n = 13) and a multiple-electrode configuration in the contralateral organ. Multiple-electrode ablations were performed with electrodes 1.5 cm apart (group 1, n = 7) or 2.0 cm apart (group 2, n = 6). The mean maximum temperature at the electrode tips was determined. After each animal was euthanized, the kidneys were removed and the ablation zones were sectioned into 5-mm transverse slices. A representative slice was stained with 2,3,5-triphenyl-2H-tetrazolium chloride. Standard ablation zone metrics were measured and differences between groups were analyzed for statistical significance.

RESULTS

The mean maximum ablation zone diameter was 3.0 cm +/- 0.6 (SD) for controls, compared with 5.0 cm +/- 0.5 for group 1 (P < .0001) and 4.4 cm +/- 1.0 for group 2 (P = .002). Mean ablation zone minimum diameter was higher for group 1 (P = .002) and group 2 (P = .03) than for controls. Isoperimetric ratios were lowest for group 2 (P = .04 vs controls) whereas the highest temperatures at the electrode tips were observed with group 1 (P = .02 vs controls).

CONCLUSION

In normal porcine kidney, multiple-electrode RF ablation produced larger zones of ablation than a cluster electrode. Efficacy was greater when electrodes were spaced 1.5 cm apart than when they were spaced 2.0 cm apart.

Microwave ablation with triaxial antennas tuned for lung: results in an in vivo porcine model

PURPOSE

To prospectively determine in swine the size and shape of coagulation zones created in normal lung tissue by using small-diameter triaxial microwave antennas and to prospectively quantify the effects of bronchial occlusion and multiple antennas on the coagulation zone.

MATERIALS AND METHODS

The study was approved by the research animal care and use committee, and all husbandry and experimental studies were compliant with the National Research Council's Guide for the Care and Use of Laboratory Animals. Twenty-four coagulation zones (three per animal) were created at thoracotomy in eight female domestic swine (mean weight, 55 kg) by using a microwave ablation system with 17-gauge lung-tuned triaxial antennas. Ablations were performed for 10 minutes each by using (a) a single antenna, (b) a single antenna with bronchial occlusion, and (c) an array of three antennas powered simultaneously. The animals were sacrificed immediately after ablation. The coagulation zones were excised en bloc and sectioned into approximately 4-mm slices for measurement of size, shape, and circularity. Analysis of variance and two-sample t tests were used to identify differences between the three ablation groups.

RESULTS

The overall mean diameters of coagulation achieved with a single antenna and bronchial occlusion (4.11 cm +/- 1.09 [standard deviation]) and with multiple-antenna arrays (4.05 cm +/- 0.69) were significantly greater than the overall mean diameter achieved with a single antenna alone (3.09 cm +/- 0.83) (P = .016 for comparison with multiple antennas, P = .032 for comparison with bronchial occlusion). No significant differences in size were seen between the coagulation zones created with bronchial occlusion and those created with multiple antennas (P = .68). The coagulation zones in all groups were very circular (isoperimetric ratio > 0.80) at cross-sectional analysis.

CONCLUSION

A 17-gauge triaxial microwave ablation system tuned for lung tissue yielded large circular zones of coagulation in vivo in porcine lungs. The coagulation zones created with bronchial occlusion and multiple antennas were significantly larger than those created with one antenna.

Radiofrequency ablation with a high-power generator: device efficacy in an in vivo porcine liver model

PURPOSE

The purpose of this study was to test the feasibility and efficacy of using a high-power generator with nondeployable electrodes to create large zones of coagulation in an in vivo porcine liver model.

METHODS

With approval from our institution's research animal care and use committee, 12 female swine (mean weight = 55 kg) were anesthetized and received RF ablation at laparotomy. Twenty-nine ablations were performed in four groups using: (i) a conventional 200-W generator and cluster electrode (n = 4), or an experimental prototype 250-W generator and (ii) a single, 17-gauge electrode (n = 9), (iii) a cluster electrode (n = 8) or (iv) three electrodes spaced 2.0 cm apart in a triangular configuration (n = 8). In the three-electrode group, power was applied by switching between electrodes using a prototype switching device. All electrodes were internally cooled. Ablation zone size, shape and generator data from each group were compared using a mixed-linear model with animals modeled as random effects.

