IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 61, no. 9,

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2014

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Rapid Prototyping Fabrication of Focused Ultrasound Transducers Yohan Kim, Adam D. Maxwell, Timothy L. Hall, Zhen Xu, Kuang-Wei Lin, and Charles A. Cain Abstract—Rapid prototyping (RP) fabrication techniques are currently widely used in diverse industrial and medical fields, providing substantial advantages in development time and costs in comparison to more traditional manufacturing processes. This paper presents a new method for the fabrication of high-intensity focused ultrasound transducers using RP technology. The construction of a large-aperture hemispherical transducer designed by computer software is described to demonstrate the process. The transducer was conceived as a modular design consisting of 32 individually focused 50.8-mm (2-in) PZT-8 element modules distributed in a 300-mm hemispherical scaffold with a geometric focus of 150 mm. The entire structure of the array, including the module housings and the hemispherical scaffold was fabricated through a stereolithography (SLA) system using a proprietary photopolymer. The PZT elements were bonded to the lenses through a quarter-wave tungsten-epoxy matching layer developed in-house specifically for this purpose. Modules constructed in this manner displayed a high degree of electroacoustic consistency, with an electrical impedance mean and standard deviation of 109 ± 10.2 Ω for the 32 elements. Time-of-flight measurements for individually pulsed modules mounted on the hemispherical scaffold showed that all pulses arrived at the focus within a 350 ns range, indicating a good degree of element alignment. Pressure profile measurements of the fully assembled transducer also showed close agreement with simulated results. The measured focal beam FWHM dimensions were 1.9 × 4.0 mm (1.9 × 3.9 mm simulated) in the transversal and axial directions respectively. Total material expenses associated with the construction of the transducer were approximately 5000 USD (as of 2011). The versatility and lower fabrication costs afforded by RP methods may be beneficial in the development of complex transducer geometries suitable for a variety of research and clinical applications.

I. Introduction

C

urrent therapeutic ultrasound ablation modalities, such as high-intensity focused ultrasound (HIFU) and histotripsy, rely on focused transducers to deliver high doses of acoustic intensity or pressure to a localized region to treat the tissue of interest. HIFU procedures require that the treatment region be heated beyond 56°C for a

Manuscript received April 29, 2014; accepted May 12, 2014. This work is supported by grants from the National Institutes of Health (R01 EB008998, R01 CA134579, and R01 DK091267) and the National Science Foundation (S10 RR022425) and a Research Scholar Grant from the American Cancer Society (RSG-13-101-01-CCE). Disclosure: C. A. Cain, Z. Xu, and T. L. Hall have financial interest and/or other relationship with HistoSonics Inc. Y. Kim, T. L. Hall, Z. Xu, K.-W. Lin, and C. A. Cain are with the Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI (e-mail: [email protected]). A. D. Maxwell is with the Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA. DOI http://dx.doi.org/10.1109/TUFFC.2014.3070 0885–3010

few seconds to cause tissue death by thermal denaturation, with focal sonication intensities varying from 1000 to 20000 W/cm2 [1], [2]. On the other hand, in histotripsy, high-pressure, low duty-cycle sonication is applied to cause mechanical fractionation or breakdown of tissue in the treatment focus by directly inciting cavitation clouds [3], [4] or boiling bubbles through shock-induced heating [5]. Typical histotripsy procedures involve peak rarefactional pressure levels of >10 MPa, and peak compressional pressures can exceed 100 MPa [6], [7]. To achieve such high levels of acoustic power and pressure output, careful consideration is required in the design of therapy transducers, whose geometry should be optimized for each intended application. Focused therapy transducers have been traditionally constructed from piezoceramic or piezocomposite materials, using spherically curved segments designed to produce ideal focusing, with resonant frequencies from 200 kHz to 5 MHz, depending on the application. Therapy transducers can range from simple single-element construction [8], to large phased arrays of up to several hundred individual elements to enable electronic focal steering [9]–[11]. Many of the ultrasound therapy transducers in use for research or clinical applications are currently provided by specialty transducer manufacturing companies. These transducers are commonly made from air-backed piezoceramic or piezocomposite elements, containing a single quarter-wavelength matching layer. For therapeutic ablation purposes, the dimensions of such transducers must be large in comparison to the operating wavelength to achieve a sufficiently high focal gain. The fabrication of custom large-aperture focused transducers is often costly and involves considerable challenges that make it difficult to iterate designs and implement updates to optimize the transducers for particular applications. For example, it is often desirable to form customized geometries for therapy transducers for integration and alignment of imaging feedback probes [12], generation of complex focal patterns [13], or maximal utilization of an available acoustic window in the body [14]. Iterative development of such transducer designs is challenging at the research level because of the high costs and lead times associated with fabrication by third-party transducer manufacturers. Although a few specialized research labs have been known to have the capabilities and resources for the fabrication of customized HIFU transducers [15], [16], the complexity and technical hurdles involved in the process make it prohibitive for many research groups to achieve such a goal. Rapid prototyping (RP) is a relatively new fabrication technique currently gaining wider acceptance as an

