MRI Navigator Echo Phase Correction for Frequency Misalignment
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Solution Overview
Problem
Current MRI techniques face challenges in accurately correcting misalignment due to center frequency variations during diffusion-weighted imaging, particularly when using navigator echoes, as they are affected by phase offsets and prolonged echo times, leading to image artifacts and blurring, especially in high-axis imaging and low magnetic field conditions.
Innovation Solution
The MRI apparatus calculates the phase change of navigator echoes within a predetermined measurement time, allowing for accurate determination of phase change relative to a reference, thereby enabling precise correction of misalignment without extending echo times or being affected by phase offsets, using a navigation controller and analyzer to analyze phase changes and calculate correction values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the echo time (TE) of the navigator echo is made long to reduce phase offset error, then measurement precision is improved, but the imaging time is elongated and T2 shine through effect increases
Solution Approach 1:
The patent changes the parameter of TE from long to short, accepting increased phase offset error in exchange for reduced imaging time and T2 shine through effect. The phase offset error is then corrected through computational methods rather than physical parameter adjustment.
Solution Approach 2:
The patent acquires a reference navigator echo before MPG pulses to establish a reference phase, then compares subsequent navigator echoes against this reference to calculate frequency variation, thereby eliminating the need for long TE to reduce phase offset.
2Reliability
If frequency measurement is performed in the middle of imaging to cope with abrupt changes, then reliability is improved, but imaging time is elongated
Solution Approach 1:
The patent acquires the reference navigator echo before the main imaging sequence begins, preparing the frequency reference in advance. This allows frequency variation to be tracked during imaging without interrupting or extending the imaging time.
Solution Approach 2:
The patent continuously monitors frequency variation by acquiring navigator echoes throughout the imaging sequence, maintaining continuous frequency correction capability without interrupting the main imaging process, thereby preserving both reliability and time efficiency.
3Measurement precision
If navigator echo is acquired before MPG pulses to avoid phase difference, then measurement precision is improved, but the phase offset error increases due to eddy current variation
Solution Approach 1:
The patent uses the reference navigator echo acquired before MPG pulses as a baseline, then continuously compares subsequent navigator echoes against this reference. This feedback mechanism allows the system to track and correct frequency variation while accounting for phase offset changes caused by eddy currents.
Solution Approach 2:
The patent transitions from a static frequency correction approach (single reference measurement) to a dynamic approach where frequency variation is continuously tracked by comparing multiple navigator echoes against the reference, allowing adaptation to changing conditions during imaging.
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 allows for high-accuracy frequency correction during imaging, reducing misalignment and improving image quality in diffusion-weighted imaging techniques like DTI, DKI, and IVIM, while preventing the elongation of imaging time and minimizing artifacts.
Implementation Method 1
a static magnetic field generator configured to generate a static magnetic field in a space where a subject is placed
Implementation Method 2
a transmitter configured to apply an RF magnetic field to the subject
Implementation Method 3
a receiver configured to receive a nuclear magnetic resonance signal generated by nuclear magnetic resonance from the subject
Implementation Method 4
a navigation analyzer configured to analyze phase change of the navigator echo during the measurement time to calculate a correction value for correcting misalignment due to the phase change
Data Source
AI summary
A navigator echo is acquired during imaging, and when frequency is corrected based on phase change, the correction is performed with high accuracy without being affected by an offset caused by variations with time. An MRI apparatus including a navigation controller is configured to control an imaging unit acquiring an NMR signal, generate the navigator echo and collect navigation data during a predetermined measurement time, prior to collection of nuclear magnetic resonance signals for reconstructing an image of a subject. The phase change of the navigator echo is analyzed during the measurement time to calculate a correction value for correcting misalignment due to the phase change with a navigation analyzer that calculates a phase change amount relative to a reference, based on a difference between the phase change of the navigator echo and the phase change of the navigator echo serving as the reference during the measurement time.