Ablation Zone Dimensions Research Articles

Patient-Specific Prediction of Immediate Phase Lung Microwave Ablation Zone Size

PurposeTo investigate the effect of patient and tumor-specific characteristics on the size of immediate phase lung microwave ablation (MWA) zone and establish a prediction model. Materials and MethodsThis institutional review board (IRB)–approved, Health Insurance Portability and Accountability Act (HIPAA)–compliant cohort included 164 lesions from 99 patients who underwent computed tomography (CT)–guided lung MWA, and the 2-dimensional elliptical ground-glass opacity ablation zone was measured. Duration, maximum temperature, tumor depth, presence of emphysema, history of ipsilateral lung ablation, surgery, and radiotherapy were recorded. K-fold cross validation with k = 5 and Least Absolute Shrinkage and Selection Operator were used to build prediction models for the major and minor axes and area of the ablation zone. ResultsThe median of immediate phase ablation duration was 2 minutes (interquartile range, 1.5–4.25 minutes) with 65 W of power for all ablations. The mean major and minor axes and area of ablation zone were 3.1 cm (SD ± 0.6), 2.0 cm (SD ± 0.5), and 5.1 cm2 (SD ± 2.1), respectively. The major and minor axes and area of immediate phase ablation zone dimensions were significantly associated with duration (P < .001, P < .001, and P < .001, respectively), maximum temperature (P < .001, P < .001, and P < .001, respectively), tumor depth (P = .387, P < .001, and P < .001, respectively), history of ipsilateral lung ablation (P = .008, P = .286, and P = .076, respectively), and lung radiotherapy (P = .001, P = .042, and P = .015, respectively). The prediction model showed R2 values for major and minor axes and area of the ablation zone to be 0.50, 0.45, and 0.53, respectively. ConclusionsDuration of ablation, maximum temperature, tumor depth, history of ipsilateral lung ablation, surgery, and radiotherapy were significantly associated with the ablation zone dimensions and size and can be used to build the prediction model to approximate the immediate phase lung MWA zone.

Changes in Microwave Ablation Zone Dimensions after Transarterial Embolization in an Ex Vivo Human Kidney Perfusion Model

PurposeTo utilize a novel ex vivo perfused human renal model and quantify microwave ablation (MWA) size differences in renal tissue when combining MWA with transarterial embolization (TAE). Materials and MethodsHuman kidneys (n = 5) declined for transplantation were obtained and connected to a fluoroscopy-compatible ex vivo perfusion system. Two ablations—1 standard MWA and 1 TAE-MWA—were performed in each kidney for 2 minutes at 100 W using a MWA system (Solero Angiodynamics). MWA alone was performed in the upper pole. In the lower pole, MWA was performed after TAE with 40–90 μm radiopaque microspheres to achieve angiographic stasis. Ablation zones of coagulative necrosis were sectioned along the long axis and segmented for maximal short-axis diameter (SAD) and long-axis diameter (LAD) measurements. ResultsA total of 10 ablations (5 MWAs and 5 TAE-MWAs) were performed in 5 human kidneys. TAE-MWA resulted in significantly increased SAD, LAD, volume, and sphericity compared with standard MWA ± SD, with mean measurements as follows (5 standard MWAs ± SD vs 5 TAE-MWAs ± SD, 2-tailed t-test): (a) SAD, 1.8 cm (SD ± 0.1) versus 2.5 cm (SD ± 0.1) (P < .001); (b) LAD, 2.9 cm (SD ± 0.3) versus 3.2 cm (SD ± 0.1) (P = .039); (c) volume, 5.0 mL (SD ± 0.5) versus 11.0 mL (SD ± 0.7) (P < .001); and (d) sphericity, 0.4 (SD ± 0.2) versus 0.6 (SD ± 0.1) (P = .049). Histology demonstrated no differences in TAE-MWA other than concentrated microspheres. ConclusionsThis ex vivo human kidney perfusion model confirmed that combined MWA-TAE significantly increased ablation size and spherical shape compared with MWA alone.

The CellFX Percutaneous Electrode System for Nanosecond Pulsed Field Ablation.

