Optimization of the Thin Waveguide for Double-Parabolic-Reflectors Ultrasonic Transducers (DPLUS) for Thermal Ablation.

OBJECTIVE: Interstitial and intracavitary ultra- sound applicators had been developed and studied for minimally invasive treatments (MIT). However, the acoustic outputs are limited by the small-size PZT. We therefore studied the acoustic waveguide (AW) applicator which enables the use of a large-size PZT, and we aimed to advance AW applicators towards thermal ablation applications. METHODS: Double parabolic reflectors wave-guided ultrasonic transducer (DPLUS) was introduced which has two parabolic reflectors for enhancing the acoustic output. Theoretical modeling was conducted for optimizing the DPLUS thin waveguide. RESULTS: Modeling results showed that optimal a/Λ (thin waveguide radius/wavelength) can be found and the optimal a depends on the excitable vibration amplitude in the thin waveguide. A local optimal a/Λ= 0.2392 was considered the best choice, which results in the optimal frequency of 2.2 MHz at the radius a of 0.6 mm. To verify this optimal frequency, experiments under two working frequencies of 1.0282 MHz and 2.2579 MHz were conducted. Temperature rise curves in the chicken breast tissue showed good agreements between experiments and modeling results, which proved the effectiveness of the modeling. In addition, experiments showed an ablated area with a diameter of 1.03±0.12 mm under continuous excitation of 2.2579 MHz and 5 s. CONCLUSION: The developed DPLUS advanced the AW applicators towards thermal ablation applications. SIGNIFICANCE: This study provides the evidence for recognizing AW applicators as a technique for thermal ablation.

Processed skin surface images acquired by acoustic impedance difference imaging using the ultrasonic interference method: a pilot study.

To clarify the potential of a novel system using the acoustic impedance difference imaging (AIDI) method for diagnosis of skin disorders, we used it on a coin and swine skin. An ultrasound wave with a central frequency of 20 MHz, emitted from a fused quartz rod with a diameter of 1.25 mm, was focused on the surface of the coin and skin samples. The difference in acoustic impedance was determined by the reflection-type interference-based acoustic impedance measurement method. The processed data were produced as greyscale images on which the maximum measured amplitudes were mapped. We applied the method to a coin. Swine skin, burned and covered with an acrylic sheet with a thickness of 0.2 mm (a few times the half-wavelength) to eliminate the undulations of the skin surface, was employed to obtain processed images from which undulation data were excluded. All the processed images obtained corresponded almost exactly with the magnified optical ones. In the processed images of swine skin, a marked difference was found after the burning procedure. The processed images obtained using the AIDI method reflected not only the undulations but also other information such as elasticity. In conclusion, our system using AIDI has the potential to become a useful modality for the diagnosis of skin disorders.

Transmission of 100-MHz-range ultrasound through a fused quartz fiber.

PURPOSE: This paper describes an investigation into direct observation of microscopic images of tissue using a thin acoustic wave guide. METHODS: First, the characteristics of the ultrasonic wave propagated in a fused quartz fiber were measured using the reflection method in order to study the insertion loss and the frequency shift of the ultrasonic wave transmitted from the transducer. Next, a receiving transducer was placed close to the end of the fiber, and the characteristics of the ultrasonic waves propagated through the acoustic coupling medium were measured using the penetration method in order to study the insertion loss and the frequency-dependent attenuation of the penetrated waves. Finally, a C-mode image was obtained by optimizing the measuring conditions using the results of the above measurements and scanning the ultrasonic beams on a target (coin) in water. RESULTS: A reflected wave with a peak frequency of approximately 220 MHz was obtained from the end of the fiber. The transmitted ultrasonic waves propagated through the acoustic coupling medium were detected with a frequency range of approximately 125-170 MHz, and the maximum detectable distance of the waves was approximately 1.2 mm within the 100-MHz frequency range. Finally, a high-frequency C-mode image of a coin in water was obtained using a tapered fused quartz fiber. CONCLUSION: The results suggest that it is necessary to improve the signal-to-noise ratio and reduce the insertion loss in the experimental system in order to make it possible to obtain microscopic images of tissue.

Double-Parabolic-Reflectors Ultrasonic Transducer With Flexible Waveguide for Minimally Invasive Treatment.

OBJECTIVE: To treat tissues that are difficult to access, ultrasound based minimally invasive treatment (MIT) is promising. However, high-power ultrasound delivery through waveguides had been difficult which can increase treatment duration. It is our effort to design the waveguide that can transmit powerful ultrasound. METHODS: The waveguide with two parabolic reflectors was proposed by us to produce high-energy-density plane wave. Use of flexible and long thin waveguide was demonstrated here. RESULTS: Double Parabolic refLectors wave-guided high-power Ultrasonic tranSducer (DPLUS) including a ϕ1 mm ×1 m Nitinol thin waveguide was fabricated. It was shown that high-power ultrasound between 1 to 2 MHz can be propagated through the thin waveguide. Low-loss waveguide material was confirmed to be important to enhance output. As ultrasound is transmitted into working medium, energy mainly flows from the side surface. Temperature of target soft tissue was demonstrated to drastically increase by 10 degree in 30 seconds. CONCLUSION: The developed DPLUS makes high-power ultrasound transmission in long and flexible thin waveguide possible. SIGNIFICANCE: The concept of DPLUS for delivering high-power ultrasound is powerful in the field of Ultrasonics.

Wideband Multimode Excitation by a Double-Parabolic-Reflector Ultrasonic Transducer.

This research presents double-parabolic-reflector wave-guided ultrasonic transducers in order to realize wideband (0-2.5 MHz), multiharmonic mode excitations (over 20 modes), and to obtain large mechanical/acoustic outputs. The double-parabolic-reflector mechanism serves as a horn structure at low frequencies and acoustic-focusing structure at high frequencies to enhance the energy density of the incident ultrasound. Upon combining simulation and experimental methods, we examined and verified the basic performance and working mechanisms of the double-parabolic-reflector waveguides: multimode excitation belongs to the harmonic modes from the thin waveguide. At the megahertz range near the thickness mode of the piezoelectric element (PZT), energy density of the incident ultrasound is enhanced by double-parabolic reflections, and the amplification ranges 10 to 40× between 1 and 2.5 MHz. At burst excitations, the amplification performance is independent of the length of the thin waveguide. Compared with conventional Langevin transducers and high-intensity focused ultrasound (HIFU) transducers, our transducers possess a wide working frequency with large mechanical/acoustic outputs and large vibration velocity amplification. By introducing these new features, our proposed method is a promising candidate for examining basic physics parameters, such as frequency dependence, in the fields of medicine, biology, industry, etc.

Double-parabolic-reflectors acoustic waveguides for high-power medical ultrasound.

High intensity focused ultrasound therapeutics are widely used to noninvasively treat various types of primary tumors and metastasis. However, ultrasound penetration depth is shallowed with increasing frequency which limits the therapeutic accuracy for deep tissues. Although acoustic waveguides are commonly inserted into tissue for localized therapy, powerful ultrasound delivery is difficult. Here, we invent double-parabolic-reflectors acoustic waveguides, where high-power ultrasound emission and large mechanical vibration enhance the therapeutic efficiency. High-energy-density ultrasound with around 20 times amplification by two parabolic reflectors propagates through the thin waveguide between 1 to 2 MHz, and wideband large mechanical vibration at the waveguide tip from 1 kHz to 2.5 MHz accelerates the therapeutics. This fundamental work serves as a milestone for future biomedical applications, from therapeutics to diagnostics. Since the high-power ability at high frequencies, our waveguide will also open up new research fields in medical, bio, physics and so on.