Lehto Lauri Juhani, Garwood Michael, Gröhn Olli, Corum Curtis Andrew
Department of Neurobiology, A. I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland; Center for Magnetic Resonance Research, University of Minnesota, 2021 6th Street SE, Minneapolis, MN 55455, USA.
Center for Magnetic Resonance Research, University of Minnesota, 2021 6th Street SE, Minneapolis, MN 55455, USA.
J Magn Reson. 2015 Mar;252:20-8. doi: 10.1016/j.jmr.2014.12.005. Epub 2015 Jan 3.
The majority of MRI phase imaging is based on gradient recalled echo (GRE) sequences. This work studies phase contrast behavior due to small off-resonance frequency offsets in brain using SWIFT, a FID-based sequence with nearly zero acquisition delay. 1D simulations and a phantom study were conducted to describe the behavior of phase accumulation in SWIFT. Imaging experiments of known brain phase contrast properties were conducted in a perfused rat brain comparing GRE and SWIFT. Additionally, a human brain sample was imaged. It is demonstrated how SWIFT phase is orientation dependent and correlates well with GRE, linking SWIFT phase to similar off-resonance sources as GRE. The acquisition time is shown to be analogous to TE for phase accumulation time. Using experiments with and without a magnetization transfer preparation, the likely effect of myelin water pool contribution is seen as a phase increase for all acquisition times. Due to the phase accumulation during acquisition, SWIFT phase contrast can be sensitized to small frequency differences between white and gray matter using low acquisition bandwidths.
大多数磁共振成像（MRI）相位成像基于梯度回波（GRE）序列。本研究使用SWIFT（一种采集延迟几乎为零的基于自由感应衰减（FID）的序列），研究大脑中由于小的失谐频率偏移引起的相位对比行为。进行了一维模拟和体模研究，以描述SWIFT中相位积累的行为。在灌注大鼠脑中进行了已知脑相位对比特性的成像实验，比较了GRE和SWIFT。此外，对一个人脑样本进行了成像。结果表明，SWIFT相位如何依赖于方向，并且与GRE具有良好的相关性，将SWIFT相位与和GRE类似的失谐源联系起来。采集时间显示与相位积累时间的回波时间（TE）类似。通过有和没有磁化传递准备的实验，髓磷脂水池贡献的可能影响在所有采集时间都表现为相位增加。由于采集过程中的相位积累，使用低采集带宽时，SWIFT相位对比可对白质和灰质之间的小频率差异敏感。