RESULTS

The high-power generator was able to increase significantly the zone of coagulation. Mean (+/-SD) ablation diameter was largest in the switched group (4.31 +/- 0.7 cm) followed by the cluster (3.98 +/- 0.5 cm) and single-electrode (3.26 +/- 0.5 cm) groups. Mean diameter in the high-power single-electrode group was no different than the low-power cluster group (3.25 +/- 0.4 cm, p = 0.98). Circularity measures were high (>0.75) in all groups.

CONCLUSIONS

Coupling a high-power generator and switching device is feasible. At higher powers, the switching device creates larger zones of ablation than cluster or single electrodes. Single-electrode ablations created with the prototype high-power generator were equivalent to those produced with the cluster electrode at conventional lower powers.

Microwave Ablation with Multiple Simultaneously Powered Small-gauge Triaxial Antennas: Results from an in Vivo Swine Liver Model

PURPOSE

To prospectively investigate the ability of a single generator to power multiple small-diameter antennas and create large zones of ablation in an in vivo swine liver model.

MATERIALS AND METHODS

Thirteen female domestic swine (mean weight, 70 kg) were used for the study as approved by the animal care and use committee. A single generator was used to simultaneously power three triaxial antennas at 55 W per antenna for 10 minutes in three groups: a control group where antennas were spaced to eliminate ablation zone overlap (n=6; 18 individual zones of ablation) and experimental groups where antennas were spaced 2.5 cm (n=7) or 3.0 cm (n=5) apart. Animals were euthanized after ablation, and ablation zones were sectioned and measured. A mixed linear model was used to test for differences in size and circularity among groups.

RESULTS

Mean (+/-standard deviation) cross-sectional areas of multiple-antenna zones of ablation at 2.5- and 3.0-cm spacing (26.6 cm(2) +/- 9.7 and 32.2 cm(2) +/- 8.1, respectively) were significantly larger than individual ablation zones created with single antennas (6.76 cm(2) +/- 2.8, P<.001) and were 31% (2.5-cm spacing group: multiple antenna mean area, 26.6 cm(2); 3 x single antenna mean area, 20.28 cm(2)) to 59% (3.0-cm spacing group: multiple antenna mean area, 32.2 cm(2); 3 x single antenna mean area, 20.28 cm(2)) larger than 3 times the mean area of the single-antenna zones. Zones of ablation were found to be very circular, and vessels as large as 1.1 cm were completely coagulated with multiple antennas.

CONCLUSION

A single generator may effectively deliver microwave power to multiple antennas. Large volumes of tissue may be ablated and large vessels coagulated with multiple-antenna ablation in the same time as single-antenna ablation.

Unintended thermal injuries from radiofrequency ablation: protection with 5% dextrose in water

OBJECTIVE

Radiofrequency ablation of hepatic tumors can lead to thermal injury of surrounding structures. Both saline and 5% dextrose in water (D5) have been used to displace these surrounding structures before radiofrequency ablation. The purpose of this study was to determine the relative effectiveness of these two fluids for protecting the diaphragm and lung during radiofrequency ablation.

MATERIALS AND METHODS

Ten female domestic swine (mean weight, 45 kg) underwent radiofrequency ablation at open surgery. Group 1 (n = 12 lesions) was pretreated with peritoneal D5 before radiofrequency ablation. Group 2 (n = 11 lesions) was pretreated with peritoneal 0.9% saline. A 2.7-mm spacer was placed between the liver surface and diaphragm in groups 1 and 2. Group 3 (n = seven lesions) served as a control group with no pretreatment regimen. Group 4, an additional control group (n = eight lesions), consisted of animals pretreated with D5 in which a larger spacer was used. After radiofrequency ablation, the animals were sacrificed and the liver, diaphragm, and lung were removed. The extent of thermal injury to the surface of each organ was recorded.

RESULTS

The animals in the D5 and saline pretreatment groups experienced fewer diaphragm injuries than the control animals (D5, p = 0.02). The smallest lesions in the lung and diaphragm were in the D5 group, followed by the saline and control groups (diaphragm, p = 0.0001; lung, p = 0.13). Diaphragm lesions were significantly smaller in the D5 and saline groups than in the control group (p = 0.0001 and 0.01, respectively).

CONCLUSION

Instillation of D5 into the peritoneal cavity before hepatic radiofrequency ablation decreases the risk and severity of diaphragm and lung injuries compared with no pretreatment or pretreatment with 0.9% saline in this animal model. Pretreatment with D5 may increase both the safety of and the number of patients eligible for treatment with thermal therapies.

