Tal Assaf, Gonen Oded
Department of Radiology, New York University School of Medicine, 660 First Avenue, New York, New York, USA.
Magn Reson Med. 2013 Oct;70(4):895-904. doi: 10.1002/mrm.24536. Epub 2012 Nov 19.
To analyze the effect of B0 field drift on multivoxel MR spectroscopic imaging and to propose an approach for its correction.
It is shown, both theoretically and in a phantom, that for ∼30 min acquisitions a linear B0 drift (∼0.1 ppm/h) will cause localization errors that can reach several voxels (centimeters) in the slower varying phase encoding directions. An efficient and unbiased estimator is proposed for tracking the drift by interleaving short (∼ T2*), nonlocalized acquisitions on the nonsuppressed water each pulse repetition time, as shown in 10 volunteers at 1.5 and 3 T.
The drift is shown to be predominantly linear in both the phantom and volunteers at both fields. The localization errors are observed and quantified in both phantom and volunteers. The unbiased estimator is shown to reliably track the instantaneous frequency in vivo despite only using a small portion of the FID.
Contrary to single-voxel MR spectroscopy, where it leads to line broadening, field drift can lead to localization errors in the longer chemical shift imaging experiments. Fortunately, this drift can be obtained at a negligible cost to sequence timing, and corrected for in post processing.
分析B0场漂移对多体素磁共振波谱成像的影响,并提出一种校正方法。
理论上和在体模中均表明,对于约30分钟的采集,线性B0漂移(约0.1 ppm/h)会导致定位误差,在变化较慢的相位编码方向上,该误差可达数个体素(厘米)。通过在每个脉冲重复时间对未抑制的水进行短时间(约T2*)、非定位采集的交错方式,提出了一种有效且无偏的估计器来跟踪漂移,1.5 T和3 T场强下的10名志愿者的实验结果证明了该方法的可行性。
在两个场强下的体模和志愿者中,漂移均主要呈线性。在体模和志愿者中均观察并量化了定位误差。尽管仅使用了一小部分自由感应衰减信号,但该无偏估计器仍能可靠地跟踪体内的瞬时频率。
与单体素磁共振波谱不同,在单体素磁共振波谱中,场漂移会导致谱线展宽,而在较长的化学位移成像实验中,场漂移会导致定位误差。幸运的是,这种漂移可以以对序列时间影响可忽略不计的成本获得,并在后期处理中进行校正。