Experimental assessment of microwave ablation computational modeling with MR thermometry

Superparamagnetic iron oxide nanoparticle enhanced percutaneous microwave ablation: Ex-vivo characterization using magnetic resonance thermometry

BACKGROUND

Percutaneous microwave ablation (pMWA) is a minimally invasive procedure that uses a microwave antenna placed at the tip of a needle to induce lethal tissue heating. It can treat cancer and other diseases with lower morbidity than conventional surgery, but one major limitation is the lack of control over the heating region around the ablation needle. Superparamagnetic iron oxide nanoparticles have the potential to enhance and control pMWA heating due to their ability to absorb microwave energy and their ease of local delivery.

PURPOSE

The purpose of this study is to experimentally quantify the capabilities of FDA-approved superparamagnetic iron oxide Feraheme nanoparticles (FHNPs) to enhance and control pMWA heating. This study aims to determine the effectiveness of locally injected FHNPs in increasing the maximum temperature during pMWA and to investigate the ability of FHNPs to create a controlled ablation zone around the pMWA needle.

METHODS

PMWA was performed using a clinical ablation system at 915 MHz in ex-vivo porcine liver tissues. Prior to ablation, 50 uL 5 mg/mL FHNP injections were made on one side of the pMWA needle via a 23-gauge needle. Local temperatures at the FHNP injection site were directly compared to equidistant control sites without FHNP. First, temperatures were compared using directly inserted thermocouples. Next, temperatures were measured non-invasively using magnetic resonance thermometry (MRT), which enabled comprehensive four-dimensional (volumetric and temporal) assessment of heating effects relative to nanoparticle distribution, which was quantified using dual-echo ultrashort echo time (UTE) subtraction MR imaging. Maximum heating within FHNP-exposed tissues versus control tissues were compared at multiple pMWA energy delivery settings. The ability to generate a controlled asymmetric ablation zone using multiple FHNP injections was also tested. Finally, intra-procedural MRT-derived heat maps were correlated with gold standard gross pathology using Dice similarity analysis.

RESULTS

Maximum temperatures at the FHNP injection site were significantly higher than control (without FHNP) sites when measured using direct thermocouples (93.1 ± 6.0°C vs. 57.2 ± 8.1°C, p = 0.002) and using non-invasive MRT (115.6 ± 13.4°C vs. 49.0 ± 10.6°C, p = 0.02). Temperature difference between FHNP-exposed and control sites correlated with total energy deposition: 66.6 ± 17.6°C, 58.1 ± 8.5°C, and 20.8 ± 9.2°C at high (17.5 ± 2.2 kJ), medium (13.6 ± 1.8 kJ), and low (8.8 ± 1.1 kJ) energies, respectively (all pairwise p < 0.05). Each FHNP injection resulted in a nanoparticle distribution within 0.9 ± 0.2 cm radially of the injection site and a local lethal heating zone confined to within 1.1 ± 0.4 cm radially of the injection epicenter. Multiple injections enabled a controllable, asymmetric ablation zone to be generated around the ablation needle, with maximal ablation radius on the FHNP injection side of 1.6 ± 0.2 cm compared to 0.7 ± 0.2 cm on the non-FHNP side (p = 0.02). MRT intra-procedural predicted ablation zone correlated strongly with post procedure gold-standard gross pathology assessment (Dice similarity 0.9).

CONCLUSIONS

Locally injected FHNPs significantly enhanced pMWA heating in liver tissues, and were able to control the ablation zone shape around a pMWA needle. MRI and MRT allowed volumetric real-time visualization of both FHNP distribution and FHNP-enhanced pMWA heating that was useful for intra-procedural monitoring. This work strongly supports further development of a FHNP-enhanced pMWA paradigm; as all individual components of this approach are approved for patient use, there is low barrier for clinical translation.

