MRI Nyquist Ghost Correction via 2D Phase Map
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Solution Overview
Problem
Existing methods for correcting Nyquist ghosts in MRI images, particularly in DWI and f-MRI, require additional reference imaging that prolongs procedure time and is susceptible to motion artifacts, reducing accuracy.
Innovation Solution
Calculating a 2D phase map using data from main imaging operations with low-order phase correction to maintain the original FOV, allowing for high-accuracy two-dimensional phase correction without the need for preliminary reference imaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If preliminary reference imaging is performed to calculate phase map for Nyquist ghost correction, then correction accuracy is improved, but imaging time is prolonged and motion artifacts increase
Solution Approach 1:
The patent merges the reference imaging and main imaging processes by calculating the phase map directly from the main imaging data itself, rather than requiring separate reference imaging. This is achieved by utilizing the odd and even echo data from the main EPI sequence to compute the phase correction map, thereby eliminating the time-consuming preliminary imaging step while maintaining correction accuracy.
Solution Approach 2:
The main imaging data serves dual purposes: it is used both for diagnostic image reconstruction and for calculating the phase correction map. This multi-functional use of the same data set eliminates the need for dedicated reference imaging, reducing total imaging time while preserving the ability to correct Nyquist ghosts effectively.
2Measurement precision
If FOV is doubled in phase encode direction to acquire reference image without aliasing, then phase map calculation accuracy is improved, but imaging time is further prolonged
Solution Approach 1:
The patent changes the approach from spatial parameter modification (doubling FOV) to computational parameter optimization (low-order phase correction algorithms). By applying low-order phase correction techniques to the original FOV data, the method achieves accurate phase map calculation without the time penalty of acquiring reference images at doubled FOV.
3Object-generated harmful factors
If complex addition is applied to DWI imaging for Nyquist ghost reduction, then ghost artifact is reduced, but signal loss and image deterioration occur
Solution Approach 1:
The patent applies preliminary phase correction to the DWI imaging data before performing complex addition. By pre-correcting the phase errors using the calculated phase map, the subsequent complex addition operation can proceed without causing signal loss or image deterioration, thereby reducing Nyquist ghosts while preserving DWI image quality and reliability.
4Object-generated harmful factors
If xky correction method is used for Nyquist ghost reduction, then most Nyquist ghosts are reduced, but high-order errors and phase encode direction errors remain uncorrected
Solution Approach 1:
The patent extends the correction approach from one-dimensional xky-space correction to two-dimensional phase space correction. By calculating and applying a 2D phase map that accounts for errors in both the readout and phase encode directions, the method achieves more complete correction of Nyquist ghosts, including high-order errors and phase encode direction errors that remain uncorrected by conventional xky correction alone.
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 enables high-speed imaging with reduced Nyquist ghosts in DWI and f-MRI, minimizing the impact of body motion and shortening the time for phase map acquisition and correction, thus improving image quality and reliability.
Implementation Method 1
a measurement unit for acquiring a plurality of pieces of data for image by using an EPI method
Implementation Method 2
reversing the applied polarity of the readout gradient magnetic field between the odd-number-th imaging and the even-number-th imaging and performing complex addition of the acquired signals on k-space
Implementation Method 3
a correction unit for correcting a Nyquist ghost included in the data for image with use of the phase map calculated by the phase map calculation unit
Data Source
AI summary
The present invention is directed to enabling high-accuracy Nyquist ghost correction without using a reference image.After at least one of a plurality of images for use in diagnosis is used to perform low-order phase correction without causing aliasing of an image, a 2D phase map including remaining high-order phase errors and phase errors in a phase encode direction is calculated. The low-order phase correction is performed on a pair of pieces of data for image obtained by inverting a readout gradient magnetic field as image data for use in 2D phase map calculation, and positive-polarity/negative-polarity errors of the readout gradient magnetic field are calculated with odd lines and even lines of the pair of pieces of data for image rearranged. In the case of DWI imaging, an image with b-value=0 can be used for 2D phase map calculation.