Improved axial resolution using pre-enhanced chirps and pulse compression

Improving the resolution of an ultrasound imaging device can have broad clinical impact. A novel pulse compression technique is developed that improves the axial resolution of an ultrasound imaging system and provides a boost in the echo signal-to-noise ratio (eSNR). The new technique, called the resolution enhancement compression (REC) technique, was validated with simulations and experimental measurements. The technique relies on exciting the source with a pre-enhanced frequency-modulated (FM) chirp that is found from application of convolution equivalence. Convolution equivalence is used to equate the pre-enhanced FM chirp convolved with the impulse response of the source of finite bandwidth with a linear FM chirp convolved with the impulse response of a source of larger bandwidth. The REC technique uses the linear chirp to compress the excitation waveform resulting in an impulse response with a bandwidth larger than the impulse response from conventional pulsing methods. Simulations and experimental measurements were conducted to validate the REC technique. Image quality was examined in terms of three metrics: the eSNR, the bandwidth, and the modulation transfer function (MTF). The simulations were conducted with a weakly-focused single-element ultrasound source with a center frequency of 5 MHz. The experimental measurements were carried out with a singleelement transducer (f/3) with a center frequency of 2.25 MHz. Measurements were taken from a planar reflector and wire targets. In simulations, the axial resolution of the ultrasound imaging system was almost doubled using the REC technique versus conventional pulsing techniques. The axial resolution measured from MTF curves was 0.14 mm and 0.27 mm, respectively. The -3-dB bandwidth was almost doubled from 47% to 96% and maximum range sidelobes were -50 dB. Experimental measurements conducted using the single-element transducer also revealed an improvement in axial resolution using the REC technique versus conventional pulsing. The axial resolution from the MTF curves was 0.31 mm and 0.44 mm, respectively. The -3-dB bandwidth was doubled from 56% to 113% and maximum range sidelobes were observed at -45 dB. In addition, a significant gain in eSNR (9.2 to 16.2 dB) was achieved in both simulations and experiments. Improvement in axial resolution, doubling of the system bandwidth, and a gain in eSNR were achieved with the REC technique with range sidelobe levels compatible with ultrasound imaging systems.