The purpose of this study was to demonstrate the safety and performance of the CellFX Percutaneous Electrode for delivering nanosecond pulsed field ablation (nsPFA) energy to soft tissues. Three different porcine tissue types were treated, namely, liver, kidney, and skeletal muscle, at treatment levels of three times greater than clinical treatment levels. The histological characteristics of the ablation zone for each of these tissues compared with that of radiofrequency (RF) ablation on day 0 and 2 days post-treatment. Ablation zone dimensions were measured during gross necropsy after tetrazolium chloride staining and compared between the nsPFA and RF groups at 2 days post-ablation. The CellFX system successfully achieved ablation and necrosis of all treatment sites in all target tissues. No evidence of thermal effects or collagen degeneration was found at any of the nsPFA treatment sites. Overall systemic tolerability was evidenced by the absence of clinically significant changes in urinalysis and serum chemistry before and after treatments.

Discrepancy Between Achieved and Vendor-Predicted Ablation Zones in the Lung: Contributing Factors.

Several factors are known to affect lung ablation zones. Questions remain as to why there are discrepancies between achieved and vendor-predicted ablation zones and what contributing factors can be modified to balance therapeutic effects with avoidance of complications. This retrospective study of lung tumour microwave ablation analyses day 1 post-treatment CT to assess the effects of lesion-specific and operator-dependent factors on ablation zones. Consecutive patients treated at a tertiary centre from 2018 to 2021 were included. All ablations were performed using a single microwave ablation device under lung isolation. The lung tumours were categorised as primary or secondary, and their "resistance" to ablation was graded according to their locations. Intraprocedural pulmonary inflation was assessed as equal to or less than the contralateral non-isolated lung. Ablation energy was categorised as high, medium, or low. Ablation zone dimensions were measured on day 1 CT and compared to vendor reference charts. Ablations with multiple needle positions or indeterminate boundaries were excluded. A total of 47 lesions in 31 patients were analysed. Achieved long axes are longer than predicted by 5mm or 14% (p < 0.01) without overall short axis discrepancy. Secondary tumours (p = 0.020), low-resistance location (p < 0.01), good lung inflation (p < 0.01), low (p = 0.003) and medium (p = 0.038) total energy produce lengthened long axes by 4-6mm or 10-19%. High total energy results in shorter than predicated short axes by 6mm or 18% (p = 0.010). We identified several factors affecting ablation zone dimensions which may have implications for ablation planning and the avoidance of complications.

Cryoablation in the liver: how accurately does the iceball predict the ablation zone?

To evaluate the accuracy with which the iceball predicts the realized ablation zone in patients undergoing cryoablation of the liver. Continuous patients who underwent cryoablation of primary or secondary malignancies of the liver were retrospectively reviewed. Iceball and ablation zone dimensions on 1month follow up imaging were collected in three orientations, the long axis (LA), perpendicular transverse (PTR), and perpendicular craniocaudal (PCC). Factors which may predict differences in the measurements were evaluated with regression analysis. Oncologic outcomes were also collected. The mean size of the iceball was 5.5 ± 1.1cm, 3.9 ± 1.1cm, and 4.4 ± 1.4cm in the LA, PTR, and PCC orientations, respectively. The mean size of the one-month ablation cavity was 4.3 ± 1.3cm, 3 ± 1.1cm, and 3 ± 1.3cm in the LA, PTR, and PCC orientations, respectively. The iceball was significantly larger than the ablation zone in all orientations (p < 0.001). When comparing HCC and non-HCC patients the Kaplan-Meier analysis of TTLP, the Kaplan Meier curves deviated significantly (p = 0.015, HR 2.26 (95%CI 1.17-4.37)). When a similar analysis was performed looking at TTP again the curves diverged significantly (p = 0.002, HR 2.4 (95%CI 1.37-4.19)). The iceball seems to overestimate the realized ablation zone by about 1cm in all orientations during hepatic cryoablation.

YENİ BİR NİTİ ŞEKİL HAFIZALI ALAŞIM TABANLI HALKA ANTEN İLE 2.45 GHz' DE EX VIVO MİKRODALGA ABLASYON UYGULAMASI

Although the most preferred treatment methods in cancer treatment are still surgery and chemotherapy, microwave ablation, one of the minimally invasive thermal ablation techniques, is increasingly used in the clinic for patients who cannot afford the risks of these treatment methods. In this study, microwave ablation with a new NiTi ring antenna was performed on a freshly slaughtered beef liver as an Ex Vivo application. Design and optimization was carried out in the CST Microwave studio. Ex Vivo MWA application was carried out at 2.45 GHz, using 50 W microwave power for 5 minutes. The lowest width of the ablation zone formed along the x-axis was 14.58 mm, the highest width was 28.61 mm, the length of the ablation area along the y-axis was 58.032 mm, and the area of the ablation zone was approximately 5.44 cm2. These results show that the proposed NiTi ring antenna has the ability to achieve a sufficient thermal lesion in terms of ablation zone dimensions.

