C.J. Gorter Center for high field MRI, Department of Radiology, Leiden University Medical Center, Leiden, The Netherlands.
Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.
Magn Reson Med. 2017 Oct;78(4):1373-1382. doi: 10.1002/mrm.26524. Epub 2016 Nov 17.
To compare methods for estimating B maps used in retrospective correction of high-resolution anatomical images at ultra-high field strength. The B maps were obtained using three methods: (1) 1D navigators and coil sensitivities, (2) field probe (FP) data and a low-order spherical harmonics model, and (3) FP data and a training-based model.
Data from nine subjects were acquired while they performed activities inducing B field fluctuations. Estimated B fields were compared with reference data, and the reductions of artifacts were compared in corrected T2* images.
Reduction of sum-of-squares difference relative to a reference image was evaluated, and Method 1 yielded the largest artifact reduction: 27 ± 15%, 20 ± 18% (mean ± 1 standard deviation) for deep breathing and combined deep breathing and hand motion activities. Method 3 performed almost as well (24 ± 18%, 15 ± 17%), provided that adequate training data were used, and Method 2 gave a similar result (21 ± 16%, 19 ± 17%).
This study confirms that all of the investigated methods can be used in retrospective image correction. In terms of image quality, Method 1 had a small advantage, whereas the FP-based methods measured the B field slightly more accurately. The specific strengths and weaknesses of FPs and navigators should therefore be considered when determining which B -estimation method to use. Magn Reson Med 78:1373-1382, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
比较在超高场强下用于高分辨率解剖图像回顾性校正的 B 图估计方法。B 图是通过三种方法获得的：（1）1D 导航仪和线圈灵敏度，（2）场探头（FP）数据和低阶球谐模型，（3）FP 数据和基于训练的模型。
在九名受试者进行诱发 B 场波动的活动时采集数据。将估计的 B 场与参考数据进行比较，并比较校正后的 T2*图像中的伪影减少情况。
相对于参考图像评估了均方根差的减少，方法 1 产生了最大的伪影减少：深吸气和深吸气加手部运动活动的 27 ± 15%，20 ± 18%（平均值 ± 1 个标准差）。方法 3 的表现几乎相同（24 ± 18%，15 ± 17%），前提是使用了足够的训练数据，而方法 2 给出了相似的结果（21 ± 16%，19 ± 17%）。
本研究证实所有研究的方法都可用于回顾性图像校正。在图像质量方面，方法 1 具有较小的优势，而基于 FP 的方法则略微更准确地测量了 B 场。因此，在确定使用哪种 B 图估计方法时，应考虑 FP 和导航仪的具体优势和弱点。磁共振医学 78:1373-1382, 2017。© 2016 国际磁共振学会。
