Three‐point dixon method enables whole‐body water and fat imaging of obese subjects

Dixon imaging techniques derive chemical shift‐separated water and fat images, enabling the quantification of fat content and forming an alternative to fat suppression. Whole‐body Dixon imaging is of interest in studies of obesity and the metabolic syndrome, and possibly in oncology. A three‐point Dixon method is proposed where two solutions are found analytically in each voxel. The true solution is identified by a multiseed three‐dimensional region‐growing scheme with a dynamic path, allowing confident regions to be solved before unconfident regions, such as background noise. 2π‐Phase unwrapping is not required. Whole‐body datasets (256 × 184 × 252 voxels) were collected from 39 subjects (body mass index 19.8‐45.4 kg/m²), in a mean scan time of 5 min 15 sec. Water and fat images were reconstructed offline, using the proposed method and two reference methods. The resulting images were subjectively graded on a four‐grade scale by two radiologists, blinded to the method used. The proposed method was found superior to the reference methods. It exclusively received the two highest grades, implying that only mild reconstruction failures were found. The computation time for a whole‐body dataset was 1 min 51.5 sec ± 3.0 sec. It was concluded that whole‐body water and fat imaging is feasible even for obese subjects, using the proposed method. Magn Reson Med 63:1659–1668, 2010. © 2010 Wiley‐Liss, Inc.     

Two‐point dixon method with flexible echo times

The two‐point Dixon method is a proton chemical shift imaging technique that produces separated water‐only and fat‐only images from a dual‐echo acquisition. It is shown how this can be achieved without the usual constraints on the echo times. A signal model considering spectral broadening of the fat peak is proposed for improved water/fat separation. Phase errors, mostly due to static field inhomogeneity, must be removed prior to least‐squares estimation of water and fat. To resolve ambiguity of the phase errors, a corresponding global optimization problem is formulated and solved using a message‐passing algorithm. It is shown that the noise in the water and fat estimates matches the Cramér‐Rao bounds, and feasibility is demonstrated for in vivo abdominal breath‐hold imaging. The water‐only images were found to offer superior fat suppression compared with conventional spectrally fat suppressed images. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast three‐dimensional dual echo dixon technique improves fat suppression in breast MRI

Purpose:

To compare qualitative and quantitative measures of the contrast‐enhanced dual‐echo Dixon technique with the commonly used standard three‐dimensional (3D) gradient echo (spectrally selective fat suppression) technique (SS‐FS) in breast MRI exams (bMRI).

Materials and Methods:

A total of 19 women, with prescheduled bMRI exam, were recruited to our study between 2006 and 2008. Dixon and standard SS‐SF techniques were used on both breasts of each patient. Image quality was rated in five categories: fat suppression quality, fat suppression uniformity, lesion margin clarity, lesion visibility, and axillary visibility. For quantitative assessment, we calculated the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) of lesion to breast, SNR efficiency, and CNR efficiency.

Results:

Of 19 patients evaluated, 13 had a primary breast malignancy and 6 had benign lesions or negative exams. Dixon images were rated higher in four of five qualitative categories (P < 0.0001) and required a shorter scan time. Dixon images yielded significantly higher SNR (43.8) and CNR (40.1) values than did 3DGRE images (SNR = 34.8, CNR = 25.3; P < 0.05). SNR efficiency (36.30) and CNR efficiency (33.79) values for Dixon images were also higher than were 3DGRE images (SNR efficiency =25.7, CNR efficiency = 19.1; P < 0.05).

Conclusion:

Dixon images were superior to the standard SS‐SF images in both qualitative and quantitative assessment of 19 bMRI exams. The Dixon technique could replace standard SS‐SF technique in bMRI exam, after our findings have been confirmed in future studies with a larger sample size. J. Magn. Reson. Imaging 2010;31:889–894. ©2010 Wiley‐Liss, Inc.     

Fast spin-echo triple-echo dixon (fTED) technique for efficient T2-weighted water and fat imaging.

Previously published fast spin-echo (FSE) implementations of a Dixon method for water and fat separation all require multiple scans and thus a relatively long scan time. Further, the minimum echo spacing (esp), a time critical for FSE image quality and scan efficiency, often needs to be increased in order to bring about the required phase shift between the water and fat signals. This work proposes and implements a novel FSE triple-echo Dixon (fTED) technique that can address these limitations. In the new technique, three raw images are acquired in a single FSE scan by replacing each frequency-encoding gradient in a conventional FSE with three consecutive gradients of alternating polarity. The timing of the three gradients is adjusted by selecting an appropriate receiver bandwidth (RBW) so that the water and fat signals for the three corresponding echoes have a relative phase shift of -180 degrees , 0 degrees , and 180 degrees , respectively. A fully automated postprocessing algorithm is then used to generate separate water-only and fat-only images for each slice. The technique was implemented with and without parallel imaging. We demonstrate that the new fTED technique enables both uniform water/fat separation and fast scanning with uncompromised scan parameters, including applications such as T(2)-weighted separate water and fat imaging of the abdomen during breath-holding.     

Multiecho time‐resolved acquisition (META): A high spatiotemporal resolution dixon imaging sequence for dynamic contrast‐enhanced MRI

Purpose

To evaluate a new dynamic contrast‐enhanced (DCE) imaging technique called multiecho time‐resolved acquisition (META) for abdominal/pelvic imaging. META combines an elliptical centric time‐resolved three‐dimensional (3D) spoiled gradient‐recalled echo (SPGR) imaging scheme with a Dixon‐based fat‐water separation algorithm to generate high spatiotemporal resolution volumes.

