The feasibility of using thermal strain imaging to regulate energy delivery during intracardiac radio-frequency ablation

A Novel Ultrasound Thermometry Method Based on Thermal Strain and Short and Constant Acoustic Bursts: Preliminary Study in Phantoms

In the field of ultrasound therapy, the estimation of temperature to monitor treatments is becoming essential. We hypothesize that it is possible to measure temperature directly using a constant acoustic power burst. Under the assumption that the acoustic attenuation does not change significantly with temperature, the thermal strain induced by such bursts presents a linear relation with temperature. A mathematical demonstration is given in the introduction. Then, simulations of ultrasound waves in a canine liver model were conducted at different temperatures (from 20 °C to 90 °C). Finally, experimental measurements on phantom samples were performed over the same temperature range. The simulation and experimental results both showed a linear relation between thermal strain and temperature. This relation may suggest the foundation of a new ultrasound-based thermometry method. The potential and limitations of the method are discussed.

Techniques for Temperature Monitoring of Myocardial Tissue Undergoing Radiofrequency Ablation Treatments: An Overview

Cardiac radiofrequency ablation (RFA) has received substantial attention for the treatment of multiple arrhythmias. In this scenario, there is an ever-growing demand for monitoring the temperature trend inside the tissue as it may allow an accurate control of the treatment effects, with a consequent improvement of the clinical outcomes. There are many methods for monitoring temperature in tissues undergoing RFA, which can be divided into invasive and non-invasive. This paper aims to provide an overview of the currently available techniques for temperature detection in this clinical scenario. Firstly, we describe the heat generation during RFA, then we report the principle of work of the most popular thermometric techniques and their features. Finally, we introduce their main applications in the field of cardiac RFA to explore the applicability in clinical settings of each method.

Fiber Bragg Grating Sensors for Millimetric-Scale Temperature Monitoring of Cardiac Tissue Undergoing Radiofrequency Ablation: A Feasibility Assessment

Radiofrequency ablation (RFA) is the most widely used technique for the treatment of cardiac arrhythmias. A variety of factors, such as the electrode tip shape, the force exerted on the tissue by the catheter and the delivered power, combine to determine the temperature distribution, and as consequence, the lesion shape and size. In this context, being able to know the temperature reached in the myocardium during the RFA can be helpful for predicting the lesion dimensions to prevent the occurrence of undesired tissue damage. The catheters used so far in such procedures provide single-point temperature measurements within the probe (by means of embedded thermocouples or thermistors), so no information regarding the temperature changes occurring in myocardial tissues can be retrieved. The aim of this study was to assess the feasibility of fiber Bragg grating sensors (FBGs) to perform multi-point and millimetric-scale temperature measurements within myocardium subjected to RFA. The assessment has been performed on ex vivo porcine myocardium specimens undergoing RFA. Data show the feasibility of the proposed solution in providing spatial temperature distribution within the myocardial tissue during the entire RFA. These high-resolved measurements may allow reconstructing the temperature distribution in the tissue. This study lays the foundations for the implementation of 3D thermal maps to investigate how the supplied power, treatment time, force of contact and irrigation flow of the catheter influence the thermal effects within the tissue.

Application of Ultrasound Thermal Imaging for Monitoring Laser Ablation in Ex Vivo Cardiac Tissue

BACKGROUND AND OBJECTIVE

Laser ablation can be used to treat atrial fibrillation by thermally isolating pulmonary veins. In this study, we evaluated the feasibility of high-resolution (<1 mm) ultrasound thermal imaging to monitor spatial temperature distribution during laser ablation on ex vivo cardiac tissue.

STUDY DESIGN/MATERIALS AND METHODS

Laser ablation (808 nm) was performed on five porcine cardiac tissue samples. A thermocouple was used to measure the interstitial tissue temperature during the laser ablation process. Tissue-strain-based ultrasound thermal imaging was conducted to monitor the spatial distribution of the temperature in the cardiac tissue. The tissue temperature was estimated from the time shifts of ultrasound signals owing to the changes in the speed of sound and was compared with the measured temperature. The temperature estimation coefficient k of porcine cardiac tissue was calculated from the estimated thermal strain and the measured temperature. The degree of tissue coagulation (temperatures > 50°C) was derived from the estimated temperature and was compared with that of the tested cardiac tissue.

RESULTS

The estimated tissue temperature using strain-based ultrasound thermal imaging at a depth of 1 mm agreed with thermocouple measurements. During the 30-second period of the laser ablation process, the estimated tissue temperature increased from 25 to 70°C at a depth of 0.1 mm, while the estimated temperature at a depth of 1 mm increased up to 46°C. Owing to the uncertainty of the coefficient k, the k value of the porcine cardiac tissue varied from 160 to 220°C with temperature changes of up to 20°C. The estimated coagulation region in the ultrasound thermal imaging was 20% wider (+0.6 mm) but 9% shallower (-0.1 mm) than the measured region of the ablated porcine cardiac tissue.

