Nonlinear coded excitation method for ultrasound contrast imaging.

Coded excitation with compression on receive is used in medical ultrasound (US) imaging to increase signal-to-noise ratio (SNR) and penetration depth. We performed a computer simulation study to investigate if chirped pulse excitation can be applied in US contrast agent imaging to increase SNR and contrast-to-tissue ratio (CTR) and, thus, reduce contrast agent destruction and tissue harmonics. A new nonlinear compression technique is proposed that selectively compresses the second harmonic component of the response. We compared a chirp of 9.4-micros duration, 2-MHz centre frequency, 45% relative bandwidth to a Gaussian pulse with equal centre frequency and bandwidth. For peak pressures between 50 and 300 kPa, we found for resonant bubbles an increase in response between 10 and 13 dB. Moreover, the axial resolution after compression was comparable to axial resolution of conventional imaging. This effect was relatively insensitive to peak excitation pressure and was largest for bubbles having resonance frequency around the centre frequency of the excitation.