Materials and Methods

Twenty‐three patients referred for hepatic metastases or renal masses were imaged using the new META sequence and a conventional fat‐suppressed 3D SPGR sequence on a 3T scanner. In 12 patients, equilibrium‐phase 3D SPGR images acquired immediately after META were used for comparing the degree and homogeneity of fat suppression, artifacts, and overall image quality. In the remaining 11 of 23 patients, DCE 3D SPGR images acquired in a previous or subsequent examination were used for comparing the efficiency of arterial phase capture in addition to the qualitative analysis for the degree and homogeneity of fat suppression, artifacts, and overall image quality.

Results

META images were determined to be significantly better than conventional 3D SPGR images for degree and uniformity of fat suppression and ability to visualize the arterial phase. There were no significant differences in artifact levels or overall image quality.

Conclusion

META is a promising high spatiotemporal resolution imaging sequence for capturing the fast dynamics of hyperenhancing hepatic lesions and provides robust fat suppression even at 3T. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.     

Myocardial fat quantification in humans: Evaluation by two‐point water‐fat imaging and localized proton spectroscopy

Proton MR spectroscopy (¹H‐MRS) has been used for in vivo quantification of intracellular triglycerides within the sarcolemma. The purpose of this study was to assess whether breath‐hold dual‐echo in‐ and out‐of‐phase MRI at 3.0 T can quantify the fat content of the myocardium. Biases, including T₁, T*₂, and noise, that confound the calculation of the fat fraction were carefully corrected. Thirty‐four of 46 participants had both MRI and MRS data. The fat fractions from MRI showed a strong correlation with fat fractions from MRS (r = 0.78; P < 0.05). The mean myocardial fat fraction for all 34 subjects was 0.7 ± 0.5% (range: 0.11–3%) assessed with MRS and 1.04 ± 0.4% (range: 0.32–2.44%) assessed with in‐ and out‐of‐phase MRI (P < 0.05). Scanning times were less than 15 sec for Dixon imaging, plus an additional minute for the acquisition used for T*₂ calculation, and 15‐20 min for MRS. The average postprocessing time for MRS was 3 min and 5 min for MRI including T*₂ measurement. We conclude that the dual echo method provides a rapid means to detect and quantifying myocardial fat content in vivo. Correction/adjustment for field inhomogeneity using three or more echoes seems crucial for the dual echo approach. Magn Reson Med 63:892–901, 2010. © 2010 Wiley‐Liss, Inc.     

T2-weighted spine imaging with a fast three-point dixon technique: comparison with chemical shift selective fat suppression

PURPOSE: To develop a phased-array coil-compatible, fast three-point Dixon (TPD) technique, and compare its performance in T2-weighted spine imaging with that of the standard chemical shift selective (CHESS) fat suppression technique. MATERIALS AND METHODS: We acquired T2-weighted spine images of 27 patients using essentially identical scanning parameters with the fast TPD technique and standard fast spin echo (FSE) with CHESS fat suppression. A phased-array coil-compatible image reconstruction algorithm was developed to generate separate water and fat images from the data acquired with the fast TPD technique. Three neuroradiologists independently scored the images from the two different techniques for uniformity of fat suppression and lesion conspicuity using a four-point system (1 = poor, 2 = fair, 3 = good, 4 = best). RESULTS: The reviewers' mean scores were 3.2 and 2.1 for the uniformity of fat suppression, and 3.0 and 2.0 for the lesion conspicuity for the fast TPD and the CHESS fat suppression techniques, respectively. The fast TPD technique was statistically superior to the CHESS technique at P < 0.0005. CONCLUSION: The fast TPD technique provides superior fat suppression and lesion conspicuity, and potentially can be used as an alternative to T2-weighted imaging of the spine.     

T1‐weighted 3D dynamic contrast‐enhanced MRI of the breast using a dual‐echo dixon technique at 3 T

Purpose:

To evaluate a single‐pass fast spoiled gradient echo (FSPGR) two‐point Dixon sequence and a gradient echo sequence with spectral fat suppression in their performance at 3 T for fat suppressed contrast‐enhanced bilateral breast imaging.

Materials and Methods:

Twenty patients were prospectively enrolled in an imaging protocol that included axial Dixon and 3D FSPGR with spectrally selective fat saturation sequences as part of patient care in this study. Qualitative analysis was performed retrospectively by two readers who scored the images for homogeneity and degree of fat saturation, severity of artifacts, and quality of normal anatomical structures. Enhancing lesions were scored according to the confidence with which American College of Radiology (ACR) BI‐RADS magnetic resonance imaging (MRI) features were identified.

Results:

The Dixon sequence showed superior fat saturation homogeneity, quality of posterior anatomical structures, and decreased artifact severity that were statistically significant (P < 0.0001). The degree of fat saturation was scored higher in the Dixon sequence, although the difference did not reach statistical significance. There were no significant differences between the 3D T1‐weighted FSPGR and Dixon groups for assessing lesion features.

Conclusion:

Our findings suggest that the Dixon technique is an effective fat suppression method for contrast‐enhanced breast MRI. The Dixon technique also seemed to provide better anatomical definition of posterior structures and improvement in severity of artifacts. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.