MRI Flip Angle Scheduling for Lower SAR and Fewer Artifacts
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Solution Overview
Problem
Existing magnetic resonance imaging (MRI) techniques face issues with power deposition and image artifacts due to high-frequency RF pulses, leading to tissue damage and blurry images, particularly in echo train pulse sequences with fixed flip angles.
Innovation Solution
A system and method for determining an optimized flip angle schedule using variable flip angles to reduce power deposition and improve image quality by calculating a flip angle schedule based on specific criteria, such as T1 and T2 relaxation times, to generate MR signals effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fixed flip angles (e.g., 180° refocusing pulses) are used in echo train pulse sequences, then the spin echo effect can be generated effectively, but power deposition increases causing tissue damage and specific absorption rate (SAR) limitations
Solution Approach 1:
The patent applies variable flip angles that dynamically change throughout the echo train rather than using fixed angles. The flip angle for each refocusing pulse is calculated based on the desired signal evolution and T2 decay characteristics, allowing the system to adapt the RF pulse strength to minimize SAR while maintaining image quality and spin echo generation.
Solution Approach 2:
The patent changes the flip angle parameter across different pulses in the echo train. By calculating optimal flip angles based on T2 relaxation times and desired signal characteristics, the system modifies this critical parameter to reduce power deposition while preserving the spin echo effect and image quality.
2Manufacturing precision
If high power RF pulses are used to maintain signal quality, then image quality can be preserved, but specific absorption rate (SAR) increases causing safety concerns and tissue heating
Solution Approach 1:
The patent calculates and applies optimized flip angles for each refocusing pulse based on T2 decay characteristics and desired signal evolution. This parameter optimization allows the system to use lower RF pulse powers while maintaining adequate signal quality, thereby reducing SAR and associated safety concerns.
Solution Approach 2:
The patent enables faster image acquisition by using variable flip angles that maintain signal quality with reduced RF power. This allows the system to complete scans more quickly or use lower power levels, reducing the time patients are exposed to RF energy and thereby reducing cumulative SAR.
3Object-affected harmful factors
If variable flip angles are used to reduce power deposition, then SAR decreases improving safety, but image acquisition time may increase
Solution Approach 1:
The patent calculates flip angles that optimize both SAR reduction and signal quality maintenance. By using T2-weighted signal evolution models, the system determines flip angle sequences that preserve adequate signal strength for image reconstruction while minimizing RF power deposition, achieving both safety and efficiency goals.
Solution Approach 2:
The patent employs a systematic approach where flip angles are periodically adjusted according to calculated optimal values based on T2 decay and signal evolution models. This structured variation in flip angles maintains signal quality throughout the echo train while reducing overall RF power deposition compared to fixed high-angle pulses.
4Ease of manufacture
If fixed flip angles are used in echo train sequences, then the pulse sequence is simple to implement, but image artifacts increase and image quality deteriorates
Solution Approach 1:
The patent implements variable flip angles that adapt to the T2 decay characteristics of the tissue being imaged. This dynamic adjustment compensates for signal loss due to T2 decay throughout the echo train, maintaining more uniform signal quality across all echoes and reducing artifacts that would otherwise appear with fixed flip angles.
Solution Approach 2:
The patent modifies the flip angle parameter across the echo train based on T2 relaxation characteristics. This parameter variation compensates for the natural signal decay in spin echo sequences, maintaining adequate signal intensity for later echoes in the train and thereby improving overall image quality and reducing artifacts.
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 optimized flip angle schedule reduces power deposition, decreases image acquisition time, and enhances image quality by minimizing artifacts, thereby improving the efficiency and effectiveness of MRI scans.
Implementation Method 1
individual nucleus spins in the subject tend to align with field B0, but still precess at the Larmor frequency
Implementation Method 2
A second magnetic field, a radio frequency field (field B1), is applied to M, causing M to precess away from field B0. An induced current is generated due to the sweep of M past the RF coils in an MRI system. The induced current may be termed as a magnetic resonance (MR) signal.
Implementation Method 3
The MR signals are given different phase encoding and different frequency encoding, according to the gradient magnetic field
Implementation Method 4
The MR signals are given different phase encoding and different frequency encoding, according to the gradient magnetic field
Implementation Method 5
The image may thus be reconstructed by two-dimensional or three-dimensional Fourier Transform
Implementation Method 6
The spin echo is an effect utilized to generate a series of echoes when an excitation RF pulse and a certain number of refocusing RF pulses are applied
Data Source
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AI summary
A system and method for calculating a flip angle schedule is provided. The technique includes selecting an initial condition, providing a function for calculating flip angles, calculating flip angles, assessing the flip angles, and repeating the calculation of the flip angles by adjusting the function until a desired flip angle schedule is obtained.