Variable Flip Angle Schedules for MRI SAR and Signal Optimization
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face challenges in determining a target flip angle schedule that balances signal-to-noise ratio and contrast ratio while minimizing specific absorption rate (SAR) and avoiding deterioration in image quality, particularly with multiple refocusing RF pulses having variable flip angles.
Innovation Solution
A method involving an iterative process on an MR imaging system using a computing device to adjust an initial flip angle schedule based on multiple criteria related to echo train parameters, such as intensity and total energy, to determine a target flip angle schedule that optimizes MR signal acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If multiple refocusing RF pulses with the same large flip angle are used, then the echo train signal intensity is maintained, but the specific absorption rate (SAR) increases
Solution Approach 1:
The patent applies variable flip angle schedules where the flip angles of refocusing RF pulses are dynamically adjusted rather than remaining constant. This allows the system to maintain adequate signal intensity while reducing the cumulative energy deposition and SAR by varying the flip angles across different pulses in the echo train.
Solution Approach 2:
The patent changes the flip angle parameter across multiple refocusing RF pulses according to specific schedules (e.g., decreasing, increasing, or non-monotonic patterns). This parameter variation enables optimization of both signal intensity and SAR by distributing energy more efficiently across the pulse sequence.
2Use of energy by moving object
If refocusing RF pulses with variable flip angles are used to decrease SAR, then the specific absorption rate is reduced, but the signal-to-noise ratio and contrast ratio deteriorate
Solution Approach 1:
The patent employs dynamic flip angle adjustment schedules that are specifically designed to maintain signal-to-noise ratio and contrast ratio while reducing SAR. The variable flip angles are optimized to preserve the central k-space signal quality (which determines contrast and SNR) while allowing lower angles for peripheral pulses that contribute less to image quality.
Solution Approach 2:
The patent applies different flip angle strategies to different portions of the echo train based on their relative importance to image quality. Central echoes that contribute most to contrast and SNR receive flip angles optimized for signal preservation, while peripheral echoes receive angles optimized for SAR reduction, achieving local optimization of both quality metrics and energy deposition.
3Measurement precision
If an iterative process with multiple criteria is implemented to determine the target flip angle schedule, then the image quality and SAR are optimized, but the computational complexity increases
Solution Approach 1:
The patent performs preliminary computational work offline to pre-calculate and store optimal flip angle schedules for various imaging conditions and tissue types. During actual MRI scanning, the system simply retrieves and applies the pre-determined schedule, significantly reducing real-time computational complexity while maintaining optimization benefits.
Solution Approach 2:
The patent implements an iterative optimization process that uses feedback from simulated or measured echo train characteristics to refine the flip angle schedule. The system evaluates multiple candidate schedules against criteria including SAR, signal intensity, and contrast, then selects the optimal schedule, creating a feedback loop that converges on the best solution.
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
The method effectively enhances signal-to-noise ratio and contrast ratio while reducing SAR, as demonstrated by the comparison of MR images generated using fixed and variable flip angle schedules, showing improved image quality with reduced energy consumption.
Implementation Method 1
Magnetic resonance imaging (MRI) systems are widely used to diagnose and treat medical conditions by exploiting a powerful magnetic field and radio frequency (RF) techniques
Implementation Method 2
The radio frequency (RF) techniques make use of an excitation RF pulse and multiple refocusing RF pulses with specific flip angles to obtain MR signals
Data Source
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AI summary
A method may include providing an initial flip angle schedule of refocusing radio frequency pulses, the refocusing radio frequency pulses being configured to generate an echo train; comparing the initial flip angle schedule with a first criterion, the first criterion relating to a first parameter relating to the echo train; determining, a first flip angle schedule based on the first comparison, the first flip angle schedule satisfying the first criterion; comparing the first flip angle schedule with a second criterion, the second criterion relating to a second parameter relating to the echo train; determining, a second flip angle schedule based on the second comparison, the second flip angle schedule satisfying the second criterion; and obtaining a magnetic resonance (MR) signal based on the second flip angle schedule.