Sten Oyre - Academia.edu (original) (raw)

Papers by Sten Oyre

Research paper thumbnail of Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR

Magnetic Resonance in Medicine, Nov 1, 1998

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stres... more Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vifro scans and computer simulations and applied to the common carotid artery of humans. In vifro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of 20.9% (vessel area) and 23.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.

Research paper thumbnail of Arterial MR Imaging Phase-Contrast Flow Measurement: Improvements with Varying Velocity Sensitivity during Cardiac Cycle1

Radiology, 2004

To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the au... more To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the authors implemented and evaluated a pulse sequence that enables automatic optimization of the velocity-encoding parameter V(enc) for individual heart phases in pulsatile flow on the basis of a rapid prescan. This sequence was prospectively evaluated by comparing velocity-to-noise ratios with those from a standard MR flow scan obtained in the carotid artery in eight volunteers. This sequence was shown to improve velocity-to-noise ratios by a factor of 2.0-6.0 in all but the systolic heart phase and was determined to be an effective technique for reducing noise in phase-contrast velocity measurements.

Research paper thumbnail of Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR

Magnetic Resonance in Medicine, 1998

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stres... more Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vifro scans and computer simulations and applied to the common carotid artery of humans. In vifro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of 20.9% (vessel area) and 23.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.

Research paper thumbnail of Left ventricular blood flow patterns in normal subjects: A quantitative analysis by three-dimensional magnetic resonance velocity mapping

Journal of the American College of Cardiology, 1995

Objectives. Magnetic resonance velocity mapping was used to investigate the hypothesis of a vorte... more Objectives. Magnetic resonance velocity mapping was used to investigate the hypothesis of a vortex motion within the left ventricle interacting with mitral valve motion and inflow velocity. Background. In vitro flow visualization studies have suggested the presence of a large anterior vortex inside the left ventricle during mitral inflow. However, to our knowledge the occurrence of this phenomenon has not been demonstrated in the human left ventricle. Methods. Magnetic resonance velocity mapping was performed in 26 healthy volunteers using a flow-adjusted gradient sequence for three-dimensional flow velocity acquisition in the long-axis plane of the left ventricle. By computer processing, the flow vectors in the left ventricle were visualized and animated dynamically. Results. The early diastolic mitral inflow was apically directed, and a large counterclockwise anterior vortex was created ~Rhin the left ventricle shortly after the onset of the mid-diastolic semiclosure of the anterior mitral leaflet. During mid-diastolic diastasis, mitral inflow ceased until the flow accelerated again at atrial systole. The final closure of the mitral valve was preceded by a smaller vortex seen at the tips of the mitral leaflets. At systolic ejection, all flow vectors were directed toward the left ventricular outflow tract. The anterior vortex had a radius of 1.62 -+ 0.24 em (mean -+ SD), and the average angular velocity (i.e., the rotation of an element about the center of the vortex within the central core) was 30.08 -+ 9.98 radians/s. The maximal kinetic energy of the anterior vortex was 4.3 x 10 -4 _+ 7.1 x 10 -s J.

Research paper thumbnail of Accurate noninvasive quantitation of blood flow, cross-sectional lumen vessel area and wall shear stress by three-dimensional paraboloid modeling of magnetic resonance imaging velocity data

Journal of the American College of Cardiology, 1998

Objectives. We present a new method in which a priori knowledge of the blood velocity fields with... more Objectives. We present a new method in which a priori knowledge of the blood velocity fields within the boundary layer at the vessel wall, combined with acquisition of high resolution magnetic resonance imaging (MRI) blood velocity data, allow exact modeling at the subpixel level.

Research paper thumbnail of Wall shear rates differ between the normal carotid, femoral, and brachial arteries: An in vivo MRI study

Journal of Magnetic Resonance Imaging, 2004

To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral ... more To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral arteries using very high resolution magnetic resonance imaging (MRI) phase contrast measurements.

