Claude Lemaire

Magnetic resonance imaging of silastic-induced canine hydrocephalus.

Nine adult beagle dogs underwent magnetic resonance imaging in a 2-Tesla small-bore unit. Six surviving dogs were followed up serially with magnetic resonance imaging after induction of hydrocephalus by injection of Silastic into the prepontine cistern or fourth ventricle. Ventricular size (Y) measured as percentage cross-sectional area of an anterior frontal slice was related to postoperative day (X) as Y = 1.54 + 4.21 x ln(X), r = 0.9596. Periventricular edema appeared initially in the superlateral angles of the frontal horns in an area that corresponded histologically to the subcallosal fasciculus. T1 relaxation time of normal white matter of 979.32 msec increased to 1813.90 msec in the area of the edema (p less than 0.0001). The T2 relaxation time of normal white matter of 83.39 msec increased to 238.26 msec in the area of the edema (p less than 0.0001). Histological changes included expansion of the extracellular space in an area comparable to the region of increased signal intensity on T2-weighted images, as well as diffuse astrocytosis in the chronically hydrocephalic dogs.

Detection of chemical waves by magnetic resonance imaging

Abstract Magnetic resonance imaging (MRI) has been used for the first time to detect chemical waves in a nonlinear colourless medium. Observation of these self-organized spatial structures has been reported previously using traditional visual and optical techniques based on changes of light absorption. The ferroin-catalyzed Belousov—Zhabotinsky (BZ) reaction is the most studied example. MRI is widely used in medical and biological research to provide proton relaxation-time-weighted images using the spin-echo technique. In the present communication we demonstrate the capability of MRI for visualizing chemical waves in the Mn 2+ -catalyzed BZ system. The dynamics of spiral waves in quasi-2D thin layers is followed and the meander motion of the spiral is examined. In three-dimensional reaction vessels we observe spherical waves and scroll rings.

Proton magnetic resonance study of the H2–He potential energy surface

Measurements of the proton spin‐lattice relaxation time (T1) carried out at 18.775 MHz in two H2–He mixtures are presented. Values of T1/ρ)∞lin, accurate to within 2%, are deduced for ten temperatures between 85 and 300 K. These data are compared with theoretical values deduced from various H2–He potential surfaces. It is shown that the data are quite capable of distinguishing between the various surfaces and that the most recent H2–He surface of Kohler and Schaefer gives an excellent representation of the data.

Measurement of the velocity of chemical waves by magnetic resonance imaging

Abstract A new method is established for the measurement of the velocity of chemical waves propagating along a tube and associated with the Belousov-Zhabotinsky (BZ) reaction. By stacking consecutive projections of magnetic resonance images, displacement-time graphs are generated as the output of the imager. The slope at any point on such a graph is the velocity of the associated wave travelling along the tube. For solutions containing agarose, the wave velocity is constant, but in solutions without agarose, abrupt changes of the velocity are observed. Preliminary studies indicate that convective instabilities are responsible for these changes.

Proton spin longitudinal relaxation time in gaseous ammonia and hydrogen chloride

New data are presented for the proton spin relaxation time in gaseous ammonia (NH3) at 300 K. The characteristic nonlinear density dependence of the relaxation time at low densities is established. Old data for gaseous hydrogen chloride (HCl) are reanalyzed. The commonly used single relaxation time aproximation for the rotational levels is shown to be inadequate to fit either the NH3 or the HCl data. Excellent fits of both sets of data are obtained using a two parameter, multilevel relaxation time approximation; the parameters deduced are the average rotational level linewidth and the dispersion. An effective intramolecular spin‐rotation coupling constant is determined from the NH3 data and it is shown to be consistent with the spin‐rotation coupling constants deduced from microwave absorption experiments. Also an angular momentum reorientation cross section is computed from the NMR data and compared to other similar cross sections obtained from microwave, ultrasonic and viscomagnetic experiments. The reo...

Detection of critical mode convection in the presence of a thermal gradient using chemical waves as a passive indicator

The detection of the lowest critical modes, γ1(1) and γ0(1), for thermal convection in a vertical tube is reported. Their visualization was through the distortion they caused to chemical waves propagating in a manganese‐catalyzed Belousov–Zhabotinsky medium. A time dependent temperature gradient was allowed to develop in the system and the dynamical behavior of spontaneously occurring trigger waves was recorded as magnetic resonance images. The work also illustrates the potential of magnetic resonance imaging to provide information on the temperature dependence of the propagation velocity of trigger waves and of the frequency of pacemaker activity.

Proton longitudinal relaxation time measurements in dilute hydrogen chloride gas

Experimental measurements of the proton longitudinal relaxation time in dilute hydrogen chloride gas are reported. The measurements were made at room temperature, at 61 MHz and for gas densities between 0.005 and 0.5 amagat. The scalar spin–rotation interaction provides the dominant relaxation mechanism. The data are well accounted for using existing relaxation theory and the single relaxation time approximation. The experiment provides a fundamental test of nuclear spin relaxation theory for a gas of heteronuclear diatomic molecules.

Evaluation of the integrity of the blood-brain barrier after meningeal trauma.

RATIONALE AND OBJECTIVES The authors used magnetic resonance imaging (MRI) techniques to examine the effect of meningeal trauma produced by cisterna magna puncture on the integrity of the blood-cerebrospinal fluid barrier (BCB) in a rat model. METHODS Intravenous gadolinium-DTPA (Gd-DTPA), a relaxation rate modifier which normally does not cross the BCB, was used as a probe to follow leakage of fluid across the BCB. After Gd-DTPA injection, cerebrospinal fluid (CSF) serial samples were obtained through the needle used to create the experimental trauma. These samples were subsequently examined in vitro by MRI to obtain their T1 relaxation rates and assayed by mass spectrometry for gadolinium and elemental iron concentrations. RESULTS The iron levels reflected the severity of puncture-related subarachnoid hemorrhage. Rats with ongoing meningeal damage showed significantly higher CSF levels of gadolinium and significantly higher CSF T1 relaxation rates than controls at all samples times over 1 hour after the puncture. Blood in the CSF could not explain these changes because the CSF iron levels did not significantly differ from control levels. CONCLUSIONS Intravenously administered Gd-DTPA can gain access to the subarachnoid space through minor defects in the BCB and cause significant increases in CSF T1 relaxation rates.