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Separation of Water and Fat Signal in Magnetic Resonance Imaging: Advances in Methods Based on Chemical Shift
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Radiology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetic resonance imaging (MRI) is one of the most important diagnostic tools of modern healthcare. The signal in medical MRI predominantly originates from water and fat molecules. Separation of the two components into water-only and fat-only images can improve diagnosis, and is the premier non-invasive method for measuring the amount and distribution of fatty tissue.

Fat-water imaging (FWI) enables fast fat/water separation by model-based estimation from chemical shift encoded data, such as multi-echo acquisitions. Qualitative FWI is sufficient for visual separation of the components, while quantitative FWI also offers reliable estimates of the fat percentage in each pixel. The major problems of current FWI methods are long acquisition times, long reconstruction times, and reconstruction errors that degrade image quality.

In this thesis, existing FWI methods were reviewed, and novel fully automatic methods were developed and evaluated, with a focus on fast 3D image reconstruction. All MRI data was acquired on standard clinical scanners.

A triple-echo qualitative FWI method was developed for the specific application of 3D whole-body imaging. The method was compared with two reference methods, and demonstrated superior image quality when evaluated in 39 volunteers.

The problem of qualitative FWI by dual-echo data with unconstrained echo times was solved, allowing faster and more flexible image acquisition than conventional FWI. Feasibility of the method was demonstrated in three volunteers and the noise performance was evaluated.

Further, a quantitative multi-echo FWI method was developed. The signal separation was based on discrete whole-image optimization. Fast 3D image reconstruction with few reconstruction errors was demonstrated by abdominal imaging of ten volunteers.

Lastly, a method was proposed for quantitative mapping of average fatty acid chain length and degree of saturation. The method was validated by imaging different oils, using gas-liquid chromatography (GLC) as the reference. The degree of saturation agreed well with GLC, and feasibility of the method was demonstrated in the thigh of a volunteer.

The developed methods have applications in clinical settings, and are already being used in several research projects, including studies of obesity, dietary intervention, and the metabolic syndrome.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 701
Keyword [en]
Magnetic resonance imaging, digital image reconstruction, chemical shift imaging, water and fat separation, Dixon method, fat suppression, quantitative MRI, whole-body MRI, fatty acid composition, fat unsaturation, triglycerides, adipose tissue, liver fat, T2* mapping
National Category
Clinical Science
Research subject
Radiology
Identifiers
URN: urn:nbn:se:uu:diva-158111ISBN: 978-91-554-8154-4 (print)OAI: oai:DiVA.org:uu-158111DiVA: diva2:437998
Public defence
2011-10-21, Hedstrandsalen, Entrance 70, Uppsala University Hospital, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2011-09-29 Created: 2011-08-31 Last updated: 2011-11-03Bibliographically approved
List of papers
1. Three-point Dixon method enables whole-body water and fat imaging of obese subjects
Open this publication in new window or tab >>Three-point Dixon method enables whole-body water and fat imaging of obese subjects
2010 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 63, no 6, 1659-1668 p.Article in journal (Refereed) Published
Abstract [en]

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. 2 pi-Phase unwrapping is not required. Whole-body datasets (256 x 184 x 252 voxels) were collected from 39 subjects (body mass index 19.8-45.4 kg/m(2)), 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.

Keyword
three-point Dixon, whole-body MRI, water and fat separation, chemical shift imaging, fat suppression
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-129498 (URN)10.1002/mrm.22385 (DOI)000278164400026 ()20512869 (PubMedID)
Available from: 2010-08-17 Created: 2010-08-17 Last updated: 2012-03-07Bibliographically approved
2. Two-point dixon method with flexible echo times
Open this publication in new window or tab >>Two-point dixon method with flexible echo times
2011 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 65, no 4, 994-1004 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
chemical shift imaging, fat suppression, two-point Dixon, water and fat separation
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-150936 (URN)10.1002/mrm.22679 (DOI)000288612000011 ()21413063 (PubMedID)
Available from: 2011-04-08 Created: 2011-04-08 Last updated: 2012-03-07Bibliographically approved
3. Three-dimensional water/fat separation and T2* estimation based on whole-image optimization: application in breathhold liver imaging at 1.5 T
Open this publication in new window or tab >>Three-dimensional water/fat separation and T2* estimation based on whole-image optimization: application in breathhold liver imaging at 1.5 T
2012 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 67, no 6, 1684-1693 p.Article in journal (Refereed) Published
Abstract [en]

The chemical shift of water and fat resonances in proton MRI allows separation of water and fat signal from chemical shift encoded data. This work describes an automatic method that produces separate water and fat images as well as quantitative maps of fat signal fraction and T2* from complex multi-echo gradient recalled datasets. Accurate water and fat separation is challenging due to signal ambiguity at the voxel level. Whole-image optimization can resolve this ambiguity, but might be computationally demanding, especially for three-dimensional (3D) data. In this work, periodicity of the model fit residual as a function of the off-resonance was utilized to modify a previously proposed formulation of the problem. This gives a smaller solution space and allows rapid optimization. Feasibility and accurate separation of water and fat signal was demonstrated in breathhold 3D liver imaging of ten volunteer subjects, with both acquisition and reconstruction times below 20 seconds.

Keyword
water and fat separation, chemical shift imaging, quantitative MRI, liver fat, T2* mapping, QPBO
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-158097 (URN)10.1002/mrm.23185 (DOI)000304086000020 ()22189760 (PubMedID)
Available from: 2011-08-31 Created: 2011-08-31 Last updated: 2017-07-12Bibliographically approved
4. Model-based mapping of fat unsaturation and chain length by chemical shift imaging: phantom validation and in vivo feasibility
Open this publication in new window or tab >>Model-based mapping of fat unsaturation and chain length by chemical shift imaging: phantom validation and in vivo feasibility
2012 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 68, no 6, 1815-1827 p.Article in journal (Refereed) Published
Abstract [en]

Knowledge about the triglyceride (fat) 1H spectrum enables quantitative determination of several triglyceride characteristics. This work describes a model-based chemical shift imaging method that separates water and fat signal and provides maps of three triglyceride quantities: fatty acid carbon chain length (CL), number of double bond pairs (ndb), and number of methylene-interrupted double bonds (nmidb). The method was validated by imaging a phantom containing ten different oils using 1.5 T and 3.0 T clinical scanners, with gas-liquid chromatography (GLC) as reference. Repeated acquisitions demonstrated high reproducibility of the method. Statistical tests of correlation and linear regression were performed to examine the accuracy of the method. Significant correlation was found at both field strengths for all three quantities, and high correlation (r2 > 0.96) was found for measuring ndb and nmidb. Feasibility of the method for in vivo imaging of the thigh was demonstrated at both field strengths. The estimates of ndb and nmidb in subcutaneous adipose tisse were in agreement with literature values, while CL appears overestimated. The method has potential use in large-scale cross-sectional and longitudinal studies of triglyceride composition, and its relation to diet and various diseases.

Keyword
water/fat separation, chemical shift imaging, quantitative MRI, fat unsaturation, triglyceride mapping, fatty acid composition
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:uu:diva-158098 (URN)10.1002/mrm.24196 (DOI)000311398600015 ()22334300 (PubMedID)
Available from: 2011-08-31 Created: 2011-08-31 Last updated: 2013-01-28Bibliographically approved

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