Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, USA; Department of Electrical and Computer Engineering, University of Arizona, Tucson, Arizona, USA.
Magn Reson Med. 2014 Jan;71(1):105-17. doi: 10.1002/mrm.24636. Epub 2013 Feb 11.
We present a theory and a corresponding method to compute high-resolution field maps over a large dynamic range.
We derive a closed-form expression for the error in the field map value when computed from two echoes. We formulate an optimization problem to choose three echo times which result in a pair of maximally distinct error distributions. We use standard field mapping sequences at the prescribed echo times. We then design a corresponding estimation algorithm which takes advantage of the optimized echo times to disambiguate the field offset value.
We validate our method using high-resolution images of a phantom at 7T. The resulting field maps demonstrate robust mapping over both a large dynamic range, and in low SNR regions. We also present high-resolution offset maps in vivo using both, GRE and multiecho gradient echo sequences. Even though the proposed echo time spacings are larger than the well known phase aliasing cutoff, the resulting field maps exhibit a large dynamic range without the use of phase unwrapping or spatial regularization techniques.
We demonstrate a novel three-echo field map estimation method which overcomes the traditional noise-dynamic range trade-off.
我们提出了一种理论和相应的方法，用于计算具有较大动态范围的高分辨率场图。
我们推导出了从两个回波计算得到的场图值的误差的封闭表达式。我们制定了一个优化问题，以选择三个回波时间，从而产生一对最大差异的误差分布。我们在规定的回波时间使用标准的场映射序列。然后，我们设计了一个相应的估计算法，利用优化的回波时间来消除场偏移值的歧义。
我们在 7T 高分辨率的幻影图像中验证了我们的方法。得到的场图在大动态范围和低 SNR 区域都表现出了稳健的映射。我们还使用 GRE 和多回波梯度回波序列在体内呈现了高分辨率的偏移图。尽管所提出的回波时间间隔大于已知的相位混叠截止值，但所得到的场图在不使用相位解缠或空间正则化技术的情况下具有较大的动态范围。
我们展示了一种新的三回波场图估计方法，克服了传统的噪声-动态范围权衡。