Evolution of a theranostic applicator for microwave ablation treatment

Abstract The purpose of this work is to further develop a novel dual-mode microwave applicator for diagnosis and thermal ablation treatment. The MR-compatible MW applicator enables differentiation of tumor tissue and healthy tissue through dielectric contrast measurements, optimizing the positioning of the applicator in the lesion. Due to the robust applicator design and the resulting permittivity tracking during ablation, even carbonized tissue can be detected. The use of operating frequencies between 5 and 10 GHz allow a noticeably lower power consumption for microwave ablation of only 20 W compared to commercially available applicators. Clinically relevant dimensions of ablation zones can be achieved and additionally monitored using MR imaging and thermometry.

Ultrasound-Guided Percutaneous Laser Ablation of the Thyroid Gland in a Swine Model: Comparison of Ablation Parameters and Ablation Zone Dimensions.

To compare laser ablation (LA) zone dimensions at two predetermined energy parameters to cover a theoretical 10 mm zone + 2 mm margin in a thyroid swine model. Approval of the Institutional Animal Care and Use Committee was obtained. After hydrodissection, an ultrasound-guided LA (Elesta Echolaser X4 with Orblaze technology, 1064nm) was performed in the periphery of the thyroid in 10 swine. Two cohorts were established to ablate a region of 10mm diameter with 2mm margin based on manufacturer's ex vivo data (n= 5 at 3W/1400J and n= 5 at 3W/1800J). The ablation zone was measured on contrast-enhanced computed tomography (CT)and compared to the pathological specimen. Euthanasia was performed 48 hours following ablation. All ablations in the 3W/1800J group achieved a diameter of 12 mm ± 1 mm in three dimensions. In the 3W/1400J group, 1 ablation reached 12 mm ± 1 mm in 2 dimensions and 4 ablations reached this size in one dimension. Maximum diameter was higher in the 3W/1800J compared to the 3W/1400J group, both on histology (1.46 cm ± 0.05 vs. 1.1 cm ± 0.0, p< 0.01) and CT (1.52 cm ± 0.04 vs. 1.18 cm ± 0.04, p< 0.01). Similar results were obtained regarding volumes, both on histology (1.12 mL ± 0.13 vs. 0.57 mL ± 0.06, p< 0.01) and CT (1.24 mL ± 0.13 vs. 0.59 mL ± 0.07, p< 0.01). Histology showed coagulation necrosis and correlated well with CT measurements. Optimal parameters to obtain a LA zone of 10 mm with 2 mm margin utilizing a single needleare 3W/1800J.

Pre-operative Assessment of Ablation Margins for Variable Blood Perfusion Metrics in a Magnetic Resonance Imaging Based Complex Breast Tumour Anatomy: Simulation Paradigms in Thermal Therapies

