Assaf Y, Cohen Y
The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, 69978, Israel.
J Magn Reson. 1998 Mar;131(1):69-85. doi: 10.1006/jmre.1997.1313.
Diffusion measurements were performed on water and N-acetyl aspartate (NAA) molecules in excised brain tissue using a wide range of b-values (up to 28.3 x 10(6) and 35.8 x 10(6) s cm-2 for water and NAA, respectively). The attenuation of the signals of water and NAA due to diffusion was measured at fixed diffusion times (tD). These measurements, in which the echo time (TE) was set to 70 ms, were repeated for several diffusion times ranging from 35 to 305 ms. Signal attenuations were fitted to mono-, bi-, and triexponential functions to obtain the apparent diffusion coefficients (ADCs) of these molecules at each diffusion time. From these experiments the following observations and conclusions were made: (1) Signal attenuation of water and NAA due to diffusion over the entire range of b values examined is not monoexponential and the extracted ADCs depend on the diffusion time; (2) In the case of water the experimental data are best fitted by a triexponential function, while for b values up to 1 x 10(6) s cm-2, a biexponential function seems to reproduce the experimental data as well as the triexponential function; (3) If only the low range of b values are fitted (up to 0.5 x 10(6) s cm-2) signal attenuation of water is monoexponential and insensitive to tD; (4) Water ADCs decreased with the increase in tD but the relative population of the fast diffusing component increases such that at a tD of 305 ms there is nearly a single population; (5) The major fast diffusion component of the water shows only very limited restriction; (6) NAA signal attenuation is biexponential and analysis of the low b-value range gives only monoexponential decay, but the obtained ADC is sensitive to the diffusion time; (7) The ADCs obtained from fitting the data with a biexponential function decrease as diffusion time increases; (8) The relative population of the slow-diffusing component decreases with increasing tD; (9) Both the fast and the slow diffusing components of NAA show a considerable restriction by what seems to be a nonpermeable barrier from which two compartments, one of 7-8 micron and one of approximately 1 micron, were calculated using the Einstein equation. It is suggested that the two compartments represent the NAA in cell bodies and in the intra-axonal space. The effect of the range of the b value used in the diffusion experiments on the results is discussed and used to reconcile some of the apparent discrepancies obtained in different experiments concerning water diffusion in brain tissue. The potential of NAA diffusion experiments to probe cellular structure is discussed.
使用广泛的b值（水和N - 乙酰天门冬氨酸（NAA）分别高达28.3×10⁶和35.8×10⁶ s cm⁻²），对切除的脑组织中的水和NAA分子进行扩散测量。在固定的扩散时间（tD）下测量水和NAA由于扩散导致的信号衰减。这些测量中，回波时间（TE）设置为70 ms，在35至305 ms的几个扩散时间内重复进行。将信号衰减拟合为单指数、双指数和三指数函数，以获得每个扩散时间这些分子的表观扩散系数（ADC）。从这些实验中得出了以下观察结果和结论：（1）在所研究的整个b值范围内，水和NAA由于扩散导致的信号衰减不是单指数的，提取的ADC取决于扩散时间；（2）对于水，实验数据最好由三指数函数拟合，而对于高达1×10⁶ s cm⁻²的b值，双指数函数似乎也能很好地再现实验数据，与三指数函数一样；（3）如果仅拟合低b值范围（高达0.5×10⁶ s cm⁻²），水的信号衰减是单指数的且对tD不敏感；（4）水的ADC随着tD的增加而降低，但快速扩散成分的相对比例增加，使得在tD为305 ms时几乎只有单一成分；（5）水的主要快速扩散成分仅显示出非常有限的受限；（6）NAA信号衰减是双指数的，对低b值范围的分析仅给出单指数衰减，但获得的ADC对扩散时间敏感；（7）用双指数函数拟合数据得到的ADC随着扩散时间增加而降低；（8）慢扩散成分的相对比例随着tD增加而降低；（9）NAA的快速和慢扩散成分都受到相当大的限制，似乎是由一个不可渗透的屏障造成的，使用爱因斯坦方程计算出两个隔室，一个为7 - 8微米，另一个约为1微米。有人认为这两个隔室分别代表细胞体和轴突内空间中的NAA。讨论了扩散实验中使用的b值范围对结果的影响，并用于协调不同实验中关于脑组织中水扩散的一些明显差异。还讨论了NAA扩散实验探测细胞结构的潜力。
