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

磁共振成像对血流非常敏感，这一特性在临床上被用于血流定量或磁共振血管造影。由于相位对比法将速度编码为所获取的MR信号的相位，具有良好的时空分辨率，目前已成为临床MR检查中血流速度量化的标准方法。然而，磁流变信号的相位是不稳定的，容易受到局部磁场强度变化和涡流等因素的影响。因此，传统的相衬法很难与回波平面成像等高速磁共振成像方法相结合。为了加快相衬法的速度，本研究提出了一种新的相速度编码技术，该技术比传统的相速度编码方法更稳定，并且可以与高速成像相结合作为准备脉冲。在该技术中，磁流变信号的速度编码相位一次转换为纵向磁化。根据信号采集后的纵向磁化强度计算信号的相位，最终得到速度信息。将该技术作为制备脉冲安装在高速磁共振成像方法中，并利用稳态流模对其速度测量精度进行了评估。得到的测量值与实际流速成正比，而比例常数取决于T1值。虽然需要用T1值对流速测量值进行校正，但该方法成功地定量了流速。由于它可以作为准备脉冲，因此实现了流速的高速测量。

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.