Noninvasive transesophageal cardiac thermal ablation using a 2-D focused, ultrasound phased array: a simulation study

Myocardial Thermal Ablation with a Transesophageal High-Intensity Focused Ultrasound Probe: Experiments on Beating Heart Models

Described here is a study of transesophageal thermal ablation of isolated and perfused beating hearts and non-human primates. An endoscope integrating a transesophageal echocardiography probe and a high-intensity focused ultrasound transducer was built and tested on five Langendorff-isolated hearts and three 30-kg baboons. B-Mode ultrasound, passive elastography and magnetic resonance imaging were performed to monitor thermal lesions. In isolated hearts, continuous and gated sonication parameters were evaluated with acoustic intensities of 9-12 W/cm². Sonication parameters of gated exposures with 12 W/cm² acoustic intensity for 5 min consistently produced visible lesions in the ventricles of isolated hearts. In animals, left atria and ventricles were exposed to repeated continuous sonications (4-15 times for 16 s) at an acoustic intensity at the surface of the transducer of 9 W/cm². Clinical states of the baboons during and after the treatment were good. One suspected lesion in the left ventricle could be evidenced by elastography, but was not confirmed by magnetic resonance imaging. The transesophageal procedure therefore has the potential to create thermal lesions in beating hearts and its safety in clinical practice seems promising. However, further technical exploration of the energy deposition in the target would be necessary before the next pre-clinical experiments.

Ultrasound-Guided Transesophageal High-Intensity Focused Ultrasound Cardiac Ablation in a Beating Heart: A Pilot Feasibility Study in Pigs

Catheter ablation for the treatment of arrhythmia is associated with significant complications and often-repeated procedures. Consequently, a less invasive and more efficient technique is required. Because high-intensity focused ultrasound (HIFU) enables the generation of precise thermal ablations in deep-seated tissues without harming the tissues in the propagation path, it has the potential to be used as a new ablation technique. A system capable of delivering HIFU into the heart by a transesophageal route using ultrasound (US) imaging guidance was developed and tested in vivo in six male pigs. HIFU exposures were performed on atria and ventricles. At the time of autopsy, visual inspection identified thermal lesions in the targeted areas in three of the animals. These lesions were confirmed by histologic analysis (mean size: 5.5 mm(2) × 11 mm(2)). No esophageal thermal injury was observed. One animal presented with bradycardia due to an atrio-ventricular block, which provides real-time confirmation of an interaction between HIFU and the electrical circuits of the heart. Thus, US-guided HIFU has the potential to minimally invasively create myocardial lesions without an intra-cardiac device.

Cardiac shear-wave elastography using a transesophageal transducer: application to the mapping of thermal lesions in ultrasound transesophageal cardiac ablation

Heart rhythm disorders, such as atrial fibrillation or ventricular tachycardia can be treated by catheter-based thermal ablation. However, clinically available systems based on radio-frequency or cryothermal ablation suffer from limited energy penetration and the lack of lesion's extent monitoring. An ultrasound-guided transesophageal device has recently successfully been used to perform High-Intensity Focused Ultrasound (HIFU) ablation in targeted regions of the heart in vivo. In this study we investigate the feasibility of a dual therapy and imaging approach on the same transesophageal device. We demonstrate in vivo that quantitative cardiac shear-wave elastography (SWE) can be performed with the device and we show on ex vivo samples that transesophageal SWE can map the extent of the HIFU lesions. First, SWE was validated with the transesophageal endoscope in one sheep in vivo. The stiffness of normal atrial and ventricular tissues has been assessed during the cardiac cycle (n = 11) and mapped (n = 7). Second, HIFU ablation has been performed with the therapy-imaging transesophageal device in ex vivo chicken breast samples (n  =  3), then atrial (left, n = 2) and ventricular (left n = 1, right n = 1) porcine heart tissues. SWE provided stiffness maps of the tissues before and after ablation. Areas of the lesions were obtained by tissue color change with gross pathology and compared to SWE. During the cardiac cycle stiffness varied from 0.5   ±   0.1 kPa to 6.0   ±   0.3 kPa in the atrium and from 1.3   ±   0.3 kPa to 13.5   ±   9.1 kPa in the ventricles. The thermal lesions were visible on all SWE maps performed after ablation. Shear modulus of the ablated zones increased to 16.3   ±   5.5 kPa (versus 4.4   ±   1.6 kPa before ablation) in the chicken breast, to 30.3   ±   10.3 kPa (versus 12.2   ±   4.3 kPa) in the atria and to 73.8   ±   13.9 kPa (versus 21.2   ±   3.3 kPa) in the ventricles. On gross pathology, the size of the lesions ranged from 0.1 to 1.5 cm(2) in the imaging plane area. Elasticity-estimated depths and widths of the lesions differed respectively with a median of 0.2 mm (first quartile Q1:  -0.8 mm; third quartile Q3: 2.6 mm) for a mean squared error (MSE) of 5.1 mm(2) and a median of 0.2 mm (Q1:  -2.7 mm; Q3: 2.7 mm) for a MSE of 11.1 mm(2) from gross pathology. We have demonstrated the feasibility of the HIFU thermal ablation monitoring using a dual therapy and imaging transesophageal device. The combination of HIFU, ultrasound imaging and SWE on the same transesophageal system could lead to a new clinical device for a safer and controlled treatment of a wide variety of cardiac arrhythmias.

