Miniaturized Intracavitary Forward-Looking Ultrasound Transducer for Tissue Ablation

Study on the heating law of thermal effect of HIFU on tissue based on piezoelectric ceramic voltage and vibration frequency

When high-intensity focused ultrasound (HIFU) is used to heat human tissues during surgery, shortening the heating time and increasing the heating rate are crucial for implementing surgery. During operations, the vibration frequencies and voltages that are suitable for piezoelectric ceramics can maximize the heating rate of HIFU. To correctly select the frequency and voltage of piezoelectric ceramics, it is necessary to understand the change law between the frequency and voltage of piezoelectric ceramics and the temperature of the heated tissue. Therefore, this study focuses on the piezoelectric ceramics in the ultrasonic transducer that produces HIFU. By coupling the piezoelectric equation, Helmholtz equation, and biological heat transfer equation of piezoelectric ceramics, the solid mechanical properties of piezoelectric ceramics and the sound field and temperature field produced by its vibration are combined to determine the influence of vibration frequency and voltage on heating time. Results show that the relation between the temperature variation of the tissue, vibration frequency, and voltage of the piezoelectric ceramics can be determined using the Rational Taylor function. The maximum voltage that the piezoelectric ceramics with different frequencies can withstand varies greatly.

Ultrasound transducers with both imaging and power output capabilities by anti-matching at backing layers

Precise ultrasound therapy requires long-term power output and imaging guidance. However, traditional therapeutic transducers do not have imaging capabilities due to the low bandwidth and long ringing. Whereas for a diagnostic transducer, 20%–30% of the energy is dissipated as heat in the backing layer, resulting in degradation of the transducer under high duty cycle and high voltage excitation. Therefore, the transducers with both power output and imaging capabilities are unmet demands for precise ultrasonic treatment. To address this problem, we propose an ultrasound transducer with both imaging and power output capabilities. An anti-matching layer is designed at the position of the backing layer of the transducer, which reflects the backward ultrasound waves to forward waves. Therefore, the majority of the energy is transmitted efficiently and little energy is dissipated in the backing layer. Finite element simulations demonstrated that a double-layer anti-matching design reflects 99.8% of the backward energy, resulting in an insertion loss of −27.7 dB. The performance was validated by a transducer prototype with transmission measurement using hydrophone and pulse-echo test. This design of transducers with both imaging and power output capabilities indicates a promising application of self-guided ultrasound therapy.

Intravascular Sono-Ablation for In-Stent Restenosis Relief: Transducer Development and the In-Vitro Demonstration

OBJECTIVE

This study aimed to propose a new clinical modality for the relief of in-stent restenosis (ISR) using focused ultrasound (FUS) ablation. In the first research stage, a miniaturized FUS device was developed for the sonification of the remaining plaque after stenting, known as one of the causes of ISR.

METHODS

This study presents a miniaturized (<2.8 mm) intravascular FUS transducer for ISR treatment. The performance of the transducer was predicted through a structural-acoustic simulation, followed by fabrication of the prototype device. Using the prototype FUS transducer, we demonstrated tissue ablation with bio-tissues over metallic stents, mimicking in-stent tissue ablation. Next, we conducted a safety test by detecting the existence of thermal damage to the arterial tissue upon sonication with a controlled dose.

RESULTS

The prototype device successfully delivered sufficient acoustic intensity (>30 W/cm2) to a bio tissue (chicken breast) through a metallic stent. The ablation volume was approximately 3.9 × 7.8 × 2.6 mm3. Furthermore, 1.5 min sonication was sufficient to obtain an ablating depth of approximately 1.0 mm, not thermally damaging the underlying artery vessel.

CONCLUSION

We demonstrated in-stent tissue sonoablation, suggesting it could be as a future ISR treatment modality.

SIGNIFICANCE

Comprehensive test results provide a key understanding of FUS applications using metallic stents. Furthermore, the developed device can be used for sonoablation of the remaining plaque, providing a novel approach to the treatment of ISR.

