NMR Imaging Short T2 Resolution via Projection Reconstruction
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Solution Overview
Problem
Conventional NMR imaging techniques fail to effectively resolve short T2 components in rocks due to lengthy echo times and interference from diffusion and magnetization, limiting the analysis of porosity and fluid properties.
Innovation Solution
The method involves generating field gradients and using CPMG pulse sequences to obtain one-dimensional projections, rotating the gradient direction, and applying inverse Radon transformations to generate NMR images that accurately represent T2 decay data, enabling the resolution of short T2 components and extraction of petrophysical information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional MRI techniques with frequency encoding and phase encoding are used, then imaging capability is achieved, but echo time becomes lengthy (several milliseconds) preventing resolution of short T2 values
Solution Approach 1:
The patent extracts and removes the gradient switching operations from the pulse sequence timing. By eliminating the need to turn gradients on and off between refocusing pulses, the method removes the source of the lengthy echo times that prevented short T2 resolution while maintaining imaging capability through projection reconstruction.
Solution Approach 2:
The patent inverts the conventional imaging approach by using projection reconstruction from multiple gradient directions instead of direct spatial encoding. This inversion allows continuous gradient application during CPMG sequences, achieving both short echo times and imaging capability through mathematical reconstruction rather than direct spatial mapping.
2Measurement precision
If higher magnetic fields are used to increase signal to noise ratio, then SNR is improved, but diffusion and induced magnetization dominate causing artificial shortening of apparent T2
Solution Approach 1:
The patent converts the harmful effect of gradient-induced diffusion dephasing into a beneficial feature by using the dephased signals as projections for image reconstruction. Instead of treating diffusion as noise to be minimized, the method utilizes the spatial encoding information contained in the dephased echoes to build images through projection reconstruction from multiple directions.
3Ease of operation
If gradient switching is performed between refocusing pulses for frequency and phase encoding, then spatial encoding is achieved, but switching time (hundreds of microseconds) increases minimum echo time
Solution Approach 1:
The patent replaces the mechanical gradient switching system with a mathematical reconstruction system. Instead of using mechanical on/off switching of gradients for spatial encoding, the method applies gradients continuously and uses projection reconstruction algorithms to achieve spatial information, eliminating the time penalty of switching operations.
4Loss of time
If slice selection gradients are turned on and off for each refocusing pulse, then slice selection is achieved, but interference with frequency encoding pulses occurs and echo time increases
Solution Approach 1:
The patent extracts and removes slice selection from the pulse sequence timing by using projection reconstruction instead. By eliminating slice selection gradients and their associated switching requirements, the method removes the source of interference with frequency encoding and the resulting echo time increases, achieving simplified gradient sequencing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for reliable imaging and analysis of short T2 components, providing detailed T2 distributions and petrophysical data, such as porosity and fluid flow permeability, without the limitations of conventional methods.
Implementation Method 1
Nuclear magnetic resonance (NMR) involves the application of a magnetic field to an object that impacts the magnetic moment (spin) of an atom in the object. In general, the magnetic field causes the atoms in the object to align along and oscillate (precess) about the axis of the applied magnetic field.
Implementation Method 2
Longitudinal relaxation due to energy exchange between the spins of the atoms and the surrounding lattice (spin-lattice relaxation) is usually denoted by a time T1 when the longitudinal magnetization has returned to a predetermined percentage (i.e., 63%) of its final value.
Implementation Method 3
Transverse relaxation that results from spins getting out of phase is usually denoted by time T2 when the transverse magnetization has lost a predetermined percentage (i.e., 63%) of its original value. The T2 measurement is often performed using a well-established Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence which utilizes an initial 90 degree excitation pulse followed by a series of 180 degree (pi) refocusing pulses.
Implementation Method 4
The T2 measurement is often performed using a well-established Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence which utilizes an initial 90 degree excitation pulse followed by a series of 180 degree (pi) refocusing pulses
Implementation Method 5
generating with NMR apparatus a field gradient along the object in a set direction
Implementation Method 6
using the one-dimensional projections, obtaining an NMR image of the object or indications thereof for each of a plurality of echoes
Data Source
AI summary
A method of investigating an object using nuclear magnetic resonance (NMR) equipment includes generating a one-dimensional projection of the object for each of a plurality of echoes utilizing echo train signal indications resulting from pulse sequences, and utilizing the plurality of one-dimensional projections, for each of the plurality of echoes, generating NMR image data for at least one location in the object. The NMR image data may be displayed. The displayed data may include a T2 decay curve, a T2 value display, a T2 distribution graph, or petrophysical data for at least one object location.