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Biomedical subjects

K Kopinga

Publications and source records attributed to K Kopinga.

At least 19 recordsLinked to original sources
PubMed2006

Curing processes in solvent-borne alkyd coatings with different drier combinations.

The concern regarding the effect of chemicals on the environment has increased considerably in recent years. Nowadays, technological developments in the coating industry are largely influenced by environmental issues and subsequent legislation. One of these issues is the tendency to replace cobalt as a catalyst with more environmentally friendly alternatives, because studies have indicated possible carcinogenicity. Not much knowledge is available on the effects of catalysts (driers) on the in-depth drying of alkyd coatings. Therefore we have studied the effect of cobalt as a primary drier combined with Ca and Zr as secondary driers on the in-depth curing of high solid solvent-borne alkyds. The profiling of the curing of alkyd coatings is performed with a new high-spatial-resolution NMR setup. In this study, two effects observed in the solvent-borne alkyd coatings are investigated. One is that when Ca and Zr are added as secondary driers the speed of the observed cross-linking front increases. Second, in the deeper un-cross-linked region below the front, the signal of the NMR profiles was found to decrease proportional to . This could be explained by the presence of slowly reacting species that diffuse into the deeper uncured region of the coating, after which they cross-link. The model describing the effect of these reactive species also indicates that the signal decrease is inversely proportional to coating thickness L, which was confirmed by additional measurements.

Journal Article↗
PubMed2005

Experimental evidence of crystallization pressure inside porous media.

Crystallization pressure of salt in porous materials is one of the mechanisms that may induce serious damage, for example, weathering of buildings and monuments of cultural heritage. Since this pressure also causes the solubility of the salt inside a porous material to differ from the bulk solubility, it can be assessed experimentally by measuring the solubility inside the pores. We show that this is possible by NMR, and study Na(2)CO(3) and Na(2)SO(4) in a series of model porous materials. Using the solubility data the crystal-liquid surface energies are estimated as gamma = 0.09 N/m for Na(2)CO(3) . 10H(2)O and gamma = 0.06 N/m for Na(2)SO(4) . 10H(2)O. For pore sizes below about 30 nm, the resulting pressure exceeds the tensile strength of typical building materials (3 MPa). No pressure is induced by the metastable Na(2)SO(4) . 7H(2)O, which suggests for this crystal a value of gamma close to zero.

Journal Article↗
PubMed2005

Ischemia-induced ADC changes are larger than osmotically-induced ADC changes in a neonatal rat hippocampus model.

Diffusion-weighted imaging (DWI) is frequently used to diagnose stroke. However, the origin of the observed reduction in the apparent diffusion coefficient (ADC) in the acute phase following ischemia is not well understood. Although cell swelling is considered to play an important role, it is unclear whether this can completely explain the large ADC decrease. We developed a method to induce in neonatal rat hippocampal slices both osmotic perturbations, which lead to cell swelling, and oxygen/glucose deprivation (OGD), which simulates ischemia. A perfusion system was used to provide the hippocampal slices with nutrients and oxygen to maintain slice viability, which was verified with the use of fluorescent dyes (live/dead staining). Upon induction of OGD, the ADC decreased to approximately 57% of the initial value within 2 hr. The ADC reduction cannot fully be explained by changes due to cell swelling, since these led only to a maximum decrease of approximately 83%. Therefore, in addition to cell swelling, other changes must contribute significantly to the ADC reduction.

Animals↗
PubMed2004

Diffusion in porous building materials with high internal magnetic field gradients.

Measuring the water diffusivity in porous building materials with NMR is hindered by the presence of large internal magnetic field gradients originating from magnetic impurities (Fe). To investigate the diffusion of water in these materials, a stimulated echo NMR technique is applied. A new analytical equation for the long-time signal decay in the presence of spatially varying internal field gradients is derived. This equation is experimentally confirmed by measurements on representative materials with large internal gradients (fired-clay brick and sintered crushed glass) and a material with very small internal gradients (glass filter). The diffusivity is determined in the long time limit, where it is constant and limited by the tortuosity of the pore structure. Tortuosities of different samples derived from the NMR data show an excellent agreement with the macroscopic tortuosities measured by electrochemical impedance spectroscopy. The developed technique can also be applied in unsaturated media, during e.g., drying, water absorption, and concentration changes. The characteristic length scales of the internal field fluctuations estimated from the model are compared with the structural length scales, whereas the magnitude of these fluctuations is compared with results of macroscopic magnetization measurements.

Algorithms↗
PubMed2004

Sodium NMR relaxation in porous materials.

The NMR relaxation of hydrogen nuclei of a fluid in a porous material is generally interpreted in terms of the Brownstein-Tarr model, in which the relaxation rate of the signal is inversely proportional to the pore size. We have investigated whether this model can be applied to the relaxation of Na nuclei in a NaCl solution in a porous material. The results indicate that the ion distribution over the pores can be obtained from an analysis of the Na NMR signal decay, if the pore sizes are roughly below 1 microm. This information is very useful for studies of combined moisture and ion transport in porous building materials.

Computer Simulation↗
PubMed2002

Random-walk simulations of NMR dephasing effects due to uniform magnetic-field gradients in a pore.

A random-walk simulation program was developed to study the effect of dephasing spins in a uniform magnetic-field gradient in a porous material. It is shown that this simulation program correctly reproduces basic nuclear magnetic resonance behavior, such as the formation of a spin echo. The spin-echo decay due to dephasing in a nonrestricted medium gives the well-known exponential relation containing the cube of time, whereas the spin-echo decay due to dephasing in a porous material gives a monoexponential decay. By varying the pore size and magnetic-field gradient, the motional averaging regime and the localization regime can be simulated. Moreover, the unknown intermediate regime is investigated. By choosing the right scaling parameters, the spin-echo decay due to dephasing in a pore can be described by one master curve for all pore sizes and gradient strengths. This master curve reveals a small intermediate regime, perfectly symmetrical around the gradient for which the dephasing length is exactly equal to the structural length of the pore.