A Model of High-Speed Endovascular Sonothrombolysis with Vortex Ultrasound-Induced Shear Stress to Treat Cerebral Venous Sinus Thrombosis

This research aims to demonstrate a novel vortex ultrasound enabled endovascular thrombolysis method designed for treating cerebral venous sinus thrombosis (CVST). This is a topic of substantial importance since current treatment modalities for CVST still fail in as many as 20% to 40% of the cases, and the incidence of CVST has increased since the outbreak of the coronavirus disease 2019 pandemic. Compared with conventional anticoagulant or thrombolytic drugs, sonothrombolysis has the potential to remarkably shorten the required treatment time owing to the direct clot targeting with acoustic waves. However, previously reported strategies for sonothrombolysis have not demonstrated clinically meaningful outcomes (e.g., recanalization within 30 min) in treating large, completely occluded veins or arteries. Here, we demonstrated a new vortex ultrasound technique for endovascular sonothrombolysis utilizing wave-matter interaction-induced shear stress to enhance the lytic rate substantially. Our in vitro experiment showed that the lytic rate was increased by at least 64.3% compared with the nonvortex endovascular ultrasound treatment. A 3.1-g, 7.5-cm-long, completely occluded in vitro 3-dimensional model of acute CVST was fully recanalized within 8 min with a record-high lytic rate of 237.5 mg/min for acute bovine clot in vitro. Furthermore, we confirmed that the vortex ultrasound causes no vessel wall damage over ex vivo canine veins. This vortex ultrasound thrombolysis technique potentially presents a new life-saving tool for severe CVST cases that cannot be efficaciously treated using existing therapies.

Evaluation of the dual-frequency transducer for controlling thermal ablation morphology using frequency shift keying signal

PURPOSE

The catheter-based ultrasound (CBUS) can reach the target tissue directly and achieve rapid treatment. The frequency shift keying (FSK) signal is proposed to regulate and evaluate tumor ablation by a miniaturized dual-frequency transducer.

METHODS

A dual-frequency transducer prototype (3 × 7 × 0.4 mm) was designed and fabricated for the CBUS applicator (OD: 3.8 mm) based on the fundamental frequency of 5.21 MHz and the third harmonic frequency of 16.88 MHz. Then, the acoustic fields and temperature field distributions using the FSK signals (with 0, 25, 50, 75, and 100% third harmonic frequency duty ratios) were simulated by finite element analysis. Finally, tissue ablation and temperature monitoring were performed in phantom and ex vivo tissue, respectively.

RESULTS

At the same input electrical power (20 W), the output acoustic power of the fundamental frequency of the transducer was 10.03 W (electroacoustic efficiencies: 50.1%), and that of the third harmonic frequency was 6.19 W (30.6%). As the third harmonic frequency duty ratios increased, the shape of thermal lesions varied from strip to droplet in simulated and phantom experimental results. The same trend was observed in ex vivo tests.

CONCLUSION

Dual-frequency transducers excited by the FSK signal can control the morphology of lesions.

SIGNIFICANCE

The acoustic power deposition of CBUS was optimized to achieve precise ablation.

Ultrawide Bandwidth High-Frequency Ultrasonic Transducers With Gradient Acoustic Impedance Matching Layer for Biomedical Imaging

The high-frequency ultrasonic transducers with larger bandwidths yield excellent imaging performance in the biomedical field. However, achieving perfect acoustic impedance matching from the piezo-element to the target medium in the operating frequency spectrum is still a challenge. Conventional matching layers are mostly fabricated by only one or two uniform materials which are limited by their acoustic property. We propose a novel composite matching layer with gradient acoustic impedance based on a 1-3 gradient composite structure and multilevel matching theory. The proposed gradient-composite matching layer applied for ultrasonic transducer provides efficient impedance matching and ultrawide bandwidth which can significantly improve the quality of biomedical imaging. The active aperture size of the matching layer is 5× 5 mm², and the overall thickness for five equivalent layers is 115 [Formula: see text]. The -6-dB bandwidth and the center frequency obtained by the ultrasonic transducer equipped with the 1-3 gradient composite matching layer are 141.7% and 22.3 MHz, respectively. The exceedingly good imaging performance of the fabricated ultrasonic transducer was demonstrated by the tungsten wire phantom and study on the biological tissues of a zebrafish and porcine eyeball. The theoretical and experimental results provide a novel train of thought for improving the quality of biomedical ultrasonic imaging.

