MRI Gradient Pre-emphasis for Eddy Current Compensation
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Solution Overview
Problem
In Steady State Free Precession (SSFP) measurement sequences, deviations in magnetic gradients from their expected form disrupt the steady state, leading to artifacts in magnetic resonance imaging due to eddy currents and transmission characteristics, affecting the accuracy of magnetic resonance image datasets.
Innovation Solution
A method is introduced to compensate for magnetic field changes using correction terms derived from the Gradient System Transfer Function (GSTF) and Gradient Impulse Response Function (GIRF), which are used to pre-emphasize gradients and create a compensation field to maintain the steady state of magnetization, thereby reducing artifacts in magnetic resonance imaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If gradient pulses are applied to create magnetic field gradients for spatial encoding, then spatial selection and encoding are achieved, but eddy currents cause deviations from nominal gradients leading to artifacts
Solution Approach 1:
The patent applies preliminary action by pre-emphasizing the gradient pulses before they are applied. The control facility modifies the gradient pulse waveform in advance based on pre-emphasis filter values, which are determined from the gradient system's transfer characteristics. This preliminary modification compensates for the expected eddy current effects, ensuring that the actual gradient closely matches the nominal gradient without requiring post-correction or additional hardware.
Solution Approach 2:
The patent implements feedback through the use of transfer characteristics and pre-emphasis filters. The system characterizes the gradient system's response (including eddy current effects) and uses this knowledge to adjust the input gradient pulses. The pre-emphasis filter values are derived from measuring the relationship between applied and actual gradients, creating a closed-loop correction system that continuously ensures accurate gradient delivery.
2Measurement precision
If pre-emphasis filters are used to compensate for eddy currents, then gradient accuracy is improved, but the system complexity increases
Solution Approach 1:
The patent replaces complex hardware corrections with software-based signal processing. Instead of adding physical components or complex circuitry to compensate for eddy currents, the system uses digital signal processing through pre-emphasis filters. The control facility applies mathematical corrections to the gradient pulse waveforms, substituting mechanical/electrical complexity with computational simplicity.
Solution Approach 2:
The patent changes the parameters of the gradient pulse waveforms through pre-emphasis filtering. By modifying the temporal characteristics of the gradient pulses (amplitude, timing, shape) based on the system's transfer characteristics, the system achieves accurate gradient delivery without changing the physical hardware. This parameter-based correction is more straightforward than hardware modifications.
3Adaptability or versatility
If multiple gradient coils are activated to create imaging gradients in different directions, then imaging coverage is improved, but deviations from nominal gradients increase due to different transmission characteristics
Solution Approach 1:
The patent applies local quality by tailoring the pre-emphasis correction to each gradient coil and direction individually. Each gradient coil has its own transfer characteristics and eddy current behavior, so the system applies direction-specific pre-emphasis filter values. This ensures that each gradient pulse is optimized for its specific spatial direction, maintaining high accuracy across all imaging directions despite the different transmission characteristics of each coil.
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
The method effectively compensates for gradient errors by applying correction terms in each sequence block, ensuring accurate imaging by maintaining the steady state of magnetization and reducing image artifacts, applicable to various magnetic resonance imaging sequences including SSFP, gradient echo, and spin echo sequences.
Implementation Method 1
A magnetic resonance installation typically has a gradient coil consisting of three separate coils, which create gradients in the x, y, and z direction
Implementation Method 2
A basic magnetic field B0 is employed in order to obtain a surplus of magnetization
Implementation Method 3
This may be flipped from the rest position by a radio frequency pulse B1
Implementation Method 4
Between the excitation pulses, the nuclear spins precess unhindered (e.g., Free Precession)
Implementation Method 5
In order to compensate for the effects of eddy currents, it is known, for example, to measure out occurrence of the eddy currents as a function of the actually desired nominal gradients
Data Source
AI summary
A method for recording a magnetic resonance image dataset includes providing a magnetic resonance sequence with a series of sequence blocks, and providing at least one correction term to compensate for a magnetic field change. The magnetic field change is produced as a change of an actual magnetic field compared to a setpoint magnetic field by gradient pulses. The magnetic field change is established via a transfer characteristic of the gradient system of the magnetic resonance installation. The at least one correction term is used to compensate for the magnetic field change, and at least one magnetic resonance image dataset is recorded with the magnetic resonance sequence using the correction term.