Background and objectivesImage-guided medical interventions facilitates precise visualization at treatment site. The conformal prediction for sparing healthy tissue fringes precisely in the vicinity of irregular tumour anatomy remains clinically challenging. Pre-clinical image-based computational modelling is imperative as it helps in enhancement of treatment quality, augmenting clinical-decision making, while planning, targeting, controlling, monitoring and assessing treatment response with an effective risk assessment before the onset of treatment in clinical settings. In this study, the influence of heat deposition rate (SAR), exposure duration, and variable blood perfusion metrics for a patient-specific breast tumour is quantified considering the tumour margins thereby suggesting need of geometrically accurate models. MethodsA three-dimensional realistic model mimicking dimensions of a female breast, comprising ~1.7 cm irregular tumour, was generated from patient specific two-dimensional DICOM format MRI images through image segmentation tools MIMICS 19.0® and 3-Matic 11.0® which is finally exported to COMSOL Multiphysics 5.2® as a volumetric mesh for finite element analysis. The Pennes bioheat transfer model and Arrhenius thermal damage model of cell-death are integrated to simulate a coupled biophysics problem. A comparative blood perfusion analysis is done to evaluate the response of tumour during heating considering thermal damage extent, including the tumour margins while sparing critical adjoining healthy tissues. ResultsThe evaluated thermal damage zones for 1 mm, 2 mm and 3 mm fringe heating region (beyond tumour boundary) reveals 0.09%, 0.21% and 0.34% thermal damage to the healthy tissue (which is <1%) and thus successful necrosis of the tumour. The iterative computational experiments suggests treatment margins < 5 mm are sufficient enough as heating beyond 3 mm fringe layer leads to higher damage surrounding the tumour approximately 1.5 times the tumour volume. Further, the heat-dosage requirements are 22% more for highly perfused tumour as compared to moderately perfused tumour with an approximate double time to ablate the whole tumour volume. ConclusionsDepending on the blood perfusion characteristics of a tumour, it is a trade-off between heat-dosage (SAR) and exposure/treatment duration to get desired thermal damage including the irregular tumour boundaries while taking into account, the margin of healthy tissue. The suggested patient-specific integrated multiphysics-model based on MRI-Images may be implemented for pre-treatment planning based on the tumour blood perfusion to evaluate the thermal ablation zone dimensions clinically and thereby avoiding the damage of off-target tissues. Thus, risks involving underestimation or overestimation of thermal coagulation zones may be minimised while preserving the surrounding normal breast parenchyma.

MR-Guided High-Power Microwave Ablation in Hepatic Malignancies: Initial Results in Clinical Routine

PurposeEvaluation of technique effectiveness, patient safety and ablation parameters of MR-guided microwave ablation in hepatic malignancies using an MR-conditional high-power microwave ablation system.Materials and MethodsInstitutional review board approval and informed patient consent were obtained. Patients who underwent MR-guided microwave ablation of hepatic malignancies in a 1.5T wide-bore scanner using a perfusion-cooled high-power microwave ablation system with a maximum generator power of 150 W were included. Ablation parameters comprising procedure durations, net ablation duration, applicator positions and ablation zone dimensions were recorded. Adverse events were classified according to the CIRSE classification system. Technique effectiveness was assessed after 1 month. Follow-up was conducted with contrast-enhanced MRI and ranged from 1 to 20 months (mean: 6.1 ± 5.4 months).ResultsTwenty-one consecutive patients (age: 63.4 ± 10.5 years; 5 female) underwent 22 procedures for 28 tumours (9 hepatocellular carcinomas, 19 metastases) with a mean tumour diameter of 14.6 ± 5.4 mm (range: 6–24 mm). Technique effectiveness was achieved in all lesions. Tumours were treated using 1.7 ± 0.7 applicator positions (range: 1–3). Mean energy and ablation duration per tumour were 75.3 ± 35.4 kJ and 13.3 ± 6.2 min, respectively. Coagulation zone short- and long-axis diameters were 29.1 ± 6.4 mm and 39.9 ± 7.4 mm, respectively. Average procedure duration was 146.4 ± 26.2 min (range: 98–187 min). One minor complication was reported. Five patients developed new tumour manifestations in the untreated liver. Local tumour progression was not observed during initial follow-up.ConclusionMR-guided high-power microwave ablation provides safe and effective treatment of hepatic malignancies with short ablation times and within acceptable procedure durations.

Accuracy of liver ablation zone prediction in a single 2450 MHz 100 Watt generator model microwave ablation system: An in human study

PurposeTo compare manufacturer provided predictions and realized ablation dimensions in the liver using one 2450MHz 100 Watt generator model microwave ablation (MWA) system. Materials and methodBetween 1/1/2015 and 2/1/2018, MWAs were performed in 86 patients who underwent a total of 103 MWAs with a single MWA system. There were 64 men and 22 women with a mean age of 63.9±9.9 (SD) years (range: 30–88 years). Demographic, procedural, and outcomes data was recorded. The manufacturer predicted ablation zone sizes in three dimensions (anterior-posterior [AP], transverse [TR], and cranial caudal [CC]) were recorded and then compared to the actual ablation zone sizes at one month follow-up imaging. ResultsMWAs were most commonly performed to treat hepatocellular carcinoma (92/103, 89.3%). Dividing the actual ablation size by the manufacturer prediction in the AP, TR, and CC directions resulted in a mean of 88.3±20.6 (SD) % (range: 33.3–156.4%), 80.2±26.5 (SD) % (range: 29.6–182.9%), and 86.7±25.1 (SD) % (range: 37–186.1%), respectively. The realized AP direction was statistically closer to the manufacturer prediction than the TR (P<0.01). Ablation Watt setting of 100 Watts resulted in more accurate predictions than the 75 or 45 Watt settings in the AP direction (P=0.03). ConclusionsThis 2450MHz 100 Watt generator MWA system manufacturer provided model fairly accurately predicts ablation zone dimensions, but tends to over predict realized dimensions in this mainly hepatocellular carcinoma, and therefore cirrhotic, cohort. The TR is the most inaccurately predicted dimension and manufacturer predictions appear to be best in the 100W setting, important aspects for interventionalists to consider during ablation planning and execution.