Research paper thumbnail of Automatic vessel segmentation using active contours in cine phase contrast flow measurements

Journal of Magnetic Resonance Imaging, 1999

The segmentation of images obtained by cine magnetic resonance (MR) phase contrast velocity mappi... more The segmentation of images obtained by cine magnetic resonance (MR) phase contrast velocity mapping using manual or semi-automated methods is a time consuming and observer-dependent process that still hampers the use of flow quantification in a clinical setting. A fully automatic segmentation method based on active contour model algorithms for defining vessel boundaries has been developed. For segmentation, the phase image, in addition to the magnitude image, is used to address image distortions frequently seen in the magnitude image of disturbed flow fields. A modified definition for the active contour model is introduced to reduce the influence of missing or spurious edge information of the vessel wall. The method was evaluated on flow phantom data and on in vivo images acquired in the ascending aorta of humans. Phantom experiments resulted in an error of 0.8% in assessing the luminal area of a flow phantom equipped with an artificial heart valve. Blinded evaluation of the volume flow rates from automatic vs. manual segmentation of gradient echo (FFE) phase contrast images obtained in vivo resulted in a mean difference of -0.9 +/- 3%. The mean difference from automatic vs. manual segmentation of images acquired with a hybrid phase contrast sequence (TFEPI) within a single breath-hold was -0.9 +/- 6%.

Research paper thumbnail of Three-dimensional visualization of velocity profiles in the human main pulmonary artery with magnetic resonance phase-velocity mapping

American Heart Journal, 1994

Detailed data on blood velocity fields in the normal human main pulmonary artery are an essential... more Detailed data on blood velocity fields in the normal human main pulmonary artery are an essential platform for discriminating physiologic from pathologic pulmonary flow patterns. Over the years, many studies have revealed quite inconsistent data mainly because of lack of suitable measuring techniques. By using combined cardiac- and respiratory-triggered magnetic resonance phase velocity mapping, very consistent data were obtained in 12 volunteers. In all subjects the location of the highest axial velocities was shifted from the inferior-right toward the superior-left part of the vessel area during the right ventricular contraction, with rapidly decreasing velocities to the inferior right evolving into retrograde flow in the deceleration phase. The mean temporal velocity profile was consistently skewed with a low flow region also toward the inferior-right vessel wall. The magnetic resonance phase shift method used in this study provided remarkably consistent high-quality data about human pulmonary artery velocity fields. This is most likely because of the use of combined cardiac and respiratory triggering.

Research paper thumbnail of Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR

Magnetic Resonance in Medicine, Nov 1, 1998

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stres... more Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vifro scans and computer simulations and applied to the common carotid artery of humans. In vifro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of 20.9% (vessel area) and 23.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.

Research paper thumbnail of Arterial MR Imaging Phase-Contrast Flow Measurement: Improvements with Varying Velocity Sensitivity during Cardiac Cycle1

Radiology, 2004

To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the au... more To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the authors implemented and evaluated a pulse sequence that enables automatic optimization of the velocity-encoding parameter V(enc) for individual heart phases in pulsatile flow on the basis of a rapid prescan. This sequence was prospectively evaluated by comparing velocity-to-noise ratios with those from a standard MR flow scan obtained in the carotid artery in eight volunteers. This sequence was shown to improve velocity-to-noise ratios by a factor of 2.0-6.0 in all but the systolic heart phase and was determined to be an effective technique for reducing noise in phase-contrast velocity measurements.

Research paper thumbnail of Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR

Magnetic Resonance in Medicine, 1998

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stres... more Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vifro scans and computer simulations and applied to the common carotid artery of humans. In vifro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of 20.9% (vessel area) and 23.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.

Research paper thumbnail of Left ventricular blood flow patterns in normal subjects: A quantitative analysis by three-dimensional magnetic resonance velocity mapping

