Safety of Multi-Line Transmit beam forming for fast cardiac imaging - a simulation study

Achieving higher frame rate is a hot topic in cardiac ultrasound. An attractive way to accomplish this is through Multi-Line Transmit (MLT) beam forming. However, transmitting into multiple directions simultaneously may pose additional safety concerns due to acoustic superposition of the MLT beams. In this study, acoustic MLT fields were compared against conventional Single-Line Transmit fields. MLT beam forming showed more than a two-fold increase in peak-compressional acoustic pressure (from 1.8 MPa to 4.6 MPa) and in the mechanical index (MI) (from 1.0 to 2.2), as well as a six-fold increase in averaged intensity. As this might bring safety issues, two modifications of standard MLT beam forming were proposed: i) transmitting the MLT beams with different pulse phases; and ii) reorganising the delay lines of the MLT transmit event. Results showed a decrease in acoustic parameters in the near field using either modification, with the latter leading to peak pressures of 2.3 MPa and an MI of 1.2. In conclusion, straightforward implementation of MLT does bring additional hazard, but small modifications of the transmit beam forming can make it safe for clinical use.