Background
High-intensity focused ultrasound (HIFU) can achieve accurate and focused deep tissue ablation through an extracorporeal emission. Cardiac ablation using HIFU applied transthoracically must overcome potential interference from intervening thoracic structures. The aim of this study was to explore the efficacy and safety of septal ablation that was induced using transthoracic HIFU.
Methods
Twenty-one canines were pretreated to improve acoustic transmission. Single ablations were induced by targeting transthoracic HIFU with acoustic power of 400 W for 3 sec at the middle and basal septum in eight canines. Extended ablations were performed to create larger lesions at the basal septum in eight more canines. The three-dimensional morphology of a basal septum lesion induced by a single ablation was analyzed. The temperature at the ablative targets was measured in the other five canines.
Results
The cardiomyocytes in the lesions underwent necrosis with a clear boundary. The three-dimensional morphology of the lesions appeared approximately as ellipsoids with a flatter endocardial side. The peak temperature at a power of 400 W for 3 sec was 93.27 ± 2.54°C, and it remained >50°C for nearly 10 sec. No procedure-related complications were observed.
Conclusions
Ultrasound-guided transthoracic HIFU has the potential to safely create small dot or large mass lesions in the septum without a thoracotomy or a catheter.
High-intensity focused ultrasound (HIFU) can noninvasively ablate internal tissues by way of an extracorporeal emission without injuring the intervening tissues. This technology has been used to treat benign and malignant tumors of the bone, liver, kidneys, breast, uterus, and prostate for >10 years, and it was recently used for renal sympathetic denervation. Previous studies have demonstrated the feasibility and ease of HIFU application for abating cardiac tissue in animal models. However, cardiac ablation using HIFU applied transthoracically might not be feasible, because of potential interference by, or damage to, intervening thoracic structures.
Reflections at the skin and soft tissue–bone interfaces should be taken into account when HIFU is transthoracically applied. Reflections may result in local temperature increases and damage to surrounding tissues if the acoustic intensity is high enough. Postfocal tissues, such as the right lung and esophagus, are sensitive to acoustic energy because of their gas containment. To deposit large amounts of energy deep in the septum without causing damage to tissues in the prefocal and postfocal regions, it is necessary to use a large aperture system with a short focal zone, which delivers acoustic energy with a beam that has a convergence angle.
The purpose of this study was to explore the acute feasibility and safety of transthoracic HIFU-induced septal ablation in a canine model.
Methods
HIFU Apparatus for Transthoracic Septal Ablation
The model JC200 HIFU tumor therapeutic system (Chongqing Haifu Technology Co. Ltd., Chongqing, China) was used. This system includes the following components: an ultrasound therapeutic transducer, an ultrasound imaging unit, a degassed water circulation unit, a motion unit, an operator console, and an ultrasound signal generator ( Figure 1 ). The therapeutic transducer has a diameter of 22 cm, a focal length of 148 mm, and an operating frequency of 1.03 MHz. The acoustic focal region is an ellipsoid with dimensions of 6 mm along the beam axis and 2 mm in the transverse direction. The acoustic power of the therapeutic transducer varies continuously from 24 to 450 W, with an acoustic intensity at the focus ranging from 1.2 to 22.5 kW/cm 2 under ideal conditions. The delivery duration time of ultrasound energy is accurately calculated in seconds. An imaging transducer with a frequency of 3.5 MHz is coaxially mounted with the therapeutic transducer as a combined treatment head, which is immersed in a therapeutic chamber filled with circulating degassed water ( Figure 2 ) to guide the localization of target tissue and to monitor the therapeutic effects in real time. The circulation of degassed water has a gas ratio < 3 ppm and a temperature range of 10°C to 40°C. The spatial position of the combined treatment head and the direction of the imaging transducer can be independently controlled. All of the manipulations were visualized and documented.
Animal Preparation
The experimental protocol was approved by the institutional ethics committee of Chongqing Medical University. Animal use and care were in compliance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals . Twenty-one healthy canines (weight, 12–15 kg; purchased from the Experimental Animal Care Center of Chongqing Medical University) of either sex were used. The canines were divided into three groups for the following treatments: group A, single ablation ( n = 8); group B, extended ablations ( n = 8); and group C, ablative temperature measurement ( n = 5).
All canines were anesthetized using an intraperitoneal injection of 3% sodium phenobarbital (30 mg/kg) and then underwent endotracheal intubation. Succinylcholine (1.5 mg/kg) was administered intravenously, and mechanical ventilation was continuously performed. Propranolol hydrochloride (initial dose, 0.75 mg/kg) was administered sublingually 1 hour before the ablation to reduce the heart rate and arrhythmia risk. Lidocaine (1.5 mg/kg) was injected intravenously 5 min before the ablation to prevent ventricular fibrillation.
