Correction of shear artifact caused by unbalanced alternating readout gradient in echo-planar Imaging

The shear artifact in echo planar images arises from the imperfections in the readout gradient. One reason is due to the improper linear shim by the readout gradient, and another is caused from the imbalance of the alternating readout gradient. The mis-shimmed readout gradient produces a static field error, and the corresponding distortion can be removed by field mapping techniques. However, since the field error caused by the unbalanced alternating readout gradient is not static, it cannot be measured by the regular field mapping techniques. A technique, based on a gradient waveform measurement, is described for the correction of the distortion resulting from the unbalanced alternating readout gradient. The unbalanced portion of the alternating gradient can be determined by a linear fitting of the phase accumulated from the unbalanced alternating gradient versus time, 2T(i) (T-i is inter-echo spacing), and converted to an 'equivalent static field error'. Then, the distortions resulting from both the off-resonance related factors and the unbalanced alternating gradient could be removed simultaneously by un-warping algorithms according to the modified field maps. This technique requires only a minor pulse sequence modification and a few additional scans to collect the necessary data on the unbalanced alternating gradient. Consequently, the image quality can be further improved.