© 2014 IEEE

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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

alternative to traditional machining methods in several industrial and biomedical fields, from the generation of visual models to the fabrication of fully functional parts and assemblies [17]–[19]. Rapid prototyping has several advantages over subtractive or formative processes such as machining and injection molding in that it is cost-effective, fast, and is able to fabricate highly complex models with no retooling requirements, substantially lowering overall manufacturing costs and facilitating design iteration. Common methods of rapid prototyping include stereolithography (SLA), selective laser sintering (SLS), and fused deposition modeling (FDM). Materials used in such machines can be conventional and proprietary polymers, elastomers, plaster, ceramic composites, and metals [20]. This paper reports a new fabrication method for highintensity ultrasound transducers using RP technology. Using the approach presented herein, transducers can be made of multiple flat (unfocused) piezoceramic elements bonded to housing modules incorporating an acoustic lens and matching layer, also providing electrical insulation and mechanical protection to the ceramic element. The individual modules can then be assembled onto a larger array scaffold, serving to align the elements to achieve any desired focal pattern. The acoustic lenses and the array scaffold can be modeled with computer-aided design (CAD) software and 3-D-printed, allowing complex features to be implemented with minimal effort. In our design process, different RP fabrication materials were evaluated for their sound speed, attenuation, and acoustic impedance. These parameters determined the formulation of a matching layer composition that best complemented the printing material for the purpose of building functional transducer prototypes. The versatility and affordability of RP manufacturing can considerably simplify the fabrication of experimental high-intensity transducers of varied geometries and aperture sizes, a capability that could potentially benefit numerous research groups in the field. Custom transducers constructed using RP fabrication have been successfully used in several research projects within our own laboratory, including the investigation of new cavitation phenomena [7], [21], and more clinically-oriented studies, such as thrombolysis, kidney stone erosion, neonatal cardiac ablation, and lymph node treatment [22]. As an example with which to demonstrate the process, we present the design and construction of a custom prototype focused ultrasound transducer using RP technology. This transducer consists of a large hemispherical aperture populated by 32 individual element modules, originally conceived to achieve maximal gain at a fixed focus for transcranial histotripsy research purposes. The transducer was entirely developed in-house through computer-aided SLA fabrication, and assembled using flat piezoceramic elements readily available commercially, at a fraction of the costs potentially involved with fabrication by specialty transducer manufacturers. Although this particular transducer was purposely built for transcranial research applications, many of the construction and assembly techniques

