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J Coremans

Publications and source records attributed to J Coremans.

8 recordsLinked to original sources
PubMed1997

A comparison between different imaging strategies for diffusion measurements with the centric phase-encoded turboFLASH sequence.

The study compared the results of three centrally reordered phase-encoded turboFLASH sequences for diffusion-weighted imaging (DWI). The sequences were conventional turboFLASH, turboFLASH with subtraction of T1-related effects, and turboFLASH with correction for T1-related effects during the imaging period only. The relative merits were studied with respect to image quality and accuracy by computer simulation and by experimental validation on phantoms and on in vivo rat brain. A T1-related underestimation of the diffusion coefficient ranging from -30% (T1 approximately 200 ms) to -5% (T1 approximately 1 s) was found to exist for the conventional sequence. Image artifacts, caused by longitudinal relaxation during the imaging period, are reflected in calculated diffusion maps. When the correction sequence is used, the artifacts and the systematic errors are reduced but longitudinal relaxation during the delay between preparation and imaging periods remains large enough to induce significant errors (-15% for T1 approximately 200 ms to -3% for T1 approximately 1 s). The subtraction sequence eliminates the influence of T1 effects on the calibrations, but leads to identical artifacts for all diffusion-weighted images.

Animals↗
PubMed1994

A method for myelin fiber orientation mapping using diffusion-weighted MR images.

In the past, the anisotropic diffusion of water molecules in white matter in the brain has been correlated to the basic symmetry of the myelin fibers: water diffuses more readily along the fiber direction than perpendicular to it. As a consequence, diffusion sensitized magnetic resonance imaging can be expected to be useful for studying the fiber orientation. In this work, we present a method for exploiting this type of information to map the fiber orientations in the image plane. It makes use of three diffusion-weighted images with sensitizing gradients along x, y and u, an axis at 45 degrees with respect to x and y. The orientation information contained in these images is summarized in a single image representing the angle between the fiber direction and a fixed axis, making use of a cyclic color scale. The method is evaluated using computer simulations for a variety of diffusion weighting strengths and signal-to-noise ratios, tested on a phantom and illustrated on an in vivo example. An extension to the determination of the fiber orientation in three dimensions is also described.

Anisotropy↗
PubMed1991

Absolute quantification of 31P muscle metabolites using NMRS with an internal standard and a high-Q, double-tuned coil.

31P-nuclear magnetic resonance spectroscopy (NMRS) is used to determine absolute concentrations of 31P metabolites in rat muscle. A technique exploiting tissue water 1H as internal concentration reference and a highly sensitive, double-tuned coil is described and evaluated experimentally. On KH2PO4 solutions of known concentration and varying coil loading it is shown to allow neutralization of systematic errors due to conductive losses, which normally range up to 20% or more. In vivo application in the determination of the absolute concentrations of ATP, PCr and Pi in eight rat thighs yields results that are in good agreement with literature values.

Adenosine Triphosphate↗
PubMed1986

Standardization of a device for continuous observation of local flow in tissue.

As our experimental set-up for continuous recording of local blood flow in the cerebral cortex of a laboratory animal with chronically implanted miniature thermistors (based on the heat clearance principle) gave satisfactory results for routine tests of pharmacological agents during anoxia, hypoxia, hypercapnia, etc., experiments, we intended to standardize the apparatus, to increase accuracy, to facilitate calibration and to enhance flexibility with respect to the operator. The exponential aspect in the thermistor resistance/temperature characteristic is linearized by applying a logarithmic converter in the thermistor amplifier. Calibration to the centigrade temperature scale is performed by a three digit numerical adaptation of two thermistor constants determined in a thermostatic-cryostatic bath (zero and slope). A heating power measuring circuit is provided so that the dissipation constant of the thermistor implanted in tissue can be obtained and the thermal conductivity of the tissue can be estimated. Linearity of the relation between cooling of the heated thermistor and local flow, for small cooling values as they are registered in vivo, is still being investigated.

Animals↗
PubMed1985

Assessment of the in vivo recording of local cerebral blood flow using a thermistor device.

In order to obtain a continuous measurement of local blood flow in the cerebral cortex of a laboratory animal using chronically implanted sensors, we have developed a device based on the heat clearance principle. Flow information is obtained from temperature measurement by means of two thermistors one of them being heated at a defined level above ambient tissue temperature; as such, cooling of the heated thermistor caused by convection phenomena in its vicinity, can be related to local perfusion rate. In a first step "in vitro" measurements were performed in order to study the behaviour, sensitivity and reliability of the device; a physical model was established explaining the results. In this paper we describe "in vivo" tests in the rabbit's brain cortex with the miniature thermistors (0.5 mm diameter) chronically implanted (at the cortical surface). Results are correlated with oxygen tension measurements using (smaller) pO2 electrodes inserted into the cortical tissue. We have observed that all sensors are well tolerated by the animals who remain symptom free. Test experiments, inducing a well known physiological effect on local blood flow, such as arterial clamping, inhalation of CO2 gas mixtures, etc., are performed. The phenomena during induced anoxic anoxia are also shown. These preliminary investigations are essential in order to attempt by future experiments the establishment of a correlation between "in vivo" recorded flow signals and the "in vitro" measured characteristics.

Animals↗
PubMed1984

A thermistor device for the continuous recording of mass transport velocity in tissue based on the heat clearance principle.

A method for the recording of mass transport velocity in tissue, related to the relative local mass flow, has been developed. It is based on the heat clearance principle, applied in the continuous mode. Thermistors are used as temperature sensors; heating is performed by a high frequency square wave, thus avoiding an additional heating coil. The cooling of the heated thermistor, with as a reference a non-heated one, is related to the local mass transport velocity. A test equipment has been built for the calibration of the device and a set of results have been obtained in test fluids with a thermal conductivity approaching the one of brain tissue (glycerin and water). A mathematical model has been set up which gave results consistent with the recorded data. From this model significant parameters could be derived which, introduced in a compensation circuit, may be used in order to linearize the relation between the measuring value and the velocity variable. In vivo results in the brain cortex of the rabbit have been obtained.

Animals↗
PubMedDate not supplied

Absolute quantification of 31P liver metabolites in rat using an external reference and a surface spoiling magnetic field gradient.

A simple method for measuring absolute concentrations of 31P metabolites in rat liver using an external reference is described. It neutralizes systematic errors due to conductive losses by calibration of the matching capacitor and selects liver signals via an electrically driven, surface spoiling magnetic field gradient. The technique avoids the chemical shift problems associated with linear field gradient localization methods at 4.7 T and allows combination with the double standard method for absolute metabolite concentration determination. Application of the method to the in vivo measurement of the absolute concentrations of ATP and P(i) in eight rat livers yields results that are in good agreement with literature values. Absolute phosphomonoester concentrations were also obtained, but no literature data were available.

Adenosine Triphosphate↗