Dual-Frequency Intravascular Sonothrombolysis: An In Vitro Study

Thrombo-occlusive disease is one of the leading causes of death worldwide. There has been active research on safe and effective thrombolysis in preclinical and clinical studies. Recently, the dual-frequency transcutaneous sonothrombolysis with contrast agents [microbubbles (MBs)] has been reported to be more efficient in trigging the acoustic cavitation, which leads to a higher lysis rate. Therefore, there is increasing interest in applying dual-frequency technique for more significant efficacy improvement in intravascular sonothrombolysis since a miniaturized intravascular ultrasound transducer typically has a limited power output to fully harness cavitation effects. In this work, we demonstrated this efficacy enhancement by developing a new broadband intravascular transducer and testing dual-frequency sonothromblysis in vitro. A broadband intravascular transducer with a center frequency of 750 kHz and a footprint size of 1.4 mm was designed, fabricated, and characterized. The measured -6-dB fractional bandwidth is 68.1%, and the peak negative pressure is 1.5 MPa under the driving voltage of 80 V_pp. By keeping one frequency component at 750 kHz, the second frequency component was selected from 450 to 650 kHz with an interval of 50 kHz. The in vitro sonothrombolysis tests were conducted with a flow model and the results indicated that the MB-mediated, dual-frequency (750+500 kHz) sonothrombolysis yields an 85% higher lysis rate compared with the single-frequency treatment, and the lysis rate of dual-frequency sonothrombolysis increases with the difference between the two frequency components. These findings suggest a dual-frequency excitation technique for more efficient intravascular sonothrombolysis than conventional single-frequency excitation.

A multi-pillar piezoelectric stack transducer for nanodroplet mediated intravascular sonothrombolysis

We aim to develop a nanodroplet (ND)-mediated intravascular ultrasound (US) transducer for deep vein thrombosis treatments. The US device, having an efficient forward directivity of the acoustic beam, is capable of expediting the clot dissolution rate by activating cavitation of NDs injected onto a thrombus. We designed and prototyped a multi-pillar piezoelectric stack (MPPS) transducer composed of four piezoelectric stacks. Each stack was made of five layers of PZT-4 plates, having a dimension of 0.85 × 0.85 × 0.2 mm3. The transducer was characterized by measuring the electrical impedance and acoustic pressure, compared to simulation results. Next, in-vitro tests were conducted in a blood flow mimicking system using the transducer equipped with an ND injecting tube. The miniaturized transducer, having an aperture size of 2.8 mm, provided a high mechanical index of 1.52 and a relatively wide focal zone of 3.4 mm at 80 Vpp, 0.96 MHz electric input. The mass-reduction rate of the proposed method (NDs + US) was assessed to be 4.1 and 4.6 mg/min with and without the flow model, respectively. The rate was higher than that (1.3-2.7 mg/min) of other intravascular ultrasound modalities using micron-sized bubble agents. The ND-mediated intravascular sonothrombolysis using MPPS transducers was demonstrated with an unprecedented lysis rate, which may offer a new clinical option for DVT treatments. The MPPS transducer generated a high acoustic pressure (~3.1 MPa) at a distance of approximately 2.2 wavelengths from the small aperture, providing synergistic efficacy with nanodroplets for thrombolysis without thrombolytic agents.

A Dual-Frequency Lens-Focused Endoscopic Histotripsy Transducer

A forward-looking miniature histotripsy transducer has been developed that incorporates an acoustic lens and dual-frequency stacked transducers. An acoustic lens is used to increase the peak negative pressure through focal gain and the dual-frequency transducers are designed to increase peak negative pressure by summing the pressure generated by each transducer individually. Four lens designs, each with an f -number of approximately 1, were evaluated in a PZT5A composite transducer. The finite-element model (FEM) predicted axial beamwidths of 1.61, 2.40, 2.84, and 2.36 mm for the resin conventional, resin Fresnel, silicone conventional, and silicone Fresnel lenses, respectively; the measured axial beamwidths were 1.30, 2.28, 2.71, and 2.11 mm, respectively. Radial beamwidths from the model were between 0.32 and 0.35 mm, while measurements agreed to within 0.2 mm. The measured peak negative was 0.150, 0.124, 0.160, and 0.160 MPa/V for the resin conventional, resin Fresnel, silicone conventional, and silicone Fresnel lenses, respectively. For the dual-frequency device, the 5-MHz (therapy) transducer had a measured peak negative pressure of 0.136 MPa/V for the PZT5A composite and 0.163 MPa/V for the PMN-PT composite. The 1.2-MHz (pump) transducer had a measured peak negative pressure of 0.028 MPa/V. The pump transducer significantly lowered the cavitation threshold of the therapy transducer. The dual-frequency device was tested on an ex vivo rat brain, ablating tissue at up to 4-mm depth, with lesion sizes as small as [Formula: see text].

Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm²) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm² (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2-1.5 MHz) and low-frequency (5-20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.