Assessment of thermochromic phantoms for characterizing microwave ablation devices

Background and Purpose

Thermochromic gel phantoms provide a controlled medium for visual assessment of thermal ablation device performance. However, there are limited studies reporting on the comparative assessment of ablation profiles assessed in thermochromic gel phantoms against those in ex vivo tissue. The objective of this study was to compare microwave ablation zones in a thermochromic tissue mimicking gel phantom and ex vivo bovine liver, and to report on measurements of the temperature dependent dielectric and thermal properties of the phantom.

Methods

Thermochromic polyacrylamide phantoms were fabricated following a previously reported protocol. Phantom samples were heated to temperatures in the range of 20 - 90 °C in a temperature-controlled water bath, and colorimetric analysis of images of the phantom taken after heating were used to develop a calibration between color changes and temperature to which the phantom was heated. Using a custom, 2.45 GHz water-cooled microwave ablation antenna, ablations were performed in fresh ex vivo liver and phantoms using 65 W applied for 5 min or 10 min ( n = 3 samples in each medium for each power/time combination). Broadband (500 MHz - 6 GHz) temperature-dependent dielectric and thermal properties of the phantom were measured over the temperature range 22 - 100 °C. Measured dielectric and thermal properties of the phantom were employed in a previously validated computational model of microwave ablation to comparatively assess model predicted extents of heating against experimental observations in the phantom.

Results

Colorimetric analysis showed that the sharp change in gel phantom color commences at a temperature of 57 °C. Short and long axes of the ablation zone in the phantom (as assessed by the 57 °C isotherm) for 65 W, 5 min ablations were aligned with extents of the ablation zone observed in ex vivo bovine liver. However, for the 65 W, 10 min setting, ablations in the phantom were on average 23.7% smaller in short axis and 7.4 % smaller in long axis than those observed in ex vivo liver. Measurements of the temperature dependent relative permittivity, thermal conductivity, and volumetric heat capacity of the phantom largely followed similar trends to published values for ex vivo liver tissue. After incorporating measured dielectric and thermal properties of the phantom, model predictions of ablation zone linear dimensions ranged between 16 - 50% larger than those observed experimentally.

Conclusion

Thermochromic tissue mimicking phantoms provide a suitable, controlled, and reproducible medium for comparative assessment of microwave ablation devices and energy delivery settings, though ablation zone size and shapes may not accurately represent ablation sizes and shapes observed in ex vivo liver tissue under similar conditions.

Computational modeling of microwave ablation with thermal accelerants

PURPOSE

To develop a computational model of microwave ablation (MWA) with a thermal accelerant gel and apply the model toward interpreting experimental observations in ex vivo bovine and in vivo porcine liver.

METHODS

A 3D coupled electromagnetic-heat transfer model was implemented to characterize thermal profiles within ex vivo bovine and in vivo porcine liver tissue during MWA with the HeatSYNC thermal accelerant. Measured temperature dependent dielectric and thermal properties of the HeatSYNC gel were applied within the model. Simulated extents of MWA zones and transient temperature profiles were compared against experimental measurements in ex vivo bovine liver. Model predictions of thermal profiles under in vivo conditions in porcine liver were used to analyze thermal ablations observed in prior experiments in porcine liver in vivo.

RESULTS

Measured electrical conductivity of the HeatSYNC gel was ∼83% higher compared to liver at room temperature, with positive linear temperature dependency, indicating increased microwave absorption within HeatSYNC gel compared to tissue. In ex vivo bovine liver, model predicted ablation zone extents of (31.5 × 36) mm with the HeatSYNC, compared to (32.9 ± 2.6 × 40.2 ± 2.3) mm in experiments (volume differences 4 ± 4.1 cm3). Computational models under in vivo conditions in porcine liver suggest approximating the HeatSYNC gel spreading within liver tissue during ablations as a plausible explanation for larger ablation zones observed in prior in vivo studies.

CONCLUSION

Computational models of MWA with thermal accelerants provide insight into the impact of accelerant on MWA, and with further development, could predict ablations with a variety of gel injection sites.

