非均匀场磁共振成像仪物理设计要点和应用定位

The Key Points in Design of Inhomogeneous Field MRI and Its Applications

阐明了单边型非均匀场的特征是垂直和平行于表面方向分别存在巨大梯度和均匀场。揭示了对样品表面层进行磁共振成像(MRI)分析的物理原理。针对这种非均匀场MRI的特定条件,对物理设计要点比如90°,180°RF脉冲、编码方式、脉冲序列等提出了要求和限制。由对静态巨大梯度引起的扩散效应的定量计算数据对回波时间提出了限制条件,从而确定对编码梯度、RF场强度的要求,以及可实际运行的RF脉冲类型。定量计算了人脑单层面成像的时间,论证了介入应用的禁戒性。指出单边型非均匀场MRI的优势在于轻巧、便携、灵活,不受场合、地域、样品尺度等的限制,可在广阔的领域以非侵入式、非破坏性方式对任意大样品的表面层进行MRI分析和测量。

For the inhomogeneous magnetic field MR imager, the polarization field is virtually uniform within horizontal degrees of freedom and decreases monotonically in a direction away from the surface. The authors describe the imaging principle, and present requirements and constraints by terms of key points of physical design such as 90° excitation pulse, 180° refocused pulse, encoding manner and pulse sequences. In addition, it reveals that the echo time is restricted by the diffusion effect on signal amplitude due to the huge statie field gradient. The data evaluated from diffusion not only restrict the usable TE, but also determine the strength of encoding gradient Gz, Gz and RF excitation field Bt and virtually usable RF pulse. The time spent for scanning one slice of human brain and inability of covering whole brain prohibit such MR imager from surgical intervention. It indicates that the advantage of the unilateral MR imager is portable. It can be used for arbitrary large sample to render MR/analysis of surface-layer in a non-invasive manner.