| 1. |
- Karlson, L, et al.
(författare)
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Complex formed in the system hydrophobically modified polyethylene glycol/methylated alpha-cyclodextrin/water. An NMR diffusometry study
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 228:1-3, s. 171-179
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous solutions hydrophobically modified polyethylene glycol (HM-PEG) forms a transient polymer network held together by intermolecular hydrophobic associations. In the present investigation we have used NMR-diffusometry to study how the addition of methylated alpha-cyclodextrin (M-alpha-CD) influences the polymer network. The addit on of M-alpha-CD resulted in an increased mean self-diffusion of HM-PEG, DHM-PEG, which is referred to a degradation of the polymer network when hydrophobic associations are disrupted due to complex formation between the hydrophobic groups of HM-PEG and M-alpha-CD. Addition of small amounts of M-alpha-CD results in a dramatic increase in DHM-PEG. Upon further addition of M-a-CD the increase in DHM-PEG is less dramatic and at excess M-alpha-CD, DHM-PEG levels off and equals the mean self-diffusion coefficient for unmodified PEG with the same molecular weight. The suggested interpretation is that the addition of the first molecules of M-alpha-CD mainly reduces the probability for hydrophobic associations inter-connecting different clusters of polymer micelles whereas at higher M-alpha-CD concentrations a disengagement of the individual clusters into separate HM-PEG molecules becomes important. (C) 2003 Elsevier B.V. All rights reserved.
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| 2. |
- Lätt, Jimmy, et al.
(författare)
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Accuracy of q-space related parameters in MRI: Simulations and phantom measurements
- 2007
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Ingår i: IEEE Transactions on Medical Imaging. - 1558-254X. ; 26:11, s. 1437-1447
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Tidskriftsartikel (refereegranskat)abstract
- The accuracy of q-space measurements was evaluated at a 3.0-T clinical magnetic resonance imaging (MRI) scanner, as compared with a 4.7-T nuclear magnetic resonance (NMR) spectrometer. Measurements were performed using a stimulated-echo pulse-sequence on n-decane as well as on polyethylene glycol (PEG) mixed with different concentrations of water, in order to obtain bi-exponential signal decay curves. The diffusion coefficients as well as the modelled diffusional kurtosis K-fit, were obtained from the signal decay curve, while the full-width at half-maximum (FWHM) and the diffusional kurtosis K were obtained from the displacement distribution. Simulations of restricted diffusion, under conditions similar to those obtainable with a clinical MRI scanner, were carried out assuming various degrees of violation of the short gradient pulse (SGP) condition and of the long diffusion time limit., The results indicated that an MRI system can not be used for quantification of structural sizes less than about 10 mu m by means of FWHM since the parameter underestimates the confinements due to violation of the SGP condition. However, FWHM can still be used as an important contrast parameter. The obtained kurtosis values were lower than expected from theory and the results showed that care must be taken when interpreting a kurtosis estimate deviating from zero.
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| 3. |
- Malmborg, Carin, et al.
(författare)
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Diffusion in an inhomogeneous system: NMR studies of diffusion in highly concentrated emulsions
- 2003
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Ingår i: Journal of Colloid and Interface Science. - 1095-7103. ; 263:1, s. 270-276
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Tidskriftsartikel (refereegranskat)abstract
- In this study the self-diffusion of different species in highly concentrated water-in-oil emulsions was investigated by means of the NMR diffusometry approach. The emulsions contained 96% aqueous solutions of salt or other additives; hept ne was used as the oil phase. The surfactants, used to stabilize the emulsion, were soybean phosphatidylcholine and C12EO4. The water drops were about 1.5 mum in diameter according to diffusion measurements performed on water. Diffusion of tetramethyl ammonium ions and glucose between the emulsion droplets was found to be negligible on the relevant time-scale (<1 s). On the contrary, acetic acid/acetate ions diffused between the droplets and had exchange times which were a function of pH. (C) 2003 Elsevier Science (USA). All rights reserved.
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| 4. |
- Malmborg, Carin, et al.
(författare)
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Mapping the intracellular fraction of water by varying the gradient pulse length in q-space diffusion MRI
- 2006
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Ingår i: Journal of Magnetic Resonance. - : Elsevier BV. - 1096-0856 .- 1090-7807. ; 180:2, s. 280-285
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Tidskriftsartikel (refereegranskat)abstract
- Finite gradient pulse lengths are traditionally considered a nuisance in q-space diffusion NMR and MRI, since the simple Fourier relation between the acquired signal and the displacement probability is invalidated. Increasing the value of the pulse length leads to an apparently smaller value of the estimated compartment size. We propose that q-space data at different gradient pulse lengths, but with the same effective diffusion time, can be used to identify and quantify components with free or restricted diffusion from multi-exponential echo decay curves obtained on cellular systems. The method is demonstrated with experiments on excised human brain white matter and a series of model systems with well-defined free, restricted, and combined free and restricted diffusion behavior. Time-resolved diffusion MRI experiments are used to map the spatial distribution of the intracellular fraction in a yeast cell suspension during sedimentation, and observe the disappearance of this fraction after a heat treatment. (c) 2006 Elsevier Inc. All rights reserved.
