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Papers by Ronald wolf
Computing Research Repository, 2004
Magnetic Resonance in Medicine, 1993
The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to defi... more The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.
The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measu... more The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.
Journal of Computer Assisted Tomography, 1994
The purpose of this study was to construct a method of vessel edge detection that correctly ident... more The purpose of this study was to construct a method of vessel edge detection that correctly identifies vessel pixels and to compare the interuser variability of cine phase contrast MR volumetric flow rates obtained with the conventional manual method and an automated method. The automated method was developed based on a magnitude image threshold value and compared with the manual method in a flow phantom (three users) and in velocity images of the portal vein (five users). The threshold value determined from the magnitude image was applied to a region of interest surrounding the vessel of interest on the magnitude image to construct a vessel edge detection mask m(x,y). The velocity images were then multiplied by the mask m(x,y) and volumetric flow rates determined using the identified vessel pixels. In the flow phantom, flow measurements with the magnitude threshold method had significantly less interuser variability compared with the manual method (p < 0.01) and were within 10% (mean 6.0%) of the actual flow versus 35% (mean 18.6%) with the manual method. Regarding flow measurements in the portal vein of six volunteers, the magnitude threshold method was significantly more precise (p < 0.01) than the manual method with a mean standard deviation between the five users of 40.4 +/- 12.9 ml/min (range 22-60 ml/min) and 110.4 +/- 32.7 ml/min (range 70-155 ml/min), respectively. The magnitude threshold method of vessel edge detection developed in this study yields flow measurements that are accurate in the model system and have significantly less interuser variability than the manual method. This method shows promise for improving the precision of cine phase contrast flow measurements.
Computing Research Repository, 2004
Magnetic Resonance in Medicine, 1993
The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to defi... more The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.
The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measu... more The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.
Journal of Computer Assisted Tomography, 1994
The purpose of this study was to construct a method of vessel edge detection that correctly ident... more The purpose of this study was to construct a method of vessel edge detection that correctly identifies vessel pixels and to compare the interuser variability of cine phase contrast MR volumetric flow rates obtained with the conventional manual method and an automated method. The automated method was developed based on a magnitude image threshold value and compared with the manual method in a flow phantom (three users) and in velocity images of the portal vein (five users). The threshold value determined from the magnitude image was applied to a region of interest surrounding the vessel of interest on the magnitude image to construct a vessel edge detection mask m(x,y). The velocity images were then multiplied by the mask m(x,y) and volumetric flow rates determined using the identified vessel pixels. In the flow phantom, flow measurements with the magnitude threshold method had significantly less interuser variability compared with the manual method (p < 0.01) and were within 10% (mean 6.0%) of the actual flow versus 35% (mean 18.6%) with the manual method. Regarding flow measurements in the portal vein of six volunteers, the magnitude threshold method was significantly more precise (p < 0.01) than the manual method with a mean standard deviation between the five users of 40.4 +/- 12.9 ml/min (range 22-60 ml/min) and 110.4 +/- 32.7 ml/min (range 70-155 ml/min), respectively. The magnitude threshold method of vessel edge detection developed in this study yields flow measurements that are accurate in the model system and have significantly less interuser variability than the manual method. This method shows promise for improving the precision of cine phase contrast flow measurements.