Dual‐echo Dixon imaging with flexible choice of echo times

Abstract

In this work, a new two-point method for water-fat imaging is described and explored. It generalizes existing two-point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two-point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient-echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two-point methods. Notably, water images with higher spatial resolution and better signal-to-noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in-and opposedphase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two-point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three-point methods in this way, especially toward the edges of larger field of views. Magn Reson Med 65:96-107, 2011. V C 2010 Wiley-Liss, Inc. Key words: water-fat separation; fat suppression; Dixon methods; multiecho acquisitions; abdominal imaging; eddy currentsAs hyperintense signal from fat may obscure underlying pathology, its partial or complete suppression is a basic requirement in various applications of magnetic resonance imaging. Its characteristics result from the comparatively short relaxation times and large chemical shifts of the dominant methylene protons and serve as the basis for its elimination.Fat suppression is often an integral part of the acquisition. Popular methods include short-tau inversion recovery, which exploits the specific relaxation times, and selective saturation, which relies on the specific chemical shifts (1,2). However, these methods all have individual drawbacks, such as longer scan times, lower signalto-noise ratio (SNR), higher specific absorption rate, or less tolerance to field inhomogeneities. Postponing the separation of water and fat signals until the reconstruction allows avoiding most of these disadvantages. So-called Dixon methods perform for this purpose measurements at different echo times to encode the chemical shift (3). Besides fat suppression, they also permit efficient water-fat imaging, providing additional diagnostic information of relevance to selected applications.Several Dixon methods have been proposed over the last two decades (4). Apart from different strategies for the separation, they are mainly characterized by the number of echoes, or points, that they sample, and by the constraints that they impose on the echo times. We focus in this work on two-and three-point methods, as multipoint methods are usually very similar to threepoint methods, and one-point methods are gene...

Liver fat reduction with niacin is influenced by DGAT-2 polymorphisms in hypertriglyceridemic patients

“…Fat-water separation images were acquired using a TFE multislice spoiled gradient echo sequence, similar to a dual-echo DIXON sequence ( 15 ). To cover the visceral fat of the entire abdomen, two separate breath-hold acquisitions were acquired, superiorly delineated by the dome of diaphragm and inferiorly by the pubic symphysis.…”

Generation of brain pseudo‐CTs using an undersampled, single‐acquisition UTE‐mDixon pulse sequence and unsupervised clustering

“…22,54 Traditionally, two-point Dixon is limited to exactly in-phase and outof-phase echo times which limit the choice of the acquisition parameters. 34 Our mDixon overcomes this limitation and allows consistent image quality for water/fat separation at flexible echo times. Therefore, the minimal echo times and spacing can be achieved based on the limitation of the MR hardware in order to minimize the acquisition time.…”

“…A typical Dixon acquisition uses RF spoiling to spoil the residual magnetization between RF excitation pulses. 34 Using this approach on the UTE-mDixon acquisition would enhance T1-weighted contrast and generate uniform, homogeneous suppression of CSF to allow for a feature that is consistently identified by the FCM.…”

“…The customized, stand-alone reconstruction is capable of performing radial encoding gridding with undersampling, as described by Stehning, Börnert et al 28 and Hu et al, 21 and in Dixon reconstruction. 34 To achieve retrospective image calibration and correction, a baseline FID and two echo images were first reconstructed without trajectory delay calibration using the stand-alone reconstruction software and by setting the trajectory delay to the default value (0 µs in this study). The retrospective, trajectory-delay correction 32 was performed using  (The MathWorks, Natick, MA) software to drive the reconstruction executables.…”

“…One of the TEs for the standard Dixon must be in-phase or out-of-phase, while the modified Dixon relaxes this requirement. 34 Therefore, minimal echo spacing may be used for the modified Dixon approach in order to achieve the minimal TR and, therefore, further minimize the total acquisition time. The R2 * map, i.e., 1/T2 * , was estimated from the primitive image series using the equation…”