<span>Background: Off-resonance effects (e.g., field inhomogeneity, susceptibility, chemical shift) cause artifacts in magnetic resonance imaging (MRI). The artifacts appear as positional shifts along the readout direction in rectilinearly sampled acquisitions. Usually, they are insignificant because of short readout times in normal spin-echo (SE) and gradient-echo (GRE) sequences. However, off-resonance artifacts sometimes appear as severe geometric distortion because of the relatively long readout time in echo planar imaging (EPI).Over the past decade, spiral imaging techniques have gained in popularity due to their short scan time and insensitivity to flow artifacts. However, off-resonance effects cause blurring artifacts in the reconstructed image. Most spiral off-resonance correction methods proposed to date are difficult to apply to correct for blurring artifacts due to the fat signals, since the fat-water frequency shift is typically much greater than that due to main magnetic field (B<sub>0</sub>) inhomogeneity across the field of view (FOV). As such, off resonance artifacts remain one of the main disadvantages of spiral imaging. Currently, off-resonance artifacts due to fat signals are most commonly avoided by use of spatially and spectrally selective radio-frequency (RF) excitation pulses (SPSP pulses) since they excite only water spins, thereby eliminating the off-resonance fat signals and thus avoiding artifact </span><span>generation. Yet, SPSP pulses may not lead to satisfactory fat signal suppression in the presence of large B<sub>0 </sub>inhomogeneity. Excitation of only water spins could be achieved through application of chemical shift presaturation pulses [e.g., CHESS pulses] prior to normal spatially selective excitation. However, the effectiveness of these frequency selective RF excitation pulses is dependent on main magnetic field homogeneity. Alternatively, Dixon techniques have been investigated for water-fat decomposition in rectilinear sampling schemes. In the original Dixon technique, water and fat images were generated by either addition or subtraction of the “in-phase” and “out-of-phase” data sets. Water and fat separation is unequivocal using this technique when magnetic field inhomogeneity is negligible over the scanned object. However, when B<sub>0 </sub>inhomogeneity cannot be neglected, the original Dixon technique fails to accurately decompose water and fat signals. Therefore, modified Dixon techniques using three data sets (i.e., three-point Dixon (3PD) technique) or four data sets were developed to correct for B<sub>0 </sub>inhomogeneity off-resonance effects and microscopic susceptibility dephasing. New versions of the Dixon technique use two data sets with B<sub>0 </sub>inhomogeneity off-resonance correction, i.e., the two-point Dixon (2PD) technique.http://www.google.com/patents?vid=USPAT7042215</span>
