Liu Ho-Ling, Huang Ju-Chuan, Wang Jiun-Jie, Wan Yung-Liang, Wai Yau-Yau
MRI Center, Chang Gung Memorial Hospital, Taoyuan 333, Taiwan.
Magn Reson Imaging. 2006 Jun;24(5):597-602. doi: 10.1016/j.mri.2005.12.016. Epub 2006 Feb 20.
Computer simulations and event-related functional MRI (ER-fMRI) experiments were performed to investigate the effects of single-trial averaging and the corresponding contrast-to-noise ratio (CNR) on the minimal resolvable hemodynamic timing difference between brain areas. Three ER-fMRI sessions with temporally delayed (250, 500 and 1,000 ms) visual stimulations between two hemifields, each with 70 repeated single trials, were examined on two subjects. From the computer simulation, the temporal resolution improved as the CNR increased, which reached 500 and 100 ms for CNRs of 1.55 and 6.44, respectively. In the ER-fMRI experiments, the measured CNR increased as more single trials were averaged. The detectability of temporal differences was positively correlated (P<.05) with the CNR in all sessions for one subject but only in the 1,000-ms session for the other subject. Temporal resolution of 1,000 ms was achieved when more than 10 trials were averaged. The 500- and 250-ms delays might be differentiable when more than 20 trials were averaged, but the results were subject-dependent. This study demonstrated that the CNR could be significantly improved by single-trial averaging, which led to an improved temporal resolution of ER-fMRI. Temporal resolution in the range of hundreds of milliseconds was subject-dependent, which might be attributed to the intrinsic spatial variations in the timing of the blood oxygenation level-dependent (BOLD) response.
进行了计算机模拟和事件相关功能磁共振成像（ER-fMRI）实验，以研究单次试验平均及其相应的对比噪声比（CNR）对脑区之间最小可分辨血流动力学时间差异的影响。在两名受试者身上检查了三个ER-fMRI实验环节，两个半视野之间有时间延迟（250、500和1000毫秒）的视觉刺激，每个环节有70次重复的单次试验。从计算机模拟结果来看，随着CNR的增加，时间分辨率提高，对于CNR分别为1.55和6.44的情况，时间分辨率分别达到500毫秒和100毫秒。在ER-fMRI实验中，随着平均的单次试验次数增加，测量得到的CNR也增加。对于一名受试者，在所有实验环节中，时间差异的可检测性与CNR呈正相关（P<0.05），但对于另一名受试者，仅在1000毫秒的实验环节中呈正相关。当平均超过10次试验时，可实现1000毫秒的时间分辨率。当平均超过20次试验时，500毫秒和250毫秒的延迟可能是可区分的，但结果因受试者而异。本研究表明，通过单次试验平均可以显著提高CNR，这导致ER-fMRI的时间分辨率得到改善。数百毫秒范围内的时间分辨率因受试者而异，这可能归因于血氧水平依赖（BOLD）反应时间的内在空间变化。
