B1 Field Mapping in High-Field MRI Systems
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Solution Overview
Problem
Non-uniformity of the excitation magnetic field B1 in high-field MRI systems leads to inconsistencies between actual and nominal flip angles, affecting the quality and accuracy of magnetic resonance images, which complicates diagnosis.
Innovation Solution
The BLOCH-SIEGERT method is used to establish a scaling factor for the excitation magnetic field, combined with a two-flip angle method to obtain and correct the spatial distribution of longitudinal relaxation times, employing a multi-shot SE-EPI sequence with an off-resonance RF pulse to accurately map the excitation magnetic field and reduce scan time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-field MRI system is used to improve image resolution and signal-to-noise ratio, then image quality is improved, but non-uniformity of the excitation magnetic field B1 increases causing inconsistency between actual and nominal flip angles
Solution Approach 1:
The patent applies preliminary action by performing a B1 field mapping measurement before the actual diagnostic imaging. The B1 map is acquired using a dedicated calibration sequence, and the resulting scaling factors are stored for use in correcting subsequent imaging sequences. This pre-characterization of the excitation field allows the system to compensate for non-uniformities without requiring real-time adjustments during patient imaging.
Solution Approach 2:
The patent employs parameter changes by introducing spatially varying scaling factors that adjust the nominal flip angles based on the measured B1 field distribution. The correction process transforms the uniform nominal flip angle parameter into a position-dependent effective flip angle parameter, thereby compensating for the non-uniform excitation field while maintaining the desired imaging contrast and quantitative accuracy.
2Manufacturing precision
If a B1 mapping method is implemented to correct excitation magnetic field non-uniformity, then flip angle accuracy is improved, but scan time increases
Solution Approach 1:
The patent applies partial action by implementing B1 mapping only in regions where quantitative accuracy is critical, such as for T1 relaxation time measurement, rather than correcting the entire image dataset. The method selectively applies correction only to the specific imaging sequences and anatomical regions where flip angle accuracy directly impacts diagnostic quality, thereby minimizing the additional time burden while maintaining accuracy where needed.
3Measurement precision
If the BLOCH-SIEGERT method with scaling factor is used to establish excitation magnetic field distribution, then spatial distribution accuracy is improved, but computational complexity increases
Solution Approach 1:
The patent applies mechanics substitution by replacing complex iterative numerical optimization methods with a direct analytical solution based on the BLOCH-SIEGERT equations. Instead of using computationally intensive gradient descent or other iterative algorithms to fit the B1 field distribution, the method uses closed-form mathematical relationships that directly calculate the scaling factors from the measured signal intensities at two different flip angles, significantly reducing computational burden while maintaining high spatial accuracy.
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 accurate spatial distribution of the excitation magnetic field, reducing scan time while maintaining image resolution and signal-to-noise ratio, thereby improving the accuracy of MRI images and correcting image intensity variations caused by magnetic field non-uniformity.
Implementation Method 1
the phase shift method, which uses an off-resonance RF pulse immediately following an excitation pulse of an imaging sequence
Implementation Method 2
Using a BLOCH-SIEGERT method to establish a scaling factor of an excitation magnetic field that excites nuclei in a subject
Implementation Method 3
a method involving two flip angles is implemented to obtain a spatial distribution of longitudinal relaxation times of the excited nuclei
Data Source
AI summary
In an MRI method and apparatus a BLOCH-SIEGERT method is used to establish a scaling factor of an excitation magnetic field that excites nuclei in a subject. A method involving two flip angles is executed to obtain a spatial distribution of longitudinal relaxation times of the excited nuclei so as to reconstruct a longitudinal relaxation time spatial distribution image, wherein the scaling factor is used to correct the two flip angles. This MRI method and apparatus cannot only obtain the spatial distribution of an excitation magnetic field accurately, but also can greatly reduce scan time, while ensuring a similar image resolution and image signal-to-noise ratio.