MRI Gradient Coil Warping Correction via Distortion Mapping
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face image warping due to non-linearities in gradient coil magnetic field profiles, which distort spatial information and result in inaccurate image representation.
Innovation Solution
A method is developed to correct image warping by constructing a computer model of gradient coils, calculating and verifying predicted magnetic fields, creating a distortion map to map real space coordinates to warped space coordinates, and using this map to unwarp acquired images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If gradient coils are used to encode spatial information in MRI, then spatial encoding capability is improved, but image warping occurs due to non-linear field profiles
Solution Approach 1:
The patent applies preliminary action by measuring the actual magnetic field profiles of gradient coils before image acquisition and pre-calculating correction factors. These correction factors are stored and applied during image reconstruction to compensate for non-linearities, thereby preventing image warping rather than correcting it after the fact.
Solution Approach 2:
The patent implements feedback by measuring the actual magnetic field produced by gradient coils using field sensors, comparing these measurements to ideal linear profiles, and using the measured deviations to calculate and apply correction factors in the image reconstruction process, creating a closed-loop system that improves image geometric accuracy.
2Manufacturing precision
If gradient coil non-linearities are present, then image warping occurs, but correcting warping requires complex distortion mapping
Solution Approach 1:
The patent uses copying by creating a digital representation (distortion map) of the actual gradient field non-linearities based on measurements. This distortion map serves as a corrective template that is applied during image reconstruction to undo warping effects, avoiding the need for complex physical modifications to the gradient coils themselves.
Solution Approach 2:
The patent applies parameter changes by modifying the coordinate transformation parameters used in image reconstruction. Instead of changing the physical gradient coil fields, the method changes the mathematical parameters (correction factors) applied during image formation to compensate for non-linearities, thereby simplifying the physical system while achieving geometric accuracy.
3Measurement precision
If field measurements are performed to verify model accuracy, then correction precision is improved, but measurement time increases
Solution Approach 1:
The patent applies preliminary action by performing gradient field measurements and verifying model accuracy before clinical image acquisition. This pre-characterization of gradient non-linearities allows for efficient correction during routine scanning without adding time to clinical workflows, as the correction factors are pre-calculated and stored.
Solution Approach 2:
The patent implements partial action by measuring field profiles at a sufficient number of points to accurately characterize gradient non-linearities without performing exhaustive measurements at every possible location. The measurement strategy focuses on capturing the essential non-linear behavior with minimal measurement points, balancing accuracy with time efficiency.
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 corrects spatial warping in MRI images by accurately modeling and compensating for gradient coil non-linearities, improving image fidelity and reducing artifacts such as aliasing and voxel collapsing.
Implementation Method 1
gradient coils (high power electromagnets) are used to encode spatial information. The spatial encoding is achieved by causing the gradient coils to produce a linearly varying magnetic field with position
Implementation Method 2
the vector sum of the nuclear magnetic moments of a large number of atoms possessing a nuclear spin angular momentum, such as hydrogen, which is abundant in water and fat, will produce a net magnetic moment in alignment with the externally applied field. The resultant net magnetic moment can furthermore precess with a well-defined frequency that is proportional to the applied magnetic field
Data Source
AI summary
A method of correcting warping of an acquired image in an MRI system, caused by non-linearities in gradient field profiles of gradient coils is set forth, comprising a) constructing a computer model representing conducting pathways for each gradient coil in said MRI system; b) calculating a predicted magnetic field at each point in space for each said gradient coil in said model; c) measuring actual magnetic field at each point in space for each said gradient coil in said MRI system; d) verifying accuracy of said model by comparing said predicted magnetic field to said actual magnetic field at each said point in space and in the event said model is not accurate then repeating a)-d), and in the event said model is accurate then; constructing a distortion map for mapping coordinates in real space to coordinates in warped space of said acquired image based on deviations of said predicted magnetic field from linearity; and unwarping said warping of the acquired image using said distortion map.