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illustrated herein could be easily translated to benefit the design of transducers suited for a variety of other research and/or clinical applications. II. Design and Simulation A. Focused Element Housings The mechanical design of the transducer components was conducted using a professional CAD package (Autodesk Inventor, Autodesk Inc., San Rafael, CA). The transducer design was based on a modular approach, in which a larger scaffold or shell is populated by identical, individually focused element modules. Previous histotripsy transducers fabricated through rapid prototyping consisted of smaller transducers containing fewer than 10 elements, which were typically directly assembled onto the transducers’ single frontal aperture. In the construction of larger arrays, however, a modular assembly method offers three main benefits: 1) Modular assembly can help improve the acoustic output consistency of the array, because each element module can be independently fabricated and individually tested before incorporation to the scaffold. 2) The ability to separately construct each module substantially simplifies the assembly procedure for the entire array structure, especially in a large hemispherical configuration. 3) In case of failure of one or more elements, this design can be easily repaired and restored to full operational capability, because failed elements can be removed and replaced with new modules. Each module consists of a housing designed to accommodate a flat piezoceramic disc coupled to an elliptical acoustic lens through a quarter wavelength matching layer. The housing incorporates several construction features leveraging RP capabilities to facilitate and improve the consistency of the assembly process (Fig. 1). The interior of the module includes four matching layer standoffs on the element mating side to ensure that the correct matching layer thickness is maintained, and four element guide pins are also present to aid in centering the piezoceramic element during assembly. A vertical indentation along the wall of the housing allows passage for the wire contacting the bottom electrode of the element. A circular channel surrounding the element mating platform allows excess matching layer material (electrically conductive) to flow downwards during assembly, preventing the potential creation of bridges between top and bottom electrodes. On the outer side of the housing, threaded patterns were specifically designed to fit into the receptacles of the scaffold array structure. A thread-stop rim is present at the bottom of the outside wall to limit the element insertion depth when threaded into the scaffold receptacle. The frontal face of each module consists of an elliptically focused lens, with four indentations around the outer perimeter of the housing designed to fit a custom tool made to facilitate the assembly process of the array. The elliptical geometry of the lens design was chosen to avoid aberration effects from spherically curved lenses

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Fig. 1. Diagrams of a single-element housing module showing details of construction features implemented by rapid prototyping. (bottom row) Cutout views show the quarter-wavelength gap between the piezoelectric element and the back of the elliptical lens, which is filled with a high-acousticimpedance matching compound during assembly.

[23], [24] and is focused at 150 mm, which is also the geometric focus of the array. Given the desired focus, the specific geometry of the elliptical shape is determined by the sound speed of the lens material and the propagation medium (water). Although individual element focusing limits the steering capabilities of the array, this approach was intentionally chosen to maximize the focal gain of the transducer. The use of acoustic lenses for ultrasound focusing provides significant cost and assembly benefits over the use of spherically shaped ceramic elements, because flat elements can be manufactured more easily and therefore are more readily available and cost-effective than curved ceramics. This design also facilitates the application of matching layers and the overall assembly process of the unit, because the lens alignment is built into the modular housing. Furthermore, the use of acoustic lenses enables a degree of design flexibility in that the same low-cost flat discs may be used for other transducer geometries, because the lens may be reshaped to enable focusing at different locations. B. Transducer Scaffold A hemispherical shape was chosen in this design to maximize the focal gain of the transducer array for the purposes of transcranial therapy research. The main transducer body consists of a scaffold with a front aperture diameter of approximately 300 mm and a geometric focal length of 150 mm (Fig. 2). The scaffold has 32 threaded receptacles populated with the individual element modules described in Section II-A. In addition, two diametrically opposed windows are available on the side of the scaffold to allow for optical imaging of the geometric focus. The inner walls

of the scaffold receptacles are threaded to match the outer wall thread pattern of the housing modules. The threaded design allows each module to be firmly attached to the scaffold without the need of using additional bolts or permanent adhesives, which also facilitates module removal if necessary. C. Linear Simulations Transient linear acoustic fields were simulated using an acoustic modeling package (Fast Object-oriented C++ Ultrasound Simulator; FOCUS) developed by McGough and colleagues [25], [26]. This acoustic simulation package enables large field calculations to be resolved efficiently through spatiotemporal decomposition techniques, and was used to estimate the linear pressure field amplitudes generated by the transducer array. Radiation apertures of similar diameter and focal distances were modeled after the individual modules and the full array geometry (Fig. 3) with the geometric focus of the transducer centered at origin of the simulation coordinates. Short source excitation pulses (

Rapid prototyping fabrication of focused ultrasound transducers.

Rapid prototyping (RP) fabrication techniques are currently widely used in diverse industrial and medical fields, providing substantial advantages in ...
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