Contrast-enhanced ultrasound imaging using capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have a nonlinear relationship between the applied voltage and the emitted signal, which is detrimental to conventional contrast enhanced ultrasound (CEUS) techniques. Instead, a three-pulse amplitude modulation (AM) sequence has been proposed, which is not adversely affected by the nonlinearly emitted harmonics. In this paper, this is shown theoretically, and the performance of the sequence is verified using a 4.8 MHz linear capacitive micromachined ultrasonic transducer (CMUT) array, and a comparable lead zirconate titanate (PZT) array, across 6-60 V applied alternating current (AC) voltage. CEUS images of the contrast agent SonoVue flowing through a 3D printed hydrogel phantom showed an average enhancement in contrast-to-tissue ratio (CTR) between B-mode and CEUS images of 49.9 and 37.4 dB for the PZT array and CMUT, respectively. Furthermore, hydrophone recordings of the emitted signals showed that the nonlinear emissions from the CMUT did not significantly degrade the cancellation in the compounded AM signal, leaving an average of 2% of the emitted power between 26 and 60 V of AC. Thus, it is demonstrated that CMUTs are capable of CEUS imaging independent of the applied excitation voltage when using a three-pulse AM sequence.

Automated summary

This study demonstrates that the nonlinear relationship between voltage and emitted signal in CMUTs can be effectively addressed by using a three-pulse amplitude modulation sequence. Experimental results show that the CMUT and PZT arrays have an average contrast-to-tissue ratio (CTR) enhancement of 37.4 dB and 49.9 dB, respectively, for B-mode and CEUS images with an applied alternating current voltage of 6-60 V. Hydrophone recordings indicate that the nonlinear emissions from CMUTs do not significantly degrade the cancellation in the compounded AM signal, with only 2% of the emitted power between 26 and 60 V AC. Therefore, it is demonstrated that CMUTs can perform CEUS imaging independently of the applied excitation voltage when using a three-pulse AM sequence.