Temperature dependence of thermal properties of ex vivo liver tissue up to ablative temperatures

Thermal properties of ex vivo bovine liver were measured as a function of temperature, by heating tissue samples in a temperature-controlled oil bath over a temperature range from about 21 °C to about 113 °C. Results evidenced temperature-dependent non-linear changes of the thermal properties, with the temperature of 100 °C representing a break point: the thermal properties increased with temperature up to 99 °C and then decreased above 100 °C. The rate of increase appeared dramatic between 90 °C and 99 °C, owing to the onset of vaporisation of water contained in the tissue. In particular, at 99 °C, the thermal conductivity reported an increase of about four times with respect to the value measured at 90 °C, whilst about a two-fold increase was reported for both the volumetric heat capacity and the thermal diffusivity. Temperatures higher than 100 °C were reached only after complete vaporisation of water contained in the tissue, resulting in about 70% loss of weight from the tissue. An overall decrease of about 71% and 63% was reported for the thermal conductivity and volumetric heat capacity, respectively, in the temperature range 101 °C–113 °C. A decrease of about 25% was reported in the measured values of the thermal diffusivity in the temperature range 101 °C–108 °C, whilst a slight increase of measured values, not statistically significant, was observed in the temperature range 108 °C–113 °C. The temperature dependent changes of the thermal parameters were modelled with non-linear regression analysis to calculate the best-fit curves interpolating measured data. The proposed regression models could be used to numerically assess the changes in the thermal properties of biological tissues at supra-physiological temperatures relevant in thermal ablation procedures, as well as their effect on the prediction of the ablation zone dimensions in computational models for treatment planning.

Microwave antennas for thermal ablation of benign adrenal adenomas

Microwave thermal ablation is under consideration as a minimally invasive modality to treat 10–20 mm benign adrenal adenomas, while preserving normally functioning adjacent adrenal tissue, and returning the gland to a normally functioning status that is under normal regulation. In contrast to applications for tumor ablation, where devices have been developed with the objective of maximizing the size of the ablation zone for treating large tumors, a challenge for adrenal ablation is to minimize thermal damage to non-targeted adrenal tissue and thereby preserve adrenal function. Here, we investigate methods for creating small spherical ablation zones of volumes in the range 0.5–4 cm3 for the treatment of benign adrenal adenomas using water-loaded microwave monopole antennas operating at 2.45 GHz and 5.8 GHz. Coupled electromagnetic and bioheat transfer simulations and experiments in ex vivo tissue were employed to investigate the effect of frequency, applied power, ablation duration, and coolant temperature on the length and width of the ablation zone. Experimental results showed that small spherical ablation zones with diameters in the range of 7.4–17.6 mm can be obtained by adjusting the applied power and ablation duration. Multi-way ANOVA analysis of the experimentally-measured ablation zone dimensions demonstrated that frequency of operation and ablation duration are the primary parameters for controlling the ablation zone length and width, respectively. Additionally, it was demonstrated that the coolant temperature provides another effective parameter for controlling the ablation zone length without affecting the ablation zone width. Our study demonstrates the feasibility of creating small spherical microwave ablation zones suitable for targeting benign adrenal adenomas.

