On the optimization of the transmitted beam in contrast-enhanced ultrasound medical imaging

The optimization of the ultrasound pulses transmitted by an echographic scanner is considered here in order to attain an insonification beam having a quasi-constant pressure profile over a large depth interval. An equalized pressure intensity allows a correct exploitation of the ultrasound contrast agent injected into the human body and, in turn, permits one to fully exploit its nonlinear response. A stochastic method of synthesis (based on the simulated annealing scheme) that is able to produce the desired transmission beam profile, acting simultaneously on the carrier frequencies of the acoustic pulses emitted by the array's transducers and on the apodizing weights, has been developed and assessed. The results obtained by the joint optimization, in terms of acoustic pressure profile, sidelobe level, and main lobe shape, are reported, discussed, and compared with those obtained by the traditional emission strategy and by the exclusive optimization of the apodizing weights or the carrier frequencies. A significant improvement of the ultrasound beam generated by the joint optimization over those generated by the exclusive optimization is pointed out.