Shim Coil Synchronization for Eddy Current Compensation
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Solution Overview
Problem
Magnetic resonance imaging is affected by eddy current fields caused by gradient coil operation, leading to reduced image quality due to production tolerances and deviations in gradient coil shape, which existing technologies struggle to fully compensate without compromising image data.
Innovation Solution
A method that modifies the gradient pulse shape to generate shim settings for the shim coil arrangement, synchronizing its activation with the gradient pulse shape to compensate eddy currents, using techniques like forming the time derivative of the gradient pulse shape to suppress eddy current fields, allowing for efficient suppression of both integral and second-order eddy current contributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If gradient coils are operated to enable magnetic resonance imaging, then image acquisition is possible, but eddy current fields are generated that reduce image quality
Solution Approach 1:
The patent applies preliminary shimming to convert the harmful eddy current fields into a manageable problem by pre-compensating for their effects. The shim coil arrangement is activated before gradient pulse application to generate compensation fields that counteract the anticipated eddy currents, thereby converting the harmful effect into a controlled parameter that can be compensated.
Solution Approach 2:
The patent implements preliminary shimming by activating the shim coil arrangement before applying gradient pulses. The control computer calculates appropriate shim currents based on the upcoming gradient sequence and applies them in advance, so that when eddy currents are generated during gradient operation, their detrimental effects are already compensated for by the pre-established compensation fields.
2Measurement precision
If shim coil arrangement is activated to compensate eddy currents, then image quality is improved, but system complexity increases
Solution Approach 1:
The patent makes the shim coil arrangement multi-functional by using it for both its traditional purpose (compensating static magnetic field inhomogeneities) and a new purpose (compensating dynamic eddy current fields). This eliminates the need for separate compensation coils, thereby improving image quality without proportionally increasing device complexity.
Solution Approach 2:
The control computer implements a feedback mechanism by monitoring the gradient pulse sequence parameters and automatically calculating the appropriate shim currents needed to compensate for the resulting eddy currents. This closed-loop control optimizes the compensation effect while managing system complexity through automated calculation rather than manual adjustment.
3Ease of manufacture
If gradient coils with production tolerances are used, then manufacturing cost is reduced, but eddy current field deviations increase
Solution Approach 1:
The patent compensates for manufacturing tolerances by dynamically adjusting the shim currents based on the actual gradient pulse sequence being applied. The control computer calculates compensation parameters that account for deviations in conductor path shape, allowing the use of gradient coils with relaxed manufacturing tolerances while maintaining acceptable eddy current compensation performance.
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 enhances the quality of magnetic resonance image data by effectively compensating eddy currents, enabling the use of gradient coils with shape deviations without image loss, and allowing for improved image acquisition in larger reception volumes.
Implementation Method 1
a magnetic resonance scanner typically has an adjustable shim coil arrangement. Such a shim coil arrangement has electrical shim coils that are fed from an amplifier with different shim currents so as to generate different compensation magnetic fields in order to improve the homogeneity.
Implementation Method 2
The emission of specific gradient pulse shapes, defined by specifications of the magnetic resonance sequence, by the gradient coils of the scanner can result in unwanted eddy current fields in a reception volume of the magnetic resonance sequence. In this context, the operation of the gradient coils can cause eddy currents in all electrically conductive components of the magnetic resonance scanner.
Implementation Method 3
a subject to be examined, in particular the body of a patient, is exposed to a relatively strong basic magnetic field with the use of a basic field magnet, for example 1.5 or 3 or 7 tesla.
Implementation Method 4
A radio-frequency antenna also emits high-frequency radio-frequency pulses, for example excitation pulses, from suitable radiators that cause the nuclear spins of specific atoms excited to resonance by these radio-frequency pulses
Data Source
AI summary
In a method for actuating a shim coil arrangement of a magnetic resonance data acquisition scanner that has a shim controller that operates said shim coil arrangement, and a gradient coil arrangement operated by a gradient controller, the gradient controller determine a gradient pulse shape, in accordance with specifications of a magnetic resonance data acquisition sequence, that is activated by the gradient coil arrangement, when the sequence is executed in said scanner. A modifies the gradient pulse shape and provides the modified gradient pulse shape to the shim controller and the shim controller generates shim settings dependent on the modified gradient pulse shape, and actuates said shim coil arrangement according to the shim settings during activation of the gradient pulse shape by the gradient coil arrangement during execution of the sequence in said scanner.