Péter Bogner

Multi-component apparent diffusion coefficients in human brain : Relationship to spin-lattice relaxation

In vivo measurements of the human brain tissue water signal decay with b‐factor over an extended b‐factor range up to 6,000 s/mm 2 reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b‐factor signal decay measurements are used to make separate measurements of the spin‐lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin‐lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b‐factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin‐lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is “fast” enough to ensure volume averaged T1 relaxation yet “slow” enough to allow for the observation of biexponential ADC decay curves over an extended b‐factor range. Magn Reson Med 44:292–300, 2000.

The existence of biexponential signal decay in magnetic resonance diffusion-weighted imaging appears to be independent of compartmentalization.

It is generally believed that the apparent diffusion coefficient (ADC) changes measured by diffusion‐weighted imaging (DWI) in brain pathologies are related to alterations in the water compartments. The aim of this study was to elucidate the role of compartmentalization in DWI via biexponential analysis of the signal decay due to diffusion. DWI experiments were performed on mouse brain over an extended range of b‐values (up to 10000 mm–2 s) under intact, global ischemic, and cold‐injury conditions. DWI was additionally applied to centrifuged human erythrocyte samples with a negligible extracellular space. Biexponential signal decay was found to occur in the cortex of the intact mouse brain. During global ischemia, in addition to a drop in the ADC in both components, a shift from the volume fraction of the rapidly diffusing component to the slowly diffusing one was observed. In cold injury, the biexponential signal decay was still present despite the electron‐microscopically validated disintegration of the membranes. The biexponential function was also applicable for fitting of the data obtained on erythrocyte samples. The results suggest that compartmentalization is not an essential feature of biexponential decay in diffusion experiments. Magn Reson Med 51:278–285, 2004.

Relationship between cellular ATP, potassium, sodium and magnesium concentrations in mammalian and avian erythrocytes

Intracellular K+/Na+ ratios of erythrocytes of various mammalian species are known to differ markedly. We have measured ATP, K+, Na+, Mg2+, H2O contents of erythrocytes of twelve mammalian and three avian species. Our results indicate that the intracellular ATP concentration in erythrocytes of different species is in close positive correlation with the K+/Na+ ratios (linear correlation coefficient, r = 0.852). Furthermore, ATP levels in erythrocytes of individual sheep with different potassium concentrations correspond with their K+/Na+ ratios (r = 0.747). Intracellular magnesium concentrations also correlate with ATP concentrations in erythrocytes of different species (r = 0.629) and in different sheep (r = 0.549).

In vivo brain edema classification: New insight offered by large b-value diffusion-weighted MR imaging

To assess the role of large b‐value diffusion weighted imaging (DWI) in the characterization of the physicochemical properties of the water in brain edema under experimental and clinical conditions.

Are there any gender differences in the hippocampus volume after head-size correction? A volumetric and voxel-based morphometric study

Previous findings on normal sexual dimorphism in hippocampal volume have not always been consistent. This study investigated gender differences in hippocampal volume using different head-size correction strategies. T1-weighted MR images were collected in 99 healthy, Caucasian, university students (66 female subjects; mean age: 23.1 ± 2.3, range: 19-31 years). Sexual dimorphism in hippocampus was investigated by automated MRI volumetry and voxel-based morphometry (VBM) using both general linear model (GLM) and proportion head-size correction strategies. Absolute hippocampal volumes were larger in men than women. After adjusting for head-size, the proportion method indicated larger hippocampi in women than men, while no gender differences were found using the GLM approach. Investigating absolute hippocampal volumes in 15 head-size matched pairs of males and females indicated no gender differences. We suggest that there is no sexual dimorphism in hippocampal size and the apparent gender differences found by the proportion method may have more to do with head-size than with sex. The GLM and proportion head-size correction strategies are not interchangeable and may yield different results. The importance of the present findings is mostly related to scientific reproducibility across MRI volumetry or VBM studies.

Lung Water and Proton Magnetic Resonance Relaxation in Preterm and Term Rabbit Pups: Their Relation to Tissue Hyaluronan

The present study was performed to investigate simultaneously total lung water, T1 and T2 relaxation times, and hyaluronan (HA) in preterm and term rabbits. Attempts were also made to establish the relationship of HA to total lung water and to T2-derived motionally distinct water fractions. Experiments were performed in fetal Pannon white rabbit pups at gestational ages of 25, 27, 29, and 31 d and at a postnatal age of 4 d. Lung tissue water content (desiccation method), T1 and T2 relaxation times (H1-NMR method), and HA concentration (radioassay) were measured, and free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. Lung water content and T1 and T2 relaxation times were highest at a gestational age of 27 d and then declined steadily during the whole study period. Similar trends and time courses were seen for the fast and slow components of the T2 relaxation curve. The T2-derived free water fraction remained unchanged at a gestational age of 25–29 d (∼67%), but increased progressively to a value of 78.5 ± 7.9% at 31 d (p < 0.001) and to 83.4 ± 9.4% at the postnatal age of 4 d (p < 0.01). Opposite changes occurred in the bound water fraction. Lung HA concentration decreased with advancing gestation from 870.8 ± 205.2 μg/g dry weight at 25 d to 162.6 ± 32.4 μg/g dry weight at 31 d (p < 0.001), but it was increased 2-fold postnatally. HA correlated positively with total lung water (r = 0.39;p < 0.001) but not with the bound water fraction. It is suggested that the physiologic lung dehydration is associated with macromolecule-related reorganization of lung water and that the role of HA in this process needs to be further investigated.

