The effect of small rotations on R₂* measured with echo planar imaging

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• 作者：Caparelli ; Elisabeth C. ; Tomasi ; Dardo ; Ernst ; Thomas
• 关键词：Effective Transverse Relaxation Rate ; Brain rotation ; Susceptibility effects
• 刊名：Neuroimage
• 出版年：2005
• 出版时间：February 15, 2005
• 年：2005
• 卷：24
• 期：4
• 页码：1164-1169
• 全文大小：631 K

文摘

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 (R₂*) and confound fMRI studies. The size of the R₂* 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?? changed R₂* more than 1.5 Hz in the phantom, and indicate that even small rotations may compromise fMRI results.