Multiple-Electrode Radiofrequency Ablation of Hepatic Malignancies: Initial Clinical Experience

OBJECTIVE

The objective of our study was to retrospectively analyze our initial clinical experience with percutaneous multiple-electrode radiofrequency ablation and evaluate its safety and efficacy for treating hepatic malignancies.

MATERIALS AND METHODS

Thirty-eight malignant hepatic tumors (mean diameter, 2.7 cm; range, 0.7-10.0 cm) in 23 patients (12 men and 11 women; mean age, 65 years; range, 40-84 years) were treated in 26 radiofrequency ablation sessions with an impedance-based multiple-electrode system. One, two, or three (mean, 2.4) 17-gauge electrodes were placed, and tumors were ablated using a combination of CT and sonography for guidance and monitoring. Electrodes were placed in close proximity (mean spacing: two electrodes, 1.0 cm; three electrodes, 1.4 cm) to treat large tumors or were used independently to treat several tumors simultaneously. Contrast-enhanced CT scans were obtained immediately after ablation to determine technical success and evaluate for complications. Follow-up CT scans at 1, 3, 6, 9, and 12 months (mean, 4 months) after ablation were obtained to assess for tumor progression and new metastases.

RESULTS

Local control was achieved in 37 of 38 tumors, 34 of which were treated in one session. Ablations created with closely spaced electrodes had a mean diameter of 4.9 cm. The total ablation time was reduced by approximately 54% compared with an equivalent number of ablations performed with a single-electrode system (1,014 vs 2,196 minutes). Three complications occurred: one death from a presumed postprocedure pulmonary embolus, one pneumothorax, and one asymptomatic perihepatic hemorrhage.

CONCLUSION

Multiple-electrode radiofrequency ablation appears to be a safe and effective means of achieving local control in large or multiple hepatic malignancies at short-term follow-up.

Microwave ablation with a single small-gauge triaxial antenna: in vivo porcine liver model

PURPOSE

To evaluate the performance of a 17-gauge triaxial antenna at microwave ablation in an in vivo porcine liver model.

MATERIALS AND METHODS

This study was approved by the institutional animal care and use committee. Thirteen female domestic pigs (mean weight, 45 kg) were used. Ablations were performed with a prototype microwave ablation system and triaxial antenna by using a constant, continuous-wave power of 68 W for 2 (n = 6), 3 (n = 6), 4 (n = 6), 5 (n = 6), 6 (n = 6), 7 (n = 13), 10 (n = 7), and 12 (n = 8) minutes. Animals were euthanized after ablation, livers were removed, and ablation zones were sliced and measured for size and roundness. A mixed linear model with animals modeled as random effects was used to test for significant differences in ablation zone metrics among time groups; post hoc tests were used to detect significant differences between time groups.

RESULTS

Mean ablation zone diameters ranged from 2.05 cm +/- 0.23 (standard deviation) at 2 minutes to 2.59 cm +/- 0.53 at 12 minutes. Thirteen (32%) of 40 ablation zones with mean maximum diameters greater than 3.0 cm were observed at the 5-12-minute time groups. No significant differences in ablation zone diameter were observed among all groups (P > .05), but a trend of increasing diameter with time was noted. Mean isoperimetric ratio (a measure of roundness) for all ablation zones was 0.88 +/- 0.02, which indicates minimal heat sinking near vessels.

CONCLUSION

The triaxial microwave ablation system is capable of creating relatively large, circular zones of ablation in minutes with minimal effects from local blood flow.

Multiple-electrode radiofrequency ablation creates confluent areas of necrosis: in vivo porcine liver results

PURPOSE

To prospectively evaluate, in vivo in pigs, an impedance-based multiple-electrode radiofrequency (RF) ablation system for creation of confluent areas of hepatic coagulation.

MATERIALS AND METHODS

The study was preapproved by the institutional research animal care and use committee. A prototype multiple-electrode RF system that enables switching between three electrically independent electrodes at impedance spikes was created. Forty-two coagulation zones (18 with single, 12 with cluster, and 12 with multiple [three single electrodes spaced 2 cm apart] electrodes) were created at laparotomy in 15 female pigs. Half the ablations were performed for 12 minutes, and half were performed for 16 minutes. The coagulation zones were excised and sliced into approximately 3-mm sections for measurement. Analysis of variance and two-sample t tests (with Bonferroni correction, alpha = .0033) were used to assess for differences between groups.

