MRI Water Fat Image Alignment via k-Space Phase Correction
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Solution Overview
Problem
Current MRI techniques face challenges in accurately reconstructing images of multiple spin species due to chemical shift artifacts, particularly when using alternating readout gradients, leading to blurring and double edges at tissue boundaries, especially at higher magnetic field strengths or reduced signal-to-noise ratios.
Innovation Solution
A correction step is implemented in the image reconstruction process by acquiring k-space data sets with one image in-phase and the other out-of-phase, using phase unwrapping algorithms to separate water and fat signals, and applying Fourier transformations to correct for chemical shift errors, allowing for accurate alignment and reconstruction of water and fat images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If alternating readout gradients are used to acquire multiple images, then image acquisition efficiency is improved, but chemical shift artifacts cause blurring and double edges at tissue boundaries
Solution Approach 1:
The patent applies preliminary phase correction to the k-space data before image reconstruction. By calculating the phase difference between images acquired with alternating gradient polarities and applying corrective phase shifts, the method pre-aligns the chemical shift artifacts before reconstruction, preventing blurring and double edges in the final images while maintaining the efficiency of alternating gradient acquisition
2Reliability
If higher magnetic field strengths are used, then signal-to-noise ratio is improved, but chemical shift artifacts are amplified causing greater misalignment
Solution Approach 1:
The patent changes the parameter being corrected from spatial position to phase angle. By transforming the alignment problem into a phase correction problem in k-space, the method can compensate for chemical shift artifacts that scale with magnetic field strength. The phase-based correction approach remains effective regardless of the field strength, allowing high-field imaging to maintain both high signal-to-noise ratio and accurate alignment
3Device complexity
If conventional image reconstruction is used without correction, then processing simplicity is maintained, but chemical shift errors cause image degradation
Solution Approach 1:
The patent introduces phase correction as an intermediary step between raw k-space data acquisition and final image reconstruction. This intermediate phase alignment process corrects chemical shift artifacts by calculating and applying phase differences between alternating gradient images, serving as a bridge that connects simple data acquisition with accurate image reconstruction without requiring complex system hardware changes
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 method effectively corrects for chemical shift errors, reducing artifacts and ensuring accurate separation and alignment of water and fat images, even at higher field strengths, thereby improving image quality and diagnostic accuracy.
Implementation Method 1
a uniform magnetic field B0 is applied to an imaged object along the z axis of a Cartesian coordinate system. The effect of the magnetic field B0 is to align the object's nuclear spins along the z axis
Implementation Method 2
the nuclei resonate at their Larmor frequencies according to the following equation: ω=γB0 where ω is the Larmor frequency
Implementation Method 3
The nuclei respond to RF pulses at this frequency by tipping their longitudinal magnetization into the transverse, x-y plane. Water, because of its relative abundance in biological tissue and the properties of its proton nuclei, is of principle concern in such imaging
Implementation Method 4
Because water and fat spins resonate at different frequencies, even when they are in the same location, their locations in the reconstructed image will be shifted with respect to each other. This is particularly problematic on the boundaries of tissues or organs where this chemical shift can cause blurring or multiple edges
Data Source
AI summary
A water and fat image are acquired using a pulse sequence in which NMR signals for one image data set are acquired with a readout gradient of one polarity and NMR signals for the other image data set are acquired with a readout gradient of the opposite polarity. A misalignment of fat signals caused by chemical shift is corrected by calculating separate water and fat image data sets in k-space and then transforming them to real space images.