Projects / Programmes
New measuring techniques and methods of magnetic resonance
| Code |
Science |
Field |
Subfield |
| 1.02.01 |
Natural sciences and mathematics |
Physics |
Physics of condesed matter |
| Code |
Science |
Field |
| P002 |
Natural sciences and mathematics |
Physics |
| P180 |
Natural sciences and mathematics |
Metrology, physical instrumentation |
magnetic resonance, microscopy, spin echo, diffusion, restricted , velocity correlation, random motion, stochastic, cumulant expansion, Gaussian process, Marcoffian, diffusive diffraction, porous , edge enhancement, ultrasound, shear wave, cement, setting times, Vicat
Researchers (7)
Organisations (2)
Abstract
With spin echo in a weak (geomagnetic) magnetic field, the strength of applied magnetic field gradient, required for self-diffusion measurement, may exceed the strength of the main magnetic field. Thus, the definition of the magnetic field gradient fails and the usual formula for self-diffusion attenuation of spin-echo is no longer valid. We have derived the generalized formula for the self-diffusion attenuation by the spin echo that valids for the strong non-uniform magnetic field. Its validity and usefulness has been demonstrated by measuring the diffusion in the geomagnetic field. In combination with magnetic resonance imaging, it provides the contrast images between substances with different self-diffusion properties.
With the use of the particle probability density we analyze the spin echo attenuation of particles, diffusing in a bounded region. It provides a mean to expand a non-uniform spin phase distribution into series of plane waves characterising the geometry and boundaries of confinement. Assuming the spin phase fluctuation and/or the randomness of spin phase distribution in sub-ensemble as a stochastic Gaussian process, we have derive an analytical expression for the echo attenuation related to the particle velocity correlation. For a diffusion in porous structure we get the expression featuring the same ''Diffusive Diffraction'' patterns as those being found and explained by P.T.Callaghan and A. Coy with the use of propagator theory. With the new approach we casts a new light to the phenomena and shows, analitically, how the diffusive diffractions appear when the sequence of finite or even modulated gradients are applied.
The enhancement of magnetic resonance image intensity near impermeable boundaries can be nicely described by above approach where the diffusional spin echoes attenuation is linked to the correlation function of molecular motion. The enhancement comes out as a discord of plane waves due to particle motion. The acquired analytical expression describes the MRI signal space distribution where the enhancement of edges depends on the intensity and the duration of gradient sequence as well as on the length of the mean squared particle displacement in restricted geometry.
The parallel measurements of setting time during the early hydration on PC cements was performed by the standard Vicat apparatus and by the home-made pulsed ultrasonic shear wave reflection USWR-2 Hardening meter. The measurements were carried out on the same samples and under the same ambient conditions. In contrast with Vicat needle test, the USWR method gives a continuous information of the stiffening and hardening process with the time via reflection changes as the rigidity of the pastes grows on hydration. The magnitudes of change in selected cement pastes (CEM I 52.5R and PC 30dz 45s) at Vicat setting times are used to determine the correlation between the two methods. With such a calibration the setting times of production samples can be satisfactorily measured with the USWR method.
