MRI Eddy Current Compensation Using Multi-Axis Shim Coils
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Solution Overview
Problem
Existing MRI techniques fail to accurately compensate for eddy currents induced in directions perpendicular to the applied gradient magnetic field, leading to image distortion and reduced signal quality due to the inability to negate cross-term effects.
Innovation Solution
An MRI apparatus and method that measures and compensates for eddy current magnetic fields in both the application direction and perpendicular directions by calculating a compensation magnetic field based on phase information from test gradient magnetic fields, allowing for the generation of a compensation magnetic field that corrects for eddy currents in all relevant directions during image reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a gradient magnetic field is applied for imaging, then imaging function is achieved, but eddy current is induced causing image quality degradation
Solution Approach 1:
The patent applies preliminary anti-action by pre-calculating and applying a compensation magnetic field that opposes the eddy current magnetic field before the eddy current fully develops. The system measures the eddy current magnetic field using test gradient magnetic fields, calculates the compensation field in advance, and applies it through the shim coil to negate the harmful effects on image quality.
Solution Approach 2:
The patent converts the harmful eddy current magnetic field into a measurable quantity that can be used for compensation. By applying test gradient magnetic fields and measuring the resulting phase differences, the system characterizes the eddy current magnetic field distribution and uses this information to generate an equal and opposite compensation field, turning the harmful effect into a correctable parameter.
2Object-affected harmful factors
If compensation magnetic field is applied to negate eddy current magnetic field, then image quality improves, but device complexity increases
Solution Approach 1:
The patent implements self-service by using the MRI apparatus's own shim coil to generate the compensation magnetic field. The system measures the eddy current magnetic field using its own gradient coils and signal detection systems, calculates the compensation parameters, and applies the correction through existing hardware components, eliminating the need for external compensation devices.
Solution Approach 2:
The patent performs preliminary action by measuring the eddy current magnetic field characteristics using test gradient magnetic fields before actual imaging. The system calculates and stores compensation parameters in advance, so that during actual imaging, the compensation can be applied efficiently using pre-computed correction fields, reducing the computational burden during real-time operation.
3Device complexity
If only eddy current magnetic field in application direction is measured, then measurement process is simple, but cross-term effects are not corrected
Solution Approach 1:
The patent extends the measurement from one dimension (application direction only) to three dimensions by measuring eddy current magnetic fields in all three spatial axes (x, y, and z directions). The system applies test gradient magnetic fields along each axis and measures the corresponding phase differences, capturing the complete spatial distribution of eddy current magnetic fields including cross-term effects, thereby enabling comprehensive compensation.
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 improves image quality by effectively compensating for eddy currents in all directions, reducing distortion and enhancing signal strength across the imaging region.
Implementation Method 1
a damping current (so-called eddy current) is induced in various structures around the gradient coil due to application of the gradient magnetic field, and this eddy current generates a magnetic field which changes spatially and temporally
Implementation Method 2
applying a compensation magnetic field for negating the magnetic field using a shim coil
Implementation Method 3
An MRI apparatus is an apparatus which measures a nuclear magnetic resonance (hereinafter, referred to as NMR) signal generated by the subject, especially, the nuclear spins which form human tissue
Data Source
AI summary
An object is to correct a magnetic field caused by an eddy current, which is generated due to application of a gradient magnetic field, not only in the application direction of the gradient magnetic field but also in a direction different from the application direction of the gradient magnetic field. In the present invention, in order to achieve this object, a compensation magnetic field which compensates for an eddy current magnetic field generated in each of an application direction of a test gradient magnetic field and a direction different from the application direction in each direction is calculated using the test gradient magnetic field. When generating an arbitrary gradient magnetic field, a compensation magnetic field which compensates for an eddy current magnetic field according to application of an arbitrary gradient magnetic field on the basis of the measured eddy current magnetic field in each direction is generated together with the arbitrary gradient magnetic field.