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| 5. |
- Malmborg, Carin, et al.
(författare)
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NMR diffusometry and the short gradient pulse limit approximation
- 2004
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Ingår i: Journal of Magnetic Resonance. - : Elsevier BV. - 1096-0856 .- 1090-7807. ; 169:1, s. 85-91
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Tidskriftsartikel (refereegranskat)abstract
- In NMR diffusometry, one often uses the short gradient pulse (SGP) limit approximation in the interpretation of data from systems with restricted diffusion. The SGP limit approximation means that the gradient pulse length, delta, is so short that the spins do not diffuse during the pulse duration, but this condition is rarely met. If the length scale of the pores corresponds to the molecular mean square displacement during the gradient pulse, the measured echo intensities become a function of the gradient pulse length. Here, we have studied highly concentrated emulsions to show how the length of the gradient pulse influences NMR diffusion experiments. We have focused on molecules confined to one pore and molecules that can migrate through the porous system. For the former the echo decays give smaller pores than the actual case and for the latter we show large changes in echo decay depending on the gradient pulse length, everything else being equal. (C) 2004 Elsevier Inc. All rights reserved.
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| 6. |
- Malmborg, Carin
(författare)
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NMR Studies of Gel-Emulsions; Diffusometry and Applications of Novel Field Gradient Techniques
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of these studies has been to investigate what information that can be obtained on gel-emulsions with nuclear magnetic resonance, NMR, as the investigating tool. The gel-emulsions are important in many technical applications such as in food products, in cosmetic and pharmaceutical formulations, and in emulsion explosives, just to name a few applications. We have worked with the NMR diffusometry technique as developed by Stejskal, Tanner and Callaghan, and have loaded the gel-emulsion water phase with simple molecules acting as model drugs, as well as ordinary drugs such as lidocaine to clarify the usefulness of this technique. In porous materials the NMR diffusometry technique measures the mean square displacement, which gives information on structure and obstruction effects. This technique can also measure the emulsion break down process. To measure the diffusion on very short time scales, as in small emulsion droplets, we have used a novel technique with modulated field gradient spin echo pulse trains. We have also applied the chemical shift imaging technique of molecular transport in gel-emulsions. With this technique we can monitor the spatial distribution of solutes as a function of time. In the interpretation of data from the application of these techniques one often uses the short gradient pulse limit approximation, but the quality of the information is a matter of some uncertainty. We have provided some insight on this problem. The NMR diffusometry technique measures diffusion in a time-window from a few 100 µs up to several seconds, and the root mean square displacement in the µm range, and have many advantages as; it is rapid, does not require any isotopic labelling or addition of disturbing probes, only small amounts of sample is needed, and it is a non-destructive method (the same sample can be repeatedly studied).
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| 7. |
- Salvati, Anna, et al.
(författare)
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Chemical shift imaging of molecular transport in colloidal systems: Visualization and quantification of diffusion processes
- 2007
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 1095-7103 .- 0021-9797. ; 308:2, s. 542-550
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic resonance imaging with chemical shift resolution is demonstrated to provide detailed information about molecular transport on the macroscopic scale in complex colloidal systems. The concentrations of species with distinct 1 H resonance lines can be quantified from spatially resolved, high-resolution, H-1 nuclear magnetic resonance spectra. The method is demonstrated by experiments on three systems with multiple simultaneous transport processes where the diffusion coefficients depend on position and/or on the concentration of other species: (1) release of poly(ethylene glycol) and imidazole from a hydrogel into an external reservoir of water, (2) migration of acetic acid and tetramethylammonium ions in a highly concentrated water-in-oil emulsion with initially non-uniform concentration of solutes, and (3) release of tetramethylammonium ions loaded into a hydrogel triggered by the diffusion of methyl green into the gel matrix.
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| 8. |
- Topgaard, Daniel, et al.
(författare)
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Restricted self-diffusion of water in a highly concentrated W/O emulsion studied using modulated gradient spin-echo NMR
- 2002
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Ingår i: Journal of Magnetic Resonance. - : Elsevier BV. - 1096-0856 .- 1090-7807. ; 156:2, s. 195-201
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Tidskriftsartikel (refereegranskat)abstract
- Restricted diffusion of water in a highly concentrated w/o emulsion was studied using pulsed field gradient spin echo techniques. The standard two-pulse version of this technique, suitable for analysis in the time domain, fails to investigate the short time-scale for diffusion inside a single emulsion droplet with radius 0.7 mum. With a pulse-train technique, originally introduced by Callaghan and Stepisnik, shorter time-scales are accessible. The latter approach is analyzed in the frequency domain and yields frequency dependent diffusion coefficients. Predictions for the outcome of the experiment were calculated in the time domain using the Gaussian phase distribution and the pore hopping formalism expressions for the echo attenuation. The results of these calculations were transformed to the frequency domain via a numerical inverse integral transform in order to compare with the experimental results. (C) 2002 Elsevier Science (USA).
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