Gran Fredrik, Jensen Jørgen Arendt
Center for Fast Ultrasound Imaging, Orsted*DTU, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
IEEE Trans Ultrason Ferroelectr Freq Control. 2006 Jul;53(7):1289-99. doi: 10.1109/tuffc.2006.1665077.
This paper investigates the possibility of flow estimation using spatio-temporal encoding of the transmissions in synthetic transmit aperture imaging (STA). The spatial encoding is based on a frequency division approach. In STA, a major disadvantage is that only a single transmitter (denoting single transducer element or a virtual source) is used in every transmission. The transmitted acoustic energy will be low compared to a conventional focused transmission in which a large part of the aperture is used. By using several transmitters simultaneously, the total transmitted energy can be increased. However, to focus the data properly, the signals originating from the different transmitters must be separated. To do so, the pass band of the transducer is divided into a number of subbands with disjoint spectral support. At every transmission, each transmitter is assigned one of the subbands. In receive, the signals are separated using a simple filtering operation. To attain high axial resolution, broadband spectra must be synthesized for each of the transmitters. By multiplexing the different waveforms on different transmitters over a number of transmissions, this can be accomplished. To further increase the transmitted energy, the waveforms are designed as linear frequency modulated signals. Therefore, the full excitation amplitude can be used during most of the transmission. The method has been evaluated for blood velocity estimation for several different velocities and incident angles. The program Field II was used. A 128-element transducer with a center frequency of 7 MHz was simulated. The 64 transmitting elements were used as the transmitting aperture and 128 elements were used as the receiving aperture. Four virtual sources were created in every transmission. By beamforming lines in the flow direction, directional data were extracted and correlated. Hereby, the velocity of the blood was estimated. The pulse repetition frequency was 16 kHz. Three different setups were investigated with flow angles of 45, 60, and 75 degrees with respect to the acoustic axis. Four different velocities were simulated for each angle at 0.10, 0.25, 0.50, and 1.00 m/s. The mean relative bias with respect to the peak flow for the three angles was less than 2%, 2%, and 4%, respectively.
本文研究了在合成孔径成像（STA）中利用发射的时空编码进行血流估计的可能性。空间编码基于频分方法。在STA中，一个主要缺点是每次发射仅使用单个发射器（表示单个换能器元件或虚拟源）。与使用大部分孔径的传统聚焦发射相比，发射的声能会较低。通过同时使用多个发射器，可以增加总发射能量。然而，为了正确聚焦数据，必须分离来自不同发射器的信号。为此，将换能器的通带划分为多个具有不相交频谱支持的子带。每次发射时，为每个发射器分配一个子带。在接收时，使用简单的滤波操作分离信号。为了获得高轴向分辨率，必须为每个发射器合成宽带频谱。通过在多次发射中对不同发射器上的不同波形进行复用，可以实现这一点。为了进一步增加发射能量，将波形设计为线性调频信号。因此，在大部分发射过程中可以使用全激励幅度。该方法已针对几种不同速度和入射角的血流速度估计进行了评估。使用了Field II程序。模拟了一个中心频率为7 MHz的128阵元换能器。64个发射元件用作发射孔径，128个元件用作接收孔径。每次发射创建四个虚拟源。通过在血流方向上进行波束形成线，提取并关联定向数据。由此估计出血流速度。脉冲重复频率为16 kHz。研究了三种不同的设置，相对于声轴的血流角度分别为45度、60度和75度。每个角度模拟了0.10、0.25、0.50和1.00 m/s的四种不同速度。相对于三种角度下的峰值血流，平均相对偏差分别小于2%、2%和4%。
