Improved range resolution and side lobe level of ultrasound echoes using nonlinear frequency-modulated excitation signal and modified compressing filter

摘要

Linear frequency modulated (LFM) pulses are used as excitation for ultrasonic imaging systems. Temporal sidelobes, due to echo compression, are generated and impart a degradation of the range resolution. This work proposes a compensated frequency-modulated (CFM) excitation signal that improves the range resolution of compressed pulses, keeping low the sidelobe levels. An experimental pulse-eco system setup was used employing a 2.25 MHz transducer (−3 dB relative bandwidth of 45%) excited with a CFM signal, with a duration of 20 µs, immersed in a water tank containing a flat aluminum reflector. The spectrum magnitude of the CFM signal was determined as the nth power of the pseudo-inverted pulse-echo system transfer function magnitude. Its phase was calculated from the group delay determined from the CFM power spectrum. The echoes were compressed with a modified Wiener filter and the best range resolution of 0.47 mm, measured in −30 dB level, was achieved. This result is superior to the latest best published result, which is 42% larger than the one from this present work.