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Comparison of different methods for combining phase-contrast images obtained with multiple coils
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.
Linköping University, Department of Biomedical Engineering. Linköping University, The Institute of Technology.ORCID iD: 0000-0001-5526-2399
Linköping University, Department of Medicine and Care, Clinical Physiology. Linköping University, Faculty of Health Sciences.
2005 (English)In: Magnetic Resonance Imaging, ISSN 0730-725X, E-ISSN 1873-5894, Vol. 23, no 7, 795-799 p.Article in journal (Refereed) Published
Abstract [en]

The ability to determine coil sensitivities implies that a method optimized in terms of maximized signal-to-noise ratio (SNR) can be applied to the combination of multiple coil images. An optimization of SNR subsequently results in a minimized variance in quantitative velocity measurements using phase-contrast imaging. When coil sensitivities are unknown, the weighted mean method, utilizing the square of the signal magnitude as weights, is suitable for combination of multiple phase images. In this study, the optimized method using estimated coil sensitivities was compared to the weighted mean method both theoretically and experimentally. It is shown that absence of noise correlation between the different coil images implies no difference between the methods regarding the variance of the phase. In the practical situation, noise correlation does exist, implying an opportunity for further reduction of phase variance using the optimized method. In vitro and in vivo studies showed, however, no significant difference between the two methods studied.

Place, publisher, year, edition, pages
2005. Vol. 23, no 7, 795-799 p.
National Category
Medical and Health Sciences
URN: urn:nbn:se:liu:diva-41882DOI: 10.1016/j.mri.2005.06.002Local ID: 59288OAI: diva2:262737
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2016-03-14
In thesis
1. Accuracy and reproducibility in phase contrast magnetic resonance imaging
Open this publication in new window or tab >>Accuracy and reproducibility in phase contrast magnetic resonance imaging
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetic resonance imaging (MRI) is an imaging modality which provides good conditions for studies of flow and motion in the cardiovascular system. When using the phase contrast imaging technique, it is possible to perform velocity measurements in moving tissues and in blood. The acquired velocity data can be used for subsequent calculation of different parameters used for the assessment of cardiovascular function. These parameters rely on the accuracy and reproducibility of the velocity measurements.

This thesis includes an introduction to phase contrast imaging and how displacement artifacts may hamper the accuracy of velocity measurements using phase contrast imaging. A method for correction of this artifact was developed using the knowledge of the times at which velocity and spatial encoding are carried out in the pulse sequence. The elaboration of this correction method included modifications of a 3D phase contrast pulse sequence and development of post processing algorithms involving streamline calculations. An alternative approach for correction of displacement artifacts is also presented where a pulse sequence, based on tripolar waveforms, inherently compensates for the artifact and, consequently, do not require any post processing algorithms.

The use of the sensitivity encoding (SENSE) method in MRI implies that the scanning time can be reduced dramatically. The SENSE technique can be combined with phase contrast imaging. An important question to answer is how the use of SENSE influences the accuracy and reproducibility in phase contrast imaging. In this thesis expressions are derived showing how the reproducibility of velocity and flow measurements are influenced when applying SENSE at different reduction factors.

As a consequence of the introduction of parallel imaging techniques, such as SENSE, modem MRI systems have the capability of assessing individual coil sensitivities. This information implies new alternatives in the combination of multiple phase images using phased array coils. Conventionally multiple phase images are combined using the weighted mean method, where the squared magnitude values are used as weights. In order to compare these two combination methods, theoretical and experimental studies were performed.

The results of this thesis show that corrupted velocity measurements caused by displacement artifacts can be alleviated using suggested correction methods or pulse sequences. This is of importance for subsequent flow analysis and visualization of stenotic and oblique flow. Phase contrast imaging in combination with SENSE provides opportunities to obtain large reductions in scanning times while keeping the increase of the reproducibility in the velocity and flow measurements within reasonable limits. In terms of phase reproducibility it is shown that the use of individual coil sensitivities in the combination of multiple phase images is equivalent to the conventional weighted mean method. In the accompanying magnitude images, however, signal variations due to coil sensitivities are compensated for when including estimated coil sensitivities.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2004. 73 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 899
National Category
Medical and Health Sciences
urn:nbn:se:liu:diva-29441 (URN)14787 (Local ID)91-85295-41-8 (ISBN)14787 (Archive number)14787 (OAI)
Public defence
2004-12-10, Wilandersalen, Universitetssjukhuset, Örebro, 10:15 (Swedish)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-12-03

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Thunberg, PerKarlsson, MattsWigström, Lars
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