Transducer design for second harmonic 3D transesophageal echocardiography

Three dimensional transesophageal echocardiography (3D TEE) provides images that can give information on the three-dimensional anatomy and function of the heart, such as the left ventricular volume and the movements of the valves. Current 3D TEE probes use fundamental imaging. We believe 3D TEE images can benefit from second harmonic (2H) imaging. In the design the transducer is divided in a transmit and receive sub-array. This ensures that the low voltage receive electronics are separated from the high voltages required for the transmitter. It enables us to optimize both the transmit and receive array for their specific acoustic role. In this paper the optimal element geometry of the transmit array is investigated. 3D FEM simulations are performed to determine the optimal element width and length. Laser interferometer and acoustic measurements are performed on a prototype. Good agreement was achieved between FEM simulations and optical measurements. The peak transmit transfer in air was 3 nm/V at 2.4 MHz for both simulation and experiment. FEM simulations of the prototype loaded by water show a peak transmit transfer of 43 kPa/V and a −6 dB bandwidth of 50%. Acoustic measurements with a hydrophone setup resulted in 55 kPa/V peak transmit transfer at 2.5 MHz and 30 kPa/V at 3 MHz. The acoustic field of an individual array element had a lateral −6 dB beamwidth of 46 mm at 40 mm depth and a peak pressure of ∼70 Pa/V. Extrapolating to the full array we obtained ∼1 MPa/120 V at 40 mm depth, which is assumed sufficient for 2H imaging. The transmit array presented here has suitable characteristics for its role in a 3D 2H TEE.