Improving Ultrasound Transmission
The fur along the acoustic path was removed with 10% soluble sodium sulfide, and the skin was degreased using 75% ethanol and degassed using an electric suction apparatus.
An artificial hydrothorax was established in each canine by injecting 300 to 350 mL normal saline (preheated at 38°C) into the pleural cavity. The injection site was at the seventh or eighth intercostal space in the left posterior-axillary line. Necessary precautions were taken to avoid pneumothorax.
Ablation Protocol
The canines were placed on the treatment bed in a left lateral recumbent position. The left chest wall was immersed in the degassed water (providing acoustic coupling between the transducer and skin) in the therapeutic chamber. A clear image of the heart was acquired by adjusting the imaging transducer and the position of the canine. The HIFU focus could be set easily on the target in the interventricular septum (IVS) using ultrasound guidance.
For the eight canines in group A, a single ablation was performed by targeting the middle and basal septum to assess the ablation feasibility. Extended ablations were performed to enlarge the volume of the lesions in group B. A spatial array of 2 × 2 targets was established at the basal septum by moving the focus and altering the ultrasound view. The targets were kept at approximately 10 mm from the valves and central fibrous body ( Figure 3 ). The distance interval of the targets was 3 mm. Single ablations were repeated five times for each target. The power used for a single ablation was 400 W (the acoustic intensity was 20 kW/cm 2 ) for 3 sec. The time interval for each ablation was 20 sec.
All ablations were performed during a pause in mechanical ventilation and monitored by echocardiography in real time. The electrocardiogram was recorded using the RM6240 Physiology Signal Collection Processing System (Chengdu Instrument Company, Chengdu, China).
Ablative Temperature Measurement
An ultrasound-guided 14-gauge needle centesis was placed in the fifth intercostal space of the canine’s right chest wall in group C for ablative temperature measurements. An optical fiber temperature sensor (Luxtron FOT Lab Kit; LumaSense Technologies, Santa Clara, CA) was inserted through the needle into the middle septum. The needle was then removed, leaving the tip of the sensor in the middle septum. Three targets were set at 2 mm around the sensor tip. The acoustic power was 400 W for 2, 3, and 4 sec. All temperature data were synchronously recorded using TrueTemp software (LumaSense Technologies).
Morphologic and Histologic Evaluation
All canines were euthanized 4 days after ablation while the electrocardiogram was recorded. Multiple tissues (skin, ribs, and lungs) along the acoustic path and cardiac tissues (main coronary arteries, valves, and chordae tendineae) were carefully collected for gross and histologic examination. The IVS was sectioned into slices (2 mm/slice) to detect lesion gross morphologic features. The lengths, widths, and thicknesses of each lesion in the basal septum were measured. Selected tissue blocks were stained with a 2% solution of triphenyltetrazolium chloride (Sigma; St. Louis, MO). The tissues also underwent histologic examination.
Three-Dimensional (3D) Morphologic Analysis
To determine the influence of heat motion on lesion shapes in the basal septum, 5 single-ablation specimens from the basal septum were collected for 3D morphologic analysis. Serial tissue sections and hematoxylin-eosin staining of each specimen were performed. Photomicrograph images of the sections were acquired and then reconstructed using 3D-doctor 4.0 software (Able Software Corporation, Lexington, MA).
Statistical Analysis
Data are expressed as mean ± SD.
Results
Overall Procedural Considerations
Each dog received an artificial hydrothorax without obvious complications. No vital sign disturbances were observed. The artificial hydrothorax successfully improved the ultrasound transmission ( Figure 4 ). All of the canines, except five from group C, underwent HIFU-induced ablation and were maintained with normal respiratory rhythm, appetite, and activity level. The canines in group C were sacrificed because of the tamponade caused by retrieving the optic fiber after the temperature measurement.
Lesion Morphologic and Histologic Features
All lesions induced by transthoracic HIFU in the IVS were excised at the end of the experiment. Gross lesion examination showed coagulation necrosis with a clear boundary and intact endocardium ( Figures 5 A and 5B). The lesion induced by a single ablation was small with a round or oval section. The larger volume lesions created by an extended ablation in the basal septum were 8.5 ± 0.8 mm in length, 7.9 ± 0.8 mm in width, and 5.3 ± 0.2 mm in depth ( Figures 5 C and 5D). Triphenyltetrazolium chloride staining further visualized the lesions, displaying a clear white area easily distinguished from the surrounding dark red tissue or a dark brown area surrounded by a white zone.
Lesion histologic evaluation displayed necrosis with a clear boundary. The cardiomyocytes in the ablative area underwent significant morphologic changes, including blurring and/or disappearance of the cytoplasmic membrane and transverse striation. Cell death features were also evident in the ablative area. The cardiomyocytes in the surrounding untreated area were normal ( Figures 6 A and 6B). The morphologic features of the endothelial nuclei remained normal in the lesions adjacent to the endocardium ( Figures 6 C and 6D).
