Faculty of Engineering, Academic Assembly, University of Toyama, 3190 Gofuku, Toyama, 930-8555, Japan.
Graduate School of Science and Engineering for Education, University of Toyama, 3190 Gofuku, Toyama, 930-8555, Japan.
J Med Ultrason (2001). 2021 Oct;48(4):417-427. doi: 10.1007/s10396-021-01113-2. Epub 2021 Jul 21.
Doppler-based methods are widely used for blood flow imaging in clinical settings. However, they inherently estimate the velocity component only in the axial direction. Therefore, various studies of angle-independent methods have been conducted. The multi-angle Doppler method is one such angle-independent method, in which the velocity vector is estimated using axial velocities obtained from multiple directions by steering an ultrasonic beam. Recently, plane wave imaging, which realizes a very high frame rate of several thousand frames per second, was applied to the multi-angle Doppler method. However, the maximum detectable velocity, i.e., the aliasing limit, was reduced depending on the number of steering angles. In the present study, the feasibility of a specific transmit sequence, namely, the repeated transmit sequence, was examined using the plane-wave multi-angle Doppler method.
In the repeated transmit sequence, plane waves were emitted to the same direction twice, after which the steering angle was changed. By repeating the same procedure, a pair of beamformed radio-frequency (RF) signals could be obtained under each beam steering angle. By applying the autocorrelation method to each pair of RF signals, the time interval between the RF signals could be kept as the pulse repetition interval (PRI). The feasibility of such a transmit sequence was examined by numerical simulation and in vivo measurement of a human carotid artery.
The simulation results showed that the maximum steering angles of over 10 degrees were not feasible with the linear array used in the present study. The feasible maximum steering angle would depend on the element pitch of the probe relative to the ultrasonic wavelength. By limiting the maximum steering angles to 5 and 10 degrees, bias errors were 9.2% and 11.3%, respectively, and root mean squared errors were 21.5% and 16.9%, respectively. Also, flow velocity vectors in a human carotid artery could be visualized with the proposed method.
The multi-angle Doppler method was implemented in plane wave imaging with the repeated transmit sequence, and the proposed method was shown to be feasible through numerical simulation and in vivo measurement of a carotid artery.
基于多普勒的方法广泛应用于临床血流成像。然而，它们本质上仅在轴向方向估计速度分量。因此，已经进行了各种角度无关方法的研究。多角度多普勒方法就是这样一种角度无关的方法，其中通过转向超声束来使用从多个方向获得的轴向速度来估计速度矢量。最近，实现每秒几千帧的非常高帧率的平面波成像是应用于多角度多普勒方法。然而，最大可检测速度（即混叠限制）会根据转向角度的数量而降低。在本研究中，使用平面波多角度多普勒方法检查了特定发射序列（即重复发射序列）的可行性。
在重复发射序列中，平面波向同一方向发射两次，然后改变转向角。通过重复相同的过程，可以在每个波束转向角下获得一对波束形成的射频（RF）信号。通过将自相关方法应用于每对 RF 信号，可以将 RF 信号之间的时间间隔保持为脉冲重复间隔（PRI）。通过数值模拟和人体颈动脉的体内测量来检查这种发射序列的可行性。
模拟结果表明，使用本研究中使用的线性阵列不可能实现超过 10 度的最大转向角。可行的最大转向角将取决于探头的元件间距与超声波波长的关系。将最大转向角限制在 5 度和 10 度，偏置误差分别为 9.2%和 11.3%，均方根误差分别为 21.5%和 16.9%。此外，还可以使用提出的方法可视化人体颈动脉中的血流速度矢量。
使用重复发射序列在平面波成象中实现了多角度多普勒方法，并且通过对颈动脉的数值模拟和体内测量证明了该方法的可行性。
