Experimental investigation of chirp coded excitation in ultrasound superharmonic imaging

Superharmonic imaging (SHI) provides improved spatial resolution and contrast detection with low reverberation artifacts by combining the third, fourth and fifth harmonics of the nonlinear received signal. The aim of this study is to increase the signal-to-noise ratio (SNR) with improved axial resolution and reduced ripple artifacts in the SHI using chirp coded excitation. Experiments are performed using a 2.25 MHz transducer mounted coaxially at a distance of 1-10 cm with a 0.2 mm hydrophone in a tank containing deionised, degassed water. The applied linear chirp signals have 10 μsec duration, and fractional bandwidths (FBW) of 20% and 40%. In the experiments, the 2.25 MHz tone-burst signal of same duration is used for comparison. The pressure level of each waveform is calibrated and the mechanical index of 0.75 is set at the focus of the transducer. The harmonic matched filtering is applied to the received signal to perform pulse compression of the individual harmonic chirp components to recover axial resolution. To get the superharmonic chirp, the third to fifth harmonic components are combined after pulse compression. To avoid ripple artifacts in the superharmonic component of the tone-burst, the envelopes of the individually filtered third, fourth and fifth harmonic signals are combined in the time domain. The results indicate that the compressed superharmonic chirp, under a 20% FBW chirp excitation, show a 35% reduction in the axial pulse width at 20 dB when compared with the superharmonic of a tone-burst. Similarly using a 40% FBW chirp excitation, 65% reduction in the pulse width of the compressed superharmonic chirp has been found when compared with the superharmonic of a tone-burst.