Michaeli Shalom, Sorce Dennis J, Springer Charles S, Ugurbil Kamil, Garwood Michael
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, MN 55455, USA.
J Magn Reson. 2006 Jul;181(1):135-47. doi: 10.1016/j.jmr.2006.04.002. Epub 2006 May 3.
Longitudinal relaxation in the rotating frame (T1rho) is the dominant mechanism during a train of adiabatic full passage (AFP) RF pulses with no interpulse intervals, placed prior to an excitation pulse. Asymptotic apparent time constants (T1rho') were measured for human occipital lobe 1H2O at 4T using brief imaging readouts following such pulse trains. Two members of the hyperbolic secant (HSn) AFP pulse family (n=1 or 4; i.e., arising from different amplitude- and frequency-modulation functions) were used. These produced two different non-monoexponential signal decays during the pulse trains. Thus, there are differing contrasts in asymptotic T1rho' maps derived from these data. This behavior is quite different than that of 1H2O signals from an aqueous protein solution of roughly the same macromolecular volume fraction as tissue. The ROI-averaged decays from the two acquisitions can be simultaneously accommodated by a two-site-exchange model for an equilibrium isochronous process whose exchange condition is modulated during the pulse. The model employs a two-spin description of dipolar interaction fluctuations in each site. The intrinsic site R1rho(identical with T1rho(-1)) value is sensitive to fluctuations at the effective Larmor frequency (omegaeff) in the rotating frame, and this is modulated differently during the two types of AFP pulses. Agreement with the data is quite good for site orientation correlation time constants characteristic of macromolecule-interacting water (site A) and bulk-like water (site B). Since R1rhoA is significantly modulated while R1rhoB is not, the intrinsic relaxographic shutter-speed for the process (identical with /R1rhoA-R1rhoB/), and thus the exchange condition, is modulated. However, the mean residence time (67 ms) and intrinsic population fraction (0.2) values found for site A are each rather larger than might be expected, suggesting a disproportionate role for the water molecules known to be "buried" within the large and concentrated macromolecules of in vivo tissue.
旋转坐标系中的纵向弛豫（T1ρ）是在激发脉冲之前施加的一系列无脉冲间隔的绝热全通过（AFP）射频脉冲期间的主要机制。使用在此类脉冲序列之后的简短成像读出，在4T下测量了人类枕叶1H2O的渐近表观时间常数（T1ρ'）。使用了双曲正割（HSn）AFP脉冲家族的两个成员（n = 1或4；即，源自不同的幅度和频率调制函数）。这些在脉冲序列期间产生了两种不同的非单指数信号衰减。因此，从这些数据得出的渐近T1ρ'图存在不同的对比度。这种行为与来自与组织具有大致相同大分子体积分数的水性蛋白质溶液的1H2O信号的行为有很大不同。来自两次采集的感兴趣区域平均衰减可以通过一个双位点交换模型同时拟合，该模型用于描述一个平衡等时过程，其交换条件在脉冲期间受到调制。该模型采用了每个位点中偶极相互作用波动的双自旋描述。本征位点R1ρ（与T1ρ（-1）相同）值对旋转坐标系中有效拉莫尔频率（ωeff）处的波动敏感，并且在两种类型的AFP脉冲期间其调制方式不同。对于与大分子相互作用的水（位点A）和类本体水（位点B）的位点取向相关时间常数，与数据的吻合度相当好。由于R1ρA受到显著调制而R1ρB未受调制，该过程的本征弛豫成像快门速度（与/R1ρA - R1ρB/相同），进而交换条件，受到调制。然而，在位点A处发现的平均停留时间（67毫秒）和本征种群分数（0.2）值都比预期的要大得多，这表明已知“埋藏”在体内组织的大而密集的大分子中的水分子起到了不成比例的作用。