CONCLUSIONS

The current study demonstrated the feasibility of temperature monitoring with the use of ultrasound thermal imaging during the laser ablation on ex vivo porcine cardiac tissue. The high-resolution ultrasound thermal imaging could map the spatial distribution of the tissue temperature. The proposed method can be used to monitor the temperature and thermal coagulation to achieve effective laser ablation for atrial fibrillation. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Temperature dependence of speed of sound and attenuation of porcine left ventricular myocardium

The temperature dependence of soft tissue acoustic properties is relevant for monitoring tissue hyperthermia. In the current work, the propagation speed and attenuation of healthy porcine left ventricular myocardium (N=5) was investigated in a frequency range relevant for clinical diagnostic imaging, i.e. 2.5-13.0MHz. Each tissue sample was held in a water bath at a temperature T=25°C, heated to 45°C, and allowed to cool back down to 25°C. Due to initial tissue swelling, the data for decreasing temperatures was considered more reliable. In this case, the slope of the phase velocity versus temperature relation was measured to be 1.10±0.04m/s/°C, and the slope of the attenuation was -0.11±0.04dB/cm/°C at 10MHz, or -0.0041±0.0015dB/cm/MHz^1.4336/°C as a function of frequency.

Temperature monitoring by channel data delays: Feasibility based on estimated delays magnitude for cardiac ablation

Ultrasound thermometry is based on measuring tissue temperature by its impact on ultrasound wave propagation. This study focuses on the use of transducer array channel data (not beamformed) and examines how a layer of increased velocity (heat induced) affects the travel-times of the ultrasound backscatter signal. Based on geometric considerations, a new equation was derived for the change in time delay as a function of temperature change. The resulting expression provides insight into the key factors that link change in temperature to change in travel time. It shows that velocity enters in combination with heating geometry: complementary information is needed to compute velocity from the changes in travel time. Using the bio-heat equation as a second source of information in the derived expressions, the feasibility of monitoring the temperature increase during cardiac ablation therapy using channel data was investigated. For an intra-cardiac (ICE) probe, using this "time delay error approach" would not be feasible, while for a trans-esophageal array transducer (TEE) transducer it might be feasible.

Improved Estimation of Ultrasound Thermal Strain Using Pulse Inversion Harmonic Imaging

Thermal (temporal) strain imaging (TSI) is being developed to detect the lipid-rich core of atherosclerotic plaques and presence of fatty liver disease. However, the effects of ultrasonic clutter on TSI have not been considered. In this study, we evaluated whether pulse inversion harmonic imaging (PIHI) could be used to improve estimates of thermal (temporal) strain. Using mixed castor oil-gelatin phantoms of different concentrations and artificially introduced clutter, we found that PIHI improved the signal-to-noise ratio of TSI by an average of 213% or 52.1% relative to 3.3- and 6.6-MHz imaging, respectively. In a phantom constructed using human liposuction fat in the presence of clutter, the contrast-to-noise ratio was degraded by 35.1% for PIHI compared with 62.4% and 43.7% for 3.3- and 6.6-MHz imaging, respectively. These findings were further validated using an ex vivo carotid endarterectomy sample. PIHI can be used to improve estimates of thermal (temporal) strain in the presence of clutter.

Quantitative evaluation of atrial radio frequency ablation using intracardiac shear-wave elastography

PURPOSE

Radio frequency catheter ablation (RFCA) is a well-established clinical procedure for the treatment of atrial fibrillation (AF) but suffers from a low single-procedure success rate. Recurrence of AF is most likely attributable to discontinuous or nontransmural ablation lesions. Yet, despite this urgent clinical need, there is no clinically available imaging modality that can reliably map the lesion transmural extent in real time. In this study, the authors demonstrated the feasibility of shear-wave elastography (SWE) to map quantitatively the stiffness of RFCA-induced thermal lesions in cardiac tissues in vitro and in vivo using an intracardiac transducer array.

METHODS

SWE was first validated in ex vivo porcine ventricular samples (N = 5). Both B-mode imaging and SWE were performed on normal cardiac tissue before and after RFCA. Areas of the lesions were determined by tissue color change with gross pathology and compared against the SWE stiffness maps. SWE was then performed in vivo in three sheep (N = 3). First, the stiffness of normal atrial tissues was assessed quantitatively as well as its variation during the cardiac cycle. SWE was then performed in atrial tissue after RFCA.

