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 ...

Quantification of hepatic steatosis is a significant unmet need for the diagnosis and treatment of patients with nonalcoholic fatty liver disease (NAFLD). MRI is capable of separating water and fat signals in order to quantify fatty infiltration of the liver (hepatic steatosis). Unfortunately, fat signal has confounding T 1 effects and the nonzero mean noise in low signal-to-noise ratio (SNR) magnitude images can lead to incorrect estimation of the true lipid percentage. In this study, the effects of bias from T 1 effects and image noise were investigated. An oil/water phantom with volume fat-fractions ranging linearly from 0% to 100% was designed and validated using a spoiled gradient echo (SPGR) sequence in combination with a chemical-shift based fat-water separation method known as iterative decomposition of water and fat with echo asymmetry and least squares estimation (IDEAL). We demonstrated two approaches to reduce the effects of T 1 : small flip angle (flip angle) and dual flip angle methods. Both methods were shown to effectively minimize deviation of the measured fat-fraction from its true value. We also demonstrated two methods to reduce noise bias: magnitude discrimination and phase-constrained reconstruction. Both methods were shown to reduce this noise bias effectively from 15% to less than 1%. Magn Reson Med 58:354 -364, 2007.

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.

MRI at 3.0 T can improve resolution and speed in musculoskeletal imaging; however, interactions between field strength and relaxation times need to be considered for optimal image contrast and signal-to-noise ratio. Scanning can be performed in shorter times at 3.0 T using single-average acquisitions. Efficient higher-resolution imaging at 3.0 T can be done by increasing the TR to account for increased T1 relaxation times and acquiring thinner slices than at 1.5 T.