Ultrasound Bandwidth Enhancement through Pulse Compression Using a CMUT Probe
Résumé
Background, Motivation and ObjectiveIn ultrafast imaging using pulse compression, the image quality is limited by the bandwidth of the ultrasonic (US) transducer. Thus extending the effective bandwidth of the US transducers could improve the performance of the current US imaging methods. Resolution Enhancement Compression (REC) allows bandwidth enhancement by exciting theultrasound transducer with frequency and amplitude modulated signals. By compressing these signals in reception with a Modified Wiener Filter (MWF), REC boosts the energy of the backscattered echoes in the frequency bands where the transducer operates inefficiently. This technique has been successfully applied on piezoelectric US probes. However, noreal study was conducted on Capacitive Micromachined Ultrasonic Transducers (CMUT). This work is focussed on the implementation of the REC on a CMUT probe in order to evaluate the resolution improvement.Statement of Contribution/MethodsThe pre-enhanced chirp, Vpre, used as the excitation for the US transducer in REC is obtained using Eq1. Vlin is a 4μs frequency modulated chirp tapered with a 20% Tukey cosine window. h1 is the Impulse Response (IR) of the transducer and h2 is the desired IR, of a bandwidth larger than the one of h1. The compression of the received signals is performedusing the MWF as in Eq2 which represents the emitted signal adjusted with regard to transducer’s bandwidth and acquisition noise. REC was implemented in simulation, by considering the experimental IR of the CMUT probe, measured using a hydrophone. Experimental results were obtained using the research platform UlaOP. The performance of theMWF compression was evaluated on a single target and an anechoic cyst centred at 22mm depth. The media was insonified using 13 plane waves with steering angles distributed between ±12°.Results/DiscussionThe objective was achieved by boosting the bandwidth of the CMUT probe by 22 % (at -6 dB and -10 dB) using the REC approach, compared to a Conventional Pulse (CP) emission (E). A better axial resolution is noticed (+ 15 % at -6 dB): respectively 210 μm and 242 μm for REC and CP (A, B). The CNR is also improved using the REC technique 7.45 dBagainst 6.36 dB for CP (C, D).