Price R R, Creasy J L, Lorenz C H, Partain C L
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN 37232-2675.
Invest Radiol. 1992 Dec;27 Suppl 2:S27-32. doi: 10.1097/00004424-199212002-00006.
After a radio frequency pulse, the decay of the magnetic resonance (MR) signal is described by two relaxation processes, T1 and T2. T1 describes the rate at which the magnetization realigns itself along the external magnetic field direction (ML), and T2 describes the rate of decay of the magnetization component along the transverse axis (MT). Magnetic resonance angiography (MRA) sequences have been developed that encode flow as changes in the apparent T1 or T2 of the moving blood relative to stationary tissues. MRA sequences typically use either time-of-flight (TOF) techniques to encode T1 or phase-contrast techniques to encode T2. TOF techniques encode flow as an apparent T1 shortening through the wash-in of fully relaxed blood from outside the image volume. The shorter T1 produces an enhancement of vascular structures relative to stationary tissues. TOF methods may use either sequential two-dimensional, three-dimensional, or multi-slab three-dimensional imaging sequences to produce a three-dimensional MRA data set. Phase-contrast methods use additional magnetic field gradients to encode flow as shifts in the phase of MT. Both TOF and phase-contrast methods use maximum intensity projection (MIP) images displayed in a cine format to aid in the visualization of three-dimensional vascular structures.
在射频脉冲之后，磁共振（MR）信号的衰减由两个弛豫过程T1和T2描述。T1描述了磁化强度沿外部磁场方向（ML）重新排列的速率，T2描述了磁化强度分量沿横轴（MT）的衰减速率。已经开发出磁共振血管造影（MRA）序列，该序列将血流编码为移动血液相对于静止组织的表观T1或T2的变化。MRA序列通常使用飞行时间（TOF）技术来编码T1或相位对比技术来编码T2。TOF技术通过从图像容积外部流入完全弛豫的血液来将血流编码为表观T1缩短。较短的T1相对于静止组织产生血管结构的增强。TOF方法可以使用顺序二维、三维或多层三维成像序列来生成三维MRA数据集。相位对比方法使用额外的磁场梯度将血流编码为MT相位的偏移。TOF和相位对比方法都使用以电影格式显示的最大强度投影（MIP）图像来辅助三维血管结构的可视化。
