Evaluation of a novel therapeutic focused ultrasound transducer based on Fermat's spiral.

The purpose of this study was to evaluate a novel phased array transducer design rule for therapeutic focused ultrasound applications. This design rule uses the discretized Fermat's spiral to determine the positioning of the transducer elements for a given number of elements and f-number. Using this principle, three variations of Fermat's spiral were generated, aimed at (1) grating lobe minimization, (2) side lobe minimization, and (3) an optimized element packing efficiency. For each spiral, sparse layouts using identical circular elements and fully populated layouts based on additional Voronoi tessellation were evaluated numerically. Evaluation criteria included the element size distribution, beam steering capabilities, focal plane pressure distribution, prefocal pressure distribution, and practical considerations. Finally, one Voronoi-tessellated design with a focal length and aperture diameter of 16 cm and a natural frequency of 1.3 MHz was evaluated experimentally through hydrophone measurements. The numerical evaluation showed that while sparse arrays possess superior beam steering capabilities for a given number of elements, the focal point quality and prefocal pressure distribution is substantially more favorable when using the Voronoi-tessellated designs. Beam steering was shown to be feasible with the tessellated designs for lateral deflections up to 10 mm and axial deflections up to 20 mm. The experimental evaluation showed that such a transducer is capable of inducing 40.00 MPa rarefactional and 237.50 MPa compressional peak pressure levels at 800 W instantaneous acoustic output power under free-field conditions, making the system potentially relevant for thermal ablation therapy, histotripsy applications, and shockwave-enhanced heating.

Dual-mode transducers for ultrasound imaging and thermal therapy.

Medical imaging is a vital component of high intensity focused ultrasound (HIFU) therapy, which is gaining clinical acceptance for tissue ablation and cancer therapy. Imaging is necessary to plan and guide the application of therapeutic ultrasound, and to monitor the effects it induces in tissue. Because they can transmit high intensity continuous wave ultrasound for treatment and pulsed ultrasound for imaging, dual-mode transducers aim to improve the guidance and monitoring stages. Their primary advantage is implicit registration between the imaging and treatment axes, and so they can help ensure before treatment that the therapeutic beam is correctly aligned with the planned treatment volume. During treatment, imaging signals can be processed in real-time to assess acoustic properties of the tissue that are related to thermal ablation. Piezocomposite materials are favorable for dual-mode transducers because of their improved bandwidth, which in turn improves imaging performance while maintaining high efficiency for treatment. Here we present our experiences with three dual-mode transducers for interstitial applications. The first was an 11-MHz monoelement designed for use in the bile duct. It had a 25x7.5 mm(2) aperture that was cylindrically focused to 10mm. The applicator motion was step-wise rotational for imaging and therapy over a 360 degrees, or smaller, sector. The second transducer had 5-elements, each measuring 3.0x3.8 mm(2) for a total aperture of 3.0x20 mm(2). It operated at 5.6 MHz, was cylindrically focused to 14 mm, and was integrated with a servo-controlled oscillating probe designed for sector imaging and directive therapy in the liver. The last transducer was a 5-MHz, 64-element linear array designed for beam-formed imaging and therapy. The aperture was 3.0x18 mm(2) with a pitch of 0.280 mm. Characterization results included conversion efficiencies above 50%, pulse-echo bandwidths above 50%, surface intensities up to 30 W/cm(2), and axial imaging resolutions to 0.2 mm. The second transducer was evaluated in vivo using porcine liver, where coagulation necrosis was induced up to a depth of 20 mm in 120 s. B-mode and M-mode images displayed a hypoechoic region that agreed well with lesion depth observed by gross histology. These feasibility studies demonstrate that the dual-mode transducers had imaging performance that was sufficient to aid the guidance and monitoring of treatment, and could sustain high intensities to induce coagulation necrosis in vivo.

In vitro evaluation of an oscillating dual-mode ultrasound probe for sector imaging and directive therapy.

Interstitial probes have been shown as effective devices to deliver high-intensity ultrasound therapy. Here, cylindrically-focused dual-mode transducers with either one or 5-elements were characterized, and a monoelement probe was evaluated in vitro. In therapy mode, the transducers were maximally efficient (> or =70%) at 5.6 MHz with surface intensities up to 20 W/cm(2). In imaging mode, fractional bandwidths were 46% and 50+/-4% (ave+/-std) for the monoelement and 5-element transducers respectively. Axial and lateral resolutions were 0.5 mm and 1.0 mm, respectively, for both transducers as measured with a point scatterer in the focal plane. After characterization, the oscillating probe was used to image and apply therapy to porcine liver. B-mode images over a 140 degrees sector were formed before and after therapy, which was applied for 90 s at each of 5 angles separated by 20 degrees (e.g. -40 degrees , -20 degrees, 0 degrees, 20 degrees, 40 degrees) to form a composite lesion. Transducer surface intensity was 18 W/cm(2). Therapy was interrupted at 125 ms intervals to collect pulse/echo data along the therapy axes. Data were displayed in real-time as an M-mode image to monitor therapy. B-mode images adequately represented the liver tissue. M-mode image data agreed well with the formation of lesions in the liver.

New piezoelectric transducers for therapeutic ultrasound.

Therapeutic ultrasound (US) has been of increasing interest during the past few years. However, the development of this technique depends on the availability of high-performance transducers. These transducers have to be optimised for focusing and steering high-power ultrasonic energy within the target volume. Recently developed high-power 1-3 piezocomposite materials bring to therapeutic US the exceptional electroacoustical properties of piezocomposite technology: these are high efficiency, large bandwidth, predictable beam pattern, more flexibility in terms of shaping and definition of sampling in annular arrays, linear arrays or matrix arrays. The construction and evaluation of several prototypes illustrates the benefit of this new approach that opens the way to further progress in therapeutic US.