Glover G H, Lai S
Stanford University School of Medicine, Department of Diagnostic Radiology, California 94305-5488, USA.
Magn Reson Med. 1998 Mar;39(3):361-8. doi: 10.1002/mrm.1910390305.
This study compares the measured activation volumes in motor cortex as well as the fluctuation noise and off-resonance characteristics for 1-, 2-, and 4-shot spiral gradient-recalled echo blood oxygen level dependent contrast functional magnetic resonance imaging (fMRI) acquisitions, under conditions of constant resolution and scan time and with two readout durations. Reconstructions were made with and without self-navigator correction. It was found that the navigator correction provided a 50% reduction in image fluctuation noise with 4-shot acquisitions, and that multishot acquisitions perform as well as single-shot techniques when self-navigation is employed. An analysis of blurring showed that off-resonance delta f causes blurring when delta f > 1/(2*Tad), where Tad is the readout duration. Off-resonance effects were readily corrected during reconstruction with retrospective linear shim, even with the longer readout duration needed for single-shot methods. With navigator and shim correction, single-shot and multishot spiral methods are highly effective for fMRI acquisitions.
本研究比较了在恒定分辨率和扫描时间条件下，以及在两种读出持续时间下，1次、2次和4次激发的螺旋梯度回波血氧水平依赖对比功能磁共振成像（fMRI）采集中运动皮层的测量激活体积、波动噪声和失谐特性。重建时采用了和未采用自导航校正。结果发现，对于4次激发采集，导航校正可使图像波动噪声降低50%，并且当采用自导航时，多次激发采集的效果与单次激发技术相当。模糊分析表明，当失谐δf > 1/(2*Tad)时（其中Tad是读出持续时间），失谐δf会导致模糊。即使对于单次激发方法所需的较长读出持续时间，在重建过程中通过回顾性线性匀场也能很容易地校正失谐效应。通过导航和匀场校正，单次激发和多次激发螺旋方法在fMRI采集中非常有效。
