MRI RF Phase Adjustment for Static Magnetic Field Inhomogeneity
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Solution Overview
Problem
In MRI imaging, especially with ultra-high magnetic fields, static magnetic field inhomogeneity leads to image quality deterioration, particularly in regions like the heart, where local Bo shimming and excitation frequency adjustments are challenging to implement effectively, especially when imaging multiple regions simultaneously.
Innovation Solution
The method calculates shimming current to reduce static magnetic field inhomogeneity in selected regions and adjusts the RF pulse irradiation phase or excitation frequency to minimize artifacts, using either phase control or frequency control based on the imaging conditions to maintain image quality without increasing operator workload.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If local Bo shimming is applied to reduce static magnetic field inhomogeneity in a selected region, then static magnetic field homogeneity in the selected region is improved, but band artifacts in SSFP sequence still overlap with the selected region
Solution Approach 1:
The invention changes the excitation frequency parameter of the RF pulse to shift the position of band artifacts. By calculating an increment of excitation frequency based on the relationship between the selected region position and band artifact positions, the band artifacts are shifted away from overlapping with the selected region, thereby resolving the contradiction between improving magnetic field homogeneity and eliminating band artifact interference
Solution Approach 2:
The invention introduces an intermediary calculation step that determines the excitation frequency increment based on the spatial relationship between the selected region and band artifacts. This intermediary parameter (frequency increment) mediates between the shimming process and the final imaging, allowing band artifacts to be shifted without directly modifying the shimming current or the anatomical position
2Object-affected harmful factors
If excitation frequency is shifted to move band artifacts away from the selected region, then band artifact overlap is reduced, but static magnetic field inhomogeneity effects increase
Solution Approach 1:
The invention performs preliminary local Bo shimming to reduce static magnetic field inhomogeneity in the selected region before applying the excitation frequency shift. This preliminary action ensures that the magnetic field is as homogeneous as possible, minimizing the negative effects that would otherwise be exacerbated by the frequency shift
3Productivity
If multiple regions are imaged using the same local Bo shimming and excitation frequency, then imaging efficiency is improved, but image quality deteriorates due to static magnetic field inhomogeneity in each region
Solution Approach 1:
The invention segments the imaging process by calculating separate excitation frequency increments for each selected region based on their respective positions and band artifact patterns. Instead of using a single global frequency shift, each region receives a customized frequency adjustment, ensuring that band artifacts do not overlap with any selected region while maintaining high imaging efficiency through automated calculation
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 effectively reduces image quality deterioration due to static magnetic field inhomogeneity across various imaging conditions, ensuring consistent and improved MRI image quality without complicating the operator's tasks.
Implementation Method 1
a compensation magnetic field that reduces static magnetic field inhomogeneity in an imaging target region using both the data is generated in a shim coil
Implementation Method 2
A MRI apparatus measures an NMR signal generated by a nuclear spin that comprises tissues of an object
Implementation Method 3
a phase encode that varies depending on the gradient magnetic field as well as frequency-encoding are provided to an NMR signal
Data Source
AI summary
In order to reduce image quality deterioration due to static magnetic field inhomogeneity according to imaging conditions without increasing an operator workload, shimming current where static magnetic field inhomogeneity of a selected region is reduced is calculated, shimming is performed for the selected region using the calculated local Bo shimming current, and then an increment (RF-Phase) in an irradiation phase of an RF pulse that excites the selected region in a state where static magnetic field inhomogeneity of the selected region is reduced or a post-adjustment excitation frequency (f0′) that is an excitation frequency is calculated. These increments (RF-Phase) in an irradiation phase and post-adjustment excitation frequency (f0′) that is an excitation frequency correspond with each other in amount.