Dual-Focus Therapeutic Ultrasound Transducer for Production of Broad Tissue Lesions

Abstract

In noninvasive high-intensity focused ultrasound (HIFU) treatment, formation of a large tissue lesion per sonication is desirable for reducing the overall treatment time. The goal of this study is to show the feasibility of enlarging tissue lesion size with a dual-focus therapeutic ultrasound transducer (DFTUT) by increasing the depth-of-focus (DOF). The proposed transducer consists of a disc- and an annular-type element of different radii of curvatures to produce two focal zones. To increase focal depth and to maintain uniform beamwidth of the elongated DOF, each element transmits ultrasound of a different center frequency: the inner element at a higher frequency for near field focusing and the outer element at a lower frequency for far field focusing. By activating two elements at the same time with a single transmitter capable of generating a dual-frequency mixed signal, the overall DOF of the proposed transducer may be extended considerably. A prototype transducer composed of a 4.1 MHz inner element and a 2.7 MHz outer element was fabricated to obtain preliminary experimental results. The feasibility the proposed technique was demonstrated through sound field, temperature and thermal dose simulations. The performance of the prototype transducer was verified by hydrophone measurements and tissue ablation experiments on a beef liver specimen. When several factors affecting the length and the uniformity of elongated DOF of the DFTUT are optimized, the proposed therapeutic ultrasound transducer design may increase the size of ablated tissues in the axial direction and, thus, decreasing the treatment time for a large volume of malignant tissues especially deep-seated targets.

Increased light penetration due to ultrasound-induced air bubbles in optical scattering media

“…Theoretically, there are no limitations on the position and size of an ultrasound focal area 16 ; thus, an ultrasound transducer can be designed to use it in conjunction with either a tightly focused light beam desirable for optical imaging or a broad uniform beam for photodynamic therapy to increase light penetration. For a deep-lying target region, we can use an ultrasound transducer specially designed for a long depth of focus to generate air bubbles along the light pathway 23 . In general, ultrasound transducers developed for HIFU surgery does not have the desired capability for imaging.…”

M-Bonacci Zone Plates for Ultrasound Focusing

“…The application of ultrasound focusing techniques is a topic of great interest in both photoacoustic imaging [24,25] and therapeutic applications [26,27,28,29]. In the latter, dual focusing profiles can be used to either treat two isolated regions simultaneously or target extensive areas with higher efficiency [30,31].…”

Multi-Focus Beamforming for Thermal Strain Imaging Using a Single Ultrasound Linear Array Transducer

“…For systematic solutions for multiple-focus field patterns, readers are referred to papers by Ebbini et al, Ibbini et al and Bostros et al (Ebbini et al 1988; Ebbini and Cain 1989; Ibbini and Cain 1990; Botros et al 1996). For interest in multi-focus transducer design, readers are referred to papers on split-focus and dual-focus transducer in the field of HIFU thermal ablation (Daum et al 1999; Jeong et al 2010; Patel et al 2008; Sasaki et al 2003; Seip at al. 2001).…”