Segmented Gradient Amplifier Calibration for Linear MRI Fields
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Solution Overview
Problem
In magnetic resonance imaging, gradient coils with uneven current distribution among segments lead to non-linear magnetic fields, causing distortions in image reconstructions, mechanical stress, and interference with shim coils, due to inaccuracies in current sensors and variations in coil resistances.
Innovation Solution
A method for calibrating gradient amplifiers by measuring and comparing magnetic field distributions generated by each coil segment, using a system control unit to equalize current deviations and apply correction factors, ensuring linear gradient fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If gradient coils are segmented into multiple coil segments powered by separate gradient amplifiers, then the gradient magnetic field can be generated more flexibly and controlled more precisely, but current sensor inaccuracies and resistance variations in individual segments lead to non-linear magnetic fields and image distortions
Solution Approach 1:
The patent measures the actual magnetic field distribution generated by each coil segment and uses this measurement feedback to calculate correction factors. These correction factors are then applied to adjust the drive currents, creating a closed-loop feedback system that compensates for sensor inaccuracies and resistance variations, thereby maintaining gradient field linearity despite segmentations.
Solution Approach 2:
The patent changes the operational parameters (drive currents) of the gradient amplifiers based on measured magnetic field distributions. By adjusting current magnitudes and phases of individual coil segments according to calculated correction factors, the system compensates for manufacturing tolerances and maintains overall gradient linearity.
2Measurement precision
If current sensors in gradient amplifiers are made as accurate as possible, then current regulation should be precise, but production-related component tolerances still cause inaccuracies and deviations between individual amplifiers
Solution Approach 1:
Instead of relying solely on sensor accuracy, the system measures the actual magnetic field produced and uses this field measurement feedback to determine correction factors. This approach bypasses sensor inaccuracies by directly measuring the effect (magnetic field) rather than relying on sensor readings of current.
Solution Approach 2:
The patent introduces magnetic field measurements as an intermediary to indirectly assess current accuracy. Rather than directly trusting current sensor readings, the system uses the magnetic field (the actual output) as a mediator to evaluate and correct current deviations across different amplifiers.
3Ease of operation
If coil segments are fed with equal currents, then the gradient-like magnetic field should be ideal, but variations in coil resistances and sensor inaccuracies prevent equal current distribution
Solution Approach 1:
The patent applies different correction factors to different coil segments based on their individual characteristics and measured performance. Instead of treating all segments uniformly, the system tailors the drive current for each segment according to its specific resistance, sensor accuracy, and measured field contribution, achieving local optimization that results in global linearity.
Solution Approach 2:
The system dynamically adjusts the drive current parameters (magnitude and phase) for each coil segment based on measured magnetic field distributions. By changing these parameters according to calculated correction factors, the system compensates for resistance variations and sensor inaccuracies, ensuring uniform magnetic field contribution from all segments.
4Manufacturing precision
If magnetic field distributions are measured and correction factors are calculated for each coil segment, then gradient field linearity can be improved, but the calibration process becomes more complex and time-consuming
Solution Approach 1:
The system performs self-calibration by automatically measuring its own magnetic field distribution, calculating correction factors, and applying them without requiring external intervention or complex manual adjustment procedures. The magnetic resonance apparatus uses its own imaging capabilities to measure the gradient field and automatically compensates for deviations.
Solution Approach 2:
The patent uses the magnetic resonance apparatus's existing imaging functions to perform the calibration measurements. The same system that images the patient is also used to measure the gradient field distribution, eliminating the need for separate dedicated measurement equipment and simplifying the overall system.
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
Reduces non-linearities in gradient magnetic fields, minimizing distortions in magnetic resonance mappings and mechanical stress, while maintaining coil integrity and improving image quality.
Implementation Method 1
Each coil segment of the plurality of coil segments is supplied with electric current (e.g., powered via a gradient amplifier (or gradient power amplifier, GPA))
Implementation Method 2
The main magnetic field, also referred to as the B0 field, is generated by a main magnet (e.g., a superconducting main magnet)
Implementation Method 3
Nuclear spins are excited in the patient by the excitation pulses generated, so that positionally encoded magnetic resonance signals are triggered
Data Source
AI summary
A method for calibrating gradient amplifiers of a magnetic resonance apparatus is provided. The magnetic resonance apparatus includes one or more gradient coils that include at least two coil segments configured to generate a gradient magnetic field with a gradient in a spatial direction, and a gradient amplifier for each of the at least two coil segments. The gradient amplifier is configured to supply the respective coil segment with electric current. A plurality of magnetic field distributions are measured, each being generated via at least one coil segment of the at least two coil segments. For measuring each magnetic field distribution of the plurality of magnetic field distributions, the at least one coil segment that generates the respective magnetic field distribution is supplied with a predetermined electric current via the respective gradient amplifier. The gradient amplifiers are calibrated via a comparison of the plurality of measured magnetic field distributions.