Catheter-based ultrasound technology for image-guided thermal therapy: current technology and applications

Catheter-based ultrasound (CBUS) is applied to deliver minimally invasive thermal therapy to solid cancer tumours, benign tissue growth, vascular disease, and tissue remodelling. Compared to other energy modalities used in catheter-based surgical interventions, unique features of ultrasound result in conformable and precise energy delivery with high selectivity, fast treatment times, and larger treatment volumes. We present a concise review of CBUS technology being currently utilized in animal and clinical studies or being developed for future applications. CBUS devices have been categorised into interstitial, endoluminal and endovascular/cardiac applications. Basic applicator designs, site-specific evaluations and possible treatment applications have been discussed in brief. Particular emphasis has been given to ablation studies that incorporate image guidance for applicator placement, therapy monitoring, feedback control, and post-procedure assessment. Examples of devices included here span the entire spectrum of the development cycle from preliminary simulation-based design studies to implementation in clinical investigations. The use of CBUS under image guidance has the potential for significantly improving precision and applicability of thermal therapy delivery.

Emerging non-cancer applications of therapeutic ultrasound

Ultrasound therapy has been investigated for over half a century. Ultrasound can act on tissue through a variety of mechanisms, including thermal, shockwave and cavitation mechanisms, and through these can elicit different responses. Ultrasound therapy can provide a non-invasive or minimally invasive treatment option, and ultrasound technology has advanced to the point where devices can be developed to investigate a wide range of applications. This review focuses on non-cancer clinical applications of therapeutic ultrasound, with an emphasis on treatments that have recently reached clinical investigations, and preclinical research programmes that have great potential to impact patient care.

Design and evaluation of a transesophageal HIFU probe for ultrasound-guided cardiac ablation: simulation of a HIFU mini-maze procedure and preliminary ex vivo trials

Atrial fibrillation (AF) is the most frequent cardiac arrhythmia. Left atrial catheter ablation is currently performed to treat this disease. Several energy sources are used, such as radio-frequency or cryotherapy. The main target of this procedure is to isolate the pulmonary veins. However, significant complications caused by the invasive procedure are described, such as stroke, tamponade, and atrioesophageal fistula, and a second intervention is often needed to avoid atrial fibrillation recurrence. For these reasons, a minimally-invasive device allowing performance of more complex treatments is still needed. High-intensity focused ultrasound (HIFU) can cause deep tissue lesions without damaging intervening tissues. Left atrial ultrasound-guided transesophageal HIFU ablation could have the potential to become a new ablation technique. The goal of this study was to design and test a minimally-invasive ultrasound-guided transesophageal HIFU probe under realistic treatment conditions. First, numerical simulations were conducted to determine the probe geometry, and to validate the feasibility of performing an AF treatment using a HIFU mini-maze (HIFUMM) procedure. Then, a prototype was manufactured and characterized. The 18-mm-diameter probe head housing contained a 3-MHz spherical truncated HIFU transducer divided into 8 rings, with a 5-MHz commercial transesophageal echocardiography (TEE) transducer integrated in the center. Finally, ex vivo experiments were performed to test the impact of the esophagus layer between the probe and the tissue to treat, and also the influence of the lungs and the vascularization on lesion formation. First results show that this prototype successfully created ex vivo transmural myocardial lesions under ultrasound guidance, while preserving intervening tissues (such as the esophagus). Ultrasound-guided transesophageal HIFU can be a good candidate for treatment of AF in the future.