Right prefrontal activation produced by arterial baroreceptor stimulation: a PET study.

This study was performed to test the hypothesis of greater right hemispheric involvement in the processing of baroreceptor stimuli. Carotid sinus baroreceptors were stimulated by rhythmically decreasing air pressure in a neck chamber, and under control conditions the thorax was stimulated in a similar manner. Changes in regional cerebral blood flow (rCBF) were measured by PET. Baroreceptor stimulation resulted in rCBF increase in the right anterior–inferior prefrontal cortex (Brodmann areas (BA) 10/44/47) and bilaterally in BA 6/8. We conclude that in at least some stages of baroreceptor information processing the right hemisphere plays a greater role than the left hemisphere.

Augmented Water Binding and Low Cellular Water Content in Erythrocytes of Camel and Camelids

We investigated a link between hemoglobin primary structure, hemoglobin hydrophobicity-hydrophilicity, and erythrocyte water content in various mammalian species. Some hemoglobin molecules, particularly those of the camel and camelids, contain more charged amino acid residues and are more hydrophilic than the hemoglobins of human and a number of other mammalian species. To test the in vivo significance of these alterations of hemoglobin primary structure, we determined the osmotically unresponsive erythrocyte water fractions in mannit solutions of various osmolarities at 4 degreesC. Among the species investigated, the size of the osmotically unresponsive erythrocyte water fraction relates in a positive linear way to hemoglobin hydrophilicity. The extreme low total erythrocyte water content of camel erythrocytes (1.1-1.3 g water/g dry mass) may be explained by a comparatively high osmotically unresponsive erythrocyte water fraction. It is proposed that alterations of hemoglobin sequences of camel and camelids may be the part of a natural selection process aimed at protecting these animals against osmotic dehydration in arid environments.

Lithium induces phosphoglucomutase activity in various tissues of rats and in bipolar patients

Phosphoglucomutase catalyses the reversible conversion of glucose-6-P and glucose-1-P. Lithium is a potent inhibitor of phosphoglucomutase in vitro, however, it is not known if phosphoglucomutase was significantly inhibited by Li+ in Li+-treated bipolar patients. Here, we demonstrate that phosphoglucomutase inhibition by chronic Li+ treatment causes alterations of glucose-phosphate levels in various tissues of rats. Also, phosphoglucomutase inhibition results in compensatory elevation of phosphoglucomutase activity in rat tissues and in leukocytes isolated from Li+-treated bipolar patients. The increase of uninhibited phosphoglucomutase activity in leukocytes of Li+-treated bipolar patients is due to the increased expression of the PGM1 gene.

Brain Adaptation to Water Loading in Rabbits as Assessed by NMR Relaxometry

The present study was undertaken to investigate the cerebral adaptation to hypoosmolar stress in adult Pannon white rabbits by applying proton nuclear magnetic resonance relaxometry. Progressive hyponatremia was induced by combined administration of hypotonic dextrose in water and 8-deamino-arginine vasopressin over a hydration period of 3, 24, and 48 h. Each group comprised five animals. After completing the hydration protocols, blood was taken to determine plasma osmolality (freezing point depression) and sodium concentration (ion-selective electrode) and, at about the same time, T2-weighted images were made. After the in vivo measurements, the animals were killed and brain tissue samples were obtained to measure water content (desiccation method) and T1 and T2 relaxation times (proton nuclear magnetic resonance method). Free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. It was shown that brain water content and T1 relaxation time remained unchanged despite the progressing hyponatremia. By contrast, T2 relaxation time increased steadily from the control value of 100.2 ± 7.7 ms to attain its maximum of 107.5 ± 8.5 ms (p< 0.05) after 48 h of hydration. Using biexponential analysis, fast and slow components of the T2 relaxation curve could be distinguished that corresponded to the bound (T21) and free (T22) water fractions. In response to hyponatremia, the bound water fraction was markedly depressed from 6.5 ± 3.0% to 3.6 ± 0.9% (3 h, p< 0.05) and 3.9 ± 0.8% (24 h, p< 0.05); then it approached the initial value of 5.3 ± 2.5% by the end of the hydration period of 48 h. It is concluded that restructuring of brain water is a contributory factor to the successful adaptation to hypotonic environment.