Abstract No. 494 Microwave ablation zone observations in a large series with recommendations for adjustments

Microwave ablation (MWA) is and established technology that has recently been applied to interventional oncology. Ablations rely on a certain power (watts) for a certain time (minutes/seconds). Charts provided by manufacturers are produced using ex-vivo bovine liver at 20°C. Results in living patients do not conform with the charts. This retrospective analysis of 121 ablation zones evaluates differences and suggests how to adjust parameters in clinical practice. 73 patients underwent MWA of 136 liver lesions in 85 procedures. Lesions measured mean 2.7 × 2.3 × 2.4 cm (SDev 1.4 × 1.2 × 1.4 cm), HCC n = 91, Cholangio n = 6, metastasis n = 39. Each lesion was treated with a single ablation. Manufacturer anticipated ablation zone and actual ablation zone measured within 24 hours on venous phase CECT images were compared. Power/time values on manufacturer charts compared to operator selected (standard (S) vs non-standard (NS) ablations); HCC vs other lesion pathology sought to assess the effect of a presumed fibrotic liver parenchyma. Overall mean anticipated ablation zone dimensions were 4.8 × 3.7 × 3.7 cm (SDev 1.1 × 0.8 × 0.8 cm) compared to actual 4.1 × 3.0 × 3.2 cm (SDev 1.2 × 0.9 × 1.1 cm) (n = 121). Differences in individual dimensions were highly significant ttest p<0.0005. Zone volumes using volume of an oval, were 37.5 ± 25 cm3 vs 24.6 ± 24.1cm3 (p<0.0001) mean 68% of predicted (range, 8.5-217%). In 107/136 lesions the zone was mean 50.4 ± 23.1% smaller, in 29/136 mean 133.6 ± 27.3% larger. In S ablations, zones were mean 64.8 ± 41.8% predicted versus 78.7 ± 40.5% predicted in NS ablations. HCC ablation zones were mean 63.5 ± 40.1% predicted versus 67.2 ± 46.1% in non-cirrhotic livers. In approximately ¾ ablations the actual ablation zone is significantly smaller than suggested based on manufacturer charts. Individualizing power/time values may improve this by inadvertent underestimation of anticipated ablation zones. Cirrhosis may exacerbate discrepancies. Larger zones did not increase complication rates. Planning of larger ablation zones is recommended to improve outcomes.

Irreversible electroporation for the treatment of localized prostate cancer: a summary of imaging findings and treatment feedback.

Imaging plays a crucial role in ablative therapies for prostate cancer (PCa). Irreversible electroporation (IRE) is a new treatment modality used for focal treatment of PCa. We aimed to demonstrate what imaging modalities can be used by descriptively reporting contrast-enhanced ultrasonography (CEUS), multiparametric magnetic resonance imaging (mpMRI), and grey-scale transrectal ultrasound (TRUS) results. Furthermore, we aimed to correlate quantitatively the ablation zone seen on mpMRI and CEUS with treatment planning to provide therapy feedback. Imaging data was obtained from two prospective multicenter trials on IRE for localized low- to intermediate-risk PCa. The ablation zone volume (AZV) seen on mpMRI and CEUS was 3D reconstructed to correlate with the planned AZV. Descriptive examples are provided using mpMRI, TRUS, and CEUS for treatment planning and follow-up after IRE. The mean AZV on T2-weighted imaging 4 weeks following IRE was 12.9 cm3 (standard deviation [SD]=7.0), 5.3 times larger than the planned AZV. Linear regression showed a positive correlation (r=0.76, P = 0.002). For CEUS the mean AZV was 20.7 cm3 (SD=8.7), 8.5 times larger than the planned AZV with a strong positive correlation (r=0.93, P = 0.001). Prostate volume is reduced over time (mean= -27.5%, SD=11.9%) due to ablation zone fibrosis and deformation, illustrated by 3D reconstruction. The role of imaging in conjunction with IRE is of crucial importance to guide clinicians throughout the treatment protocol. CEUS and mpMRI may provide essential treatment feedback by visualizing the ablation zone dimensions and volume.

Percutaneous Spinal Ablation in a Sheep Model: Protective Capacity of an Intact Cortex, Correlation of Ablation Parameters with Ablation Zone Size, and Correlation of Postablation MRI and Pathologic Findings.

