Biomedical subjects

D LeBihan

Publications and source records attributed to D LeBihan.

6 recordsLinked to original sources

PubMed1994

Estimation of the effective self-diffusion tensor from the NMR spin echo.

The diagonal and off-diagonal elements of the effective self-diffusion tensor, Deff, are related to the echo intensity in an NMR spin-echo experiment. This relationship is used to design experiments from which Deff is estimated. This estimate is validated using isotropic and anisotropic media, i.e., water and skeletal muscle. It is shown that significant errors are made in diffusion NMR spectroscopy and imaging of anisotropic skeletal muscle when off-diagonal elements of Deff are ignored, most notably the loss of information needed to determine fiber orientation. Estimation of Deff provides the theoretical basis for a new MRI modality, diffusion tensor imaging, which provides information about tissue microstructure and its physiologic state not contained in scalar quantities such as T1, T2, proton density, or the scalar apparent diffusion constant.

Animals↗

PubMed1994

MR diffusion tensor spectroscopy and imaging.

This paper describes a new NMR imaging modality--MR diffusion tensor imaging. It consists of estimating an effective diffusion tensor, Deff, within a voxel, and then displaying useful quantities derived from it. We show how the phenomenon of anisotropic diffusion of water (or metabolites) in anisotropic tissues, measured noninvasively by these NMR methods, is exploited to determine fiber tract orientation and mean particle displacements. Once Deff is estimated from a series of NMR pulsed-gradient, spin-echo experiments, a tissue's three orthotropic axes can be determined. They coincide with the eigenvectors of Deff, while the effective diffusivities along these orthotropic directions are the eigenvalues of Deff. Diffusion ellipsoids, constructed in each voxel from Deff, depict both these orthotropic axes and the mean diffusion distances in these directions. Moreover, the three scalar invariants of Deff, which are independent of the tissue's orientation in the laboratory frame of reference, reveal useful information about molecular mobility reflective of local microstructure and anatomy. Inherently tensors (like Deff) describing transport processes in anisotropic media contain new information within a macroscopic voxel that scalars (such as the apparent diffusivity, proton density, T1, and T2) do not.

Animals↗

PubMedDate not supplied

Hyperthermia system combined with a magnetic resonance imaging unit.

Magnetic resonance imaging (MRI) has recently been proposed as a method to monitor, noninvasively, temperature, blood flow, and cell metabolism during oncologic hyperthermia (HT). To heat and "image" simultaneously, it is necessary to combine a HT device and a MRI unit. As a demonstrative example of the problems associated with implementing such a system, a mini-annular phased array hyperthermia applicator was combined with a 0.5-T whole body MRI unit. With the aid of filters, baluns, and switches, the HT applicator and the MRI unit were made compatible. The overall system was tested using a muscle-equivalent, cylindrically shaped polyacrylamide gel phantom. No interference between the HT device and the MRI unit was observed. Noninvasive temperature images, with a resolution better than 1 degree C/cm, were obtained from images of molecular diffusion recorded before and during heating.

Humans↗

PubMedDate not supplied

On the accuracy of noninvasive thermometry using molecular diffusion magnetic resonance imaging.

Temperature measurement using magnetic resonance imaging (MRI) of water self-diffusion is investigated. Diffusion images and derived temperatures are obtained in polyacrylamide gel phantom. The temperatures measured from MRI are compared with those from temperature probes to verify their accuracy. In general, the difference between temperatures determined from MRI diffusion images over 0.3 cm3 regions of interest and from temperature probes were 0.2 degrees C. It is concluded that current MRI technology allows noninvasive temperature tomography that is comparable with invasive thermometry with respect to temperature accuracy, has spatial and time resolutions that would be useful in hyperthermic oncology.

Body Temperature↗

PubMedDate not supplied

MR of slow CSF flow in the spine.

PURPOSE: To evaluate a slow-flow MR sequence in normal CSF flow and in CSF flow disturbance in cases of spinal stenosis. METHOD: The method was tested for flow sensitivity and applied to 67 sites of spinal canal compromise. RESULTS: Phantom studies show that flow can be depicted at a velocity of 0.5-1 mm/sec. On clinical images, stagnant CSF is black, flowing CSF is bright. Typically, in high-grade (90%-100%) stenosis, CSF above and below the site of spinal canal compromise (SCC) is black. With intermediate stenosis (50%-89%), CSF above the SCC remains white but becomes black distal to the SCC. Low-grade stenosis shows only localized flow disturbances. CONCLUSION: This easy-to-use sequence can solidify the MR diagnosis of high-grade stenosis when a distinct flow pattern is recognized. Flow patterns for intermediate and low-grade stenosis are less reliable.

Cerebrospinal Fluid↗

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↗