Performance of multi-layer 2-D transducer arrays

In medical ultrasound imaging, 2-D array transducers have become essential to implement dynamic focusing and phase-correction in the elevation dimension as well as real-time volumetric scanning. Unfortunately, 2-D array elements have poor sensitivity because their small size results in a small clamped capacitance and a large electrical impedance near resonance. An N layer structure of PZT connected acoustically in series and electrically in parallel has a reduced impedance, resulting in greater sensitivity. A 3×43 phased-array transducer was fabricated with 3 layer PZT-5H material using thick film hybrid technology. Each element had a thickness of 0.66 mm and an area of 0.37×3.5 mm. The MLC array elements had an impedance of 100 Ω at series resonance of 2.25 MHz and a minimum round-trip insertion loss of 24.0 dB. This was compared to an 800 Ω impedance and 34.1 dB insertion loss for elements of a single layer control array. These results were consistent with KLM modeling. B-scan images were made of cysts in a tissue-mimicking phantom and of the left kidney in vivo. The images clearly showed a higher sensitivity for the MLC array compared to the control. The authors believe this is the first example of a 2-D array using MLC technology to significantly improve the transducer sensitivity