Despite the growing use of percutaneous ablation therapy for the treatment of metastatic spine disease, several issues have yet to be fully addressed. Our aims were to determine whether the vertebral body cortex protects against ablation-induced spinal cord injury; correlate radiofrequency, cryo-, and microwave ablation parameters with resulting spinal ablation zone dimensions and describe normal spinal marrow postablation changes on MR imaging. Ten thoracolumbar vertebrae in 3 sheep were treated with radiofrequency ablation, cryoablation, or microwave ablation under fluoroscopic guidance. Technique parameters were chosen to produce ablation zones that exceeded the volume of the vertebral bodies in sheep 1 and were confined to the vertebrae in sheep 2 and 3. Expected ablation zone dimensions were based on data provided by the device manufacturers. Postablation MR imaging was performed at 48 hours (sheep 1) or 7 days (sheep 2 and 3). In sheep 1, cryoablation and microwave ablations extended into the spinal canal and caused histologically confirmed neurologic injury, but radiofrequency ablation did not. The mean difference between the lengths of the radiofrequency ablation zone dimensions measured on gross pathology compared with those expected was 9.6 ± 4.1 mm. The gross pathologic cryo- and microwave ablation zone dimensions were within 1 mm of those expected. All modalities produced a nonenhancing ablation zone with a rim of enhancement, corresponding histologically to marrow necrosis and hemorrhagic congestion. An intact cortex appears to protect against radiofrequency ablation-induced spinal cord injury, but not against non-impedance-based modalities. Ablation dimensions produced by microwave and cryoablation are similar to those expected, while radiofrequency ablation dimensions are smaller. Ablation of normal marrow produces a rim of enhancement at the margin of the ablation zone on MR imaging.

Ablation zone visualization enhancement by periodic contrast-enhancement computed tomography during microwave ablation

Intra-procedural contrast-enhanced computed tomography (CECT) has been proposed to monitor the growth of thermal ablations. The primary challenge with multiple CT acquisitions is reducing radiation dose while maintaining sufficient image quality. The purpose of this study was to evaluate the feasibility of applying local highly constrained backprojection reconstruction (HYPR-LR) on periodic CECT images acquired with low-dose protocols, and to determine whether the ablations visible on CT were commensurate to gross pathology. Low-dose (CTDIvol≤1.49mGy), temporal CECT volumes were acquired during microwave ablation on normal porcine liver. HYPR processing was performed on each volume after image registration. Ablation signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were collected to evaluate the degree of enhancement of image quality and ablation zone visualization. Ablation zones were manually segmented on HYPR and non-HYPR images and compared spatially using Dice's coefficient. The dimensions of ablation zones were also compared to gross pathology by correlation and dimensional differences. The SNR and CNR of ablation zones were increased after HYPR processing. The manually segmented ablation zone was highly similar to gross pathology with a Dice coefficient of 0.81 ± 0.03, while the low-dose CECT had a smaller Dice coefficient of 0.72 ± 0.05. Both HYPR and low-dose CECT had high correlation to gross pathology (0.99 and 0.94, respectively), but the variance of measurements were lower after HYPR processing compared to unprocessed images. The relative difference in area, length of long axis, and length of short axis for HYPR image were 13.1 ± 5.6%, 9.7 ± 4.2%, and 15.2 ± 2.8%, which were lower than those for low-dose CECT at 37.5 ± 6.0%, 17.7 ± 2.8%, and 28.9 ± 5.4%. HYPR processing applied to periodic CECT images can enhance ablation zone visualization. HYPR processing may potentially enable CECT in real-time ablation monitoring under strict regulation of radiation dose.

Analysis of iodinated contrast delivered during thermal ablation: is material trapped in the ablation zone?

Intra-procedural contrast-enhanced CT (CECT) has been proposed to evaluate treatment efficacy of thermal ablation. We hypothesized that contrast material delivered concurrently with thermal ablation may become trapped in the ablation zone, and set out to determine whether such an effect would impact ablation visualization. CECT images were acquired during microwave ablation in normal porcine liver with: (A) normal blood perfusion and no iodinated contrast, (B) normal perfusion and iodinated contrast infusion or (C) no blood perfusion and residual iodinated contrast. Changes in CT attenuation were analyzed from before, during and after ablation to evaluate whether contrast was trapped inside of the ablation zone. Visualization was compared between groups using post-ablation contrast-to-noise ratio (CNR). Attenuation gradients were calculated at the ablation boundary and background to quantitate ablation conspicuity. In Group A, attenuation decreased during ablation due to thermal expansion of tissue water and water vaporization. The ablation zone was difficult to visualize (CNR = 1.57 ± 0.73, boundary gradient = 0.7 ± 0.4 HU mm−1), leading to ablation diameter underestimation compared to gross pathology. Group B ablations saw attenuation increase, suggesting that iodine was trapped inside the ablation zone. However, because the normally perfused liver increased even more, Group B ablations were more visible than Group A (CNR = 2.04 ± 0.84, boundary gradient = 6.3 ± 1.1 HU mm−1) and allowed accurate estimation of the ablation zone dimensions compared to gross pathology. Substantial water vaporization led to substantial attenuation changes in Group C, though the ablation zone boundary was not highly visible (boundary gradient = 3.9 ± 1.1 HU mm−1). Our results demonstrate that despite iodinated contrast being trapped in the ablation zone, ablation visibility was highest when contrast is delivered intra-procedurally. Therefore, CECT may be feasible for real-time thermal ablation monitoring.