MRI-guided microwave ablation and albumin-bound paclitaxel for lung tumors: Initial experience

Magnetic resonance-guided microwave ablation (MRI-guided MWA) is a new, minimally invasive ablation method for cancer. This study sought to analyze the clinical value of MRI-guided MWA in non-small cell lung cancer (NSCLC). We compared the precision, efficiency, and clinical efficacy of treatment in patients who underwent MRI-guided MWA or computed tomography (CT)-guided microwave ablation (CT-guided MWA). Propensity score matching was used on the prospective cohort (MRI-MWA group, n = 45) and the retrospective observational cohort (CT-MWA group, n = 305). To evaluate the advantages and efficacy of MRI-guided MWA, data including the accuracy of needle placement, scan duration, ablation time, total operation time, length of hospital stay, progression-free survival (PFS), and overall survival (OS) were collected and compared between the two groups. The mean number of machine scans required to adjust the needle position was 7.62 ± 1.69 (range 4–12) for the MRI-MWA group and 9.64 ± 2.14 (range 5–16) for the CT-MWA group (p < 0.001). The mean time for antenna placement was comparable between the MRI and CT groups (54.41 ± 12.32 min and 53.03 ± 11.29 min, p = 0.607). The microwave ablation time of the two groups was significantly different (7.62 ± 2.65 min and 9.41 ± 2.86 min, p = 0.017), while the overall procedure time was comparable (91.28 ± 16.69 min vs. 93.41 ± 16.03 min, p = 0.568). The overall complication rate in the MRI-MWA group was significantly lower than in the CT-MWA group (12% vs. 51%, p = 0.185). The median time to progression was longer in the MRI-MWA group than in the CT-MWA group (11 months [95% CI 10.24–11.75] vs. 9 months [95% CI 8.00–9.99], p = 0.0003; hazard ratio 0.3690 [95% CI 0.2159–0.6306]). OS was comparable in both groups (MRI group 26.0 months [95% CI 25.022–26.978] vs. CT group 23.0 months [95% CI 18.646–27.354], p = 0.18). This study provides hitherto-undocumented evidence of the clinical effects of MRI-guided MWA on patients with NSCLC and determines the relative safety and efficiency of MRI- and CT-guided MWA.

Fat Quantification Imaging and Biophysical Modeling for Patient-Specific Forecasting of Microwave Ablation Therapy

Computational tools are beginning to enable patient-specific surgical planning to localize and prescribe thermal dosing for liver cancer ablation therapy. Tissue-specific factors (e.g., tissue perfusion, material properties, disease state, etc.) have been found to affect ablative therapies, but current thermal dosing guidance practices do not account for these differences. Computational modeling of ablation procedures can integrate these sources of patient specificity to guide therapy planning and delivery. This paper establishes an imaging-data-driven framework for patient-specific biophysical modeling to predict ablation extents in livers with varying fat content in the context of microwave ablation (MWA) therapy. Patient anatomic scans were segmented to develop customized three-dimensional computational biophysical models and mDIXON fat-quantification images were acquired and analyzed to establish fat content and determine biophysical properties. Simulated patient-specific microwave ablations of tumor and healthy tissue were performed at four levels of fatty liver disease. Ablation models with greater fat content demonstrated significantly larger treatment volumes compared to livers with less severe disease states. More specifically, the results indicated an eightfold larger difference in necrotic volumes with fatty livers vs. the effects from the presence of more conductive tumor tissue. Additionally, the evolution of necrotic volume formation as a function of the thermal dose was influenced by the presence of a tumor. Fat quantification imaging showed multi-valued spatially heterogeneous distributions of fat deposition, even within their respective disease classifications (e.g., low, mild, moderate, high-fat). Altogether, the results suggest that clinical fatty liver disease levels can affect MWA, and that fat-quantitative imaging data may improve patient specificity for this treatment modality.

Image-based computer modeling assessment of microwave ablation for treatment of adrenal tumors

PURPOSE

To assess the feasibility of delivering microwave ablation for targeted treatment of aldosterone producing adenomas using image-based computational models.

