Multi-transmit beam forming for fast cardiac imaging

A very high temporal resolution is critical to reach a better understanding of short-lived cardiac phases which have potential diagnostic value. To increase the frame rate, as an alternative to parallel receive beam forming, it has been proposed to transmit multiple lines simultaneously (i.e. multi-line transmit (MLT)) although this approach has received less attention due to potential cross-talk artifacts between beams. In this study, the point spread function (PSF) of different number of MLT beams (i.e. 2, 4, 6) was compared to that of conventional beam forming by computer simulation in order to find optimal settings to develop an MLT system for fast cardiac imaging. To reduce the cross-talk artifacts, different combinations of transmit and receive apodizations were tested using windowing functions: Rectangular, Hanning, Tukey and Hamming. Results showed that the cross-talk became more severe as the number of the MLT beams increased. Apodizations could significantly reduce these artifacts. In particular a Hanning-Tukey (α = 0.3) combination reduced these artifacts below the -40dB level for all MLT systems tested. With this apodization scheme, no marked differences between Single line transmit (SLT) and 6MLT phantom images could be observed. This study showed that the 6MLT imaging system has a competitive image quality to SLT but with a 6 times higher frame rate. In addition, the MLT approach can be combined with (multiple) parallel receive beam forming to increase frame rate further. With these methods, a frame rate of approximately up to 450Hz can be achieved to generate a 90° sector image without significant loss in image quality.