Deoni Sean C L, Rutt Brian K, Jones Derek K
Centre for Neuroimaging Research, Institute of Psychiatry, King's College London, London, UK.
J Magn Reson Imaging. 2008 Jun;27(6):1421-9. doi: 10.1002/jmri.21079.
To investigate the effect of chemical exchange and multicomponent relaxation on the rapid T(2) mapping method, DESPOT2 (driven equilibrium single pulse observation of T(2)) and the steady-state free precession (SSFP) sequence upon which it is based. Although capable of rapid T(2) determination, an assumption implicit of the method is single-component relaxation. In many biological tissues (such as white and gray matter), it is well established that the T(2) decay curve is more accurately described by the summation of more than one relaxation species.
The effects of exchange were first incorporated into the general SSFP magnetization expressions and its effect on the measured SSFP signal investigated using Bloch-McConnell simulations. Corresponding imaging experiments were performed to support the presented theory.
Simulations show the measured multicomponent SSFP signal may be expressed as a linear summation of signal from each species under usual imaging conditions where the repetition time is much less than T(2). Imaging experiments performed using dairy cream demonstrate strong agreement with the presented theory. Finally, using a dairy cream model, we demonstrate quantification of multicomponent relaxation from multiangle SSFP data for the first time, showing good agreement with reference spin-echo values.
SSFP and DESPOT2 may provide a new method for investigating multicomponent systems, such as human brain, and disease processes, such as multiple sclerosis.
研究化学交换和多组分弛豫对快速T(2)成像方法、DESPOT2（驱动平衡单脉冲T(2)观测法）及其所基于的稳态自由进动（SSFP）序列的影响。尽管该方法能够快速测定T(2)，但其隐含的一个假设是单组分弛豫。在许多生物组织（如白质和灰质）中，T(2)衰减曲线由不止一种弛豫成分的总和来更准确地描述，这一点已得到充分证实。
首先将交换效应纳入通用的SSFP磁化表达式，并使用布洛赫 - 麦康奈尔模拟研究其对测量的SSFP信号的影响。进行了相应的成像实验以支持所提出的理论。
模拟表明，在重复时间远小于T(2)的常规成像条件下，测量的多组分SSFP信号可表示为各成分信号的线性总和。使用乳脂进行的成像实验与所提出的理论高度吻合。最后，使用乳脂模型，我们首次展示了从多角度SSFP数据对多组分弛豫进行定量分析，结果与参考自旋回波值高度一致。
SSFP和DESPOT2可能为研究多组分系统（如人脑）以及疾病过程（如多发性硬化症）提供一种新方法。
