Asymmetric Gradient Coil Calibration for MRI Concomitant Field Correction
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Solution Overview
Problem
High-performance MRI systems with asymmetric gradient coils face challenges in accurately calibrating concomitant gradient field corrections, leading to image artifacts and reduced image quality due to non-linear spatial variations in magnetic fields.
Innovation Solution
A method is developed to determine phase difference measurements using bipolar gradient waveforms applied to gradient coils, allowing for the calculation of gradient coil constants and the generation of compensatory waveforms to correct concomitant gradient fields, which are then applied to compensate for these effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If high gradient amplitude coils are used to achieve higher image quality and spatial resolution, then image quality and spatial resolution are improved, but non-linearity in the gradient field strength increases and becomes more severe farther from isocenter
Solution Approach 1:
The system performs preliminary calibration measurements to determine actual gradient coil constants before imaging. Compensatory gradient waveforms are pre-calculated based on these measured constants to counteract the non-linear effects before the actual imaging sequence is executed.
Solution Approach 2:
The system changes the gradient waveform parameters by applying compensatory gradients that are calculated based on the measured coil constants. This modifies the effective gradient field to correct for non-linearities while maintaining the desired imaging gradient strength.
2Power
If asymmetric gradient coils are used to achieve compact design and high gradient amplitude, then device size is reduced and gradient amplitude is improved, but accurate calibration of gradient coil parameters becomes more difficult
Solution Approach 1:
The system performs self-calibration by using the asymmetric gradient coils themselves to generate measurable phase differences in a phantom. The calibration process uses the coils' own operational characteristics to determine their specific constants, eliminating the need for external calibration equipment.
Solution Approach 2:
The patent replaces traditional mechanical calibration methods with an electromagnetic field-based measurement approach. Instead of using physical measurement tools, the system uses MRI signal phase differences generated by the gradient coils themselves to determine calibration constants.
3Ease of operation
If conventional gradient coil calibration methods are used, then calibration process is simple, but image artifacts occur and image quality is reduced due to uncorrected concomitant gradient fields
Solution Approach 1:
The system implements a feedback loop where calibration measurements are taken to determine actual coil constants, which then inform the calculation of compensatory waveforms. This measured-feedback approach ensures that the correction parameters accurately reflect the actual coil performance, eliminating artifacts while maintaining operational simplicity.
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 accurately correcting concomitant gradient field errors, ensuring precise calibration of asymmetric gradient coils and reducing artifacts, even with high-amplitude gradient fields, and is applicable across a range of gradient amplitudes.
Implementation Method 1
Gradient coils in the MRI system produce gradients which distorts this uniform magnetic field. In general, the gradients cause the change in field strength of the magnetic field in the patient body from one point to another.
Implementation Method 2
determining a plurality of first phase difference measurements between two acquisitions using a plurality of first bipolar gradient waveforms applied to a first gradient coil axis or direction
Implementation Method 3
A first gradient coil axis' constant is determined based on the plurality of first phase different measurements. With the determined first gradient coil (axis) constant, compensatory gradient waveforms related to that gradient coil axis are determined.
Implementation Method 4
the compensatory gradient waveforms are applied along with target gradient waveforms to compensate for a concomitant gradient field
Data Source
AI summary
A method for correcting concomitant gradient field effects in a magnetic resonance imaging (MRI) system includes determining a plurality of first phase difference measurements between two acquisitions using a plurality of first bipolar gradient waveforms applied to a first gradient coil. A first gradient coil constant is determined based on the plurality of first phase difference measurements and compensatory gradient waveforms are determined based on the first gradient coil constant. The compensatory gradient waveforms are applied to the gradient coils along with target gradient waveforms to compensate for a concomitant gradient field.