Acoustic output of multi-line transmit beamforming for fast cardiac imaging: a simulation study

Achieving higher frame rates in cardiac ultrasound could unveil short-lived myocardial events and lead to new insights on cardiac function. Multi-line transmit (MLT) beamforming (i.e., simultaneously transmitting multiple focused beams) is a potential approach to achieve this. However, two challenges come with it: first, it leads to cross-talk between the MLT beams, appearing as imaging artifacts, and second, it presents acoustic summation in the near field, where multiple MLT beams overlap. Although several studies have focused on the former, no studies have looked into the implications of the latter on acoustic safety. In this paper, the acoustic field of 4-MLT was simulated and compared with single-line transmit (SLT). The findings suggest that standard MLT does present potential concerns. Compared with SLT, it shows a 2-fold increase in mechanical index (MI) (from 1.0 to 2.3), a 6-fold increase in spatial-peak pulse-average intensity (I^sppa) (from 99 to 576 W·cm^-2) and a 12-fold increase in spatial-peak temporalaverage intensity (I^spta) (from 119 to 1407 mW·cm^-2). Subsequently, modifications of the transmit pulse and delay line of MLT were studied. These modifications allowed for a change in the spatio-temporal distribution of the acoustic output, thereby significantly decreasing the safety indices (MI = 1.2, I^sppa = 92 W·cm^-2 and I^spta = 366 mW·cm^-2). Accordingly, they help mitigate the concerns around MLT, reducing potential tradeoffs between acoustic safety and image quality.