Magnetic resonance specimen evaluation using multiple pulsed field gradient sequences with a wavenumber magnitude local minimum and restricted compartment estimation
Inventors
Ozarslan, Evren • Basser, Peter J.
Assignees
US Department of Health and Human Services
Publication Number
US-8704515-B2
Publication Date
2014-04-22
Expiration Date
2029-08-11
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Abstract
Using pulsed-field-gradient (PFG) sequences, the sizes of the pores in ordered porous media can be estimated from the “diffraction” pattern that the signal attenuation curves exhibit. A different diffraction pattern is observed when the experiment is extended to a larger number (N) of diffusion gradient pulse pairs. Differences in the characteristics of attenuation curves also permit distinguishing different pore shapes and distributions using the N-PFG technique. Using an even number of PFG pairs, an approximation to the average pore size can be obtained even when the sample contains pores with a broad distribution of sizes. Multi-PFG sequences can also be used to differentiate free and multi-compartment diffusion, and to estimate compartment sizes and orientations, and to distinguish microscopic and ensemble anisotropy.
Core Innovation
The invention relates to methods and apparatus for magnetic resonance based evaluation of heterogeneous specimens using pulsed-field-gradient (PFG) sequences. It particularly addresses the use of multiple PFG sequences to analyze porous media by examining the diffusion-induced diffraction patterns present in nuclear magnetic resonance (NMR) signal attenuation curves. By employing multi-PFG sequences, the invention enables estimation of pore sizes, distinguishing pore shapes and distributions, and differentiating between free and restricted diffusion compartments.
The disclosed methods overcome limitations of conventional single-PFG experiments, which require high gradient strengths and long diffusion times to observe diffraction dips, and often fail to provide accurate pore size estimations in specimens with a broad distribution of pore sizes. Multi-PFG sequences, especially those with even numbers of gradient pairs, enable estimation of average pore size at lower gradient strengths and shorter diffusion times by exploiting local minimum and zero-crossing features in the magnetic resonance wavenumber signal. The techniques also support estimation of compartment sizes, orientations, and anisotropies from NMR signals acquired across varied pulse sequence parameters.
Claims Coverage
The claims include four independent claims directed to computer-readable medium methods and apparatus for estimating restricted compartment size distributions, and methods for applying multi-PFG sequences with varied gradient angles to obtain such estimates.
Generation of restricted compartment size distribution estimate from multi-PFG signals
Obtaining recorded magnetic resonance signal amplitude as a function of magnetic resonance wavevector in response to a multi-PFG sequence including at least two PFG sequences, and generating an estimate of the distribution of restricted compartment sizes within the sample based on the determination of magnetic resonance wavenumber magnitudes associated with local minima or sign transitions in the recorded signal.
Estimation of mean restricted compartment size from multi-PFG signal features
Providing an estimate of a mean value of a restricted compartment size distribution based on local minimum or transition points of recorded magnetic resonance signal amplitude, applicable to sequences with odd or even numbers of PFG sequences, and associated with radii of cylindrical or spherical compartments or planar compartment separations.
Determination of restricted compartment dimensions from angularly varied multi-PFG sequences
Applying multiple PFG sequences with varied angles between field gradients, recording magnetic resonance signal amplitude as a function of these angles, determining magnetic resonance wavenumber magnitudes linked to local minima or sign changes, and providing estimates of restricted compartment dimensions, including diffusion distance or compartment radius.
Providing estimates of restricted compartment orientations and imaging
From recorded signals with varied angular or diffusion parameters, providing estimates of restricted compartment orientations for one or more compartment distributions, and displaying images based on compartment properties as a function of specimen location.
The independent claims encompass methods and computer-readable media for using multi-PFG magnetic resonance signals, focusing on detecting local minima and zero-crossings in wavenumber-dependent signal amplitudes to estimate restricted compartment size distributions, mean values, dimensions, orientations, and to support specimen evaluation and imaging.
Stated Advantages
Enables estimation of average pore size more robustly, accurately, and precisely even when pores have a broad size distribution.
Reduces required gradient strength by approximately half compared to single-PFG experiments, facilitating measurement of smaller pore sizes.
Permits measurement at shorter diffusion times, making experiments more practical and reducing total acquisition time.
Allows distinguishing different pore shapes and distributions using the multi-PFG technique.
Discriminates restricted diffusion from free or multi-compartment Gaussian diffusion by analyzing angular dependence of the signal.
Documented Applications
Determination of pore sizes and shapes in ordered porous media, including biological specimens with heterogeneous pore size distributions.
Differentiation of free and multi-compartment restricted diffusion in tissues or materials.
Estimation of compartment sizes, orientations, and anisotropies in specimens, including biological tissues such as nerve fibers or cellular structures.
Diagnosis, monitoring, segmentation, classification, and imaging of tissue based on restricted compartment properties derived from multi-PFG magnetic resonance measurements.
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