| 11. |
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| 12. |
- Dyverfeldt, Petter, et al.
(författare)
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On MRI turbulence quantification
- 2009
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Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 0730-725X .- 1873-5894. ; 27:7, s. 913-922
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent flow, characterized by velocity fluctuations, accompanies many forms of cardiovascular disease and may contribute to their progression and hemodynamic consequences. Several studies have investigated the effects of turbulence on the magnetic resonance imaging (MRI) signal. Quantitative MRI turbulence measurements have recently been shown to have great potential for application both in human cardiovascular flow and in engineering flow. In this article, potential pitfalls and sources of error in MRI turbulence measurements are theoretically and numerically investigated. Data acquisition strategies suitable for turbulence quantification are outlined. The results show that the sensitivity of MRI turbulence measurements to intravoxel mean velocity variations is negligible, but that noise may degrade the estimates if the turbulence encoding parameter is set improperly. Different approaches for utilizing a given amount of scan time were shown to influence the dynamic range and the uncertainty in the turbulence estimates due to noise. The findings reported in this work may be valuable for both in vitro and in vivo studies employing MRI methods for turbulence quantification.
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| 13. |
- Dyverfeldt, Petter, et al.
(författare)
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Pulse wave velocity with 4D flow MRI: Systematic differences and age-related regional vascular stiffness
- 2014
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Ingår i: Magnetic Resonance Imaging. - : Elsevier. - 0730-725X .- 1873-5894. ; 32:10, s. 1266-1271
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The objective of this study was to compare multiple methods for estimation of PWV from 4D flow MRI velocity data and to investigate if 4D flow MRI-based PWV estimation with piecewise linear regression modeling of travel-distance vs. travel time is sufficient to discern age-related regional differences in PWV. Methods: 4D flow MRI velocity data were acquired in 8 young and Solder (age: 23 +/- 2 vs. 58 +/- 2 years old) normal volunteers. Travel-time and travel-distance were measured throughout the aorta and piecewise linear regression was used to measure global PWV in the descending aorta and regional PWV in three equally sized segments between the top of the aortic arch and the renal arteries. Six different methods for extracting travel-time were compared. Results: Methods for estimation of travel-time that use information about the whole flow waveform systematically overestimate PWV when compared to methods restricted to the upslope-portion of the waveforms (p less than 0.05). In terms of regional PWV, a significant interaction was found between age and location (p less than 0.05). The age-related differences in regional PWV were greater in the proximal compared to distal descending aorta. Conclusion: Care must be taken as different classes of methods for the estimation of travel-time produce different results. 4D flow MRI-based PWV estimation with piecewise linear regression modeling of travel-distance vs. travel time can discern age-related differences in regional PWV well in line with previously reported data.
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| 14. |
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| 15. |
- Engström, Maria, et al.
(författare)
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Projection screen or video goggles as stimulus modality in functional magnetic resonance imaging
- 2005
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Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 0730-725X .- 1873-5894. ; 23:5, s. 695-699
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this study was to investigate the reliability of functional magnetic resonance imaging (fMRI) by using either a projection screen or video goggles as stimulus modality. A sequence of visual stimuli were presented to the same subject at different occasions. The sequence was optimized with a genetic algorithm. In five sessions the stimuli were presented using a projection screen viewed through a mirror in the head coil and in five sessions using video goggles. Failure to detect visual activation in the medial left hemisphere was observed in sessions using the projection screen as stimulus modality. Decreased thresholds for P values and cluster size resulted in activation outside the occipital lobe and did not significantly increase activated areas in this region. Results in this study indicate that presentation of fMRI tasks with visual routes is more reliable with direct input through video goggles than with the conventional use of projection screens. Failure to detect crucial visual areas has severe consequences for tumor surgery in the visual cortex. Inferior visual impression might also have negative consequences for cognitive tests with high demand on attention and perception.
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| 16. |
- Fahlström, Markus, et al.
(författare)
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A mathematical model for temporal cerebral blood flow response to acetazolamide evaluated in patients with Moyamoya disease
- 2024
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Ingår i: Magnetic Resonance Imaging. - : Elsevier. - 0730-725X .- 1873-5894. ; 110, s. 35-42
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Tidskriftsartikel (refereegranskat)abstract
- Background: Paired cerebral blood flow (CBF) measurement is usually acquired before and after vasoactive stimulus to estimate cerebrovascular reserve (CVR). However, CVR may be confounded because of variations in time-to-maximum CBF response (tmax) following acetazolamide injection. With a mathematical model, CVR can be calculated insensitive to variations in tmax, and a model offers the possibility to calculate additional model-derived parameters. A model that describes the temporal CBF response following a vasodilating acetazolamide injection is proposed and evaluated.Methods: A bi-exponential model was adopted and fitted to four CBF measurements acquired using arterial spin labelling before and initialised at 5, 15 and 25 min after acetazolamide injection in a total of fifteen patients with Moyamoya disease. Curve fitting was performed using a non-linear least squares method with a priori constraints based on simulations.Results: Goodness of fit (mean absolute error) varied between 0.30 and 0.62 ml·100 g-1·min-1. Model-derived CVR was significantly higher compared to static CVR measures. Maximum CBF increase occurred earlier in healthy- compared to diseased vascular regions.Conclusions: The proposed mathematical model offers the possibility to calculate CVR insensitive to variations in time to maximum CBF response which gives a more detailed characterisation of CVR compared to static CVR measures. Although the mathematical model adapts generally well to this dataset of patients with MMD it should be considered as experimental; hence, further studies in healthy populations and other patient cohorts are warranted.