Modelling of endoluminal and interstitial ultrasound hyperthermia and thermal ablation: applications for device design, feedback control and treatment planning

Endoluminal and catheter-based ultrasound applicators are currently under development and are in clinical use for minimally invasive hyperthermia and thermal ablation of various tissue targets. Computational models play a critical role in device design and optimisation, assessment of therapeutic feasibility and safety, devising treatment monitoring and feedback control strategies, and performing patient-specific treatment planning with this technology. The critical aspects of theoretical modelling, applied specifically to endoluminal and interstitial ultrasound thermotherapy, are reviewed. Principles and practical techniques for modeling acoustic energy deposition, bioheat transfer, thermal tissue damage, and dynamic changes in the physical and physiological state of tissue are reviewed. The integration of these models and applications of simulation techniques in identification of device design parameters, development of real time feedback-control platforms, assessing the quality and safety of treatment delivery strategies, and optimisation of inverse treatment plans are presented.

MRIgHIFU: a tool for image-guided therapeutics

High-intensity focused ultrasound (HIFU) provides focal delivery of mechanical energy deep into the body. This energy can be used to elevate the tissue temperature to such a degree that ablation is achieved. The elevated temperature can also be used to release drugs from temperature-sensitive carriers or activate therapeutic molecules using mechanical or thermal energy. Lower dose exposures modify the vasculature to allow large molecules to diffuse from blood in the surrounding tissue for local drug delivery. The energy delivery can be targeted and monitored using magnetic resonance imaging (MRI). The online image guidance and monitoring provides treatment delivery that is customized to each patient such that optimal, effective treatment can be achieved. This ability to localize and customize treatment delivery may further enhance the future potential of targeted drugs that are personalized for each patient. This review examines the rapid development of MRI-guided HIFU (MRIgHIFU) methods over the past few years and discuss their future potential.

Optimal ultrasonic array focusing in attenuative media

This paper presents a parametric study on the efficiency of ultrasound focusing in an attenuative medium, using phased arrays. Specifically, an analytical model of ultrasound wave focusing in a homogeneous, isotropic and attenuative fluid with point sources is presented. Calculations based on the model have shown that in an attenuative medium, an optimum frequency exists for the best focusing performance for a particular size of aperture and focal distance. The effect of different f numbers on the focusing performance in the attenuative medium is further investigated. The information obtained from the analytical model provides insights into the design and installation of a phased transducer array for energy efficient wave focusing.

Feasibility of in vivo transesophageal cardiac ablation using a phased ultrasound array

Over 2.2 million Americans suffer from atrial fibrillation making it one of the most common arrhythmias. Cardiac ablation has shown a high rate of success in treating paroxysmal atrial fibrillation. Prevailing modalities for this treatment are catheter based radio-frequency ablation or surgery. However, there is measurable morbidity and significant costs and time associated with these invasive procedures. Due to these issues, developing a method that is less invasive to treat atrial fibrillation is needed. In the development of such a device, a transesophageal ultrasound applicator for cardiac ablation was designed, constructed and evaluated. A goal of this research was to create lesions in myocardial tissue using a phased array. Based on multiple factors from array simulations, transesophageal imaging devices and throat anatomy, a phased ultrasound transducer that can be inserted into the esophagus was designed and tested. In this research, a two-dimensional sparse phased array with the aperture size of 20.7 mm x 10.2 mm with flat tapered elements as a transesophageal ultrasound applicator was fabricated and evaluated with in vivo experiments. Five pigs were anesthetized; the array was passed through the esophagus and positioned over the heart. The array was operated for 8-15 min at 1.6 MHz with the acoustic intensity of 150-300 W/cm(2) resulting in both single and multiple lesions on atrial and ventricular myocardium. The average size of lesions was 5.1 +/- 2.1 mm in diameter and 7.8 +/- 2.5 mm in length. Based on the experimental results, the array delivered sufficient power to the focal point to produce ablation while not grossly damaging nearby tissue outside the target area. These results demonstrate a potential application of the ultrasound applicator to transesophageal cardiac surgery in atrial fibrillation treatment.

