Ngo Giang-Chau, Wong Chelsea N, Guo Steve, Paine Thomas, Kramer Arthur F, Sutton Bradley P
Department of Bioengineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.
Beckman Institute of Technology for Advanced Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.
J Magn Reson Imaging. 2017 Jan;45(1):207-214. doi: 10.1002/jmri.25347. Epub 2016 Jun 14.
To evaluate the potential for bias in functional magnetic resonance imaging (fMRI) aging studies resulting from age-related differences in magnetic field distributions that can impact echo time and functional contrast.
Magnetic field maps were taken on 31 younger adults (age: 22 ± 2.9 years) and 46 older adults (age: 66 ± 4.5 years) on a 3T scanner. Using the spatial gradients of the magnetic field map for each participant, an echo planar imaging (EPI) trajectory was simulated. The effective echo time, time at which the k-space trajectory is the closest to the center of k-space, was calculated. This was used to examine both within-subject and across-age-group differences in the effective echo time maps. The blood oxygenation level-dependent (BOLD) percent signal change resulting from those echo time shifts was also calculated to determine their impact on fMRI aging studies.
For a single subject, the effective echo time varied as much as ±5 msec across the brain. An unpaired t-test between the effective echo time across age groups resulted in significant differences in several regions of the brain (P < 0.01). The difference in echo time was only ∼1 msec, however, which is not expected to have an important impact on BOLD fMRI percent signal change (<4%).
Susceptibility-induced magnetic field gradients induce local echo-time shifts in gradient echo fMRI images, which can cause variable BOLD sensitivity across the brain. However, the age-related differences in BOLD signal are expected to be small for an fMRI study at 3T.
1 J. Magn. Reson. Imaging 2017;45:207-214.
评估功能磁共振成像（fMRI）衰老研究中，因磁场分布的年龄相关差异可能导致的偏差，这种差异会影响回波时间和功能对比度。
在一台3T扫描仪上，对31名年轻成年人（年龄：22±2.9岁）和46名年长成年人（年龄：66±4.5岁）进行磁场图采集。利用每位参与者磁场图的空间梯度，模拟了回波平面成像（EPI）轨迹。计算有效回波时间，即k空间轨迹最接近k空间中心的时间。这用于检查有效回波时间图在个体内和跨年龄组的差异。还计算了由这些回波时间偏移导致的血氧水平依赖（BOLD）信号百分比变化，以确定它们对fMRI衰老研究的影响。
对于单个受试者，整个大脑的有效回波时间变化高达±5毫秒。年龄组间有效回波时间的独立样本t检验在大脑的几个区域产生了显著差异（P<0.01）。然而，回波时间的差异仅约为1毫秒，预计这对BOLD fMRI信号百分比变化不会有重要影响（<4%）。
敏感性诱导的磁场梯度在梯度回波fMRI图像中引起局部回波时间偏移，这可能导致大脑中BOLD敏感性的变化。然而，对于3T的fMRI研究，预计BOLD信号的年龄相关差异较小。
1 J.Magn.Reson.Imaging 2017;45:207 - 214。
