Goerke Ute, van de Moortele Pierre-Francois, Ugurbil Kamil
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA.
Magn Reson Med. 2007 Oct;58(4):754-62. doi: 10.1002/mrm.21369.
The origin of the stimulus/task-induced signal changes in spin echo (SE) functional MRI (fMRI) at high magnetic fields is dynamic averaging due to diffusion in the presence of field gradients surrounding deoxyhemoglobin-containing microvasculature. The same mechanism is expected to be operative in stimulated echoes (STE). Compared to SE-fMRI, however, STE-fMRI has the potential for larger diffusion weighting and consequently larger stimulus/task-induced signal changes as a result of an additional delay, the mixing time, T(M). In the present study, functional signal changes were quantified for both primary echo (PRE) and STE as a function of echo and mixing time. The relative blood oxygenation level dependent (BOLD) signal changes in STE were larger than in PRE at the same echo time and increased with both mixing and echo time. The contrast-to-noise ratio (CNR) of the STE, however, is close to the CNR of the PRE, indicating an increase of physiological noise with longer mixing times. In addition, the signal attenuation due to diffusion in the presence of magnetic field gradients near blood vessels was modeled using Monte Carlo simulations. They support the hypothesis that the sensitivity of the STE to fluctuations of susceptibility-induced magnetic field gradients near microvasculature is enhanced as a result of an extended diffusion time.
在高磁场下自旋回波（SE）功能磁共振成像（fMRI）中，刺激/任务诱导信号变化的起源是在含脱氧血红蛋白的微脉管系统周围存在场梯度的情况下，由于扩散导致的动态平均。预计相同的机制在刺激回波（STE）中也起作用。然而，与SE-fMRI相比，STE-fMRI由于额外的延迟即混合时间T(M)，具有更大扩散加权的潜力，因此刺激/任务诱导的信号变化更大。在本研究中，对主回波（PRE）和STE的功能信号变化作为回波和混合时间的函数进行了量化。在相同回波时间下，STE中相对血氧水平依赖（BOLD）信号变化大于PRE中的变化，并且随混合时间和回波时间增加。然而，STE的对比噪声比（CNR）接近PRE的CNR，表明随着混合时间延长，生理噪声增加。此外，使用蒙特卡罗模拟对血管附近存在磁场梯度时由于扩散导致的信号衰减进行了建模。它们支持这样的假设，即由于扩散时间延长，STE对微脉管系统附近磁化率诱导磁场梯度波动的敏感性增强。
