Dual-Echo Dixon Imaging with Flexible Choice of Echo Times Holger Eggers, 1 * Bernhard Brendel, 1 Adri Duijndam, 2 and Gwenael Herigault 2 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 ef- ficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multi- gradient-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 opposed- phase images is demonstrated to be replaceable by a synthe- sis 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 to- ward 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 currents As hyperintense signal from fat may obscure underlying pathology, its partial or complete suppression is a basic requirement in various applications of magnetic reso- nance imaging. Its characteristics result from the compa- ratively 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 acquisi- tion. Popular methods include short-tau inversion recov- ery, which exploits the specific relaxation times, and selective saturation, which relies on the specific chemi- cal shifts (1,2). However, these methods all have individ- ual drawbacks, such as longer scan times, lower signal- to-noise ratio (SNR), higher specific absorption rate, or less tolerance to field inhomogeneities. Postponing the separation of water and fat signals until the reconstruc- tion 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 addi- tional 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 three- point methods, and one-point methods are generally too error-prone without a priori information (5). Both two- and three-point methods originally required so-called in-phase and opposed-phase echo times at which the water and fat signals after demodulation to baseband are parallel and antiparallel in the complex plane, respectively (3,6). The three-point methods were then gradually generalized to allow flexible echo times (7–9). Thus, they do not restrict the angle or phase between the water and fat signals at the echo times to certain values anymore. In this way, they provide more freedom in sequence design and enable in particular a trade-off between SNR gains from the acquisition and SNR losses in the separation. We will explicitly refer to Reeder’s method (9) in this work, also as flexible three- point method. By contrast, the existing two-point meth- ods still require at least one in-phase echo time. For instance, Ma’s method (10), as the original Dixon method (3), assumes one in-phase and one opposed-phase echo time, and Xiang’s method (11) assumes one in-phase and one partially opposed-phase echo time. Sampling only two instead of three echoes is espe- cially desirable in time-critical applications, such as ab- dominal imaging in single breathholds. However, con- straints on the echo times may actually render dual-echo acquisitions slower than triple-echo acquisitions (12). We, therefore, aim at eliminating such constraints in dual-echo Dixon imaging in this work. In the following sections, we first derive a flexible two-point method by generalizing Xiang’s semiflexible two-point method. We then characterize its noise per- formance and investigate, representative for model errors, the effect of fat signal dephasing and decay and of their consideration in the separation by a more accu- rate spectral model of fat. We also propose the applica- tion of this method for an eddy current correction in tri- ple-echo acquisitions. We present selected results of abdominal imaging in single breathholds at 1.5 T and 3.0 T, and we compare the new two-point method with existing two- and three-point methods. 1 Philips Research, Hamburg, Germany. 2 Philips Healthcare, Best, The Netherlands. *Correspondence to: Holger Eggers, Ph.D., Philips Research Europe, Sector Imaging Systems and Interventions, Roentgenstrasse 24-26, 22335 Hamburg, Germany. E-mail: holger.eggers@philips.com Received 29 January 2010; revised 8 June 2010; accepted 29 June 2010. DOI 10.1002/mrm.22578 Published online 21 September 2010 in Wiley Online Library (wileyonlinelibrary.com). Magnetic Resonance in Medicine 65:96–107 (2011) V C 2010 Wiley-Liss, Inc. 96