Journal of the American College of Cardiology, 1995

Objectives. Magnetic resonance velocity mapping was used to investigate the hypothesis of a vorte... more Objectives. Magnetic resonance velocity mapping was used to investigate the hypothesis of a vortex motion within the left ventricle interacting with mitral valve motion and inflow velocity. Background. In vitro flow visualization studies have suggested the presence of a large anterior vortex inside the left ventricle during mitral inflow. However, to our knowledge the occurrence of this phenomenon has not been demonstrated in the human left ventricle. Methods. Magnetic resonance velocity mapping was performed in 26 healthy volunteers using a flow-adjusted gradient sequence for three-dimensional flow velocity acquisition in the long-axis plane of the left ventricle. By computer processing, the flow vectors in the left ventricle were visualized and animated dynamically. Results. The early diastolic mitral inflow was apically directed, and a large counterclockwise anterior vortex was created ~Rhin the left ventricle shortly after the onset of the mid-diastolic semiclosure of the anterior mitral leaflet. During mid-diastolic diastasis, mitral inflow ceased until the flow accelerated again at atrial systole. The final closure of the mitral valve was preceded by a smaller vortex seen at the tips of the mitral leaflets. At systolic ejection, all flow vectors were directed toward the left ventricular outflow tract. The anterior vortex had a radius of 1.62 -+ 0.24 em (mean -+ SD), and the average angular velocity (i.e., the rotation of an element about the center of the vortex within the central core) was 30.08 -+ 9.98 radians/s. The maximal kinetic energy of the anterior vortex was 4.3 x 10 -4 _+ 7.1 x 10 -s J.

Research paper thumbnail of Accurate noninvasive quantitation of blood flow, cross-sectional lumen vessel area and wall shear stress by three-dimensional paraboloid modeling of magnetic resonance imaging velocity data

Journal of the American College of Cardiology, 1998

Objectives. We present a new method in which a priori knowledge of the blood velocity fields with... more Objectives. We present a new method in which a priori knowledge of the blood velocity fields within the boundary layer at the vessel wall, combined with acquisition of high resolution magnetic resonance imaging (MRI) blood velocity data, allow exact modeling at the subpixel level.

Research paper thumbnail of Wall shear rates differ between the normal carotid, femoral, and brachial arteries: An in vivo MRI study

Journal of Magnetic Resonance Imaging, 2004

To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral ... more To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral arteries using very high resolution magnetic resonance imaging (MRI) phase contrast measurements.

Research paper thumbnail of Automatic vessel segmentation using active contours in cine phase contrast flow measurements

Journal of Magnetic Resonance Imaging, 1999

The segmentation of images obtained by cine magnetic resonance (MR) phase contrast velocity mappi... more The segmentation of images obtained by cine magnetic resonance (MR) phase contrast velocity mapping using manual or semi-automated methods is a time consuming and observer-dependent process that still hampers the use of flow quantification in a clinical setting. A fully automatic segmentation method based on active contour model algorithms for defining vessel boundaries has been developed. For segmentation, the phase image, in addition to the magnitude image, is used to address image distortions frequently seen in the magnitude image of disturbed flow fields. A modified definition for the active contour model is introduced to reduce the influence of missing or spurious edge information of the vessel wall. The method was evaluated on flow phantom data and on in vivo images acquired in the ascending aorta of humans. Phantom experiments resulted in an error of 0.8% in assessing the luminal area of a flow phantom equipped with an artificial heart valve. Blinded evaluation of the volume flow rates from automatic vs. manual segmentation of gradient echo (FFE) phase contrast images obtained in vivo resulted in a mean difference of -0.9 +/- 3%. The mean difference from automatic vs. manual segmentation of images acquired with a hybrid phase contrast sequence (TFEPI) within a single breath-hold was -0.9 +/- 6%.

Research paper thumbnail of Three-dimensional visualization of velocity profiles in the human main pulmonary artery with magnetic resonance phase-velocity mapping

American Heart Journal, 1994

Detailed data on blood velocity fields in the normal human main pulmonary artery are an essential... more Detailed data on blood velocity fields in the normal human main pulmonary artery are an essential platform for discriminating physiologic from pathologic pulmonary flow patterns. Over the years, many studies have revealed quite inconsistent data mainly because of lack of suitable measuring techniques. By using combined cardiac- and respiratory-triggered magnetic resonance phase velocity mapping, very consistent data were obtained in 12 volunteers. In all subjects the location of the highest axial velocities was shifted from the inferior-right toward the superior-left part of the vessel area during the right ventricular contraction, with rapidly decreasing velocities to the inferior right evolving into retrograde flow in the deceleration phase. The mean temporal velocity profile was consistently skewed with a low flow region also toward the inferior-right vessel wall. The magnetic resonance phase shift method used in this study provided remarkably consistent high-quality data about human pulmonary artery velocity fields. This is most likely because of the use of combined cardiac and respiratory triggering.