High Frequency Piezo-Composite Transducer Array Designed For Ultrasound Scanning Applications

A 20 MHz high density linear array transducer is presented in this paper. This array has been developed using an optimized ceramicpolymer composite material. The electro-mechanical behaviour of this composite, especially designed for high frequency applications , is characterised and the results are compared to theoretical predictions . To support this project, a new method of transducer simulation has been implemented. This simulation software takes into account the elementary boundary phenomena and allows prediction of inter-element coupling modes in the array. The model also yields realistic computed impulse responses of transducers . A miniature test device and water tank have been constructed to perform elementary acoustic beam pattern measurements . It is equipped with highly accurate motion controls and a specific needle-shaped target has been developed . The smallest displacement available in the three main axes of this system is 10 microns. The manufacturing of the array transducer has involved high precision dicing and micro interconnection techniques.The flexibility of the material provides us with the possibility of curving and focusing the array transducer . Performance of this experimental array are discussed and compared to the theoretical predictions . The results demonstrate that such array transducers will allow high quality near field imaging .This work presents the efforts to extend the well known advantages of composite piezoelectric transducers to previously unattainable frequencies. INTRODUCTION Over the last years, constant development and technological progress of ultrasound scanners have led to significant improvements in image quality. New applications, including intravascular,superficial and ophthalmic ultrasound imaging have in parallel focussed the primary requirements in terms of higher spatial resolution associated with good penetration and doppler sensitivity. To meet these objectives, operating frequency has been increased to the range of 10 to 20MHz , and in some cases, above 50MHz with single element or annular array [1]. This work represents the continuation of the development of high frequency transducer arrays [2]. Preliminary studies have covered the design and fabrication of high frequency ceramic array. This paper is specifically related to the development of arrays made from piezoelectric composite material, designed to operate at nominal frequecy up to 15 20 MHz. The objective aims to push the performance of composite material in this upper range of frequency and to combine the competitive characteristics of this technology in term of sensitivity and resolution. GEOMETRIC AND ACOUSTIC DESIGN OF THE ARRAY Arrays have been designed to comply with high frequency imaging applications. Main characteristics are described hereafter. Specifications of the transducer: Nominal frequency: 20MHz Shape of array: linear Nb of elements: 128 Elevation focus: 12mm Pitch: 110μm Elevation height: 2.5mm Bandwidth: 50% Cross coupling: ≤ -30 dB The 12mm geometrical focus is based on clinical experiences to optimize penetration and lateral resolution. Focal parameters (-6dB) have following values: focal point:10.6mm, focal length:9.84mm, and focal width (at 10.6mm): 0.28mm. 0-7803-3615-1 (C) 1996 IEEE 1996 IEEE ULTRASONICS SYMPOSIUM-943 1996 IEEE ULTRASONICS SYMPOSIUM S A t i TX N 3 6 2 2 Electrical conditions are defined as follow: 100 Vcc negative excitation pulse 2 meters of 50pF/m coaxial cable tunning coils.( serial or parallel inductor ) TRANSDUCER MODELLING Uni-dimensional models for piezoelectric transducer do not include element interactions and take only into account the thickness mode. Usually, designer considers the ceramic vibration as the pure mode, and the crosstalk negligeable , thus, classical models (1D model ) such as Mason or KLM are sufficient to predict transducer behaviour. Model Transducer array elements have to be modeled by a bidimensional method; however, in the past, it has been demonstrated that when the ratio W/T (width/thickness) is smaller or equal to 0.6 [3], the uni-dimensional model still works,.except for neighboring element contribution. Fig 1a,1b. Based on the works carried out by Pappalardo and Lamberti [4] each array element is considered to be loaded on each lateral side by a semi-infinite medium. In this condition, the lateral contribution is considered as comparable to the contribution of a Lamb wave zeroeth order mode [5], these modes radiate energy into the medium, and their velocity is nearly equal to 2000 m/s. In order to calculate transducer waveforms, the 2D model takes into account the induced longitudinal vibration of the adjacent elements. Fig.2a,2b,2c,2d