The impact of frequency on the performance of microwave ablation

Purpose: The use of higher frequencies in percutaneous microwave ablation (MWA) may offer compelling interstitial antenna design advantages over the 915 MHz and 2.45 GHz frequencies typically employed in current systems. To evaluate the impact of higher frequencies on ablation performance, we conducted a comprehensive computational and experimental study of microwave absorption and tissue heating as a function of frequency.Methods: We performed electromagnetic and thermal simulations of MWA in ex vivo and in vivo porcine muscle at discrete frequencies in the 1.9–26 GHz range. Ex vivo ablation experiments were performed in the 1.9–18 GHz range. We tracked the size of the ablation zone across frequency for constant input power and ablation duration. Further, we conducted simulations to investigate antenna feed line heating as a function of frequency, input power, and cable diameter.Results: As the frequency was increased from 1.9 to 26 GHz the resulting ablation zone dimensions decreased in the longitudinal direction while remaining relatively constant in the radial direction; thus at higher frequencies the overall ablation zone was more spherical. However, cable heating at higher frequencies became more problematic for smaller diameter cables at constant input power.Conclusion: Comparably sized ablation zones are achievable well above 1.9 GHz, despite increasingly localised power absorption. Specific absorption rate alone does not accurately predict ablation performance, particularly at higher frequencies where thermal diffusion plays an important role. Cable heating due to ohmic losses at higher frequencies may be controlled through judicious choices of input power and cable diameter.

Hepatic Microwave Ablation Zone Size: Correlation with Total Energy, Net Energy, and Manufacturer-Provided Chart Predictions

PurposeTo determine whether total energy (TE) reaching the microwave (MW) applicator or net energy (NE) exiting the applicator (after correcting for reflectivity) correlates better with hepatic MW ablation zone dimensions than manufacturer-provided chart predictions. Materials and MethodsSingle-applicator, nonoverlapping ablations of 93 liver tumors (0.7–5.9 cm) were performed in 52 adult patients. TE and NE were recorded for each ablation. Long axis diameter (LAD), short axis diameter (SAD), and volume (V) of each ablation zone were measured on magnetic resonance imaging or computed tomography after the procedure and retrospectively compared with TE; NE; and manufacturer-provided chart predictions of LAD, SAD, and V using correlation and regression analyses. ResultsFor treated tumors, mean (± SD) TE and NE were 49.8 kJ (± 22.7) and 36.4 kJ (± 19.4). Mean LAD, SAD, and V were 5.8 cm (± 1.3), 3.7 cm (± 0.8), and 44.1 cm3 (± 25.4). Correlation coefficients (95% confidence interval) with LAD, SAD, and V were 0.46 (0.28, 0.61), 0.52 (0.36, 0.66), and 0.52 (0.36, 0.66) for TE; 0.42 (0.24, 0.58), 0.55 (0.39, 0.68), and 0.53 (0.36, 0.66) for NE; and 0.51 (0.34, 0.65), 0.63 (0.49, 0.74), and 0.60 (0.45, 0.73) for chart predictions. Using regression analysis and controlling for TE, SAD was 0.34 cm larger in patients with cirrhosis than in patients without cirrhosis. ConclusionsCorrecting for reflectivity did not substantially improve correlation of energy values with MW ablation zone size parameters and did not outperform manufacturer-provided chart predictions. Correlations were moderate and variable using all methods. The results suggest a disproportionate influence of tissue factors on MW ablation results.