Iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL): Application with fast spin‐echo imaging

Abstract

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 ...

Temperature Dependence of Relaxation Times in Proton Components of Fatty Acids

“…However, as we observed in earlier spectrometer experiments (results not shown), the components other than CH 2 and CH 3 have diŠerent temperature sensitivities in their relaxation times. To consider the eŠect from these components in imaging temperature, it would be preferable to separate signal using, for example, a multipoint Dixon-type technique, such as iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL), 11 which requires a speciˆc signal model with a number of components with certain phase angles among them. Moreover, investigation of the temperature dependence of such components is an interesting topic that requires examination.…”

Dixon techniques for water and fat imaging

“…In particular, NSA becomes zero for water-only and fat-only images when water and fat are equal. The SNR performance from this analysis forms the basis for the IDEAL (iterative decomposition of water and fat with echo asymmetry and least-squares estimation) technique (65,66). The essential component of the IDEAL technique is the combination of the iterative least-squares approach for water/fat separation (18) and the optimal sampling scheme under which NSA is independent of the water/fat ratios.…”

Field map estimation with a region growing scheme for iterative 3‐point water‐fat decomposition

“…1b and c show R-curves for 2 sets of asymmetric TE n and for various W:F ratios: TE n ϭ [Ϫ1.8 ms, 0, 2.5 ms] and TE n ϭ [Ϫ0.38 ms, 1.14 ms, 2.66 ms]; the latter corresponds to phase shifts of [Ϫ /6, /2, 7 /6], which have been shown to have ideal noise performance in related studies (8,16). In Fig.…”