Ultrasound imaging is a well established tool for medical diagnosis and nondestructive testing and inspection of materials. Ultrasound imaging systems use transducers to create short high frequency acoustic pulses. The acoustic pulses propagate into the object under investigation. At locations where there is a change in acoustic properties, such as the boundary between two different tissue layers, part of the ultrasound energy is reflected. The reflected echos are detected by the transducer and processed to produce a two dimensional image of the underlying structures. The ability of the imaging system to detect small or subtle structures is primarily determined by the ability of the transducer to focus the ultrasound energy. Focusing the ultrasound beam can be achieved by shaping the transducer, by using an acoustic lens, or by using an array of transducers. Most modern ultrasound imaging systems employ transducer arrays.
The ultrasound energy produced by an array is focused by introducing time delays to the signals delivered to (or received from) individual array elements so that signals transmitted to (or received from) the desired region in space constructively interfere while signals outside this region destructively interfere. How well an array achieves this constructive interference is determined by the radiation pattern of the array. The radiation pattern can be visualized as a plot of the amplitude of the signal transmitted or received by the array as a function of position in space. An example of a radiation pattern is shown in FIG. 1, where the radiation pattern has been plotted as a contour plot using a logarithmic scale (dB) normalized to the peak pressure. The peak in the radiation pattern corresponds to the desired region in space over which the energy will be transmitted and from which the energy will be received. The width of the main peak in the radiation pattern is inversely proportional to the width of the array and determines the resolution of the imaging system. The non-zero amplitude of the radiation pattern away from the peak is caused by imperfect destructive interference and results in transmission and reception of unwanted energy. Transducer arrays are designed with the goal of obtaining a peak in the radiation pattern that is as narrow as possible while minimizing the amplitude of energy in the radiation pattern away from the main peak.
Ultrasound transducer arrays are fabricated by cutting a series of narrow grooves or kerfs through a bulk transducer substrate such as lead zirconate titanate (PZT). The grooves are used to mechanically and electrically isolate the array elements. To provide mechanical support to the narrow array elements, the grooves are often filled with a soft polymer material. The resulting array is essentially a composite structure consisting of alternating layers of PZT ceramic and polymer. Transducer arrays have also been fabricated by forming an electrode pattern on the surface of a ceramic-polymer composite fabricated using other methods, or on a polymer transducer substrate such as poly(vinylidene fluoride) (PVDF).
The grooves or kerfs of most ceramic arrays are machined using a thin diamond wheel. Other machining techniques such as laser machining or ultrasonic machining have also been used to separate the array elements. Although these techniques work well for arrays designed to operate below 10 MHz, they are not suitable for machining the extemely small and closely spaced elements required for high frequency imaging. They are also difficult to use with new single crystal relaxor ferroelectric materials such as lead zirconate niobate-lead titanate (PZN-PT) and lead magnesium niobate-lead titanate (PMN-PT) which are brittle and prone to chipping and cracking.