Journal Article↗
PubMed2000

Dealing with the subvoxel vessel position relative to the reconstruction voxel grid in 2D MR quantitative flow measurements.

A method is introduced that quantifies the error in 2D MR Quantitative Flow measurements induced by the position of the vessel relative to the reconstruction voxel grid, called the subvoxel vessel position. In this method, the vessel area and the volume flow rate are determined for all possible subvoxel vessel positions resulting in a mean value with standard deviation. Since the subvoxel vessel position in standard MR image reconstruction is completely arbitrary, the standard deviation can be considered as a measure of its random error contribution. Simulation studies and in vivo measurements show that our method can be used to quantify and subsequently eliminate this random error. It is further quantitatively shown that, for low noise levels, Fourier interpolation to a higher reconstruction matrix also decreases the random error. We conclude that the precision of a 2D MR Quantitative Flow measurement is improved either by using our method or by reconstruction to a higher matrix.

Aorta↗
PubMed1999

Venous signal suppression in 3D dynamic Gd-enhanced carotid artery imaging using the eigenimage filter.

A three-dimensional dynamic gadolinium-enhanced carotid artery imaging protocol with 10 sec per phase was evaluated with respect to the acquisition of an arterial-only phase after contrast bolus injection. Subsequently, the eigenimage filter was used to suppress any venous signal based on a difference in arterial and venous temporal enhancement patterns. From 63 consecutive scans of the carotid bifurcation, venous enhancement in the maximal arterial phase was found to be absent in 43%, weak in 19%, and strong in 38% of cases. Our eigenimage filter successfully suppressed the low signal veins in 100% and the high signal veins in 67% of cases. The number of acquired high-quality arterial-only images increased from 43% without to 87% with the use of the filter. In conclusion, even when a dynamic scan cannot resolve the short physiological delay between arterial and venous enhancement, the eigenimage filter can effectively be used to suppress the veins. Magn Reson Med 42:307-313, 1999.

Carotid Artery, Internal↗
PubMed1999

Separation of haemodynamic flow waves measured by MR into forward and backward propagating components.

Physiological information on the action of the heart and on the reflection sites in the arterial system can be derived respectively from the forward and the backward propagating pressure or flow wave components. Earlier work on the separation of these components was exclusively based on invasive measurements of pressure or flow. In this study magnetic resonance (MR), which is a non-invasive imaging technique, was used to measure the blood flow waveform simultaneously at multiple positions along a vessel. Linear one dimensional transmission-line theory was used to separate the flow waves into forward and backward propagating components. First results, obtained from the thoracic aorta of five healthy male volunteers, consistently showed a negative reflection with a delay of about 100 ms between the foot of the forward and the foot of the backward propagating flow wave. Our model, consisting of a single vessel segment with constant diameter and wall properties, was validated by the excellent agreement between the vessel area as calculated from the flow data using the law of mass conservation and as directly measured with a different independent MR technique.

Aorta, Thoracic↗
PubMed1996

Determination of moisture profiles in porous building materials by NMR.

Because moisture in porous building materials can give rise to several kinds of damage, a detailed knowledge of the moisture transport is essential. for such studies it is important to measure dynamic moisture profiles quantitatively. Nuclear Magnetic Resonance (NMR) offers a powerful technique to measure such profiles in a nondestructive way. Because of the large amount of paramagnetic ions present in many of these materials (0.1-5% Fe) standard NMR imaging equipment cannot be used. A NMR apparatus will be described that was especially developed to study the moisture transport in porous building materials. At the moment, one-dimensional moisture profiles in samples with a diameter of 20 mm can be measured with an accuracy of 1% and a resolution of 1 mm. It takes about 40 s to determine the moisture content at a specific position.

Construction Materials↗
PubMed1993

Dynamic response of a neonatal catheter-manometer system in situ.

OBJECTIVE: The purpose of this study was to develop, validate, and apply a flush-pulse method to determine the dynamic response of a neonatal catheter-manometer system (CMS) in situ. METHODS: In the flush-pulse method, the opened fast-flush valve of the CMS is closed; as a result, the fluid column in the CMS is impacted. This procedure can be done without affecting the net flow of infusion fluid. We validated the method in laboratory conditions by comparing 14 paired results obtained with this method to the results obtained using a generally accepted step-response method. The measurable values are the resonance frequency (fr) and the damping coefficient (delta). The analysis of the flush-pulse method in situ is complicated by the patient's blood pressure wave. A remedy for this problem that is based on the first derivative of the pressure signal has been developed. The flush-pulse method is applied 14 times in situ. RESULTS: In laboratory settings, the fr ranged from 12.5 to 64.0 Hz and delta ranged from 0.14 to 0.32. The correlation coefficient was 0.99 for fr and 0.91 for delta. We found four overdamped systems in situ (delta > 1). In other systems fr values between 8.5 and 41.0 Hz and delta values between 0.16 and 0.72 were observed. The dynamic response in situ appeared to deteriorate with time due to routine intensive care procedures. CONCLUSIONS: The flush-pulse method proved to be a valid test for determining the dynamic response. The results obtained in situ emphasize the need for a regular evaluation of the dynamic response of the neonatal CMS in order to assess the shape of the pressure wave.

Blood Pressure↗