Passive diamagnetic shimming, using shaped pyrolytic graphite covered in plastic, significantly reduces the signal loss and distortion artefact in EPI images. See Wilson, J., Jenkinson, M. and Jezzard, P., "Optimisation of Static Field Homogeneity in Human Brain Using Diamagnetic Passive Shims", Magnetic Resonance in Medicine, 48, 906-914 (2002)