Parsons Edward C, Does Mark D, Gore John C
Department of Applied Physics, Yale University, New Haven, Connecticut, USA.
Magn Reson Med. 2006 Jan;55(1):75-84. doi: 10.1002/mrm.20732.
The theory of temporal diffusion spectra is reviewed. In contrast to q-space spectroscopy, which measures the displacement spectrum of spins in a spatial domain, the spectral density of the velocity correlation function (VCF) in the temporal domain is considered. It is demonstrated that casting diffusion in this domain may facilitate measurements of microscopic geometry and the decomposition of the diffusion signal into components due to disperse flow and restricted diffusion. An oscillating gradient (OG) method of diffusion spectroscopy was developed and implemented. Microscopic pore sizes, surface-to-volume ratios (S/Vs), and diffusion path tortuosities were extracted from model systems using this method. Cases are discussed in which this type of experiment may allow the characterization of pore geometry when spatial domain experiments fail. OGs may be combined with imaging sequences to map complex patterns of diffusion and flow. Moreover, scalar apparent diffusion coefficient (ADC) measurements in complex biological systems may be subtly dependent on specific pulse sequence parameters. Thus, scalar ADC measurements using gradient pulses with different frequency spectra may give different results. Conversely, the frequency dependence of motion-sensitizing gradient pulses may be exploited to deduce the origin of ADC changes.
本文回顾了时间扩散谱理论。与在空间域中测量自旋位移谱的q空间光谱学不同，这里考虑的是时间域中速度相关函数（VCF）的谱密度。结果表明，在该域中进行扩散分析可能有助于测量微观几何结构，并将扩散信号分解为由于分散流和受限扩散引起的分量。开发并实施了一种振荡梯度（OG）扩散光谱法。使用该方法从模型系统中提取了微观孔径、表面积与体积比（S/V）和扩散路径曲折度。讨论了在空间域实验失败时，这类实验可用于表征孔隙几何结构的情况。振荡梯度可与成像序列相结合，以绘制复杂的扩散和流动模式。此外，复杂生物系统中的标量表观扩散系数（ADC）测量可能会微妙地依赖于特定的脉冲序列参数。因此，使用具有不同频谱的梯度脉冲进行标量ADC测量可能会得出不同的结果。相反，运动敏感梯度脉冲的频率依赖性可用于推断ADC变化的起源。
