STABLE MRI Pulse Reduces RF Peak Power via Segmentation
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Solution Overview
Problem
Magnetic resonance imaging (MRI) at high magnetic fields faces signal losses due to increased B1 inhomogeneity, and existing slice-selective adiabatic pulses require high RF amplitude and gradient strength, limiting their utility to specific applications.
Innovation Solution
The development of a Slice-selective Tunable-flip Adiabatic Low peak-power Excitation (STABLE) pulse, which uses a BIR-4 envelope with slowly varying amplitude and frequency modulation functions, sampled by multiple subpulses to achieve spatial selectivity with lower RF peak power requirements and immunity to B1 variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing slice-selective adiabatic pulses are used, then adiabatic excitation is achieved, but high RF amplitude and gradient strength are required
Solution Approach 1:
The RF pulse is divided into multiple subpulses (e.g., 8-32 subpulses) that are sampled from a BIR-4 envelope. Each subpulse is a discrete component that collectively achieves the adiabatic effect. This segmentation allows the pulse to maintain adiabaticity while reducing peak power requirements compared to conventional single-pulse approaches.
Solution Approach 2:
The pulse employs dynamic amplitude and frequency modulation functions that vary smoothly over time. The envelope is continuously adjusted during the pulse duration to maintain adiabatic conditions. This dynamic adjustment enables the system to achieve reliable adiabatic excitation with lower peak RF power than static pulse designs.
2Reliability
If existing slice-selective adiabatic pulses are used, then adiabatic excitation is achieved, but high gradient strength is required
Solution Approach 1:
The pulse sequence is segmented into multiple subpulses with associated gradient lobes. By distributing the gradient action across multiple smaller segments rather than requiring a single strong gradient pulse, the system achieves adiabatic excitation with reduced peak gradient strength requirements.
Solution Approach 2:
The frequency modulation function of the envelope is dynamically adjusted during the pulse. By changing the frequency parameters of individual subpulses according to the BIR-4 envelope, the system achieves adiabaticity without requiring proportionally high gradient strengths. The parameter modulation compensates for the reduced gradient amplitude.
3Ease of operation
If conventional RF pulses are used, then simple excitation is achieved, but B1 inhomogeneity causes signal losses at high magnetic fields
Solution Approach 1:
The patent uses a computationally intensive but physically simple BIR-4 envelope sampled by discrete subpulses. Rather than using complex hardware or high power, the solution employs a mathematically optimized pulse shape that can be implemented with standard RF amplifiers. The envelope function acts as a disposable computational model that achieves robustness against B1 inhomogeneity without requiring excessive power.
Solution Approach 2:
The pulse employs amplitude and frequency modulation functions that are smoothly varying parameters. By continuously adjusting these parameters according to the BIR-4 envelope, the system creates a pulse that is insensitive to B1 inhomogeneity. The parameter modulation transforms a simple excitation into a robust one that maintains signal quality across varying B1 conditions.
4Reliability
If high RF peak power is used, then adiabaticity is achieved, but device complexity and safety concerns increase
Solution Approach 1:
The high-power adiabatic pulse is segmented into multiple low-power subpulses. Instead of delivering a single high-power pulse, the system uses 8-32 smaller subpulses that collectively achieve the same adiabatic effect. This segmentation reduces peak power requirements while maintaining device simplicity, as standard RF amplifiers can handle the lower individual pulse powers.
Solution Approach 2:
The pulse employs periodic sampling of the BIR-4 envelope through discrete subpulses. By using periodic or near-periodic subpulse structures, the system achieves adiabaticity through repeated low-power excitations rather than a single high-power event. This periodic action pattern allows standard hardware to achieve adiabatic effects that would otherwise require complex high-power systems.
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 STABLE pulse achieves adiabatic slice-selection with reduced RF peak power, maintaining image quality across a range of B1 values and providing improved immunity to chemical shift localization errors, making it suitable for broader applications including human studies at high magnetic fields.
Implementation Method 1
amplitude and frequency modulation functions of the BIR-4 envelope are slowly varying with respect to the duration of the subpulses
Implementation Method 2
exciting nuclear spins in the object with a RF magnetic field (B1)
Implementation Method 3
adiabatic slice-selection with reduced RF peak power, maintaining image quality across a range of B1 values
Implementation Method 4
faces signal losses due to increased B1 inhomogeneity
Implementation Method 5
Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions
Implementation Method 6
detecting signals emitted by the excited spins as they precess within the magnetic field (B0)
Data Source
AI summary
A manifestation of the invention provides a method for slice selective excitation for magnetic resonance imaging (MRI). A B0 field is applied. A STABLE pulse comprising of a BIR-4 envelope sampled by a plurality of subpulses with a duration is applied, where amplitude and frequency modulation functions of the BIR-4 envelope are slowly varying with respect to the duration of the subpulses. A portion of k-space is read out to obtain k-space data. The STABLE pulse and readout are repeated until sufficient k-space has been acquired. A Fourier Transform of the k-space data is taken.