RESULTS

A large increase in stiffness was observed in ablated ex vivo regions (average shear modulus across samples in normal tissue: 22 ± 5 kPa, average shear-wave speed (ct): 4.5 ± 0.4 m s(-1) and in determined ablated zones: 99 ± 17 kPa, average ct: 9.0 ± 0.5 m s(-1) for a mean shear modulus increase ratio of 4.5 ± 0.9). In vivo, a threefold increase of the shear modulus was measured in the ablated regions, and the lesion extension was clearly visible on the stiffness maps.

CONCLUSIONS

By its quantitative and real-time capabilities, Intracardiac SWE is a promising intraoperative imaging technique for the evaluation of thermal ablation during RFCA.

Quantitative ultrasound imaging for monitoring in situ high-intensity focused ultrasound exposure

Quantitative ultrasound (QUS) imaging is hypothesized to map temperature elevations induced in tissue with high spatial and temporal resolution. To test this hypothesis, QUS techniques were examined to monitor high-intensity focused ultrasound (HIFU) exposure of tissue. In situ experiments were conducted on mammary adenocarcinoma tumors grown in rats and lesions were formed using a HIFU system. A thermocouple was inserted into the tumor to provide estimates of temperature at one location. Backscattered time-domain waveforms from the tissue during exposure were recorded using a clinical ultrasonic imaging system. Backscatter coefficients were estimated using a reference phantom technique. Two parameters were estimated from the backscatter coefficient (effective scatterer diameter (ESD) and effective acoustic concentration (EAC). The changes in the average parameters in the regions corresponding to the HIFU focus over time were correlated to the temperature readings from the thermocouple. The changes in the EAC parameter were consistently correlated to temperature during both heating and cooling of the tumors. The changes in the ESD did not have a consistent trend with temperature. The mean ESD and EAC before exposure were 120 ± 16 μm and 32 ± 3 dB/cm3, respectively, and changed to 144 ± 9 μm and 51 ± 7 dB/cm3, respectively, just before the last HIFU pulse was delivered to the tissue. After the tissue cooled down to 37 °C, the mean ESD and EAC were 126 ± 8 μm and 35 ± 4 dB/cm3, respectively. Peak temperature in the range of 50-60 °C was recorded by a thermocouple placed just behind the tumor. These results suggest that QUS techniques have the potential to be used for non-invasive monitoring of HIFU exposure.

Mean scatterer spacing estimation in normal and thermally coagulated ex vivo bovine liver

The liver has been hypothesized to have a unique arrangement of microvasculature that presents as an arrangement of quasiperiodic scatterers to an interrogating ultrasound pulse. The mean scatterer spacing (MSS) of these quasiperiodic scatterers has been proposed as a useful quantitative ultrasound biomarker for characterizing liver tissue. Thermal ablation is an increasingly popular method for treating hepatic tumors, and ultrasonic imaging approaches for delineating the extent of thermal ablation are in high demand. In this work, we examine the distribution of estimated MSS in thermally coagulated bovine liver and normal untreated bovine liver ex vivo. We estimate MSS by detecting local maxima in the spectral coherence function of radio frequency echoes from a clinical transducer, the Siemens VFX 9L4 transducer operating on an S2000 scanner. We find that normal untreated bovine liver was characterized by an MSS of approximately 1.3 mm. We examined regions of interest 12 mm wide laterally, and ranging from 12 mm to 18 mm axially, in 2 mm increments. Over these parameters, the mode of the MSS estimates was between 1.25 and 1.37 mm. On the other hand, estimation of MSS in thermally coagulated liver tissue yields a distribution of MSS estimates whose mode varied between 0.45 and 1.0 mm when examining regions of interest over the same sizes. We demonstrate that the estimated MSS in thermally coagulated liver favors small spacings because the randomly positioned scatterers in this tissue are better modeled as aperiodic scatterers. The submillimeter spacings result from the fact that this was the most probable spacing to be estimated if the discretely sampled spectral coherence function was a uniformly random two-dimensional function.

