Multiecho chemical shift-based water-fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shiftbased water-fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T* 2 in the presence of fat incorrectly estimate the T* 2 decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) water-fat separation and simultaneous T* 2 estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self-calibration algorithms. The improvement in water-fat separation and T* 2 estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans. Key words: water-fat separation; R* 2 measurement; T * 2 measurement; fat spectrum; fat quantification; fat spectral peak Multiecho chemical shift-based water-fat separation methods have seen a recent increase in clinical use (1-6), particularly in challenging applications where inhomogeneous magnetic fields cause failure of conventional fat saturation methods. Dixon (1) first used in-phase (IP) and out-of-phase (OP) images to analytically calculate the water and fat images, in the so-called the 2-point Dixon method. Glover (2) and Glover and Schneider (3) then extended the idea to collect three echoes such that the water-fat separation can be performed with the correction for B 0 field inhomogeneity. In the last decade, numerous variations have been proposed based on the 2-point and 3-point Dixon methods. These previous methods assumed a relatively simple signal representation that models both water and fat as a single resonant frequency. For most applications, this is a satisfactory model and excellent qualitative water-fat separation can be achieved.Although water is well modeled by a single frequency, this is not true for fat. In general, it is assumed that fat resonates at a single frequency ϳ3.5 ppm downfield from water (approximately 210 Hz at 1.5T, and 420 Hz at 3T). However, it is well known that fat has a number of spectral peaks (7-17). In particular, the spectral peak from olefinic proton (5.3 ppm) is close to the water resonant frequency, which will manifest as a baseline level of signal within adipose tissue on the separated water images (2,14,16). This effect is also commonly seen on images acquired with either conventional fat saturation (18) or spatial-spectral excitation (19). In general, this small signal within the fatty tissues is cl...

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 ...

Purpose:To describe and demonstrate the feasibility of a novel multiecho reconstruction technique that achieves simultaneous water-fat decomposition and T2* estimation. The method removes interference of water-fat separation with iron-induced T2* effects and therefore has potential for the simultaneous characterization of hepatic steatosis (fatty infiltration) and iron overload. Materials and Methods:The algorithm called "T2*-IDEAL" is based on the IDEAL water-fat decomposition method. A novel "complex field map" construct is used to estimate both R2* (1/T2*) and local B 0 field inhomogeneities using an iterative least-squares estimation method. Water and fat are then decomposed from source images that are corrected for both T2* and B 0 field inhomogeneity. Results:It was found that a six-echo multiecho acquisition using the shortest possible echo times achieves an excellent balance of short scan and reliable R2* measurement. Phantom experiments demonstrate the feasibility with high accuracy in R2* measurement. Promising preliminary in vivo results are also shown. Conclusion:The T2*-IDEAL technique has potential applications in imaging of diffuse liver disease for evaluation of both hepatic steatosis and iron overload in a single breath-hold.

Purpose: To combine gradient-echo (GRE) imaging with a multipoint water-fat separation method known as "iterative decomposition of water and fat with echo asymmetry and least squares estimation" (IDEAL) for uniform waterfat separation. Robust fat suppression is necessary for many GRE imaging applications; unfortunately, uniform fat suppression is challenging in the presence of B 0 inhomogeneities. These challenges are addressed with the IDEAL technique. Materials and Methods:Echo shifts for three-point IDEAL were chosen to optimize noise performance of the water-fat estimation, which is dependent on the relative proportion of water and fat within a voxel. Phantom experiments were performed to validate theoretical SNR predictions. Theoretical echo combinations that maximize noise performance are discussed, and examples of clinical applications at 1.5T and 3.0T are shown. Results:The measured SNR performance validated theoretical predictions and demonstrated improved image quality compared to unoptimized echo combinations. Clinical examples of the liver, breast, heart, knee, and ankle are shown, including the combination of IDEAL with parallel imaging. Excellent water-fat separation was achieved in all cases. The utility of recombining water and fat images into "in-phase," "out-of-phase," and "fat signal fraction" images is also discussed. Conclusion:IDEAL-SPGR provides robust water-fat separation with optimized SNR performance at both 1.5T and 3.0T with multicoil acquisitions and parallel imaging in multiple regions of the body. GRADIENT-ECHO (GRE) imaging is a rapid MRI method that is used for a variety of applications throughout the body. T 1 -weighted (T 1 W) spoiled gradient-echo (SPGR) sequences are of particular importance for postcontrast imaging in many areas of the body, including the abdomen (1) and breast (2). Noncontrast-enhanced T 1 W SPGR imaging is also highly valuable for assessing cartilage morphology (3).Many T 1 W GRE imaging applications require suppression of fat signal. Fat is bright in these sequences and can potentially obscure underlying pathologies, such as tumor or inflammation. Unfortunately, reliable and uniform fat suppression can be challenging in areas of main field (B 0 ) and RF (B 1 ) inhomogeneities. Examples of challenging applications include imaging of the extremities and areas with unfavorable geometry (e.g., the brachial plexus), off-isocenter imaging, and large field of view (FOV) imaging. Other fat-suppression methods, such as short TI inversion recovery (STIR) (4), are incompatible with rapid GRE imaging because of the need for a long inversion time (approximately 200 msec). Spectral-spatial or water-selective pulses can be combined with GRE imaging, but they are lengthy. Although the fat-water discrimination achieved by these pulses is insensitive to B 1 inhomogeneities, they are still sensitive to B 0 inhomogeneities (5).In 1984 Dixon (6) first described "in-and out-ofphase" imaging, a method that acquires two images at different echo times (TEs), thereby exploiting th...

