A 35 MHz linear ultrasonic array for medical imaging

Ultrasound backscatter microscope (UBM) imaging systems operating above 20 MHz are capable of achieving the axial and lateral resolutions needed for applications in ophthalmology and dermatology. Unfortunately these systems rely upon mechanically scanned single element transducers and suffer from a fixed focus, low frame rate and cumbersome scanning hardware. High frequency arrays are therefore desirable for several reasons, including the ability to dynamically focus the sound beam, increase frame rates and clinical convenience. This study investigates the design tradeoffs involved in the development of a high frequency (35 MHz) 64-element linear array. This array was designed primarily for human eye and skin imaging, and features monolithic elements mechanically diced out of a fine grain high density PZT-5H ceramic. Array elements were spaced with a 50 μm pitch, interconnected via a flexible circuit and matched to the 50 Ω system electronics via a 85 Ω transmission line coaxial cable. Several prototype arrays were constructed with promising results. An average center frequency of 34 MHz with a -6 dB bandwidth of at least 45% per element was achieved. The maximum combined electrical and acoustical crosstalk for nearest and next nearest elements was less than -29 dB, and the average 40 dB pulse length was 105 ns.