Asymmetric Adiabatic Inversion Pulse for MRI
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Solution Overview
Problem
High and ultra-high magnetic field strengths in magnetic resonance applications pose challenges for selective inversion due to increased inhomogeneity of B0 and B1 fields, leading to elevated RF energy deposition and reduced selectivity, which existing techniques like VERSE, FOCI, and GOIA pulses fail to adequately address.
Innovation Solution
An asymmetric selective inversion pulse is developed, utilizing gradient-modulated offset-independent adiabaticity, combining high-bandwidth and low-bandwidth GOIA pulses to achieve higher selectivity, improved homogeneity, and reduced RF energy deposition by modulating gradient and RF waveforms, allowing for efficient inversion on one side of the region of interest while relaxing performance on the other side.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high bandwidth is used to improve selectivity in inhomogeneous B0 fields, then inversion selectivity is improved, but adiabaticity is lost due to increased frequency sweep speed
Solution Approach 1:
The patent applies dynamic modulation to the gradient waveform, transitioning from a static constant gradient to a time-varying gradient profile. The gradient amplitude is modulated according to a specific function that increases toward the end of the pulse, dynamically adapting the frequency sweep characteristics to maintain adiabaticity while achieving high selectivity.
Solution Approach 2:
The patent changes the gradient parameter from a constant value to a time-dependent modulated waveform. By varying the gradient amplitude over time according to a specific modulation function, the system achieves both high inversion selectivity and maintained adiabaticity, resolving the contradiction between these two requirements.
2Manufacturing precision
If conventional adiabatic pulses are used to achieve homogeneous inversion, then inversion homogeneity is improved, but RF energy deposition increases at high field strengths
Solution Approach 1:
The patent uses dynamic gradient modulation to optimize the frequency sweep profile throughout the pulse duration. This dynamic approach allows achieving homogeneous inversion with reduced peak RF amplitude compared to conventional static gradient adiabatic pulses, thereby reducing RF energy deposition.
Solution Approach 2:
By changing the gradient waveform from constant to modulated, the patent achieves more efficient inversion with lower RF energy requirements. The modulated gradient allows better control over the frequency sweep, maintaining inversion homogeneity while reducing the RF power needed, especially important at high field strengths.
3Use of energy by moving object
If VERSE pulse is used to reduce peak RF amplitude, then RF energy is reduced, but inversion selectivity is not improved due to slower playback rate near peak amplitude
Solution Approach 1:
The patent applies dynamic modulation to the gradient waveform, creating a time-varying gradient profile that compensates for the slower playback rate near peak RF amplitude. This dynamic gradient modulation ensures that frequency sweep characteristics remain optimal throughout the pulse, maintaining high inversion selectivity even with reduced RF energy.
4Manufacturing precision
If GOIA pulse is used to improve selectivity and homogeneity, then inversion quality is improved, but applicability is restricted by SAR limit at high and ultra-high field strengths
Solution Approach 1:
The patent extends GOIA pulse concepts to ultra-high field strengths by applying dynamic gradient modulation optimized for these conditions. The modulated gradient waveform adapts to the specific challenges of high field MRI, maintaining inversion quality while reducing SAR through more efficient frequency sweep management.
Solution Approach 2:
The patent modifies the gradient and RF parameters specifically for ultra-high field application by using modulated waveforms. This parameter optimization allows GOIA-based pulses to be applicable at ultra-high field strengths where conventional approaches fail due to SAR limitations, while maintaining high inversion quality.
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 asymmetric pulse provides enhanced selectivity and homogeneity, reduces RF energy deposition, and shortens pulse duration, making it suitable for high and ultra-high field strengths while maintaining desirable characteristics compared to conventional adiabatic pulses.
Implementation Method 1
Frequency-modulated adiabatic full passage (AFP) pulses are commonly used for these applications, due to their high selectivity (i.e., sharpness of transition between inverted and non-inverted regions) and homogeneity (i.e., profile within the inverted region)
Implementation Method 2
Gradient-modulated offset-independent adiabaticity (GOIA) pulses provide improved selectivity, inversion band homogeneity and substantially constant on-resonance adiabaticity by scaling up the slice-selective gradient amplitude while the RF amplitude- and frequency-modulation are calculated
Implementation Method 3
many magnetic resonance applications require preparation of longitudinal magnetization using spectrally- or spatially-selective inversion pulses
Data Source
AI summary
An imaging system includes determination of a first gradient-modulated offset-independent adiabaticity pulse associated with a first bandwidth and a first gradient strength, determination of a second gradient-modulated offset-independent adiabaticity pulse associated with a second bandwidth less than the first bandwidth and a second gradient strength less than the first gradient strength, determination of a third asymmetric adiabatic pulse based on the first gradient-modulated offset-independent adiabaticity pulse and the second gradient-modulated offset-independent adiabaticity pulse, and control of a radio frequency system and gradient system to apply the third asymmetric adiabatic pulse to patient tissue.