T1, T2, and magnetization transfer (MT) measurements were performed in vitro at 3 T and 37 degrees C on a variety of tissues: mouse liver, muscle, and heart; rat spinal cord and kidney; bovine optic nerve, cartilage, and white and gray matter; and human blood. The MR parameters were compared to those at 1.5 T. As expected, the T2 relaxation time constants and quantitative MT parameters (MT exchange rate, R, macromolecular pool fraction, M0B, and macromolecular T2 relaxation time, T2B) at 3 T were similar to those at 1.5 T. The T1 relaxation time values, however, for all measured tissues increased significantly with field strength. Consequently, the phenomenological MT parameter, magnetization transfer ratio, MTR, was lower by approximately 2 to 10%. Collectively, these results provide a useful reference for optimization of pulse sequence parameters for MRI at 3 T.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.20605

T1, T2 relaxation and magnetization transfer in tissue at 3T

Translational motion in solution, either diffusion or fluid flow, is at the heart of chemical and biochemical reactivity. Nuclear Magnetic Resonance (NMR) provides a powerful non-invasive technique for studying the phenomena using magnetic field gradient methods. Describing the physical basis of measurement techniques, with particular emphasis on diffusion, balancing theory with experimental observations and assuming little mathematical knowledge, this is a strong, yet accessible, introduction to the field. A detailed discussion of magnetic field gradient methods applied to Magnetic Resonance Imaging (MRI) is included, alongside extensive referencing throughout, providing a timely, definitive book to the subject, ideal for researchers in the fields of physics, chemistry and biology.

/pdf/nmr-studies-of-translational-motion-1y45lcz4az.pdf

NMR Studies of Translational Motion

A new method for mapping diffusivity profiles in tissue is presented. The Bloch-Torrey equation is modified to include a diffusion term with an arbitrary rank Cartesian tensor. This equation is solved to give the expression for the generalized Stejskal-Tanner formula quantifying diffusive attenuation in complicated geometries. This makes it possible to calculate the components of higher-rank tensors without using the computationally-difficult spherical harmonic transform. General theoretical relations between the diffusion tensor (DT) components measured by traditional (rank-2) DT imaging (DTI) and 3D distribution of diffusivities, as measured by high angular resolution diffusion imaging (HARDI) methods, are derived. Also, the spherical tensor components from HARDI are related to the rank-2 DT. The relationships between higher- and lower-rank Cartesian DTs are also presented. The inadequacy of the traditional rank-2 tensor model is demonstrated with simulations, and the method is applied to excised rat brain data collected in a spin-echo HARDI experiment.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.10596

Generalized diffusion tensor imaging and analytical relationships between diffusion tensor imaging and high angular resolution diffusion imaging

This is a tutorial describing how the time-dependent diffusion coefficient D(t) of mobile molecules confined in pores or cells carries information about the confining geometry. At early times, D(t) = D0[1 − (4/9)(S/Vp)] gives, irrespective of details, the pore surface to volume ratio (S/Vp), which is a measure of microscopic length. Here D0 is the bulk unrestricted diffusion coefficient. At long times, D(t) reaches a limiting value D0/α at a rate that depends on macroscopic length scales characteristic of the medium. The geometrical parameter “tortuosity” α plays an important role in various transport processes in porous media, such as the electrical conductivity. Extensions of these concepts to permeable pore walls, fluid identification by combining diffusion with relaxation, restricted diffusion in inhomogeneous magnetic fields leading to edge enhancement, and fractal surfaces are briefly described. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part A 23A: 1–21, 2004

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cmr.a.20017

Time-dependent diffusion coefficient as a probe of geometry

Multisite exchange models have been applied frequently to quantify measurements of transverse relaxation and diffusion in living tissues. Although the simplicity of such models is attractive, the precise relationship of the model parameters to tissue properties may be difficult to ascertain. Here, we investigate numerically a two-compartment exchange (Karger) model as applied to diffusion in a system of randomly packed identical parallel cylinders with permeable walls, representing cells with permeable membranes, that may serve particularly as a model for axons in the white matter of the brain. By performing Monte Carlo simulations of restricted diffusion, we show that the Karger model may provide a reasonable coarse-grained description of the diffusion-weighted signal in the long time limit, as long as the cell membranes are sufficiently impermeable, i.e. whenever the residence time in a cell is much longer than the time it takes to diffuse across it. For larger permeabilities, the exchange time obtained from fitting to the Karger model overestimates the actual exchange time, leading to an underestimated value of cell membrane permeability.

Monte Carlo study of a two-compartment exchange model of diffusion

An analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr-Purcell Meiboom-Gill and pulsed-field-gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.

Water dynamics in human blood via combined measurements of T2 relaxation and diffusion in the presence of gadolinium.

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Water dynamics in human blood via combined measurements of T2 relaxation and diffusion in the presence of gadolinium.