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| 17. |
- Filippov, Andrei, et al.
(författare)
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Dynamic properties of water in silicalite-1 powder
- 2012
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Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 0730-725X .- 1873-5894. ; 30:7, s. 1022-1031
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Tidskriftsartikel (refereegranskat)abstract
- Self-diffusion of D2O in partially filled silicalite-1 crystals was studied at 25 degrees C by H-2 nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10(-7) to similar to 10(-10) m(2)/s, which include values much higher and lower than that of bulk water (similar to 10(-9) m(2)/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10-200 ms. A two-site Karger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.
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| 18. |
- Filippov, Andrei, PhD, 1957-, et al.
(författare)
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Dynamics of ethylammonium nitrate near PTFE surface
- 2022
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Ingår i: Magnetic Resonance Imaging. - : Elsevier. - 0730-725X .- 1873-5894. ; 85, s. 102-107
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Tidskriftsartikel (refereegranskat)abstract
- Self-diffusion of ions in the protic ionic liquid ethylammonium nitrate (EAN) was studied by 1H NMR pulsed field gradient techniques between 294 and 393 K in the presence of a PTFE insert in a 5-mm NMR tube. At all temperatures, the bulk diffusion of ions (measured by 1H and 15N NMR) can be described by a unique diffusion coefficient. The presence of solid hydrophobic surfaces of PTFE induces regions of EAN in their vicinity, where diffusion of ions, both cations and anions, is reduced compared to the bulk values. An additional line-shape analysis in 1H NMR spectra showed that local mobility of ethylammonium cations in the surface layers near PTFE is also reduced.
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| 19. |
- Filippov, Andrei, et al.
(författare)
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Lateral diffusion in equimolar mixtures of natural sphingomyelins with dioleoylphosphatidylcholine
- 2012
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Ingår i: Magnetic Resonance Imaging. - : Elsevier. - 0730-725X .- 1873-5894. ; 30:3, s. 413-421
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Tidskriftsartikel (refereegranskat)abstract
- Cellular membranes of mammals are composed of a complex assembly of diverse phospholipids. Sphingomyelin (SM) and phosphatidylcholine (PC) are important lipids of eukaryotic cellular membranes and neuronal tissues, and presumably participate in the formation of membrane domains, known as "rafts," through intermolecular interaction and lateral microphase decomposition. In these two-dimensional membrane systems, lateral diffusion of lipids is an essential dynamic factor, which might even be indicative of lipid phase separation process. Here, we used pulsed field gradient nuclear magnetic resonance to study lateral diffusion of lipid components in macroscopically oriented bilayers composed of equimolar mixtures of natural SMs of egg yolk, bovine brain, bovine milk and dipalmitoylphosphatidylcholine (DPPC) with dioleoylphosphatidylcholine (DOPC). In addition, differential scanning calorimetry was used as a complementary technique to characterize the phase state of the lipid bilayers. In fully liquid bilayers, the lateral diffusion coefficients in both DOPC/DPPC and DOPC/SM systems exhibit mean values of the pure bilayers. For DOPC/SM bilayer system, this behavior can be explained by a model where most SM molecules form short-lived lateral domains with preferential SM-SM interactions occurring within them. However, for bilayers in the presence of their low-temperature gel phase, lateral diffusion becomes complicated and cannot simply be understood solely by a simple change in the liquid phase decomposition.
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| 20. |
- Gaeta, Stephen, et al.
(författare)
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Fixed volume particle trace emission for the analysis of left atrial blood flow using 4D Flow MRI
- 2018
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Ingår i: Magnetic Resonance Imaging. - : ELSEVIER SCIENCE INC. - 0730-725X .- 1873-5894. ; 47, s. 83-88
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Tidskriftsartikel (refereegranskat)abstract
- 4D Flow MRI has been used to quantify normal and deranged left ventricular blood flow characteristics on the basis of functionally distinct flow components. However, the application of this technique to the atria is challenging due to the presence of continuous inflow. This continuous inflow necessitates plane-based emission of particle traces from the inlet veins, leading to particles that represents different amounts of blood, and related quantification errors. The purpose of this study was to develop a novel fixed-volume approach for particle tracing and employ this method to develop quantitative analysis of 4D blood flow characteristics in the left atrium. 4D Flow MRI data were acquired during free-breathing using a navigator-gated gradient-echo sequence in three volunteers at 1.5 T. Fixed-volume particle traces emitted from the pulmonary veins were used to visualize left atrial blood flow and to quantitatively separate the flow into two functionally distinct flow components: Direct flow = particle traces that enter and leave the atrium in one heartbeat, Retained flow = particle traces that enter the atrium and remains there for one cardiac cycle. Flow visualization based on fixed-volume traces revealed that, beginning in early ventricular systole, flow enters the atrium and engages with residual blood volume to form a vortex. In early diastole during early ventricular filling, the organized vortical flow is extinguished, followed by formation of a second transient atrial vortex. Finally, in late diastole during atrial contraction, a second acceleration of blood into the ventricle is seen. The direct and retained left atrial flow components were between 44 and 57% and 43-56% of the stroke volume, respectively. In conclusion, fixed volume particle tracing permits separation of left atrial blood flow into different components based on the transit of blood through the atrium.
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