RESULTS

At 12 minutes, the mean multiple-electrode coagulation was significantly larger than the mean single-electrode coagulation (minimum diameter, 2.8 vs 1.6 cm; maximum diameter, 4.2 vs 2.0 cm; volume, 22.1 vs 6.7 cm(3); P < .0033 for all comparisons). The mean maximum diameter achieved at 12 minutes with multiple electrodes was significantly larger than that achieved with the cluster electrode (4.2 vs 2.9 cm, P = .02). At 16 minutes, the mean multiple-electrode coagulation (minimum diameter, 3.2 cm; maximum diameter, 4.2 cm; volume, 29.1 cm(3)) was significantly larger than the mean single-electrode (minimum diameter, 1.7 cm; maximum diameter, 2.2 cm; volume, 7.1 cm(3); P < .0033 for all comparisons) and cluster-electrode (minimum diameter: 2.3 cm, P = .007; maximum diameter: 3.2 cm, P = .005; volume: 13.1 cm(3), P = .001) coagulations.

CONCLUSION

Compared with the single and cluster systems used as controls, the multiple-electrode RF ablation system enabled the creation of significantly larger coagulation zones.

Multiple-electrode radiofrequency ablation: simultaneous production of separate zones of coagulation in an in vivo porcine liver model

PURPOSE

A multiple-electrode radiofrequency (RF) system was developed based on switching between electrodes that allows for the simultaneous use of as many as three electrically independent electrodes. The purpose of this study was to determine if each multiple-electrode ablation zone is identical to an ablation zone created with conventional single-electrode mode.

MATERIALS AND METHODS

Nine female domestic pigs (mean weight, 90 kg) were used for this study. A prototype monopolar multiple-electrode RF ablation system was created with use of an RF generator and an electronic switching algorithm. A maximum of three electrodes can be used simultaneously by switching between electrodes at each impedance spike (30 omega greater than baseline levels). A total of 39 zones of ablation were created at open laparotomy in pig livers with use of a conventional single electrode (n = 9), two single electrodes simultaneously (n = 6 ablations; 12 ablation zones), or three single electrodes simultaneously (n = 6 ablations; 18 ablation zones). RF electrodes were spaced in separate lobes of the liver when multiple zones of coagulation were created simultaneously. Animals were euthanized after RF ablation, livers were removed, and ablation zones were sectioned and measured.

RESULTS

Zones of coagulation created simultaneously with two or three electrodes were equivalent to ablation zones created with use of conventional single-electrode ablation. No significant differences were observed among control animals treated with a single electrode, those with two separate zones of ablation created simultaneously, and those with three simultaneously created ablation zones in terms of mean (+/-SD) minimum diameter (1.6 cm +/- 0.6, 1.6 cm +/- 0.5, and 1.7 cm +/- 0.4, respectively), maximum diameter (2.0 cm +/- 0.5, 2.3 cm +/- 0.5, 2.2 cm +/- 0.5, respectively), and volume (6.7 cm3 +/- 3.7, 7.4 cm3 +/- 3.8, and 7.8 cm3 +/- 3.9; P > .30, analysis of variance, pairwise t-test comparisons).

CONCLUSIONS

A rapid-switching multiple-electrode RF system was able to simultaneously create as many as three separate ablation zones of equivalent size compared with single-electrode controls. This system would allow physicians to simultaneously treat multiple tumors, substantially reducing procedure time and anesthesia risk.

Hepatic hemorrhage caused by percutaneous tumor ablation: radiofrequency ablation versus cryoablation in a porcine model

PURPOSE

To determine the extent of hepatic hemorrhage caused by percutaneous cryoablation performed with a small-diameter cryoablation probe compared with that caused by percutaneous radiofrequency (RF) ablation in a porcine model.

MATERIALS AND METHODS

The study was pre-approved by the institutional research animal care and use committee, and husbandry and experiments complied with National Institutes of Health standards for care and use of laboratory animals. Percutaneous hepatic ablation was performed in 18 domestic pigs (mean weight, 45 kg) by using a 17-gauge (1.5-mm-diameter) RF electrode (n = 6), a cluster of three RF electrodes (n = 6), or a 13-gauge (2.4 mm-diameter) cryoprobe (n = 6). Ablation was performed in four sites per liver. Total blood loss, minimum lesion diameter, maximum lesion diameter, and lesion volume were determined for each group and compared by using analysis of variance.

