Navigator-Based Correction for Simultaneous Multislice MR Imaging
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Solution Overview
Problem
Simultaneous multislice echo planar imaging (EPI) techniques face challenges in correcting for B0 drift and N/2 ghosting effects, as conventional methods are inadequate for slice-specific corrections and can lead to increased ghosting artifacts due to changes in navigator data conditions during imaging.
Innovation Solution
The method employs phase-encoded and non-phase-encoded navigator sequences to correct for B0 drift and N/2 ghosting effects by determining phase shifts between odd and even echoes, using dynamic off-resonance in k-space and slice-specific GRAPPA techniques, allowing for improved image reconstruction and reduced artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional non-simultaneous multislice EPI sequences are used with navigator echoes for correction, then B0 drift and N/2 ghosting can be corrected, but the imaging process is time-consuming and productivity is reduced
Solution Approach 1:
The patent divides the imaging process into simultaneous multislice acquisition with separate navigator echoes for each slice. Each slice is imaged independently with its own navigator correction, allowing parallel processing while maintaining correction accuracy. This segmentation enables faster imaging compared to sequential methods while preserving image quality through slice-specific corrections.
Solution Approach 2:
Navigator echoes are acquired immediately before each imaging readout sequence for each slice, providing real-time correction data before the actual imaging occurs. This preliminary acquisition of correction information ensures that B0 drift and N/2 ghosting are corrected for each slice without requiring post-processing time, thus maintaining both image quality and imaging speed.
2Measurement precision
If navigator echoes are acquired for each slice in simultaneous multislice EPI, then slice-specific corrections can be applied, but the complexity of the imaging sequence increases
Solution Approach 1:
The patent implements separate navigator echo acquisitions for each slice in the simultaneous multislice sequence. Each slice has its own navigator echoes acquired with specific phase encoding gradients, allowing independent correction of B0 drift and N/2 ghosting for each slice. This segmentation approach maintains correction accuracy while organizing the complex sequence into manageable slice-specific units.
Solution Approach 2:
The patent employs dynamic adjustment of phase encoding gradients for navigator echoes based on the slice position and imaging parameters. The navigator sequence is dynamically configured for each slice to optimize correction while minimizing added complexity. This dynamic approach allows the system to adapt to different imaging conditions without requiring a completely different sequence design.
3Measurement precision
If phase-encoded and non-phase-encoded navigator sequences are used, then both B0 drift and N/2 ghosting can be corrected, but the amount of navigator data increases
Solution Approach 1:
The patent divides navigator data acquisition into two separate types: phase-encoded navigators for N/2 ghosting correction and non-phase-encoded navigators for B0 drift correction. Each type of navigator is acquired selectively based on the specific correction needed for each slice. This segmentation reduces the total navigator data volume compared to acquiring all types for all slices, while maintaining comprehensive correction capability.
Solution Approach 2:
The patent applies different navigator types locally based on the specific correction requirements of each slice. Phase-encoded navigators are used where N/2 ghosting is problematic, while non-phase-encoded navigators are used for B0 drift correction. This localized approach ensures that correction capability is optimized for each slice's specific conditions without unnecessarily increasing the overall navigator data volume.
Data Source
AI summary
A magnetic resonance method and system are provided for providing improved simultaneous multislice echo planar imaging (EPI) with navigator-based correction of image data for B0 drift and N/2 ghosting. The correction is based on two types of multi-echo phase-encoded navigator sequences having opposite readout gradient polarities, and optionally also uses a non-phase-encoded navigator sequence. One or more navigator sequences can be generated between each RF excitation pulse and the subsequent EPI readout sequence. A dynamic off-resonance in k-space technique can be used to correct for B0 drift, and a modified slice GRAPPA technique that is based on odd and even kernels can provide slice-specific correction for N/2 ghosting effects for the EPI MR image data sets. Various patterns of navigator sequences and/or interpolation of navigator data can be used to improve accuracy of the image data corrections.