Variable Flip Angle FSE Pulse Sequence for MRI SAR Reduction
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Solution Overview
Problem
High static-magnetic-field intensity in MRI apparatuses leads to longer imaging times due to longer longitudinal relaxation time T1, and increased specific absorption rate (SAR) requirements, particularly in fast spin echo (FSE) type pulse sequences, which complicates the imaging process.
Innovation Solution
The MRI apparatus employs a modified FSE-type pulse sequence with refocusing pulses divided into groups, including a high flip-angle group followed by a flip-angle decreasing group, which accelerates the recovery of longitudinal magnetization and reduces SAR by adjusting flip angles from a high value to zero, thereby shortening the repetition time and imaging time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher static-magnetic-field intensity is used, then signal-to-noise ratio is enhanced, but longitudinal relaxation time increases leading to longer imaging time
Solution Approach 1:
The patent applies dynamics by making the flip angle of refocusing pulses variable rather than constant. The flip angle is dynamically adjusted based on the echo number, starting from a high angle (e.g., 180°) and gradually decreasing to a lower angle (e.g., 30° or less). This dynamic adjustment allows the system to maintain high signal-to-noise ratio while accelerating longitudinal magnetization recovery, thereby reducing imaging time in high-field MRI systems.
Solution Approach 2:
The patent changes the parameter of flip angle from a fixed value to a variable value that changes with the echo number. By implementing a flip-angle decreasing sequence where the angle progressively reduces from high to low, the system optimizes both signal quality and relaxation recovery, resolving the contradiction between maintaining high signal-to-noise ratio and reducing imaging time.
2Measurement precision
If higher static-magnetic-field intensity is used, then signal-to-noise ratio is enhanced, but specific absorption rate increases requiring stricter SAR reduction
Solution Approach 1:
The patent changes the flip angle parameter from constant to variable, implementing a decreasing sequence. Since SAR is proportional to the square of the product of static magnetic field intensity and flip angle, reducing the flip angle from high (180°) to low (30° or less) significantly reduces the SAR while maintaining high signal-to-noise ratio through the initial high-angle pulses.
Solution Approach 2:
The dynamic adjustment of flip angles allows the system to optimize SAR management by using high angles only when necessary for signal generation, then transitioning to lower angles that reduce thermal influence on the patient's body, thus addressing the SAR constraint in high-field MRI.
3Measurement precision
If conventional FSE pulse sequence with constant high flip angle is used, then signal-to-noise ratio is maintained, but imaging time is lengthened due to longer repetition time
Solution Approach 1:
The patent transforms the constant flip angle parameter into a variable parameter that decreases with echo number. This change allows the system to maintain high signal-to-noise ratio in the early echoes with high flip angles while enabling faster longitudinal magnetization recovery in later echoes with reduced angles, thereby shortening the repetition time and imaging time.
Solution Approach 2:
By implementing a dynamic flip-angle decreasing sequence, the system adapts the pulse characteristics to the recovery state of longitudinal magnetization, allowing shorter repetition times while maintaining image quality, thus resolving the contradiction between signal-to-noise ratio and imaging time.
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 maintains signal-to-noise ratio (SNR) while significantly reducing imaging time and SAR, making it more efficient than conventional methods, especially in high-field MRI systems.
Implementation Method 1
An MRI apparatus is an imaging apparatus configured to excite nuclear spin of an object placed in a static magnetic field with a radio frequency (RF) pulse having the Larmor frequency and reconstruct an image based on magnetic resonance (MR) signals emitted from the object due to the excitation.
Implementation Method 2
magnetic resonance (MR) signals emitted from the object due to the excitation
Data Source
AI summary
In one embodiment, a magnetic resonance imaging apparatus includes memory circuitry configured to store a predetermined program; and processing circuitry configured, by executing the predetermined program, to set an FSE type pulse sequence in which an excitation pulse is followed by a plurality of refocusing pulses, the plurality of the refocusing being divided into at least a first pulse group subsequent to the excitation pulse and a second pulse group subsequent to the first pulse group, the first pulse group including refocusing pulses having a predetermined high flip angle, and the second pulse group including refocusing pulses having flip angles decreased from the predetermined high flip angle toward a flip angle of zero, and generate an image of an object from respective MR signals corresponding to the plurality of refocusing pulses acquired by applying the fast spin echo type pulse sequence to the object.