Damage law and mechanism of coal-rock joint structure induced by liquid nitrogen at low temperature.

The width and degree of connectivity of coal-rock joints directly affect the seepage capacity of flow energy such as gas. To study the damage law and mechanism of the coal-rock joint structure under the action of liquid nitrogen, two methods of liquid nitrogen unloaded and liquid nitrogen freeze-thaw were used to carry out damage modification experiments on coal-rock with different water saturation. Using OLS4000 laser confocal microscope and MH-25 universal testing machine to conduct electron microscope scanning and uniaxial compression test, measure the joint width expansions and Young's modulus of the coal-rock surface before and after the test, establish a physical and mechanical model of freeze-thaw damage to analyze the ice-wedge expansion stress influence on the damage of coal-rock joint structure and establish damage criterion. The research results show that the ice-wedge expansion stress, confining pressure, and temperature stress in the joint jointly affect the structural damage of coal-rock joints, and the ice-wedge expansion stress contributes the most. With the increase of water saturation, the damage to the coal-rock joint structure intensifies, and the ice-wedge expansion stress under the water saturation state has the most obvious influence on the damage to the coal-rock joint structure. The damage criterion constructed by the freeze-thaw damage physical-mechanical model can reveal the damage mechanism of the effect of ice-wedge expansion stress on the coal-rock joint structure. This paper has certain practical significance for the safety and stability evaluation of rock engineering in cold and arid regions and provides new ideas for effectively extracting clean energy such as coalbed methane and preventing rock bursts.

Assessment of left ventricular 2D flow pathlines during early diastole using spatial modulation of magnetization with polarity alternating velocity encoding: a study in normal volunteers and canine animals with myocardial infarction.

A high-temporal resolution 2D flow pathline analysis method to study early diastolic filling is presented. Filling patterns in normal volunteers (n = 8) and canine animals [baseline (n = 1) and infarcted (n = 6)] are studied. Data are acquired using spatial modulation of magnetization with polarity alternating velocity encoding, which permits simultaneous quantification of 1D blood velocities (using phase contrast encoding) and myocardial strain (using spatial modulation of magnetization tagging and harmonic phase analysis) at high-temporal resolution of 14 ms within a single breath hold. Virtual emitter particles, released from the mitral valve plane every time frame during rapid filling, are tracked to depict the 2D pathlines on the imaged plane. The pathline regional distribution is compared with myocardial longitudinal strains and to regional pressure gradients. Quantitative analysis of net kinetic energy of pathlines is finally performed. Our results demonstrate a linear correlation (r(2) = 0.85) between pathline spatial distribution and myocardial strain. Peak net kinetic energy of 0.06 ± 0.01 mJ in normal volunteers, 0.043 mJ in baseline dog, 0.143 ± 0.03 mJ in infarcted dogs with nominal flow dysfunction, and 0.016 ± 0.007 mJ in infarcted dogs with severe flow dysfunction is observed. In conclusion, 2D pathline analysis provides a direct regional assessment of early diastolic filling patterns and is sensitive to abnormalities in early diastolic filling.

Assessment of early diastolic strain-velocity temporal relationships using spatial modulation of magnetization with polarity alternating velocity encoding (SPAMM-PAV).

A novel MR imaging technique, spatial modulation of magnetization with polarity alternating velocity encoding (SPAMM-PAV), is presented to simultaneously examine the left ventricular early diastolic temporal relationships between myocardial deformation and intra-cavity hemodynamics with a high temporal resolution of 14 ms. This approach is initially evaluated in a dynamic flow and tissue mimicking phantom. A comparison of regional longitudinal strains and intra-cavity pressure differences (integration of computed in-plane pressure gradients within a selected region) in relation to mitral valve inflow velocities is performed in eight normal volunteers. Our results demonstrate that apical regions have higher strain rates (0.145 ± 0.005 %/ms) during the acceleration period of rapid filling compared to mid-ventricular (0.114 ± 0.007 %/ms) and basal regions (0.088 ± 0.009 %/ms), and apical strain curves plateau at peak mitral inflow velocity. This pattern is reversed during the deceleration period, when the strain-rates in the basal regions are the highest (0.027 ± 0.003 %/ms) due to ongoing basal stretching. A positive base-to-apex gradient in peak pressure difference is observed during acceleration, followed by a negative base-to-apex gradient during deceleration. These studies shed insight into the regional volumetric and pressure difference changes in the left ventricle during early diastolic filling.