Development of high-speed velocity measurement technique using phase contrast MRI

磁共振(MR)成像对血流很敏感，这一特性在临床上被用于血流定量或MR血管成像。由于相位对比法将血流速度编码为采集到的磁共振信号的相位，具有良好的空间和时间分辨率，现已成为临床磁共振检查中定量血流速度的标准方法。然而，磁共振信号的相位是不稳定的，容易受到许多因素的影响，如磁场强度的局部变化和涡流。因此，常规的相衬成像方法很难与回波平面成像等高速磁共振成像方法相结合。为了加速相位对比法，本文提出了一种新的相速度编码技术，该技术比传统方法更稳定，能够与高速成像结合作为准备脉冲，该技术将MR信号的速度编码相位一次转换为纵向磁化。由信号采集后的纵向磁化强度计算出信号的相位，最终提取速度信息。该技术被安装在高速磁共振成像方法中作为准备脉冲，并使用稳定流动体模评估其速度测量的准确性。测量结果与实际流速成正比，而比例常数取决于T1值。虽然需要用T1值对速度测量进行校正，但该技术成功地定量了流速。由于它可以用作制备脉冲，因此新技术实现了对流速的高速测量。

Magnetic resonance (MR) imaging is sensitive to blood flow and this property is clinically utilized to flow quantitation or MR angiography. Since the phase contrast method that encodes velocity to the phase of acquired MR signal has fine spatial and temporal resolution, it is now standard method to quantitate the blood flow velocity in clinical MR examinations. The phase of MR signal, however, is unstable and susceptible to many factors such as local variation of magnetic field strength and eddy currents. Therefore, conventional phase contrast method is difficult to combine with high speed MR imaging method like echo planar imaging. To accelerate the phase contrast method, we propose in this study a new technique of phase-velocity encoding that is more stable than conventional method and is able to combine with high speed imaging as a preparation pulse.In the proposed technique, the velocity encoded phase of MR signal is once converted to longitudinal magnetization. The phase of signal is calculated from the longitudinal magnetization after the signal acquisition and the velocity information is finally retrieved. The technique was installed in a high speed MR imaging method as preparation pulse and its accuracy of velocity measurement was assessed using a steady flow phantom. The resulting measurement was proportional to the actual flow velocity, while the proportional constant depended on the T1 value. Though the correction of velocity measurements using T1 value was required, the proposed technique successfully quantitated the flow velocity. Since it can be used as preparation pulse, the new technique realizes a high-speed measurement of flow velocity.