Observation of magnetic structural universality and jamming transition with NMR.

Nuclear magnetic resonance (NMR) has been instrumental in deciphering the structure of proteins. Here we show that transverse NMR relaxation, through its time-dependent relaxation rate, is distinctly sensitive to the structure of complex materials or biological tissues at the mesoscopic scale, from micrometers to tens of micrometers. Based on the ideas of universality, we show analytically and numerically that the time-dependent transverse relaxation rate approaches its long-time limit in a power-law fashion, with the dynamical exponent reflecting the universality class of mesoscopic magnetic structure. The spectral line shape acquires the corresponding non-analytic power law singularity at zero frequency. We experimentally detect the change in the dynamical exponent as a result of the transition into maximally random jammed state characterized by hyperuniform correlations. The relation between relaxational dynamics and magnetic structure opens the way for noninvasive characterization of porous media, complex materials and biological tissues.

On the design of filters for Fourier and oSVD-based deconvolution in bolus tracking perfusion MRI.

OBJECT: Bolus tracking perfusion evaluation relies on the deconvolution of a tracers concentration time-courses in an arterial and a tissue voxel following the tracer kinetic model. The object of this work is to propose a method to design a data-driven Tikhonov regularization filter in the Fourier domain and to compare it to the singular value decomposition (SVD)-based approaches using the mathematical equivalence of Fourier and circular SVD (oSVD). MATERIALS AND METHODS: The adaptive filter is designed using Tikhonov regularization that depends on only one parameter. Using a simulation, such an optimal parameter that minimizes the sum of statistical and systematic error is determined as a function of the first moment difference between the tissue and the arterial curve and the contrast to noise ratios of the input data (CNR( a ) in arteries and CNR( t ) in tissue). The performance of the method is evaluated and compared to oSVD in simulations and measured data. RESULTS: The proposed method yields a smaller flow underestimation especially for high flows when compared to the oSVD approach with constant threshold. However, this improvement comes to the price of an increased uncertainty of the flow values. The translation of the Tikhonov regularization parameter to an adaptive oSVD-threshold is in good agreement with the literature. CONCLUSION: The proposed method is a comprehensive approach for the design of data-driven filters that can be easily adapted to specific needs.

High-resolution three-dimensional 19F-magnetic resonance imaging of rat lung in situ: evaluation of airway strain in the perfluorocarbon-filled lung.

Perfluorocarbons (PFC) are biologically and chemically inert fluids with high oxygen and CO(2) carrying capacities. Their use as liquid intrapulmonary gas carriers during liquid ventilation has been investigated. We established a method of high resolution 3D-(19)F-MRI of the totally PFC-filled lung. The goal of this study was to investigate longitudinal and circumferential airway strain in the setting of increasing airway pressures on 3D-(19)F-MR images of the PFC-filled lung. Sixteen female Wistar rats were euthanized and the liquid perfluorocarbon FC-84 instilled into their lungs. 3D-(19)F-MRI was performed at various intrapulmonary pressures. Measurements of bronchial length and cross-sectional area were obtained from transversal 2D images for each pressure range. Changes in bronchial area were used to determine circumferential strain, while longitudinal strain was calculated from changes in bronchial length. Our method of 3D-(19)F-MRI allowed clear visualization of the great bronchi. Longitudinal strain increased significantly up to 31.1 cmH(2)O. The greatest strain could be found in the range of low airway pressures. Circumferential strain increased strongly with the initial pressure rise, but showed no significant changes above 10.4 cmH(2)O. Longitudinal strain was generally higher in distal airways, while circumferential strain showed no difference. Analysis of mechanical characteristics showed that longitudinal and circumferential airway expansion occurred in an anisotropic fashion. Whereas longitudinal strain still increased with higher pressures, circumferential strain quickly reached a 'strain limit'. Longitudinal strain was higher in distal bronchi, as dense PFCs gravitate to dependent, in this case to dorso-basal parts of the lung, acting as liquid positive end expiratory pressure.