Chemical shift based methods are often used to achieve uniform water-fat separation that is insensitive to B o inhomogeneities. Many spin-echo (SE) or fast SE (FSE) approaches acquire three echoes shifted symmetrically about the SE, creating time-dependent phase shifts caused by water-fat chemical shift. This work demonstrates that symmetrically acquired echoes cause artifacts that degrade image quality. According to theory, the noise performance of any water-fat separation method is dependent on the proportion of water and fat within a voxel, and the position of echoes relative to the SE. To address this problem, we propose a method termed "iterative decomposition of water and fat with echo asymmetric and least-squares estimation" (IDEAL). This technique combines asymmetrically acquired echoes with an iterative least-squares decomposition algorithm to maximize noise performance. Theoretical calculations predict that the optimal echo combination occurs when the relative phase of the echoes is separated by 2/3, with the middle echo centered at /2؉k (k ؍ any integer), i.e., (-/6؉k, /2؉k, 7/6؉k). Only with these echo combinations can noise performance reach the maximum possible and be independent of the proportion of water and fat. Key words: fat suppression; fast spin echo; magnetic resonance imaging; water-fat separation; asymmetric echoes; brachial plexus Reliable and uniform fat suppression is essential for accurate diagnoses in many areas of MRI. This is particularly true for sequences such as fast spin-echo (FSE) imaging, in which fat is bright and may obscure underlying pathology. Although conventional fat saturation may be adequate for areas of the body with a relatively homogeneous B o field, there are many applications in which fat saturation routinely fails. This is particularly true for extremity imaging, off-isocenter imaging, large field of view (FOV) imaging, and challenging areas such as the brachial plexus and skull base, as well as many others. Short-TI inversion recovery (STIR) imaging provides uniform fat suppression, but at a cost of a reduced signal-to-noise ratio (SNR) and mixed contrast that is dependent on T 1 (1). This latter disadvantage limits STIR imaging to T 2 -weighted (T 2 W) applications, and current T 1 -weighted (T 1 W) applications rely solely on conventional fat-saturation methods. Another fat-suppression technique used with FSE is the application of spectral-spatial pulses; however, this method is also sensitive to field inhomogeneities (2,3)."In and out of phase" imaging was first described by Dixon (4) in 1984, and was used to exploit the difference in chemical shifts between water and fat in order to separate water and fat into separate images. Glover (5) and Glover and Schneider (6) further refined this approach in 1991 with a three-point method that accounts for B o field inhomogeneities. Hardy et al. (7) first applied this method to FSE imaging by acquiring three images with the readout centered at the SE for one image, and symmetrically before and after the SE in the ...
This work describes a new approach to multipoint Dixon fatwater separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single-and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fatsilicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed. Steady-state free precession (SSFP) is a rapid, short-TR imaging technique that offers specific advantages over short-TR gradient-echo techniques, including a high signal-to-noise ratio (SNR) and favorable contrast behavior. This is especially true for the visualization of fluid, because its contrast depends upon both T 1 and T 2 (1-3). However, the use of SSFP has been limited by the fact that fluid and fat both appear bright on SSFP images. This characteristic of SSFP may cause abnormalities to appear similar to normal fat and thus obscure underlying pathology.The application of a Dixon fat-water separation method to SSFP imaging could potentially provide homogeneous and reliable separation of fat and water from SSFP images (4,5). Current methods for SSFP fat suppression include fluctuating equilibrium magnetic resonance (FEMR), linear combination SSFP, and fat-suppressed SSFP. However, all of these techniques are sensitive to field heterogeneities (6 -8). The notion of combining Dixon methods with SSFP is challenging for several reasons. First, SSFP requires short repetition times (TRs) to prevent image degradation from field heterogeneities (1,2). This constraint limits TE increments to values that are smaller than those traditionally used in three-point Dixon methods (5). In addition, resonant frequency offsets from chemical shift and field heterogeneities produce additional phase shifts that are unique to SSFP and are problematic for Dixon fat-water decomposition techniques (2,6).The application of Dixon imaging in fast spin-echo (FSE) sequences has also been limited because the acquisition of echoes at different time shifts with respect to the SE increases the spacing between successive refocusing pulses (echo spacing) (9). Increasing the echo spacing reduces the number of echoes that can be collected in a time that maintains acceptable blurring from T 2 decay (10), offsetting the scan time benefits of FSE. A fat...
Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
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