In this work, a new two-point method for water-fat imaging is described and explored. It generalizes existing two-point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two-point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient-echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two-point methods. Notably, water images with higher spatial resolution and better signal-to-noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in-and opposedphase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two-point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three-point methods in this way, especially toward the edges of larger field of views. Magn Reson Med 65:96-107, 2011. V C 2010 Wiley-Liss, Inc. Key words: water-fat separation; fat suppression; Dixon methods; multiecho acquisitions; abdominal imaging; eddy currentsAs hyperintense signal from fat may obscure underlying pathology, its partial or complete suppression is a basic requirement in various applications of magnetic resonance imaging. Its characteristics result from the comparatively short relaxation times and large chemical shifts of the dominant methylene protons and serve as the basis for its elimination.Fat suppression is often an integral part of the acquisition. Popular methods include short-tau inversion recovery, which exploits the specific relaxation times, and selective saturation, which relies on the specific chemical shifts (1,2). However, these methods all have individual drawbacks, such as longer scan times, lower signalto-noise ratio (SNR), higher specific absorption rate, or less tolerance to field inhomogeneities. Postponing the separation of water and fat signals until the reconstruction allows avoiding most of these disadvantages. So-called Dixon methods perform for this purpose measurements at different echo times to encode the chemical shift (3). Besides fat suppression, they also permit efficient water-fat imaging, providing additional diagnostic information of relevance to selected applications.Several Dixon methods have been proposed over the last two decades (4). Apart from different strategies for the separation, they are mainly characterized by the number of echoes, or points, that they sample, and by the constraints that they impose on the echo times. We focus in this work on two-and three-point methods, as multipoint methods are usually very similar to threepoint methods, and one-point methods are gene...

SW MR imaging depicts significantly more small hemorrhagic lesions than does conventional GRE MR imaging and therefore has the potential to improve diagnosis of DAI.

Purpose To assess cardiac motion-induced signal loss in diffusion-weighted magnetic resonance imaging (DWI) of the liver using dynamic DWI. Materials and methods Three volunteers underwent dynamic coronal DWI of the liver under breathholding, in the diastolic (DWI diast ) or systolic (DWI syst ) cardiac phase, and with motion probing gradients (MPGs) in phase encoding (P, left-right), frequency encoding (M, superior-inferior), or slice select (S, anterior-posterior) direction. Liver-to-background contrasts (LBCs) of DWI syst were compared to those of DWI diast , for both the left and right liver lobes, using nonparametric tests. Signal decrease ratios (SDRs) were calculated as (1 (median 3.35) were significantly lower (P < 0.0001) than those of DWI diast (median 4.84). In the right liver lobe, LBCs of DWI syst (median 4.17) were also significantly lower (P < 0.0001) than those of DWI diast (median 5.35). SDRs of the left and right liver lobes were 25.5% and 17.3%, respectively. In DWI syst , the significantly lowest (P < 0.05) LBCs were observed in the M direction (left liver lobe) and P direction (right liver lobe) of MPGs. Conclusion Signal intensity of both liver lobes are affected by cardiac motion in DWI. In the left liver lobe, signal loss especially occurs in the superior-inferior direction of MPGs, whereas in the right lobe, signal loss especially occurs in the left-right direction of MPGs.

The aim of this study was to investigate the possible correlation between the 1H MRS mobile lipid signal, necrosis and lipid droplets in C6 rat glioma. First, the occurrence of necrosis and lipid droplets was determined during tumor development, by a histological analysis performed on 34 rats. Neither necrosis nor lipid droplets were observed before 18 days post-implantation. At later stages of development, both necrosis and lipid droplets were apparent, the lipid droplets being mainly located within the necrotic areas. Using a second group of eight rats, a temporal correlation was evidenced between mobile lipid signal detected by in vivo single-voxel one- (136 ms echo time) and two-dimensional J-resolved 1H MR spectroscopy, and the presence of necrosis and lipid droplets on the histological sections obtained from the brains of the same rats. Finally, spatial distribution of the mobile lipid signal was analyzed by chemical-shift imaging performed on a third group of eight animals, at the end of the tumor growth. The spectroscopic image corresponding to the resonance of mobile lipids had its maximum intensity in the center of the tumor where necrotic regions were observed on the histological sections. These necrotic areas contained large amounts of lipid droplets. All these results suggest that mobile lipids detected in vivo by 1H MRS (136 ms echo time) in C6 rat brain glioma arise mainly from lipid droplets located in necrosis.

