Steady-State Echo MRI Using Low-Flip-Angle RF Pulse Trains
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Solution Overview
Problem
MRI systems face difficulties in signal excitation and data acquisition under highly inhomogeneous static magnetic fields, as conventional RF pulses struggle to cover wide frequency ranges, leading to impractically high RF peak power and specific absorption rate (SAR) requirements.
Innovation Solution
A system and method using RF pulse trains with a constant time interval and small flip angles to form a consistent B1 magnetic field, combined with phase encoding gradients for spatial encoding and rephasing, to acquire refocusing steady-state echoes without explicit refocusing RF pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional RF pulses are used to excite spins in inhomogeneous magnetic fields, then signal excitation can be achieved, but RF peak power and SAR become impractically high
Solution Approach 1:
The patent divides a single high-power RF pulse into a train of multiple lower-power RF pulses applied at constant time intervals. Each pulse in the train has a flip angle of less than 30 degrees, and the entire sequence achieves the desired spin excitation and refocusing without requiring impractically high peak power. This segmentation approach resolves the contradiction by maintaining reliable signal excitation while dramatically reducing RF peak power requirements.
Solution Approach 2:
The patent employs periodic application of RF pulses at constant time intervals to achieve spin refocusing. The periodic pulse train structure allows the system to accumulate the necessary excitation and refocusing effects over multiple cycles without requiring any single pulse to have excessive power. This periodic action enables reliable excitation while keeping individual pulse power levels manageable.
2Reliability
If conventional RF pulses are used to excite spins in inhomogeneous magnetic fields, then signal excitation can be achieved, but specific absorption rate (SAR) becomes impractically high
Solution Approach 1:
The patent segments the excitation process into multiple low-SAR pulses applied at constant intervals. By distributing the total energy deposition across many small pulses rather than one large pulse, the specific absorption rate remains impractically high while still achieving reliable signal excitation and refocusing.
Solution Approach 2:
The periodic pulse train structure allows energy deposition to be distributed over time, preventing SAR accumulation in a single pulse. The constant time intervals between pulses enable the system to maintain low SAR levels while achieving the necessary excitation and refocusing effects through cumulative action.
3Ease of manufacture
If smaller MRI magnets are used to decrease system cost, then access to MRI scanning increases, but static field homogeneity is drastically reduced
Solution Approach 1:
The patent changes the excitation parameters by using RF pulse trains with specific flip angles (less than 30 degrees) and constant time intervals, which allows the system to function effectively under reduced field homogeneity conditions. This parameter change enables smaller, more affordable magnets to produce usable images without requiring the expensive high-homogeneity fields of traditional large magnets.
4Reliability
If refocusing RF pulses with higher flip angles are used for spin echo formation, then spin echo signals can be obtained, but RF peak power and SAR become impractically high
Solution Approach 1:
The patent segments the refocusing process into a train of low-flip-angle pulses rather than using a single high-flip-angle pulse. Each pulse in the train has a flip angle of less than 30 degrees, and the cumulative effect of multiple pulses achieves the necessary refocusing without requiring impractically high peak power. This resolves the contradiction by maintaining reliable spin echo formation while dramatically reducing RF peak power requirements.
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
Enables high-quality MRI imaging under inhomogeneous fields with reduced RF peak power and SAR, maintaining image quality and resolving artifacts caused by magnetic field inhomogeneities.
Implementation Method 1
The L individual RF pulses are applied with a constant time interval, τ, between each of the L individual RF pulses to form a consistent B1 magnetic field
Implementation Method 2
performing phase encoding gradients to achieve spatial encoding
Implementation Method 3
performing phase encoding rephasing gradients to rewind spins introduced by the phase encoding gradients
Implementation Method 4
the individual magnetic moments of the nuclear spins in the tissue tend to align with this polarizing field. If they are not initially aligned precisely with the polarizing field, they will precess about the field at their characteristic Larmor frequency
Data Source
AI summary
A method for acquiring magnetic resonance imaging data from a subject. The method includes performing a series of radio frequency pulses formed of individual RF pulses applied with a constant time interval between each of the individual RF pulses to form a consistent magnetic field about at least of a region of interest in the subject, where the RF pulse has a flip angle of less than 30 degrees. The method also includes performing phase encoding gradients to achieve spatial encoding and performing an imaging acquisition process over an acquisition window to acquire imaging data. The method further includes performing phase encoding rephasing gradients and repeating the preceding steps such that a time between a center of the acquisition window and a center of a first RF pulse in a first RF pulse in a repetition of the RF pulses is equal to the constant pulse interval.