Optimizing Transmit Sequence for Ultrafast Ultrasound Imaging
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A simulation tool developed for estimating the acoustic properties of different transmit beams and sequences, based on FieldSim and the linear simulator Field2, is presented. For defocused and unfocused beams, some inaccuracies in the axial intensity profiles were observed. A probable cause is differences in elevation focusing properties. Estimates of MI and Ispta were comparable. For focusing beams there were larger differences. More optimal focusing in the simulations and highly non-linear propagation in the measurements are considered the main causes. Overall, most simulations were more conservative than the measurements, making the tool useful for estimating boundaries. However, simulation focusing will have to be adjusted for more accurate intensity fields and power estimates. Limitations induced by standard safety requirements were established for focusing, plane and diverging transmit beam forming. Transducer surface temperature measurements concluded that a 2.9MHz two-cycle with 4kHz PRF would be just within the 10C thermal rise limit. Axial measurements of diverging, plane and focusing waves established that plane and diverging beams were not limited by MI or Ispta within 35V, resulting in surface temperature being the active constraint. The focused beam had a higher intensity making Ispta the restriction parameter for non-scanning modes. In conclusion, there is a larger available workspace when using defocused or unfocused beam forming for non-scanned operation. If scanning is applied surface temperature is expected to be the main restriction in all three cases. Further, it was found that the unfocused and defocused beams behave close to linearly. Non-linear effects are essential in cardiac b-mode imaging. Loss in harmonics suggest that defocused beams may not be suitable for this purpose, even when utilizing coherent compounding. Regarding pulsed Doppler blood flow imaging linearity will not be an issue since the second harmonic content is not used. Thus, there may be some advantage in using defocused and unfocused beams for obtaining more exact velocity measurements by utilizing the larger available workspace.