Retrospective correction of B ₀-field-induced geometric distortions in multislice echo-planar images: A 3D solution

A method has been developed to utilize a 3D B ₀ fieldmap, with a multi-volume-of-interest segmentation map, to quantify and correct geometric distortions in echo-planar images. The purpose is to provide accurate co-registration of anatomical MRI to functional MRI time course sequences. A data structure capable of extracting and reporting the necessary information forms a central part of the solution. Images were obtained from a 1.5 Tesla scanner with an experimental y-gradient insert coil. Two 3D-gradient echo sequences supply the data needed to calculate the B ₀ map across the volume. Segmentation of the volume into brain/background produces the data needed for the phase unwrapping and volume(s) of interest generation, from which the global B ₀ variation map is obtained. Subsequent EPI acquisition yields the fMRI time-course information. Tests were carried out on a phantom and a human volunteer engaged in a motor task (finger-tapping). Strong distortions were measured, and subsequently corrected, particularly near the petrous bone/mastoid air cells and in the frontal and maxillary sinuses. Additionally, a strong eddy current resulting from the unshielded y-gradient was detected. The method facilitates geometric distortion correction through an imaging volume, containing multiple regions of interest within a slice, starting from a single starting point.