The information in an ultrasound image depends on the frequency that is used. In a clinical examination it may therefore be beneficial to generate ultrasound images acquired at multiple frequencies, which is difficult to achieve with conventional transducers. Capacitive micromachined ultrasonic transducers (CMUTs) offer a frequency response that is tunable by the bias voltage. In this study we investigate this frequency tunability for ultrasonic imaging. We characterized a CMUT array operated at bias voltages up to three times higher than the collapse-voltage. All elements of the array were connected to a single transmit and receive channel through a bias circuit. We quantified the transmit-receive and transmit sensitivity as a function of frequency for a range of bias voltages. Impulse response measurements show that the center frequency is modifiable between 8.7 MHz and 15.3 MHz with an applied bias voltage of −50 V to −170 V. The maximum transmit sensitivity is 52 kPa/V at a center frequency of 9.0 MHz with an applied bias voltage of −105 V. The −3 dB transmit range in center frequency accessible with the variable bias voltage is 6.7–15.5 MHz. This study shows that a collapse-mode CMUT can operate efficiently at multiple center frequencies when the driving pulse and the bias voltage are optimized. We demonstrate the usefulness of frequency tuning by comparing images at different optimal combinations of driving frequency and bias voltage, acquired by linearly moving the transducer across a tissue mimicking phantom.

The size of the features, and their relative distance to the probe, vary a lot in the intracardiac echocardiography application thus challenging the design of the probe. Therefore it may be beneficial to design a versatile probe which can produce both a large image to provide overview for navigation, and a smaller but detailed anatomic image on the structures of interest. This could be achieved by a probe whose frequency range of operation can be tuned - on the fly - to the specific task. Our goal is to develop a forward-looking catheter which can change its imaging frequency in the range 5 MHz - 15 MHz, allowing for both high penetration and high resolution intracardiac imaging within a single device. Our design comprises a capacitive micromachined ultrasonic transducer (CMUT) array operated in collapse-mode, which allows tuning of the imaging frequency. Custom-made front-end electronics is integrated in a catheter tip close to the CMUT for improved performance. In this paper, we report on the frequency-agility of the fabricated collapse-mode 1-D CMUT array.