Using ultrasound CBE imaging without echo shift compensation for temperature estimation

Clinical trials have demonstrated that hyperthermia improves cancer treatments. Previous studies developed ultrasound temperature imaging methods, based on the changes in backscattered energy (CBE), to monitor temperature variations during hyperthermia. Echo shift, induced by increasing temperature, contaminates the CBE image, and its tracking and compensation should normally ensure that estimations of CBE at each pixel are correct. To obtain a simplified algorithm that would allow real-time computation of CBE images, this study evaluated the usefulness of CBE imaging without echo shift compensation in detecting distributions in temperature. Experiments on phantoms, using different scatterer concentrations, and porcine livers were conducted to acquire raw backscattered data at temperatures ranging from 37°C to 45°C. Tissue samples of pork tenderloin were ablated in vitro by microwave irradiation to evaluate the feasibility of using the CBE image without compensation to monitor tissue ablation. CBE image construction was based on a ratio map obtained from the envelope image divided by the reference envelope image at 37°C. The experimental results demonstrated that the CBE image obtained without echo shift compensation has the ability to estimate temperature variations induced during uniform heating or tissue ablation. The magnitude of the CBE as a function of temperature obtained without compensation is stronger than that with compensation, implying that the CBE image without compensation has a better sensitivity to detect temperature. These findings suggest that echo shift tracking and compensation may be unnecessary in practice, thus simplifying the algorithm required to implement real-time CBE imaging.

High-power CMUTs: design and experimental verification

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array.

First in vivo use of a capacitive micromachined ultrasound transducer array-based imaging and ablation catheter

OBJECTIVES

The primary objective was to test in vivo for the first time the general operation of a new multifunctional intracardiac echocardiography (ICE) catheter constructed with a microlinear capacitive micromachined ultrasound transducer (ML-CMUT) imaging array. Secondarily, we examined the compatibility of this catheter with electroanatomic mapping (EAM) guidance and also as a radiofrequency ablation (RFA) catheter. Preliminary thermal strain imaging (TSI)-derived temperature data were obtained from within the endocardium simultaneously during RFA to show the feasibility of direct ablation guidance procedures.

METHODS

The new 9F forward-looking ICE catheter was constructed with 3 complementary technologies: a CMUT imaging array with a custom electronic array buffer, catheter surface electrodes for EAM guidance, and a special ablation tip, that permits simultaneous TSI and RFA. In vivo imaging studies of 5 anesthetized porcine models with 5 CMUT catheters were performed.

RESULTS

The ML-CMUT ICE catheter provided high-resolution real-time wideband 2-dimensional (2D) images at greater than 8 MHz and is capable of both RFA and EAM guidance. Although the 24-element array aperture dimension is only 1.5 mm, the imaging depth of penetration is greater than 30 mm. The specially designed ultrasound-compatible metalized plastic tip allowed simultaneous imaging during ablation and direct acquisition of TSI data for tissue ablation temperatures. Postprocessing analysis showed a first-order correlation between TSI and temperature, permitting early development temperature-time relationships at specific myocardial ablation sites.

CONCLUSIONS

Multifunctional forward-looking ML-CMUT ICE catheters, with simultaneous intracardiac guidance, ultrasound imaging, and RFA, may offer a new means to improve interventional ablation procedures.

The link between tissue elasticity and thermal dose in vivo

The objective of this study was to investigate in vivo the relationship between stiffness and thermal dose. For this purpose, shear wave elastography (SWE)--a novel ultrasound-based technique for real-time mapping of the stiffness of biological soft tissues--is performed in temperature-controlled experiments. Experiments were conducted on nine anesthetized rats. Their right leg was put in a thermo-regulated waterbath. The right leg of each animal was heated at one particular temperature between 38 °C and 48.5 °C for 15 min to 3 h. Shear waves were generated in the muscle using the acoustic radiation force induced by a linear ultrasonic probe. The shear wave propagation was imaged in real time by the probe using an ultrafast scanner prototype (10,000 frames s⁻¹). The local tissue stiffness was derived from the shear wave speed. Two optical fiber sensors were inserted into the muscle to measure in situ the temperature. Stiffness was found to increase strongly during the experiments. When expressed as a function of the thermal dose, the stiffness curves were found to be the same for all experiments. A thermal dose threshold was found at 202 min for an eightfold stiffness increase. Finally, the time-temperature relationship was established for different stiffness ratios. The slope of the time-temperature relationship based on stiffness measurements was found identical to the one obtained for cell death in the seminal paper on the thermal dose by Sapareto and Dewey in 1984 (Int. J. Radiat. Oncol. Biol. Phys. 10 787-800). The present results highlight the stiffness increase as a good indicator of thermal necrosis. SWE imaging can be used in vivo for necrosis threshold determination in thermal therapy.

Thermal strain imaging: a review

Thermal strain imaging (TSI) or temporal strain imaging is an ultrasound application that exploits the temperature dependence of sound speed to create thermal (temporal) strain images. This article provides an overview of the field of TSI for biomedical applications that have appeared in the literature over the past several years. Basic theory in thermal strain is introduced. Two major energy sources appropriate for clinical applications are discussed. Promising biomedical applications are presented throughout the paper, including non-invasive thermometry and tissue characterization. We present some of the limitations and complications of the method. The paper concludes with a discussion of competing technologies.