Biomedical subjects

D DesPres

Publications and source records attributed to D DesPres.

8 recordsLinked to original sources

PubMed1994

Water diffusion and acute stroke.

The occlusion of the middle cerebral artery was used as an experimental acute stroke model in 30 cats. The diffusion of water was followed by diffusion-sensitized MRI between 1 and 15 h after induction of stroke. It is demonstrated that images representing the trace of the diffusion tensor provide a much more accurate delineation of affected area than images representing the diffusion in one direction only. The reason is that the strong contrast caused by the anisotropy and orientation of myelin fibers is completely removed in the trace of the diffusion tensor. The trace images show a small contrast between white and gray matter. The diffusion coefficient of white matter is decreased in acute stroke to approximately the same extent as gray matter. It is further shown that the average lifetime of water in extra and intracellular space is shorter than 20 ms both for healthy and ischemic tissue indicating that myelin fibers are permeable to water. The anisotropy contrast did not change before or after induction of stroke, nor after sacrifice. Together, these observations are consistent with the view that the changes in water diffusion during acute stroke are directly related to cytotoxic oedema, i.e., to the change in relative volume of intra- and extracellular spaces. Changes in membrane permeability do not appear to contribute significantly to the changes in diffusion.

Acute Disease↗

PubMed1994

Comparison of EPI gradient-echo contrast changes in cat brain caused by respiratory challenges with direct simultaneous evaluation of cerebral oxygenation via a cranial window.

A gradient-echo echo planar imaging (EPI) sequence has been used to generate images of cat brain during respiratory challenges. Direct spectrophotometric measurements have been made simultaneously in order to correlate the changes in oxygen saturation as measured by spectrophotometry with the image intensity changes seen in the gradient-echo images. When blood volume remains approximately constant, as derived from the spectrophotometry data, good correlation is seen between calculated plots made of changes in the transverse relaxation rate, delta R2*, and the oxygen saturation as measured by spectrophotometry for much of the time course of the respiratory challenges of anoxia, apnea and hypercapnia. In some cases, the correlation is poorer during the recovery periods of the apnea and anoxia challenges. Those lower correlations can often be accounted for by changes in blood volume, which also affects the NMR relaxation rate. These results contribute to the understanding of the image intensity changes seen during functional brain imaging studies in humans.

Animals↗

PubMed1994

Evaluation of restricted diffusion in cylinders. Phosphocreatine in rabbit leg muscle.

Interpretation of NMR diffusion data is complicated in the presence of diffusion barriers. In this study, restricted diffusion in cylinders is evaluated. A method is presented to determine the diameter of the cylinder and the unrestricted diffusion coefficient from the dependence of the trace of the diffusion tensor with diffusion time. This method is valid even if the orientation of the cylinders is unknown and varies within the sample. An example is given for the diffusion of phosphocreatine in the cylindrically shaped fibers of rabbit skeletal muscle.

Animals↗

PubMed1993

In vivo proton spectroscopy and spectroscopic imaging of [1-13C]-glucose and its metabolic products.

Metabolism of [1-13C]-glucose was studied in situ in cat brain using gradient-enhanced proton-detected heteronuclear spectroscopy. Proton detection of [1-13C]-glucose, [3-13C]-lactate, 4-[13C]-glutamine, 4-[13C]-glutamate and the combined signals 2-[13C]-glutamate/glutamine and 3-[13C]-glutamate/glutamine was achieved, despite the fact that some of the associated proton resonances are close to the water signal. Two-dimensional [1H-13C]-spectra demonstrate the possibility of in situ spectral assignment with 1H sensitivity and 13C resolution. Spectroscopic images of glucose and its metabolic products were also acquired, showing the possibility to study spatial dependence of metabolism.

Animals↗

PubMed1991

Restricted and anisotropic displacement of water in healthy cat brain and in stroke studied by NMR diffusion imaging.

The occlusion of the middle cerebral artery in cat brain was used as an experimental stroke model to investigate the physical basis of the recently reported lowered diffusion constant of water in acute infarcted brain tissue (Moseley et al., Magn. Reson. Med. 14, 330, 1990). The original findings were confirmed in this study of 12 animals investigated with the diffusion-sensitized stimulated echo sequence. The following additional results were obtained: First, the onset of significant lowering of the diffusion constant in the stroke area varied significantly (up to 2.5 h depending on the animal). Second, the affected area is much more clearly outlined in diffusion-weighted images than in T2-weighted images, even in the period between 3 to 12 h following occlusion. Third, for diffusion times between 50 and 2000 ms. the diffusion constant of water is independent of diffusion time in healthy tissue, as well as in the stroke area. Fourth, the diffusion anisotropy is similar in healthy and in stroke area and remains similar regardless of the diffusion time used.

Animals↗

PubMed1990

In vivo NMR diffusion spectroscopy: 31P application to phosphorus metabolites in muscle.

Apparent diffusion coefficients (Da) of individual metabolites can be studied in vivo by diffusion NMR spectroscopy using an echo sequence sensitized to molecular motion. The methods are based on the echo attenuation due to phase dispersion resulting from incoherent displacement during the diffusion time. As the displacement of metabolites by diffusion in vivo can be affected by compartment size, temperature, adsorption processes, etc., the presented methods are potentially useful in studying such phenomena in vivo. Here, the methods are applied to phosphocreatine in the rat quadriceps muscle. It is demonstrated that the displacement of phosphocreatine resembles free diffusion for short diffusion times but becomes limited as a result of boundaries due to compartmentation for longer diffusion times. The limit of the displacement indicates an apparent average size of 44 microns of the compartment in the direction of the diffusion gradient. As the gradient was applied approximately parallel (angle less than 25 degrees) to the muscle fiber, this result indicates that phosphocreatine moves freely in the cytosol but is limited by the boundaries of the muscle cells. Error analyses are performed with regard to motion artifacts and gradient performance. The methods were tested extensively for distilled water and free metabolites.

Animals↗

PubMedDate not supplied

Measuring random microscopic motion of water in tissues with MR imaging: a cat brain study.

Cat brain images sensitized to incoherent motion by additional gradient pulses were obtained on a 4.7 T magnetic resonance unit equipped with shielded gradient coils. The apparent diffusion coefficient of water in gray and white matter was accurately determined and imaged from the signal attenuation curve obtained as a function of gradient strength. Contrast in calculated diffusion images differed from typical T2-weighted contrast. Furthermore, in gray matter and in areas containing flowing CSF the attenuation curve was found to be biexponential. These results are interpreted in terms of a simple voxel model with microcirculation and diffusion contributions.

Animals↗

PubMedDate not supplied

MR of diffusion slowing in global cerebral ischemia.

PURPOSE: To investigate the causal connections between ischemia and the hyperintensity in diffusion-weighted MR images that has been associated with it. METHODS: Diffusion-weighted and T2-weighted MR imaging were used in a feline global cerebral ischemia/reperfusion model. Single 30-minute vascular occlusions followed by reperfusion were studied. Global occlusions were used to avoid interpretive complications associated with the temporally unstable hemodynamics of the penumbral zones around focal occlusions and the possible growth of the ischemic and penumbral regions with time. RESULTS: Diffusion-weighted hyperintensity and the associated diffusional slowing were not attributable exclusively to the cessation of blood flow because: 1) it does not appear abruptly at the onset of ischemia; 2) it resolves slowly early in reperfusion; and 3) it reappears after prolonged reperfusion. CONCLUSION: The times during which diffusion-weighted hyperintensity is manifested during ischemia, and recovers with reperfusion, point to a role for energy metabolism failure.

Animals↗