MRI Gradient Waveform Correction for Eddy Current Distortion
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Solution Overview
Problem
Conventional MRI techniques struggle to correct image quality degradation due to distortion in the gradient magnetic field distribution caused by eddy currents, especially when the imaging region is off-center, as they only compensate for primary components and require significant waiting times or lack corresponding shim coil channels for higher-order components.
Innovation Solution
A magnetic resonance imaging method and apparatus that includes a signal acquisition unit, an image generating unit, a position acquiring unit, and a correction unit, which transforms the gradient magnetic field waveform based on positional information and the time constant of the eddy-current magnetic field to cancel the eddy-current-induced magnetic field, thereby compensating for both primary and higher-order components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional eddy-current compensation is used to compensate for only the primary component of the magnetic field induced by eddy current, then the compensation method is simple, but it cannot compensate for the secondary and higher-order components, causing image quality degradation when the imaging region is off-center
Solution Approach 1:
The patent transforms the gradient magnetic field waveform parameters based on the imaging region position and eddy-current time constants to compensate for both primary and higher-order magnetic field components. This parameter transformation approach achieves comprehensive compensation without requiring additional hardware, resolving the contradiction between compensation completeness and method simplicity
Solution Approach 2:
The patent calculates and applies correction waveforms before actual imaging to preemptively compensate for eddy-current effects. By performing preliminary correction based on predetermined time constants and imaging position, the system eliminates higher-order components before they cause image degradation, maintaining both simplicity and precision
2Manufacturing precision
If higher-order shim coils are used to compensate for the secondary and higher-order components of the magnetic field induced by eddy current, then compensation completeness is improved, but a waiting time of several seconds is needed and channel correspondence issues arise
Solution Approach 1:
The patent replaces the mechanical/shim-coil-based compensation system with a waveform transformation system. Instead of physically adjusting higher-order shim coils (which require waiting time and channel correspondence), the system uses computational transformation of the gradient waveform to achieve the same compensation effect instantaneously, eliminating the time loss while maintaining completeness
Solution Approach 2:
The patent introduces a correction waveform as an intermediary between the gradient magnetic field and the imaging process. This intermediary waveform carries the compensation information for higher-order components without requiring direct physical intervention from shim coils, thus avoiding waiting time and channel correspondence problems while achieving complete compensation
3Manufacturing precision
If the gradient magnetic field waveform is transformed based on imaging region position and eddy-current time constant, then both primary and higher-order components are compensated, but the calculation complexity increases
Solution Approach 1:
The patent performs preliminary calculation of transformation parameters based on predetermined eddy-current time constants and imaging position. By pre-calculating the correction waveforms and storing them for rapid retrieval and application, the system reduces real-time calculation complexity while maintaining high precision in gradient magnetic field distribution
Solution Approach 2:
The patent uses parameter transformation of the gradient waveform based on imaging position and time constants. This mathematical transformation approach achieves precise compensation of higher-order components through systematic parameter adjustments rather than complex iterative calculations, reducing computational complexity while maintaining accuracy
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 effectively improves image quality by achieving a target gradient magnetic field distribution, reducing distortion and image degradation even when the imaging region is off-center, and allows for efficient compensation of eddy-current-induced magnetic field components without the need for lengthy waiting times or mismatched shim coil channels.
Implementation Method 1
A known cause of image quality degradation of MRI is distortion of a gradient magnetic field distribution. Ideally, the gradient magnetic field is distributed in the slice selection direction, the phase encoding direction and the frequency encoding direction in such a manner that the magnetic field intensity linearly varies with the position in the direction of application
Implementation Method 2
In actuality, however, a pulse current supplied to a gradient magnetic field coil causes an eddy current. The eddy current induces a magnetic field, and the magnetic field is added to the gradient magnetic field to cause distortion of the gradient magnetic field distribution
Implementation Method 3
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation
Data Source
AI summary
According to one embodiment, an MRI apparatus includes a signal acquisition unit, an image generating unit, a position acquiring unit, and a correction unit. The signal acquisition unit acquires a magnetic resonance signal produced from an object by applying a gradient magnetic field and an RF pulse in an imaging space where the object is placed. The image generating unit reconstructs image data on the object based on the magnetic resonance signal. The position acquiring unit acquires an imaging region as positional information in the imaging space. The correction unit brings a distribution of the gradient magnetic field close to a target distribution by transforming a waveform of the gradient magnetic field based on the positional information and a time constant of an eddy-current magnetic field to cancel the eddy-current magnetic field.