Caparelli Elisabeth C, Tomasi Dardo, Ernst Thomas
Medical Department, Brookhaven National Laboratory, Building 490, Upton, NY 11973, USA.
Neuroimage. 2005 Feb 15;24(4):1164-9. doi: 10.1016/j.neuroimage.2004.11.011. Epub 2005 Jan 5.
Several modern MRI techniques, such as functional MRI (fMRI), rely on the detection of microscopic changes in magnetic susceptibility. However, differences in magnetic susceptibility between brain tissue, bone, and air also produce local magnetic field gradients that may interfere with the contrast of interest, particularly at high field strengths. Since the magnetic field distribution depends on the orientation of the human head in the MRI scanner, head rotations can change the effective transverse relaxation rate (R(2)) and confound fMRI studies. The size of the R(2) changes produced by small head rotations was estimated from a brain-shaped gel-phantom at 4 T, by measuring the signal decay at 96 different echo times. Similar measurements were carried out in a human study. Rotations larger than 2 degrees changed R(2)* more than 1.5 Hz in the phantom, and indicate that even small rotations may compromise fMRI results.
几种现代磁共振成像（MRI）技术，如功能磁共振成像（fMRI），依赖于对磁化率微观变化的检测。然而，脑组织、骨骼和空气之间的磁化率差异也会产生局部磁场梯度，这可能会干扰感兴趣的对比度，尤其是在高场强下。由于磁场分布取决于人体头部在MRI扫描仪中的方向，头部旋转会改变有效横向弛豫率（R(2)*）并混淆fMRI研究。通过在4 T下测量脑形凝胶模型在96个不同回波时间的信号衰减，估算了小角度头部旋转产生的R(2)*变化的大小。在一项人体研究中也进行了类似的测量。在模型中，大于2度的旋转使R(2)*变化超过1.5 Hz，这表明即使是小角度旋转也可能影响fMRI结果。
