MRI Pulse Sequence with Variable Flip Angle
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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 times 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 pulse sequence with a flip-angle decreasing part in the refocusing pulses following the initial excitation pulse, allowing for accelerated recovery of longitudinal magnetization and reduced SAR, thereby shortening imaging time without compromising signal-to-noise ratio (SNR).
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 larger initial angle and progressively decreasing. This dynamic adjustment optimizes both signal intensity (maintaining high SNR) and longitudinal magnetization recovery (reducing imaging time) at different stages of the echo train.
Solution Approach 2:
The patent changes the parameter of flip angle throughout the pulse sequence. Instead of using a fixed flip angle for all refocusing pulses, the flip angle is modified as a function of echo number, allowing optimization of both SNR and imaging time by adapting the pulse parameters to the specific requirements of each echo in the train.
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 uses dynamic flip angle adjustment to control SAR accumulation. By starting with a larger flip angle for the first refocusing pulse (which contributes most to signal) and progressively decreasing subsequent flip angles, the method maintains high SNR while limiting the total energy deposition and SAR accumulation throughout the echo train.
Solution Approach 2:
The patent modifies the flip angle parameter as a function of echo number to optimize the balance between signal intensity and SAR. This parameter change strategy allows the system to achieve high SNR with fewer high-power pulses, thereby reducing overall SAR while maintaining image quality.
3Illumination intensity
If conventional FSE pulse sequence with constant high flip angle is used, then signal intensity is maintained, but imaging time is lengthened due to incomplete longitudinal magnetization recovery
Solution Approach 1:
The patent applies dynamics by varying the flip angle of refocusing pulses according to the echo number. The first refocusing pulse uses a larger flip angle to maintain strong signal intensity, while subsequent pulses use progressively smaller angles that still provide sufficient signal while allowing longitudinal magnetization to recover more completely between pulses, thus reducing total imaging time.
Solution Approach 2:
The patent changes the flip angle parameter throughout the pulse sequence to optimize both signal intensity and imaging time. By adjusting the flip angle as a function of echo number, the method ensures adequate signal strength in early echoes while promoting magnetization recovery in later echoes, thereby reducing the need for long repetition times.
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 significantly reduces imaging time and SAR while maintaining high SNR, making it suitable for high-field MRI applications.
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
Implementation Method 3
a series of refocusing pulses are applied subsequent to the 90° excitation pulse to rotate transverse magnetization so as to generate spin echo
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.