Comparative Analysis of CMUT and PZT Transducers for Coded Excitation

Coded excitation (CE) signals have the potential to greatly improve image quality and penetration depth by increasing the signal-to-noise ratio (SNR). However, CE can increase the operating temperature of a transducer beyond the safety limits defined by the International Electrotechnical Commission (IEC), and this has often limited the use of CE with Lead Zirconate Titanate (PZT) probes. Capacitive Micro-machined Ultrasonic Transducers (CMUTs) are often claimed to have low heat generation compared to PZT transducers. It is here hypothesized that CMUTs have a lower heat generation and, therefore, can generate CE signals more efficiently while operating within the IEC temperature limits. A custom-made CMUT and a commercial PZT transducer were compared. Two tests, still air and simulated use test on phantom, as defined by the IEC were conducted. Temperature rise for the two probes was measured at different pulse repetition frequencies
$(f_{prf})$
and signal durations. A simple chirp using the SARUS research scanner was employed for imaging and a synthetic transmit aperture having 12 emissions with an F-number of −1 was used as imaging sequence. The PZT probe failed the still air test, reaching the IEC temperature limit of
$\boldsymbol{27} \ ^{\circ} \mathbf{C}$
within 3 minutes of acquisition, when a CE of 8
$\mu \mathbf{s}$
at
$f_{prf}$
of 4 kHz was used, while the CMUT probe showed a rise of only 3
$^{\circ} \mathbf{C}$
over 30 minutes. Simulated use tests on phantoms showed that the CMUT was able to emit a 5 times longer signal as compared to the PZT transducer. It was concluded from the results that due to better thermal efficiency, CMUTs are more advantageous for CE compared to the PZTs and therefore they can fully benefit from CE imaging.