Efficient single-shot Z-shim EPI via spatial and temporal encoding

Echo planar imaging (EPI) is used widely for neurological fMRI studies. However, in regions of magnetic field inhomogeneity—particularly near the nasal sinuses—EPI suffers from signal dropout. Z-shim methods are one approach to recover this lost signal, where a z gradient is employed prior to the EPI readout echo train to counter phase accumulation in the signal dropout region. Typically, two separate images of the same region are acquired under different z-shim gradient strengths, and then combined into a composite image. Single-shot z-shim methods give improved temporal resolution, but with a cost of longer echo trains, which increases geometric distortion. Here, we combine parallel MR imaging (pMRI) with a single-shot z-shim method to achieve short echo train lengths. We also employ temporal encoding to counter Nyquist ghost effects, which improves the pMRI calibration. We demonstrate the method with phantom images, and evaluate its suitability for fMRI applications.