Purpose: To identify and quantify the potential sources of motion in carotid artery imaging. Materials and Methods:Two healthy volunteers and 12 patients (20 -75 years old) with atherosclerotic disease were scanned on a Philips Intera 1.5T system. A single-shot balanced-fast field echo (bFFE) sequence was used to acquire real-time axial views of the carotid artery wall (three images per second). A three-step acquisition protocol was performed to analyze the three types of motion (arterial pulsation, breathing, and swallowing) separately. The isocenter carotid artery motion amplitude in either the x or y direction was measured. Radial variation in the carotid lumen between the systolic and diastolic phases was analyzed. Motion frequency was reported for each patient.Results: Significant motion related to arterial pulsation (amplitude ϭ 0.27-0.93 mm, mean ϭ 0.6, SD ϭ 0.19), breathing (amplitude ϭ 0.5-3.6 mm, mean ϭ 1.56, SD ϭ 0.99)), and swallowing (amplitude ϭ 1.4 -9.2 mm, mean ϭ 4.7, SD ϭ 2.4) were visualized. Conclusion:Pulsation, breathing, and swallowing are sources of significant motion in the carotid artery wall. Such motion should be considered in the future to improve carotid artery image quality.

Online MR temperature monitoring during radiofrequency (RF) ablation of cardiac arrhythmias may improve the efficacy and safety of the treatment. MR thermometry at 1.5 T using the proton resonance frequency (PRF) method was assessed in 10 healthy volunteers under normal breathing conditions, using a multi-slice, ECG-gated, echo planar imaging (EPI) sequence in combination with slice tracking. Temperature images were post-processed to remove residual motion-related artifacts. Using an MR-compatible steerable catheter and electromagnetic noise filter, RF ablation was performed in the ventricles of two sheep in vivo. The standard deviation of the temperature evolution in time (TSD) was computed. Temperature mapping of the left ventricle was achieved at an update rate of approximately 1 Hz with a mean TSD of 3.6 ± 0.9 °C. TSD measurements at the septum showed a higher precision (2.8 ± 0.9 °C) than at the myocardial regions at the heart-lung and heart-liver interfaces (4.1 ± 0.9 °C). Temperature rose maximally by 9 °C and 16 °C during 5 W and 10 W RF applications, respectively, for 60 s each. Tissue temperature can be monitored at an update rate of approximately 1 Hz in five slices. Typical temperature changes observed during clinical RF application can be monitored with an acceptable level of precision.

TRON offers sharp diffusion-weighted images of the liver using an efficient scan time, making it an excellent alternative to RT and FB DWI. The moderate to poor agreement in ADCs of liver metastases between TRON and RT DWI requires further investigation.

Purpose: To compare the image quality of two variants of a three-dimensional (3D) gradient echo sequence (GRE) for hepatic MRI.Materials and Methods: Thirty-nine patients underwent hepatic MRI on a 3.0 Tesla (T) magnet (Intera Achieva; Philips Medical Systems). The clinical protocol included two variants of a 3D GRE with fat suppression: (i) a ''centric'' approach, with elliptical centric k-space ordering and (ii) an ''enhanced'' approach using linear sampling and partial Fourier in both the slice and phase encoding direction. ''Centric'' and ''Enhanced'' 3D GRE images were obtained both precontrast (n ¼ 32) and after gadoxetic acid injection (n ¼ 39). Two reviewers jointly reviewed MR images for anatomic sharpness, overall contrast, homogeneity, and absence of artifacts. The liver-to-lesion signal difference ratio (SDR) was measured. Paired sample Wilcoxon test and paired t-tests were used.Results: Enhanced 3D GRE images performed better than centric 3D GRE images with respect to anatomic sharpness (P ¼ 0.0156), overall contrast (P ¼ 0.0195), homogeneity (P < 0.0001), and absence of artifacts (P ¼ 0.0003) on precontrast images. For postcontrast MRI, enhanced 3D GRE images showed better quality in terms of overall contrast (P ¼ 0.0195), homogeneity (P < 0.0001), and absence of artifacts (P ¼ 0.009). Liver-tolesion SDR on enhanced 3D GRE images (0.48 6 0.13) was significantly higher than that of conventional 3D GRE images (0.40 6 0.19, P ¼ 0.0004) on postcontrast images, but not on precontrast images. Conclusion:The enhanced 3D GRE sequence available on our scanner provided better hepatic image quality than the centric variant, without compromising lesion contrast.