RESULTS

Mean blood loss was 11.11 mL +/- 11.47 (standard deviation), 105.29 mL +/- 175.58, and 28.06 mL +/- 30.97 with the single RF electrode, RF electrode cluster, and cryoablation probe, respectively. Mean minimum and maximum lesion diameters were largest with the RF electrode cluster (2.40 and 3.98 cm, respectively), followed by the cryoablation probe (2.38 and 3.94 cm) and single RF electrode (1.49 and 2.63 cm). Mean minimum and maximum lesion diameters were significantly different between the single RF electrode and the RF electrode cluster, as well as between the single RF electrode and the cryoablation probe (P < .001). Mean lesion volume was largest for the RF electrode cluster (24.03 cm3), followed by those for the cryoablation probe (17.46 cm3) and single RF electrode (9.05 cm3) (single RF electrode vs cryoablation probe, P < .05). Lesion volumes were not significantly different with the RF electrode cluster versus the single RF electrode (P = .052) or with the RF electrode cluster versus the cryoablation probe (P = .381).

CONCLUSION

Mean blood loss from percutaneous cryoablation in this model was between that for RF ablation with the single electrode and that for RF ablation with the electrode cluster.

Radiofrequency versus microwave ablation in a hepatic porcine model

PURPOSE

To compare microwave (MW) and radiofrequency (RF) ablation in a hepatic porcine model.

MATERIALS AND METHODS

Institutional animal research committee approval was obtained. Nineteen pigs were divided into groups based on time of sacrifice (group A, immediate; group B, 2 days; group C, 28 days; group D, 28 days). Groups A, B, and C each underwent a combination of RF and MW ablation. Group D underwent either four MW or four RF ablations. Ablation was performed with a prototype MW device (915 Mhz, 40 W, 10 minutes) and a commercial RF system (150 W, 10 minutes, 3-cm deployment). Computed tomography (CT) was performed in groups B and C at 2 days and in group C at 28 days. Group D underwent serial laboratory testing. Specimens were serially sectioned, and short-axis diameter and length of each were measured. The percentage deflection caused by local blood vessels (heat-sink effect) was also measured in group A. Likelihood ratio tests and unpaired t tests were used for statistical analyses as appropriate.

RESULTS

MW ablation zones were longer at days 0, 2, and 28 (P < .05), but short-axis diameter was not different from that with RF ablation at any time point (P > .05). Local blood vessels caused 3.5% +/- 5.3 (standard deviation) deflection at MW ablation compared with 26.2% +/- 27.9 at RF ablation (P < .05). MW and RF ablation zones were indistinguishable at CT or pathologic evaluation. Laboratory test results were similar between RF ablation-only animals and MW ablation-only animals, with the exception of a slightly higher alkaline phosphatase levels at day 2 in RF ablation-only animals (P < .02).

CONCLUSION

MW and RF ablation zones are similar in pathologic appearance and imaging characteristics. Increased length with MW ablation is likely caused by the length of the radiating segment of the antenna. MW ablation may be less affected by the heat-sink effect that is thought to contribute to local recurrence after RF ablation.

Large-volume radiofrequency ablation of ex vivo bovine liver with multiple cooled cluster electrodes

Three methods of creating large thermal lesions with cool-tip cluster electrodes were compared. Three cluster electrodes were arranged 4 cm apart in a triangular array. Eight lesions were created ex vivo in fresh bovine liver (from a butcher) with each method: sequential ablation (three electrodes, 12 minutes each); simultaneous activation of electrodes (12 minutes); and rapid switching of power between electrodes (12 minutes), for which an electronic computer-controlled switch was developed. For sequential, rapid switching, and simultaneous methods, lesion volumes were 137.5 cm(3)+/- 22.2, 116.4 cm(3)+/- 15.2, and 22.3 cm(3)+/- 6.4 (P < .05), respectively, and final temperatures at lesion center were 80 degrees C +/- 5, 97 degrees C +/- 8, and 41 degrees C +/- 3 (P < .001), respectively. Because of electrical interference between electrodes, simultaneous method led to little heating at the center between the electrodes and created small discontinuous lesions. Rapid switching created large round lesions by employing multiple electrodes concurrently, which substantially reduced treatment time and resulted in more effective heating between electrodes.

