Interleaved Navigator Echoes for Multi-Slice Gradient Echo MRI
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current techniques for reducing artifacts in echo planar imaging (EPI) are inadequate, particularly in simultaneous multi-slice EPI, where phase errors and constant magnetic field drifts lead to N/2 ghosting artifacts and reduced image quality, especially due to limited access to navigator data and restricted temporal resolution.
Innovation Solution
An interleaved method is employed where MR measurement and navigator MR measurement sequences are executed, with phase-encoded gradient pulses to generate time-parallel and time-sequential gradient echoes, allowing for slice-specific data separation and modification using navigator data to correct phase errors and drifts, and employing partial parallel acquisition techniques for improved reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If EPI measurement sequence is used to improve imaging speed, then productivity is improved, but phase errors and N/2 ghosting artifacts occur due to alternating gradient pulse polarity
Solution Approach 1:
The patent introduces navigator echoes as an intermediary mechanism to measure and correct phase errors. These navigator echoes are acquired between the alternating polarity gradient echoes and provide reference data for phase correction, mediating between the fast EPI sequence and the need for phase accuracy
Solution Approach 2:
The patent implements a feedback mechanism where phase errors measured by navigator echoes are used to correct the phase of imaging data. The correction process continuously adjusts for drift and timing errors, creating a closed-loop system that maintains phase accuracy throughout the EPI sequence
2Productivity
If simultaneous multi-slice EPI is used to improve productivity, then imaging speed is improved, but access to navigator data is limited and temporal resolution is reduced
Solution Approach 1:
The patent segments the simultaneous multi-slice EPI sequence by inserting navigator echoes between alternating polarity gradient echoes within each slice acquisition. This segmentation allows navigator data to be acquired without interrupting the overall simultaneous multi-slice imaging process
Solution Approach 2:
The patent performs preliminary phase error measurement using navigator echoes acquired during the EPI sequence execution. By measuring phase errors in advance within the same sequence, the correction can be applied immediately to the imaging data without requiring separate measurement time
3Stability of the object's composition
If constant magnetic field is used to align nuclear magnetization, then imaging stability is improved, but temporal drift occurs due to heating and mechanical vibration
Solution Approach 1:
The patent implements feedback correction where phase drift measured by navigator echoes is used to adjust and correct the phase of imaging data. This continuous monitoring and correction compensates for magnetic field drift caused by heating and mechanical vibration
Solution Approach 2:
The patent converts the harmful effect of magnetic field drift into a measurable signal through navigator echoes. The drift-induced phase changes are captured by the navigator data and then used as correction information to eliminate the drift's negative impact on image quality
4Productivity
If gradient pulses with alternating polarity are used to encode spatial information, then productivity is improved, but N/2 ghosting artifacts occur due to phase offset between even and odd echoes
Solution Approach 1:
The patent introduces navigator echoes as an intermediary reference that measures the phase offset between even and odd gradient echoes. These navigator measurements mediate the correction process by providing the specific phase error values needed to align the alternating polarity echoes
Solution Approach 2:
The patent performs preliminary measurement of phase offsets using navigator echoes acquired during the EPI sequence. By measuring the phase difference between even and odd echoes in advance, the correction can be applied to eliminate N/2 ghosting artifacts before image reconstruction
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 N/2 ghosting artifacts and constant magnetic field drifts, enhancing image quality and temporal resolution by enabling precise phase modification and correction, even in challenging EPI applications like SMS EPI.
Implementation Method 1
The constant magnetic field aligns the nuclear magnetization of the examination object; in particular a polarization of the nuclear spin magnetization takes place in the direction of the constant magnetic field
Implementation Method 2
Radio-frequency (RF) pulses can then be radiated in order to deflect the nuclear magnetization from its rest position in the direction of the constant magnetic field, i.e. in order to excite the nuclear magnetization
Implementation Method 3
The gradient pulses create gradient magnetic fields (gradient fields) that are superimposed on the constant magnetic field
Implementation Method 4
The subsequent relaxation of the nuclear magnetization creates RF signals, so-called echoes
Data Source
AI summary
A number of repetitions of a magnetic resonance measurement sequence and a number of repetitions of a navigator magnetic resonance measurement sequence are executed in a interleaved manner. Each repetition of the magnetic resonance measurement sequence includes the time-parallel creation of gradient echoes for measurement of magnetic resonance data. Each repetition of the navigator magnetic resonance measurement sequence includes the radiating of RF excitation pulse, the activation of at least one gradient pulse train for time-sequential creation of gradient echoes, and the read out of the gradient echoes as navigator magnetic resonance data. The magnetic resonance data are modified based on the navigator magnetic resonance data. This enables an N/2 ghosting artifact and/or a constant magnetic field drift and/or a movement artifact to be reduced. Such techniques can be applied in conjunction with simultaneous multi-slice echo planar magnetic resonance imaging, SMS EPI. Diffusion-weighted magnetic resonance imaging also is possible.