METHODS

We curated an anonymized dataset of diagnostic ¹¹C-metomidate PET/CT images of 14 patients with aldosterone producing adenomas (APA). A semi-automated approach was developed to segment the APA, adrenal gland, and adjacent organs within 2 cm of the APA boundary. The segmented volumes were used to implement patient-specific 3D electromagnetic-bioheat transfer models of microwave ablation with a 2.45 GHz directional microwave ablation applicator. Ablation profiles were quantitatively assessed based on the extent of the APA target encompassed by an ablative thermal dose, while limiting thermal damage to the adjacent normal adrenal tissue and sensitive critical structures.

RESULTS

Across the 14 patients, adrenal tumor volumes ranged between 393 mm³ and 2,395 mm³. On average, 70% of the adrenal tumor volumes received an ablative thermal dose of 240CEM43, while limiting thermal damage to non-target structures, and thermally sparing 83.5-96.4% of normal adrenal gland. Average ablation duration was 293 s (range: 60-600 s). Simulations indicated coverage of the APA with an ablative dose was limited when the axis of the ablation applicator was not well aligned with the major axis of the targeted APA.

CONCLUSIONS

Image-based computational models demonstrate the potential for delivering microwave ablation to APA targets within the adrenal gland, while limiting thermal damage to surrounding non-target structures.

A Computational Study on Magnetic Nanoparticles Hyperthermia of Ellipsoidal Tumors

The modelling of magnetic hyperthermia using nanoparticles of ellipsoid tumor shapes has not been studied adequately. To fill this gap, a computational study has been carried out to determine two key treatment parameters: the therapeutic temperature distribution and the extent of thermal damage. Prolate and oblate spheroidal tumors, of various aspect ratios, surrounded by a large healthy tissue region are assumed. Tissue temperatures are determined from the solution of Pennes’ bio-heat transfer equation. The mortality of the tissues is determined by the Arrhenius kinetic model. The computational model is successfully verified against a closed-form solution for a perfectly spherical tumor. The therapeutic temperature and the thermal damage in the tumor center decrease as the aspect ratio increases and it is insensitive to whether tumors of the same aspect ratio are oblate or prolate spheroids. The necrotic tumor area is affected by the tumor prolateness and oblateness. Good comparison is obtained of the present model with three sets of experimental measurements taken from the literature, for animal tumors exhibiting ellipsoid-like geometry. The computational model enables the determination of the therapeutic temperature and tissue thermal damage for magnetic hyperthermia of ellipsoidal tumors. It can be easily reproduced for various treatment scenarios and may be useful for an effective treatment planning of ellipsoidal tumor geometries.

Transcervical microwave ablation in type 2 uterine fibroids via a hysteroscopic approach: analysis of ablation profiles

Type 2 uterine fibroids are challenging to resect surgically as ≥ 50% volume of myoma lies within the myometrium. A hysteroscopic approach for ablating fibroids is minimally-invasive, but places a considerable burden on the operator to accurately place the ablation applicator within the target. We investigated the sensitivity of transcervical microwave ablation outcome with respect to position of the ablation applicator within 1 - 3 cm type 2 fibroids.Methods:A finite element computer model was developed to simulate 5.8 GHz microwave ablation of fibroids and validated with experiments inex vivotissue. The ablation outcome was evaluated with respect to applicator insertion angles (30°, 45°, 60°) , depth and offset from the fibroid center (±2 mm for 3 cm fibroid and ±1 mm for 1 cm fibroid) with 35 W and 15 W applied power for 3 cm and 1 cm fibroids, respectively. Power deposition was stopped when thermal dose of 40 cumulative equivalent minutes at 43 °C (CEM43) was accrued in adjacent myometrium.Results:Within the range of all evaluated insertion angles, depths and offsets, the ablation coverage was less sensitive to variation in angle as compared to depth and offset, and ranged from 34.9 - 83.6% for 3 cm fibroid in 140 - 400 s and 34.1 - 67.9% for 1 cm fibroid in 30 - 50 s of heating duration. Maximum achievable ablation coverage in both fibroid cases reach ∼ 90% if thermal dose is allowed to exceed 40 CEM43 in myometrium.Conclusion:The study demonstrates the technical feasibility of transcervical microwave ablation for fibroid treatment and the relationship between applicator position within the fibroid and fraction of fibroid that can be ablated while limiting thermal dose in adjacent myometrium.