Microwave Ablation with Loop Antenna: In Vivo Porcine Liver Model

PURPOSE

To determine the effectiveness of tissue ablation with a loop microwave antenna in various configurations in porcine liver tissue.

MATERIALS AND METHODS

Microwave energy was applied for 7 minutes at 60 W in six porcine livers (mean weight, 68.2 kg) by using single (n = 7) or dual 2.7-cm loop microwave probes in parallel (n = 9) or orthogonal (n = 9) configurations. Volume, diameter, shape, and temperature of the zone of necrosis and the presence of viable tissue inside the loop were determined and compared by means of factorial analysis of variance.

RESULTS

Mean lesion volume and maximum diameter, respectively, were 32.2 cm(3) +/- 14.4 (SD) and 4.6 cm +/- 1.4 for lesions ablated with parallel probes (parallel lesions), 29.5 cm(3) +/- 8.1 and 4.3 cm +/- 0.6 for lesions ablated with orthogonal probes (orthogonal lesions), and 6.4 cm(3) +/- 1.9 and 3.4 cm +/- 0.62 for lesions ablated with single probes (single lesions) (P <.05, single vs parallel and orthogonal lesions). Mean minimum diameter was greatest for orthogonal lesions (3.5 cm +/- 0.53; P =.017, parallel vs orthogonal lesions). Orthogonal lesions had the highest mean internal temperature (97.2 degrees C) versus parallel (91.9 degrees C) and single (60.0 degrees C) lesions. All orthogonal lesions heated to 60 degrees C in comparison to eight of nine parallel and four of seven single lesions. The mean time to reach 60 degrees C was shortest for orthogonal lesions (93.3 seconds) versus parallel (123.8 seconds) and single (263.0 seconds) lesions. Orthogonal lesions were the most spherical. Viable tissue was present in the center of five of seven single, six of nine parallel, and zero of nine orthogonal lesions.

CONCLUSION

Loop microwave antennas allow precise control and effective ablation of targeted tissue, particularly in the orthogonal configuration.

Multiple probe radiofrequency ablation: pilot study in an animal model

PURPOSE

Radiofrequency ablation (RFA) is becoming increasingly popular for the minimally invasive treatment of benign and malignant tumors. Currently available systems are limited to the use of a single probe because of electrical interactions between probes. The purpose of this study was to test a new prototype multiple probe generator with a built-in switching mechanism to determine if multiple zones of necrosis could be formed simultaneously without a significant penalty in terms of lesion size and procedure time.

MATERIALS AND METHODS

A dual probe generator was created by modifying a commercially available system into an alternating monopolar system with an external electronic switch controlled by a temperature feedback loop. A total of 20 radiofrequency (RF) lesions (conventional single probe, n = 10; switched dual probe, n = 10) were created in the livers of six adult pigs (temperature, 100 degrees C; 10-minute ablation). Lesions were excised and examined for volume, minimum diameter, and maximum diameter.

RESULTS

The time to target temperature was slightly greater for dual (3.5 minutes) versus single ablations (2.7 minutes). However, this resulted in only a 48 second (6.5%) longer total ablation time. There was no significant difference between conventional single and dual lesions for lesion volume (13.6 +/- 9.3 cm(3) versus 13.7 +/- 7.0 cm(3); P >.05), minimum diameter (1.63 +/- 0.56 cm(3) versus 1.61 +/- 0.53; P >.05) or maximum diameter (3.3 +/- 0.84 versus 3.4 +/- 0.55, P >.05).

CONCLUSION

A multiple probe RFA system that can simultaneously ablate multiple areas in the liver is feasible. If multiple probe units become clinically available, large or irregularly shaped lesions could be treated more effectively than with conventional single probe units, and multiple tumors could be ablated simultaneously, thus potentially decreasing procedure time and anesthetic complications.

