Phase Correction for Echo-Planar Imaging Nyquist Ghosts
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Solution Overview
Problem
Existing MR imaging techniques, such as echo-planar imaging (EPI), face challenges in correcting Nyquist ghosts due to inconsistencies between odd and even lines of k-space, particularly in scenarios like diffusion EPI, where synthesized k-space frames often include artifacts, making it difficult to achieve accurate ghost correction.
Innovation Solution
The method involves converting EPI reference frames with opposite readout polarities to image-space, identifying and aligning corresponding k-space pixels, determining a phase difference relationship, and applying phase corrections to align the frames, which are then converted back to k-space for effective ghost correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If rapid gradient switching is used to enable high-speed EPI acquisition, then image acquisition speed is improved, but inconsistencies between odd and even lines of k-space occur resulting in Nyquist ghosts
Solution Approach 1:
The patent applies preliminary phase correction to the reference EPI frames before using them for ghost correction. By pre-aligning the phase of odd and even k-space lines in the reference frames through iterative optimization, the system ensures that the reference data used for ghost correction is itself free from Nyquist ghost artifacts, enabling accurate correction of the main EPI image
2Reliability
If two reference EPI scans with opposite readout polarities are used for Nyquist ghost correction, then ghost correction capability is improved, but artifacts in synthesized k-space frames reduce correction accuracy
Solution Approach 1:
The patent employs an iterative self-correction mechanism where the reference EPI frames are repeatedly refined through phase correction using their own odd and even k-space line inconsistencies. The system uses the reference frames themselves to generate and apply phase correction factors, progressively eliminating artifacts within the reference data until convergence is achieved, making the reference frames artifact-free for subsequent ghost correction
Solution Approach 2:
The patent implements a feedback loop where phase correction factors are continuously refined by comparing odd and even k-space lines, applying corrections, and re-evaluating the reference frames. This iterative feedback process adjusts the phase correction parameters based on the measured inconsistencies, progressively reducing artifacts in the synthesized k-space frames until the reference data achieves sufficient quality for accurate ghost correction
Data Source
AI summary
Systems and methods include conversion of a first frame of k-space data acquired using a first initial readout polarity to first hybrid (kx, y)-space data, conversion of a second frame of k-space data acquired using a second initial readout polarity to second hybrid (kx, y)-space data, determination of a relationship between phase difference and y-position based on phase differences between a plurality of pixels located at kx=a of first hybrid (kx, y)-space data and a plurality of pixels at kx=b of second hybrid (kx, y)-space data, where a and b are constants, modification of the second hybrid (kx, y)-space data based on the relationship, conversion of the modified second hybrid (kx, y)-space data to a modified second frame of k-space data, generation of two single-polarity readout k-space frames based on the first frame of k-space data and the modified second frame of k-space data, and correction of a third frame of EPI image data based on the two single-readout polarity k-space frames.