Gran Fredrik, Jensen Jørgen Arendt
Center for Fast Ultrasound Imaging, Ørsted DTU, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
IEEE Trans Ultrason Ferroelectr Freq Control. 2007 Oct;54(10):2070-81. doi: 10.1109/tuffc.2007.502.
In this paper a method for designing waveforms for temporal encoding in medical ultrasound imaging is described. The method is based on least squares optimization and is used to design nonlinear frequency modulated signals for synthetic transmit aperture imaging. By using the proposed design method, the amplitude spectrum of the transmitted waveform can be optimized, such that most of the energy is transmitted where the transducer has large amplification. To test the design method, a waveform was designed for a BK8804 linear array transducer. The resulting nonlinear frequency modulated waveform was compared to a linear frequency modulated signal with amplitude tapering, previously used in clinical studies for synthetic transmit aperture imaging. The latter had a relatively flat spectrum which implied that the waveform tried to excite all frequencies including ones with low amplification. The proposed waveform, on the other hand, was designed so that only frequencies where the transducer had a large amplification were excited. Hereby, unnecessary heating of the transducer could be avoided and the signal-to-noise ratio could be increased. The experimental ultrasound scanner RASMUS was used to evaluate the method experimentally. Due to the careful waveform design optimized for the transducer at hand, a theoretic gain in signal-to-noise ratio of 4.9 dB compared to the reference excitation was found, even though the energy of the nonlinear frequency modulated signal was 71% of the energy of the reference signal. This was supported by a signal-to-noise ratio measurement and comparison in penetration depth, where an increase of 1 cm was found in favor for the proposed waveform. Axial and lateral resolutions at full-width half-maximum were compared in a water phantom at depths of 42, 62, 82, and 102 mm. The axial resolutions of the nonlinear frequency modulated signal were 0.62, 0.69, 0.60, and 0.60 mm, respectively. The corresponding axial resolutions for the reference waveform were 0.58, 0.65, 0.62, and 0.60 mm, respectively. The compression properties of the matched filter (mismatched filter for the linear frequency modulated signal) were tested for both waveforms in simulation with respect to the Doppler frequency shift occurring when probing moving objects. It was concluded that the Doppler effect of moving targets does not significantly degrade the filtered output. Finally, in vivo measurements are shown for both methods, wherein the common carotid artery on a 27-year-old healthy male was scanned.
本文描述了一种用于医学超声成像中时间编码波形设计的方法。该方法基于最小二乘法优化，用于设计合成发射孔径成像的非线性调频信号。通过使用所提出的设计方法，可以优化发射波形的幅度谱，使得大部分能量在换能器具有大放大倍数的地方发射。为了测试该设计方法，为BK8804线性阵列换能器设计了一个波形。将得到的非线性调频波形与先前在合成发射孔径成像的临床研究中使用的具有幅度渐变的线性调频信号进行了比较。后者具有相对平坦的频谱，这意味着该波形试图激发所有频率，包括那些放大倍数低的频率。另一方面，所提出的波形被设计成仅激发换能器具有大放大倍数的频率。由此，可以避免换能器不必要的发热，并提高信噪比。实验性超声扫描仪RASMUS用于通过实验评估该方法。由于针对手头的换能器进行了精心的波形设计，即使非线性调频信号的能量是参考信号能量的71%，与参考激励相比仍发现理论上的信噪比增益为4.9 dB。这得到了穿透深度的信噪比测量和比较的支持，其中发现所提出的波形的穿透深度增加了1 cm。在深度为42、62、82和102 mm的水模中比较了半高全宽处的轴向分辨率和横向分辨率。非线性调频信号的轴向分辨率分别为0.62、0.69、0.60和0.60 mm。参考波形的相应轴向分辨率分别为0.58、0.65、0.62和0.60 mm。针对两种波形在模拟中测试了匹配滤波器（线性调频信号的失配滤波器）在探测运动物体时出现多普勒频移方面的压缩特性。得出结论，运动目标的多普勒效应不会显著降低滤波输出。最后，展示了两种方法的体内测量结果，其中对一名27岁健康男性的颈总动脉进行了扫描。
