Water–fat separation with bipolar multiecho sequences

Abstract

Multiecho sequences provide an efficient means to acquire multiple echoes in a single repetition, which has found applications in spectroscopy, relaxometry, and water-fat separation. By replacing the fly-back gradients in unipolar multiecho sequences with alternating readout gradients, bipolar multiecho sequences greatly reduce both echo-spacing and repetition interval. This offers many attractive advantages, such as shorter scan times, higher SNR efficiency, more robust field map estimation, reduced motion-induced artifacts, and less sensitivity to short T * 2 . However, the alternating readout gradients cause several technical problems, including delay effects and image misregistrations, which prevent direct application of existing water-fat separation methods. This work presents solutions to address these problems, including a post-processing method that shifts k -space data to correct k -space echo misalignment, an image warping method that utilizes a low-resolution field map to remove field-inhomogeneity-induced misregistration, and a k -space water-fat separation method that eliminates chemicalshift-induced artifacts in separated water and fat images. In addition, a noise amplification factor, which characterizes the noise present in separated images, is proposed to serve as a useful guideline for choosing imaging parameters or regularization parameters in the case of ill-conditioned separation.

Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds

“…This was accomplished in our experiments by the acquisition of reference lines without phase encoding [7,9]. Correction of higher order phase errors, field-inhomogeneity induced misregistration and chemical shift induced misregistration were not carried out but can be implemented according to Lu et al [21] and are expected to improve the separation quality. The chemical shift induced misregistration is suppressed by selecting the BW/pixel such that the shift of the metabolite with highest absolute value of the chemical shift is maximally half of a pixel.…”

Optimized in-phase and opposed-phase MR imaging for accurate detection of small fat or water fractions: theoretical considerations and experimental application in emulsions

“…It should be mentioned that this is only an option for phantoms or body regions without significant motion, especially if fat/water distribution maps are calculated on a pixelby-pixel basis. If phase images are included in the calculation an appropriate echo spacing [31] or bipolar readout gradients 10 For the emulsions of series 1, the maximum appeared at a nominal α 25% higher than α E,fat [42,43] could further increase the SNR. However, the complexity of the image-or raw-data post-processing increases.…”

Fast volumetric spatial-spectral MR imaging of hyperpolarized 13C-labeled compounds using multiple echo 3D bSSFP

“…This condition was satisfied in our experiments by using a readout bandwidth corresponding to 976 Hz/pixel. Remaining inconsistencies due to the alternating readout gradient can be corrected according to the proposals of Lu et al [26] and Brodsky et al [27]. The displayed experiments did not include a correction for field inhomogeneities, as the reconstruction of the relatively small sample proved to be stable enough after careful shimming.…”