Abstract
To develop a postprocessing algorithm that corrects geometric distortions due to spatial variations of the static magnetic field amplitude, B0 , and effects from relaxation during signal acquisition in EPI. An analytic, complex point-spread function is deduced for k-space trajectories of EPI variants and applied to corresponding acquisitions in a resolution phantom and in human volunteers at 3 T. With the analytic point-spread function and experimental maps of B0 (and, optionally, the effective transverse relaxation time, ) as input, a point-spread function matrix operator is devised for distortion correction by a Thikonov-regularized deconvolution in image space. The point-spread function operator provides additional information for an appropriate correction of the signal intensity distribution. A previous image combination algorithm for acquisitions with opposite phase blip polarities is adapted to the proposed method to recover destructively interfering signal contributions. Applications of the proposed deconvolution-based distortion correction ("DecoDisCo") algorithm demonstrate excellent distortion corrections and superior performance regarding the recovery of an undistorted intensity distribution in comparison to a multifrequency reconstruction. Examples include full and partial Fourier standard EPI scans as well as double-shot center-out trajectories. Compared with other distortion-correction approaches, DecoDisCo permits additional deblurring to obtain sharper images in cases of significant effects. Robust distortion corrections in EPI acquisitions are feasible with high quality by regularized deconvolution with an analytic point-spread function. The general algorithm, which is publicly released on GitHub, can be straightforwardly adapted for specific EPI variants or other acquisition schemes.
Highlights
Echo-planar imaging[1] is the most widely used fast MRI technique, especially in functional MRI, diffusion-weighted MRI or studies of tissue perfusion
Applications may be limited by the performance of the receive coil, such as in functional MRI (fMRI) at laminar resolution, where the FOV may be of the order of the size of the coil elements.[6,7,8]
We introduce a Point-spread function (PSF) method for deconvolution-based distortion correction
Summary
Echo-planar imaging[1] is the most widely used fast MRI technique, especially in functional MRI (fMRI), diffusion-weighted (dw) MRI or studies of tissue perfusion. The in-plane resolution may be degraded by blurring related to transverse relaxation with an effective rate R∗2 = 1∕T2∗ during image acquisition.[2] Even more prominent are geometric distortions due to spatially varying offsets of the Larmor frequency, Δ 0 = 2 Δ 0 Such offsets may result from the presence of compounds of different magnetic susceptibility in the imaged object or from eddy currents induced by switching magnetic field gradients.[2,3] As R∗2 and Δ 0 increase with the amplitude of the static magnetic field, B0, the related image artifacts become increasingly prominent at higher field. They can be mitigated with parallel imaging techniques.[4,5] applications may be limited by the performance of the receive coil, such as in fMRI at laminar resolution, where the FOV may be of the order of the size of the coil elements.[6,7,8]
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