Microwave ablation of lung tumors: A probabilistic approach for simulation-based treatment planning

PURPOSE

Microwave ablation (MWA) is a clinically established modality for treatment of lung tumors. A challenge with existing application of MWA, however, is local tumor progression, potentially due to failure to establish an adequate treatment margin. This study presents a robust simulation-based treatment planning methodology to assist operators in comparatively assessing thermal profiles and likelihood of achieving a specified minimum margin as a function of candidate applied energy parameters.

METHODS

We employed a biophysical simulation-based probabilistic treatment planning methodology to evaluate the likelihood of achieving a specified minimum margin for candidate treatment parameters (i.e., applied power and ablation duration for a given applicator position within a tumor). A set of simulations with varying tissue properties was evaluated for each considered combination of power and ablation duration, and for four different scenarios of contrast in tissue biophysical properties between tumor and normal lung. A treatment planning graph was then assembled, where distributions of achieved minimum ablation zone margins and collateral damage volumes can be assessed for candidate applied power and treatment duration combinations. For each chosen power and time combination, the operator can also visualize the histogram of ablation zone boundaries overlaid on the tumor and target volumes. We assembled treatment planning graphs for generic 1, 2, and 2.5 cm diameter spherically shaped tumors and also illustrated the impact of tissue heterogeneity on delivered treatment plans and resulting ablation histograms. Finally, we illustrated the treatment planning methodology on two example patient-specific cases of tumors with irregular shapes.

RESULTS

The assembled treatment planning graphs indicate that 30 W, 6 min ablations achieve a 5-mm minimum margin across all simulated cases for 1-cm diameter spherical tumors, and 70 W, 10 min ablations achieve a 3-mm minimum margin across 90% of simulations for a 2.5-cm diameter spherical tumor. Different scenarios of tissue heterogeneity between tumor and lung tissue revealed 2 min overall difference in ablation duration, in order to reliably achieve a 4-mm minimum margin or larger each time for 2-cm diameter spherical tumor.

CONCLUSIONS

An approach for simulation-based treatment planning for microwave ablation of lung tumors is illustrated to account for the impact of specific geometry of the treatment site, tissue property uncertainty, and heterogeneity between the tumor and normal lung.

How large is the periablational zone after radiofrequency and microwave ablation? Computer-based comparative study of two currently used clinical devices

Purpose:

To compare the size of the coagulation (CZ) and periablational (PZ) zones created with two commercially available devices in clinical use for radiofrequency (RFA) and microwave ablation (MWA), respectively.

Methods:

Computer models were used to simulate RFA with a 3-cm Cool-tip applicator and MWA with an Amica-Gen applicator. The Arrhenius model was used to compute the damage index (Ω). CZ was considered when Ω> 4.6 (>99% of damaged cells). Regions with 0.6<Ω< 2.1 were considered as the PZ (tissue that has undergone moderate sub-ablative hyperthermia). The ratio of PZ volume to CZ volume (PZ/CZ) was regarded as a measure of performance, since a low value implies achieving a large CZ while keeping the PZ small.

Results:

Ten-min RFA (51 W) created smaller periablational zones than 10-min MWA (11.3 cm³ vs. 17.2 22.9 cm³, for 60 100 W MWA, respectively). Prolonging duration from 5 to 10 min increased the PZ in MWA more than in RFA (2.7 cm³ for RFA vs. 8.3–11.9 cm³ for 60–100 W MWA, respectively). PZ/CZ for RFA were relatively high (65–69%), regardless of ablation time, while those for MWA were highly dependent on the duration (increase of up to 25% between 5 and 10 min) and on the applied power (smaller values as power was raised, 102% for 60 W vs. 81% for 100 W, both for 10 min). The lowest PZ/CZ across all settings was 56%, obtained with 100 W-5 min MWA.

Conclusions:

Although RFA creates smaller periablational zones than MWA, 100 W-5 min MWA provides the lowest PZ/CZ.