Nonlinear contrast imaging with Capacitive Micromachined Transducers

Advantages of capacitive micromachined ultrasonic transducers (CMUTs) such as wide frequency bandwidth could be used in nonlinear contrast imaging. However, the driving electrostatic force induces a nonlinear behavior of the CMUT generating thus undesirable harmonic components. Consequently, the use of CMUT for harmonic imaging, with or without contrast agents becomes challenging. To insure their exploitation for nonlinear imaging, a linear and a nonlinear method that compensate the CMUT nonlinear behavior are presented. Optimal parameters of cancellation components were estimated using a needle hydrophone positioned at 5 mm from the source. Alternatively, a wire (perfect reflector) was used and the reflected signal was measured using a single element 5-MHz transducer positioned perpendicularly. A fundamental excitation signal of 2.5 MHz Gaussian pulse was transmitted. The harmonic generation from a 1/2000 diluted solution of Sonovue microbubbles was measured with and without compensation. Compensation methods allow a harmonic reduction of approximately 20 dB when one element or 32 interconnected elements are excited. Second harmonic bubbles responses to CMUT compensated excitations are similar to bubbles echoes from PZT excitation. Results demonstrate the need for compensation methods in nonlinear imaging and the opportunity to use CMUT with traditional harmonic contrast imaging techniques.