MRI Apparatus Frequency Adjustment for Non-Uniform Field Correction
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Solution Overview
Problem
In magnetic resonance imaging (MRI), two-dimensional selective excitation methods face challenges due to non-uniform static magnetic fields, leading to deviations in resonance frequencies, which cause deformation of excitation profiles and result in poor image quality.
Innovation Solution
Measuring non-uniformity in the static magnetic field within the selective excitation region and adjusting the irradiation frequency accordingly, using a shim gradient magnetic field to correct for these deviations, ensures that the irradiation frequency matches the resonance frequency, maintaining the desired excitation profile.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a two-dimensional selective excitation method (SS method) is used to restrict excitation to a specific region, then signal from outside the region is suppressed and image quality is improved, but the excitation profile deforms due to resonance frequency deviation caused by non-uniform static magnetic field
Solution Approach 1:
The patent applies a frequency shift to the RF irradiation based on the measured resonance frequency distribution in the target region. By changing the irradiation frequency parameter to match the actual resonance frequency (center frequency) of the region of interest, the excitation profile maintains its desired shape and accuracy despite magnetic field non-uniformity. This parameter adjustment resolves the contradiction by adapting the excitation frequency to the specific regional characteristics.
Solution Approach 2:
The patent performs a prescan measurement of the resonance frequency distribution in the target region before applying the two-dimensional selective excitation. This preliminary action allows the system to determine the actual resonance frequency (center frequency) of the region, which is then used to adjust the RF irradiation frequency. By performing this measurement and adjustment in advance, the system ensures accurate excitation profile without deformation during the actual imaging process.
2Productivity
If the irradiation frequency is determined based on prescan from the entire imaging region, then the imaging process is efficient, but a deviation of tens to 100 Hz occurs locally between the set irradiation frequency and the resonance frequency due to dispersion
Solution Approach 1:
The patent divides the imaging process into two stages: a prescan stage that covers the entire imaging region for efficiency, and a target region analysis stage that focuses on the specific region of interest for frequency precision. By segmenting the measurement scope, the system maintains overall imaging efficiency while achieving high frequency accuracy in the relevant region. The prescan provides a baseline, and the targeted frequency measurement refines the parameter for the actual excitation.
Solution Approach 2:
The patent applies different frequency determination strategies to different regions: a general prescan frequency for the entire imaging region and a localized center frequency for the specific target region. By prioritizing local quality (frequency accuracy) in the region of interest while maintaining overall efficiency through the prescan, the system resolves the contradiction between productivity and measurement precision. The local frequency measurement ensures accurate excitation where it matters most.
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-quality imaging even with non-uniform static magnetic fields by accurately matching the irradiation frequency with the resonance frequency, preventing unwanted excitation and improving image fidelity.
Implementation Method 1
resonance frequency of nuclear magnetization obtained from signals collected by a scan performed before the main imaging
Implementation Method 2
an irradiation frequency of an RF used for main imaging is usually determined on the basis of a resonance frequency of nuclear magnetization
Implementation Method 3
an oscillating gradient magnetic field
Data Source
AI summary
A high-quality image is obtained using a two-dimensional selective excitation method even if the static magnetic field is not uniform. Therefore, non-uniformity of a static magnetic field of a region to be focused in particular in a selective excitation region excited by 2DRF is measured, and a result of the measurement is reflected in an imaging sequence using the 2DRF. For example, a resonance frequency of magnetization obtained from the measurement result is set as an irradiation frequency of the 2DRF. In addition, a shim gradient magnetic field is applied so as to correct the non-uniformity of the magnetization obtained from the measurement result. These are applied only in the imaging sequence using the 2DRF, and an irradiation frequency and a shim gradient magnetic field set in a conventional method are used in other imaging sequences.