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Accuracy and reproducibility in phase contrast magnetic resonance imaging
Linköping University, Department of Biomedical Engineering. Linköping University, Department of Medicine and Care. Linköping University, The Institute of Technology.
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 899
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-29441Local ID: 14787ISBN: 91-85295-41-8 (print)OAI: oai:DiVA.org:liu-29441DiVA: diva2:250256
Public defence
2004-12-10, Wilandersalen, Universitetssjukhuset, Örebro, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-12-03
List of papers
1. Correction for acceleration-induced displacement artifacts in phase contrast imaging
Open this publication in new window or tab >>Correction for acceleration-induced displacement artifacts in phase contrast imaging
Show others...
2000 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 43, no 5, 734-738 p.Article in journal (Refereed) Published
Abstract [en]

The acceleration-induced displacement artifact impairs the accuracy of MR velocity measurements. This study proposes a post processing method for correction of this artifact. Velocity measurements were performed in a flow phantom containing a constriction. Velocity curves were obtained from streamlines parallel to the frequency, phase, and slice directions, respectively. The acceleration-induced displacement artifact was most prominent when the frequency encoding direction was aligned with the flow direction. After correction, velocity assignment improved and a more accurate description of the flow was obtained. In vivo measurements were performed in the aorta in a patient with a repaired aortic coarctation. The correction method was applied to velocity data along a streamline parallel to the frequency encoding direction. The result after correction was a new location of the peak velocity and improved estimates of the velocity gradients.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-26712 (URN)10.1002/(SICI)1522-2594(200005)43:5<734::AID-MRM16>3.0.CO;2-A (DOI)11306 (Local ID)11306 (Archive number)11306 (OAI)
Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2017-12-13
2. Correction for displacement artifacts in 3D phase contrast imaging
Open this publication in new window or tab >>Correction for displacement artifacts in 3D phase contrast imaging
2002 (English)In: Journal of Magnetic Resonance Imaging, ISSN 1053-1807, E-ISSN 1522-2586, Vol. 16, no 5, 591-597 p.Article in journal (Refereed) Published
Abstract [en]

Purpose

To correct for displacement artifacts in 3D phase contrast imaging.

Materials and Methods

A 3D phase contrast pulse sequence was modified so that displacements of velocity measurements were restricted to one direction. By applying a postprocessing method, displaced measurements could be traced back to their accurate positions. Flow studies were performed using a phantom that generated flow through a stenosis, directed oblique relative to the phase and frequency encoding directions. Velocity profiles and streamline visualization were used to compare displaced and corrected velocity data to a reference.

Results

Velocity profiles obtained from the original measurement showed skewed profiles due to the displacement artifact, both at close proximity to the orifice as well as further downstream. After correction, concordance with the reference improved considerably.

Conclusion

The displacement artifact, which restricts the accuracy of phase contrast measurements, can be corrected for using the proposed method. Correction of the phase contrast velocity data may improve the accuracy of subsequent flow analysis and visualization.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-26708 (URN)10.1002/jmri.10187 (DOI)11301 (Local ID)11301 (Archive number)11301 (OAI)
Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2017-12-13
3. Optimization of gradient waveforms for velocity measurements and concurrent reduction of displacement artifacts in phase contrast imaging
Open this publication in new window or tab >>Optimization of gradient waveforms for velocity measurements and concurrent reduction of displacement artifacts in phase contrast imaging
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The purpose of the present study was to develop gradient waveforms for phase contrast magnetic resonance imaging that provides concurrent reduction of displacement artifacts. A framework was developed which was based on a linear combination of three individual gradients, each gradient having a trapezoidal shape. The amplitude of each gradient trapezoid was determined by the desired values of the zeroth, first and second moments of the combined tripolar waveform. All calculations of moments of the tripolar gradient waveform were performed with the expansion point set equal to the echo time. The times of spatial and velocity encoding are simultaneous in all three encoding directions, implying elimination of the displacement artifact. The proposed phase contrast pulse sequence using tripolar gradients was compared to a conventional phase contrast pulse sequence based on bipolar gradients. The echo time increased with the use of tripolar gradients compared to the conventional pulse sequence. Compensation for the increase in scan time can be achieved with the use of parallel imaging techniques.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-85889 (URN)
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2016-03-14
4. Accuracy and reproducibility in phase contrast imaging using SENSE
Open this publication in new window or tab >>Accuracy and reproducibility in phase contrast imaging using SENSE
2003 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 50, no 5, 1061-1068 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study was to evaluate the accuracy and reproducibility of phase contrast imaging using the sensitivity encoding (SENSE) method at different reduction factors. Analytical expressions were derived that state how reproducibility is influenced for velocity and flow measurements. Computer simulations, and in vitro and in vivo studies were performed in order to validate these expressions and to assess how accuracy is affected when different reduction factors are applied. It was shown that reproducibility depends on the reduction and geometry factors. Since the geometry factor varies spatially, so does the reproducibility for phase contrast imaging. In areas with high geometry factors, the standard deviation (SD) may become so large that aliasing occurs. The accuracy of phase contrast imaging is not influenced directly when SENSE is used, but may be indirectly influenced due to high SDs of the measured phase that may subsequently cause aliasing. The current results show that it is possible to achieve accurate flow measurements even at high reduction factors. By taking the geometry factor into account, it may be possible to find areas where phase contrast imaging is accurate even at high reduction factors.

Keyword
Flow quantification, Parallel MRI, Phase contrast imaging, Reproducibility, SENSE
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-46443 (URN)10.1002/mrm.10634 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
5. Comparison of different methods for combining phase-contrast images obtained with multiple coils
Open this publication in new window or tab >>Comparison of different methods for combining phase-contrast images obtained with multiple coils
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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-41882 (URN)10.1016/j.mri.2005.06.002 (DOI)59288 (Local ID)59288 (Archive number)59288 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13

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