MR Gradient Sequences with Decay Duration for Eddy Current Artifact Reduction
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Solution Overview
Problem
Eddy currents induced by gradient switching operations in magnetic resonance tomography (MRT) systems lead to distortions and artifacts in imaging, particularly in high-gradient applications like diffusion-weighted imaging and Ultra Short Echo Time (UTE) sequences, due to the rapid changes in magnetic fields.
Innovation Solution
A method is introduced where a decay duration is inserted after the ramp portion of an MR sequence, allowing eddy currents to decay before applying the excitation pulse, with the decay duration determined by an algorithm based on system parameters to minimize artifacts, and the repetition duration can be extended or ramp duration shortened to accommodate this decay time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If gradient field strength is increased to enable high-resolution imaging and diffusion-weighted imaging, then imaging resolution and application capability are improved, but eddy current artifacts increase causing image distortion
Solution Approach 1:
The patent applies preliminary action by inserting a decay duration between the gradient ramp and the excitation pulse. This allows eddy currents to decay before the actual imaging process begins, preventing artifacts while maintaining high gradient strengths for improved resolution and diffusion-weighted imaging capability.
Solution Approach 2:
The patent segments the gradient sequence into distinct portions: a ramp portion for gradient switching, a decay portion for eddy current dissipation, and a constant portion for imaging. This segmentation allows each portion to be optimized independently, resolving the contradiction between high gradient strength and artifact reduction.
2Speed
If gradient switching speed is increased to reduce readout time and minimize T2* decay, then imaging speed and T2* preservation are improved, but eddy current generation increases causing image artifacts
Solution Approach 1:
The patent uses preliminary action by allowing eddy currents to decay during a dedicated decay duration before the excitation pulse. This prevents artifacts from rapid gradient switching while maintaining high readout speeds and minimal T2* decay during the actual imaging process.
Solution Approach 2:
The patent applies local quality by having different gradient characteristics in different time portions: rapid switching during the ramp portion for speed, followed by a decay period for artifact reduction, then a stable constant portion for high-quality imaging. Each portion has optimized properties for its specific function.
3Object-affected harmful factors
If decay duration is extended to allow complete eddy current dissipation, then artifact reduction is improved, but total sequence duration increases reducing productivity
Solution Approach 1:
The patent applies partial action by implementing a decay duration that is sufficient to reduce eddy current artifacts to acceptable levels, rather than waiting for complete dissipation. This partial decay approach achieves adequate artifact reduction while minimizing the extension of total sequence duration and maintaining imaging productivity.
Solution Approach 2:
By performing the decay during a dedicated preliminary period before the actual imaging, the patent allows artifact reduction to occur without interfering with the main imaging process. This maintains productivity by keeping the decay portion separate and optimized for its specific purpose.
4Object-affected harmful factors
If repetition duration is extended to accommodate decay time, then eddy current decay is improved, but time efficiency decreases
Solution Approach 1:
The patent uses partial action by extending the repetition duration only as much as necessary to achieve adequate eddy current decay, rather than using excessive time. This balances artifact reduction with time efficiency, minimizing the loss of imaging time while still achieving sufficient decay to prevent artifacts.
Solution Approach 2:
The patent optimizes the repetition duration parameter to find the optimal balance between decay time and imaging speed. By carefully tuning this parameter, the system achieves adequate artifact reduction while minimizing the time penalty, maintaining overall 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 reduces or prevents artifacts caused by eddy currents, allowing for higher resolution imaging with strong gradient strengths by ensuring eddy currents have decayed before image acquisition, thereby maintaining image quality and enabling the use of high-gradient applications without distortion.
Implementation Method 1
Changes in the magnetic field can produce eddy currents in conductive structures. This is also the case, in particular, in metal structures of an MRT apparatus. Thus, for example, eddy currents can arise at the flanks of gradient switching operations.
Implementation Method 2
According to Lenz's law, eddy currents generate magnetic fields that counteract the cause, in this case, the gradient.
Implementation Method 3
a decay duration is inserted after the ramp portion of an MR sequence, allowing eddy currents to decay before applying the excitation pulse
Data Source
AI summary
An MR sequence with a repetition in which a constant portion of the gradient strength adjoins a ramp portion. In the constant portion, an HF (high frequency) pulse is carried out. At the beginning of the constant portion, a specifiable decay duration is waited until the excitation pulse is carried out. During the decay duration, eddy current effects decay away.