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With the wide acceptance of ultrasound medical imaging as the non-invasive diagnostic modality of choice, sonography equipment must offer the tools to complete the diagnosis, including multi-frequency operation for difficult-to-image patients. The trade-off between greater depth of penetration at low frequency for large organs and the improved detail resolution at high frequency is an essential capability that necessitates wideband transducer design and matching system hardware. This paper presents a phased array transducer design with variable ceramic thickness in the elevation direction. The design offers tow major contributions: first, -6dB round trip fractional bandwidth is increased by as much as 120 percent. This is done by controlling the thickness of the crystal from the middle to the outer edge. Since each sampling point in the crystal resonates freely at half wavelength in its fundamental mode, extended bandwidth is achieved for the single element in the phased array. This method has considerable advantage over the usual methods, such as backing the transducer with a matched lossy material. The drawback to backing the transducer is that the acoustic power consumed by the backing represents a severe insertion loss, especially if optimum bandwidth is desired. The second contribution of this design is the use of software to control the elevation slice thickness with axial symmetry around the 2D imaging plane. This is done by controlling the excitation frequency on transmit, and filtering on receive, thereby controlling the transmit and receive apertures independently during imaging. Compared to the standard elevation sampled 1.5D or 2D arrays with an increased number of hardware system channels and extensive cable wires needed, the new design offers simplicity and cost effectiveness. This represents a key development, especially with the advent of second harmonic imaging, both from a point of view of bandwidth requirement and slice thickness on receive. This paper also discusses other advantages of the design, presents experimental and simulation results, and shows a Schlieren video segment of the performance versus standard uniform thickness.
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