Mehta R C, Pike G B, Enzmann D R
Department of Radiology, Stanford University School of Medicine, CA 94305-5105, USA.
Top Magn Reson Imaging. 1996 Aug;8(4):214-30.
Magnetic resonance imaging has traditionally used the T1 and T2 relaxation times and proton density (PD) of tissue water (hydrogen protons) to manipulate contrast. Magnetization transfer (MT) is a new form of tissue contrast based on the physical concept that tissues contain two or more separate populations of hydrogen protons: a highly mobile (free) hydrogen (water) pool, Hr, and an immobile (restricted) hydrogen pool, Hr, the latter being those protons bound to large macromolecular proteins and lipids, such as those found in such cellular membranes as myelin. Direct observation of the Hr magnetization pool is normally not possible because of its extremely short T2 time (< 200 microseconds). But saturation of the restricted pool will have a detectable effect on the mobile (free) proton pool. Saturation of the restricted pool decreases the signal of the free pool by transferring the restricted pool's saturation. Exchange of magnetization between the free and restricted hydrogen protons is a substantial mechanism for spin-lattice (T1) relaxation in tissues and the physical basis of MT. Through an appropriately designed pulse sequence, magnetization transfer contrast (MTC) can be produced. MT contrast is different from T1, T2, and PD, and it likely reflects the structural integrity of the tissue being imaged. A variety of clinically important uses of MT have emerged. In this clinical review of the neuroradiological applications of MT, we briefly review the physics of MT, the appearance of normal brain with MT, and the use of MT as a method of contrast enhancement/background suppression and in tissue characterization, such as evaluation of multiple sclerosis and other white-matter lesions and tumors. The role of MT in small-vessel visualization on three-dimensional time-of-flight magnetic resonance angiography and in head and neck disease and newer applications of MT are also elaborated.
传统上，磁共振成像利用组织水（氢质子）的T1和T2弛豫时间以及质子密度（PD）来控制对比度。磁化传递（MT）是一种基于物理概念的新型组织对比度，该概念认为组织包含两个或更多个单独的氢质子群体：一个高度可移动（自由）的氢（水）池，Hr，和一个不可移动（受限）的氢池，Hr，后者是那些与大分子蛋白质和脂质结合的质子，例如在髓鞘等细胞膜中发现的质子。由于其极短的T2时间（<200微秒），通常无法直接观察到Hr磁化池。但是受限池的饱和将对可移动（自由）质子池产生可检测的影响。受限池的饱和通过转移受限池的饱和来降低自由池的信号。自由和受限氢质子之间的磁化交换是组织中自旋晶格（T1）弛豫的重要机制以及MT的物理基础。通过适当设计的脉冲序列，可以产生磁化传递对比度（MTC）。MT对比度不同于T1、T2和PD，它可能反映了所成像组织的结构完整性。MT已经出现了多种临床上重要的用途。在本次关于MT神经放射学应用的临床综述中，我们简要回顾了MT的物理学、MT下正常脑的表现，以及MT作为对比度增强/背景抑制方法和在组织特征化中的应用，例如对多发性硬化症和其他白质病变及肿瘤的评估。还阐述了MT在三维时间飞跃磁共振血管造影中小血管可视化以及在头颈部疾病中的作用和MT的新应用。