The relation between dietary intake and adipose tissue composition of selected fatty acids in US women

We compared fatty acid amounts in adipose tissue with fatty acid intake calculated from 2 separate weeks of diet recording and two food-frequency questionnaires for 140 participants in the Nurses' Health Study. Our results showed that the amounts of polyunsaturated and trans fatty acids in adipose tissue reflect dietary intake and confirm those of previous studies. The correlation between the polyunsaturated fatty acid content of adipose tissue and polyunsaturated fatty acid intake from the average of the two food-frequency questionnaires was 0.40; this correlation for trans fatty acids was also 0.40. Linolenic acid amounts in adipose tissue were also moderately correlated with intake from the average of the food-frequency questionnaires (r = 0.34). An estimate of trans fatty acid intake from vegetable sources correlated much more strongly with adipose trans fatty acids than did an estimate of trans fatty acids from animal sources. Adipose tissue aspirates can be used to indicate intake of exogenous fatty acids.

Evaluation of a food frequency questionnaire-food composition approach for estimating dietary intake of inorganic arsenic and methylmercury

Inorganic arsenic intake in 969 men and women and methylmercury intake in 785 men and women from across the United States were assessed by a semiquantitative food frequency questionnaire, in combination with a database for the content of those elements in foods, and by toenail concentrations of arsenic and mercury. In addition, empirical weights for foods on the dietary questionnaire were derived from multivariate regression models to estimate associations between diet and toenail arsenic and mercury levels, independent of the assumptions about inorganic arsenic and methylmercury in foods, which are based upon limited residue measurements. The use of empirical weights significantly improved the correlation of arsenic consumption with toenail arsenic levels (r = 0.33, P = 0.0001), compared with the weak correlation obtained using the food residue method to calculate intake (r = 0.15, P = 0.0001). Mercury consumption computed using empirical weights yielded a significant correlation with toenail arsenic (r = 0.42, P = 0.001), similar to the correlation using energy-adjusted intake calculated from food residue tables (r = 0.35, P = 0.001). These results illustrate the potential use of empirically derived weights for foods in estimating toenail levels of selected heavy metals and support the validity of published food residue data that are used to estimate mercury consumption.

Intake of trans fatty acids and risk of coronary heart disease among women

Trans isomers of fatty acids, formed by the partial hydrogenation of vegetable oils to produce margarine and vegetable shortening, increase the ratio of plasma low-density-lipoprotein to high-density-lipoprotein cholesterol, so it is possible that they adversely influence risk of coronary heart disease (CHD). To investigate this possibility, we studied dietary data from participants in the Nurses' Health Study. We calculated intake of trans fatty acids from dietary questionnaires completed by 85,095 women without diagnosed CHD, stroke, diabetes, or hypercholesterolaemia in 1980. During 8 years of follow-up, there were 431 cases of new CHD (non-fatal myocardial infarction or death from CHD). After adjustment for age and total energy intake, intake of trans isomers was directly related to risk of CHD (relative risk for highest vs lowest quintile 1.50 [95% Cl 1.12-2.00], p for trend = 0.001). Additional control for established CHD risk factors, multivitamin use, and intakes of saturated fat, monounsaturated fat, and linoleic acid, dietary cholesterol, vitamins E or C, carotene, or fibre did not change the relative risk substantially. The association was stronger for the 69,181 women whose margarine consumption over the previous 10 years had been stable (1.67 [1.05-2.66], p for trend = 0.002). Intakes of foods that are major sources of trans isomers (margarine, cookies [biscuits], cake, and white bread) were each significantly associated with higher risks of CHD. These findings support the hypothesis that consumption of partially hydrogenated vegetable oils may contribute to occurrence of CHD.

Vitamin A supplementation and plasma retinol levels: a randomized trial among women

Although dietary intake of vitamin A has little, if any, overall effect on blood retinol in generally well-nourished populations, subgroups may exist that would be responsive to supplementation. The hypothesis that vitamin A supplementation increases blood retinol in apparently well-fed individuals with lower than usual blood levels was tested in female health workers, with relatively low blood retinol values, who were randomly assigned to receive vitamin A (10,000 IU daily) or placebo. After 4 weeks the mean change in plasma retinol was -0.4 micrograms/dl for the group receiving placebo and +4.1 micrograms/dl (an increase of 9% over base-line values) for the group receiving vitamin A (P = .02). The results were similar when the base-line retinol level and several other covariates were considered. Thirteen women who had initially received placebo were then switched to vitamin A for 4 weeks. These women experienced a mean increase of 5.3 micrograms/dl in plasma retinol (P = .04). Responses to vitamin A supplementation tend to be greater among women with lower previous total vitamin A intake, as assessed by questionnaire [Spearman rank correlation coefficient (r) = 0.50; P = .01].