New design for an endoesophageal sector- based array for the treatment of atrial fibrillation: a parametric simulation study

Atrial fibrillation (AF) is the most frequent and sustained cardiac arrhythmia affecting humans. The electrical isolation by ablation of the pulmonary veins (PV) in the left atrium (LA) of the heart has proved to be an effective cure for the AF. The ablation consists mainly of the formation of a localized circumferential thermal coagulation of the cardiac tissue surrounding the PVs. In this article, a parametric study was carried out to establish an optimal configuration of endesophageal ultrasound phased arrays intended to treat the AF. The devices are spherical-surface sections truncated at 15 mm, with a depth of 4 mm, and they are cut in concentric-rings, each composed of independently driven sectors. The number of independent elements (N(e)) was minimized for different values of ratio of pressure amplitude of the secondary lobe over the main lobe (eta) of 0.35, 0.4, 0.45, and 0.5 inside a volume of interest (VOI). After assuming a Cartesian system with the origin in the center of the device, the VOI was defined as the prism enclosed by the coordinates (-12, 10, -9) mm and (12, 37, 9) mm. The VOI has its center at (0, 23.5, 0) mm and is large enough to contain all the targets identified in the Visible Human Project Male specimen. Operating at 1 MHz, eta and N(e)were calculated in function of the element size and focal length (F). Four devices for each value of eta were found. After keeping values of F and normalized dimensions of the independent elements in terms of wavelength, higher frequencies were considered: 1.25 MHz, 1.5 MHz, and 2 MHz. In total, 16 device configurations were obtained. Realistic modeling of lesion formation in the heart chamber showed that the 16 configurations were able to produce the typical lesion used to treat the AF while preserving surrounding structures. At higher frequencies, lower power was required, and a greater number of array elements was required. For an exposure of 5 s and a maximum temperature of 70 degrees C, the average (+/-s.d.) acoustical intensity at transducer surface varied from 22.3(+/-5.8) W/cm(2) for a device with F = 98 mm at 1 MHz to 5.8(+/-1.2) W/cm(2) for a device with F = 186 mm at 2 MHz, while requiring 319 and 2093 elements, respectively, and achieving values of eta of 0.5 and 0.41, respectively. For the intended application, the selected devices implied a better focusing when compared with more traditional planar 2-D arrays, while requiring less power and fewer independent elements.

Lateral mode coupling to reduce the electrical impedance of small elements required for high power ultrasound therapy phased arrays

A method that uses lateral coupling to reduce the electrical impedance of small transducer elements in generating ultrasound waves was tested. Cylindrical, radially polled transducer elements were driven at their length resonance frequency. Computer simulation and experimental studies showed that the electrical impedance of the transducer element could be controlled by the cylinder wall thickness, while the operation frequency was determined by the cylinder length. Acoustic intensity (averaged over the cylinder diameter) over 10 W / cm(2) (a therapeutically relevant intensity) was measured from these elements.

Circumferential lesion formation around the pulmonary veins in the left atrium with focused ultrasound using a 2D-array endoesophageal device: a numerical study

Atrial fibrillation (AF) is the most frequently sustained cardiac arrhythmia affecting humans. The electrical isolation by ablation of the pulmonary veins (PVs) in the left atrium (LA) of the heart has been proven as an effective cure of AF. The ablation consists mainly in the formation of a localized circumferential thermal coagulation of the cardiac tissue surrounding the PVs. In the present numerical study, the feasibility of producing the required circumferential lesion with an endoesophageal ultrasound probe is investigated. The probe operates at 1 MHz and consists of a 2D array with enough elements (114 x 20) to steer the acoustic field electronically in a volume comparable to the LA. Realistic anatomical conditions of the thorax were considered from the segmentation of histological images of the thorax. The cardiac muscle and the blood-filled cavities in the heart were identified and considered in the sound propagation and thermal models. The influence of different conditions of the thermal sinking in the LA chamber was also studied. The circumferential ablation of the PVs was achieved by the sum of individual lesions induced with the proposed device. Different scenarios of lesion formation were considered where ultrasound exposures (1, 2, 5 and 10 s) were combined with maximal peak temperatures (60, 70 and 80 degrees C). The results of this numerical study allowed identifying the limits and best conditions for controlled lesion formation in the LA using the proposed device. A controlled situation for the lesion formation surrounding the PVs was obtained when the targets were located within a distance from the device in the range of 26 +/- 7 mm. When combined with a maximal temperature of 70 degrees C and an exposure time between 5 and 10 s, this distance ensured preservation of the esophageal structures, controlled lesion formation and delivery of an acoustic intensity at the transducer surface that is compatible with existing materials. With a peak temperature of 70 degrees C, the device and setup presented here induced highly localized lesions with a lesion volume varying from 10 +/- 4 to 18 +/- 7 mm(3) for an ultrasound exposure between 5 and 10 s, respectively, while the intensity varied from 26 +/- 7 to 20 +/- 6 W cm(-2).