Yablonskiy D A, Haacke E M
Mallinckrodt Institute of Radiology, Washington University Medical School, St. Louis, Missouri 63110.
Magn Reson Med. 1994 Dec;32(6):749-63. doi: 10.1002/mrm.1910320610.
This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.
本文致力于研究在存在静磁场不均匀性的情况下生物组织中核磁共振信号行为的理论。我们开发了一种方法，用于在扩散现象可忽略的静态去相位状态下对核磁共振信号进行解析描述。该方法已被应用于评估存在血管网络（用于功能成像）、骨髓（针对两种特定的小梁结构，不对称和柱状）以及铁氧体造影剂时的核磁共振信号。所有研究的系统都有一些共同的行为。如果回波时间TE小于给定系统的已知特征时间tc，那么信号以一个与TE二次方相关的自变量呈指数衰减。这等同于一个作为TE线性函数的R2弛豫率。在相反的情况下，当TE大于tc时，核磁共振信号遵循简单的指数衰减，且弛豫率不依赖于回波时间。在此时间间隔内，R2是以下各项的线性函数：a）产生磁场的物体所占的体积分数σ，b）磁场Bo或仅铁氧体颗粒的物体磁矩，以及c）物体与介质之间的磁化率差Δχ。
