Morgan Paul S, Bowtell Richard W, McIntyre Dominick J O, Worthington Brian S
Academic Radiology, University of Nottingham, University Hospital, Nottingham, England.
J Magn Reson Imaging. 2004 Apr;19(4):499-507. doi: 10.1002/jmri.20032.
To derive and implement a method for correcting spatial distortion caused by in vivo inhomogeneous static magnetic fields in echo-planar imaging (EPI).
The reversed gradient method, which was initially devised to correct distortion in images generated by spin-warp MRI, was adapted to correct distortion in EP images. This method provides point-by-point correction of distortion throughout the image. EP images, acquired with a 3 T MRI system, of a phantom and a volunteer's head were used to test the correction method.
Good correction was observed in all cases. Spatial distortion in the uncorrected images ranged up to 4 pixels (12 mm) and was corrected successfully.
The correction was improved by the application of a nonlinear interpolation scheme. The correction requires that two EP images be acquired at each slice position. This increases the acquisition time, but an improved signal-to-noise ratio (SNR) is seen in the corrected image. The local SNR gain decreases with increasing distortion. In many EPI acquisition schemes, multiple images are averaged at each slice position to increase the SNR; in such cases the reversed gradient correction method can be applied with no increase in acquisition duration.
推导并实施一种用于校正回波平面成像（EPI）中体内不均匀静磁场引起的空间畸变的方法。
最初设计用于校正自旋扭曲MRI生成图像中畸变的反向梯度法，被用于校正EP图像中的畸变。该方法对整个图像的畸变进行逐点校正。使用3T MRI系统采集的体模和志愿者头部的EP图像来测试校正方法。
在所有情况下均观察到良好的校正效果。未校正图像中的空间畸变高达4个像素（12毫米），并成功得到校正。
通过应用非线性插值方案改进了校正效果。该校正需要在每个切片位置采集两幅EP图像。这增加了采集时间，但在校正后的图像中可见信噪比（SNR）有所提高。局部SNR增益随畸变增加而降低。在许多EPI采集方案中，在每个切片位置对多幅图像进行平均以提高SNR；在这种情况下，可以应用反向梯度校正方法而不增加采集持续时间。
