Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
Magn Reson Med. 2010 Apr;63(4):902-9. doi: 10.1002/mrm.22267.
Quantitative MRI measures of multiexponential T(2) relaxation and magnetization transfer were acquired from six samples of excised and fixed rat spinal cord and compared with quantitative histology. MRI and histology data were analyzed from six white matter tracts, each of which possessed unique microanatomic characteristics (axon diameter and myelin thickness, in particular) but a relatively constant volume fraction of myelin. The results indicated that multiexponential T(2) relaxation characteristics varied substantially with variation of microanatomy, while the magnetization transfer characteristics remained close to constant. The most-often-cited multiexponential T(2) relaxation metric, myelin water fraction, varied by almost a factor of 2 between two regions with myelin volume fractions that differed by only approximately 12%. Based on the quantitative histology, the proposed explanation for this variation was intercompartmental water exchange, which caused the underestimation of myelin water fraction and T(2) values and is, presumably, a greater factor in white matter regions where axons are small and myelin is thin. In contrast to the multiexponential T(2) relaxation observations, magnetization transfer metrics were relatively constant across white matter tracts and concluded to be relatively insensitive to intercompartmental water exchange.
从六个切除和固定的大鼠脊髓样本中获取了多指数 T(2)弛豫和磁化传递的定量 MRI 测量值，并与定量组织学进行了比较。对六个白质束的 MRI 和组织学数据进行了分析，每个白质束都具有独特的微观解剖特征（尤其是轴突直径和髓鞘厚度），但髓鞘的体积分数相对恒定。结果表明，多指数 T(2)弛豫特征随微观解剖的变化而有很大差异，而磁化传递特征则保持接近恒定。最常引用的多指数 T(2)弛豫指标，髓鞘水分数，在髓鞘体积分数相差仅约 12%的两个区域之间相差近 2 倍。基于定量组织学，这种变化的原因是隔室间水交换，这导致髓鞘水分数和 T(2)值的低估，并且据推测，在轴突较小和髓鞘较薄的白质区域，这是一个更大的因素。与多指数 T(2)弛豫观察结果相反，磁化传递指标在白质束之间相对恒定，并且被认为对隔室间水交换相对不敏感。
