Correcting for Gradient Imperfections in Ultra- Short Echo Time Imaging

Abstract

Unlike Cartesian-based acquisition methods, which are immune to a relatively wide range of imperfections in the applied spatial gradients, radial techniques are susceptible to even very slight delays or miscalibrations in the readout gradients. A delay in one or both of the principal readout gradients induces phase shifts in the resulting projections which are view-dependent, leading to image artifacts caused by inconsistent k-space data. If the gradient delays are known, then such problems can be corrected either by applying phase offsets to the projections in image space, or by shifting the data appropriately in k-space prior to gridding. Various strategies have been used to measure such delays. However, in the context of ultra-short echo time (UTE) methods, in which data is collected during the up-ramp of the readout gradient, delays are not the only source of gradient imperfection. Indeed, as demonstrated in this study, even when a trapezoidal gradient shape is prescribed, the assumption of a linear ramp-up leading to a flat gradient plateau may be incorrect. Here we describe a simple, robust technique for mapping the k-space trajectories during ramp-up of the readout gradients in a UTE pulse sequence. Whereas other methods exist for mapping k-space trajectories, particularly for spiral-type acquisitions, the present technique has certain advantages in the context of UTE imaging. We require a short, one-time calibration scan involving only a small modification to the main UTE imaging sequence. The shape of the ramp-up portion of the readout gradients is accurately measured, and this measurement is robust with respect to B0 inhomogeneity.