Robust fat suppression techniques are required for many clinical applications. Multi-echo water-fat separation methods are relatively insensitive to B 0 field inhomogeneity compared to the fat saturation method. Estimation of this field inhomogeneity, or field map, is an essential and important step, which is well known to have ambiguity. For an iterative water-fat decomposition method recently proposed, ambiguities still exist, but are more complex in nature. They were studied by analytical expressions and simulations. To avoid convergence to incorrect field map solutions, an initial guess closer to the true field map is necessary. This can be achieved using a region growing process, which correlates the estimation among neighboring pixels. Further improvement in stability is achieved using a low-resolution reconstruction to guide the selection of the starting pixels for the region growing. The proposed method was implemented and shown to significantly improve the algorithm's immunity to field inhomogeneity. Magn Reson Med 54: 1032-1039, 2005.

Three encoding strategies for the measurement of flow velocities in arbitrary directions with phase-contrast magnetic resonance imaging are presented; their noise and dynamic range performance are compared by means of theoretical analysis and computer simulation. A six-point measurement strategy is shown to be quite inefficient in terms of velocity variance per unit time. A simple four-point method exhibits equal dynamic range; its noise depends on flow direction but on average is equal to that of the six-point method. An alternate, balanced four-point method has noise that is direction independent and has, depending on implementation, possibly lower noise levels. Either four-point method is more efficient and is preferred over the six-point approach.

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Purpose: To develop a chemical-shift-based imaging method for fat quantification that accounts for the complex spectrum of fat, and to compare this method with MR spectroscopy (MRS). Quantitative noninvasive biomarkers of hepatic steatosis are urgently needed for the diagnosis and management of nonalcoholic fatty liver disease (NAFLD). Materials and Methods:Hepatic steatosis was measured with "fat-fraction" images in 31 patients using a multiecho chemical-shift-based water-fat separation method at 1.5T. Fat-fraction images were reconstructed using a conventional signal model that considers fat as a single peak at -210 Hz relative to water ("single peak" reconstruction). Fat-fraction images were also reconstructed from the same source images using two methods that account for the complex spectrum of fat; precalibrated and self-calibrated "multipeak" reconstruction. Single-voxel MRS that was coregistered with imaging was performed for comparison.Results: Imaging and MRS demonstrated excellent correlation with single peak reconstruction (r 2 ϭ 0.91), precalibrated multipeak reconstruction (r 2 ϭ 0.94), and self-calibrated multipeak reconstruction (r 2 ϭ 0.91). However, precalibrated multipeak reconstruction demonstrated the best agreement with MRS, with a slope statistically equivalent to 1 (0.96 Ϯ 0.04; P ϭ 0.4), compared to self-calibrated multipeak reconstruction (0.83 Ϯ 0.05, P ϭ 0.001) and single-peak reconstruction (0.67 Ϯ 0.04, P Ͻ 0.001). Conclusion:Accurate spectral modeling is necessary for accurate quantification of hepatic steatosis with MRI.