Fast Concomitant Gradient Field Correction for Spiral Scans

Introduction: A major drawback to non-Cartesian imaging is the off-resonance blurring effects. Common linear techniques that remove the distortion effects are not sufficient, because the high order concomitant gradient fields are nontrivial for low fields and for scans that are more than 5 cm off isocenter [1]. Previous correction algorithms either are slow or do not take into account the known effects of concomitant gradient fields [3-5]. In this work, two fast and accurate algorithms to account for the concomitant field effects are presented and analyzed: one in the frequency domain [4] and the other in the spatial domain [3]. Methods: The concomitant gradient field distortion can be approximated as a quadratic phase function: f(x,y,z,t) = K * gm (t) * C(x,y,z), where K is a constant scaling factor, gm(t) is the total gradient magnitude for the spiral trajectory, and C(x,y,z) is the spatial location effects of the concomitant gradient fields [2]. In order to reduce the computation time, the correction algorithm is performed post gridding since the value gm 2 is smoothly varying as a function of k-space location. The blurring correction was performed in both the k-space domain and the image domain for comparison. In the k-space domain approach, a series of images were generated by focusing each image with different center frequencies. The series was then spliced together for a final corrected image. In the spatial domain approach, the image was convolved with a spatial varying correction convolution kernel. For speed, these kernels can be precomputed and stored in a look-up table.