Structural Analysis of Relaxation Curves in MRI

In magnetic resonance imaging, the gradient recalled echo sequence preserves information about spatial heterogeneities of magnetic field within a voxel, providing additional opportunity for classification of biological tissues. All the information, composed of physically meaningful parameters, like proton density, spin–spin relaxation time T ₂, gradients of magnetic field and spin–spin relaxation, effective relaxation time \(T_{2}^{*}\), and many others, is encoded in the shape of a relaxation curve, which is more complicated than a pure monoexponent, traditionally observed in spin echo sequences. The previous work [A. Protopopov, Appl. Magn. Reason. 48, 255-274 (2017)], introduced the theory and basic algorithms for separation of those parameters. The present work further expands this theory to the case of spin–spin relaxation gradients, improves reliability of the algorithms, introduces physical explanation of the phenomenon previously known as “multiexponentiality”, and presents new validation of the algorithms on volunteers. The entire approach may be named the structural analysis of relaxation curves.     

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature T_g, whether there is an underlying thermodynamic transition at some finite temperature below T_g, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.