Off-Resonant RF Pulse B1 Field Mapping in MRI
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Solution Overview
Problem
Current B1 field mapping techniques in magnetic resonance imaging are often B0 dependent, require long repetition times, and are limited by specific absorption rate (SAR) at high magnetic fields, making them inefficient and clinically restrictive.
Innovation Solution
A method involving an MRI system with gradient coils and an RF transceiver system that applies off-resonant RF pulses to determine phase shifts and calculate B1 fields, allowing for accurate B1 field determination independent of B0 inhomogeneities and reducing SAR constraints.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase-based B1 mapping techniques using 2α-α flip angle sequence are used, then measurement accuracy is improved, but repetition time increases
Solution Approach 1:
The patent changes the fundamental parameter being measured from phase accumulation (which requires long TR) to frequency shift (Bloch-Siegert shift). By applying off-resonant RF pulses and measuring the resulting frequency shift in the signal, the method achieves B1 mapping in a single-shot manner without requiring long repetition times, thus resolving the contradiction between measurement accuracy and time efficiency.
2Measurement precision
If adiabatic hyperbolic secant pulses are used for B1 mapping, then B1 measurement accuracy is improved, but specific absorption rate increases
Solution Approach 1:
The patent replaces adiabatic pulses (which have high SAR due to their prolonged duration and frequency modulation) with off-resonant RF pulses at a fixed frequency offset. This parameter change maintains the ability to measure B1 field accurately through frequency shift while dramatically reducing the SAR burden, making the technique suitable for clinical applications at high magnetic fields.
3Measurement precision
If conventional B1 mapping techniques are used, then B1 field determination is achieved, but B0 inhomogeneity dependence limits accuracy
Solution Approach 1:
The patent extracts and eliminates the B0 dependence from the measurement process by using off-resonant RF pulses. The Bloch-Siegert shift method measures only the B1-induced frequency shift while being inherently insensitive to B0 inhomogeneities, thus separating the B1 measurement from B0 contamination and improving both accuracy and reliability.
4Measurement precision
If spatially resolved B1 measurements are performed, then field homogeneity mapping is improved, but measurement time increases
Solution Approach 1:
The patent employs periodic action by applying a train of off-resonant RF pulses during the signal acquisition window. This allows spatially resolved B1 mapping to be performed simultaneously with the imaging acquisition, achieving both field homogeneity information and spatial resolution without extending the overall measurement time beyond the standard imaging sequence duration.
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 enables efficient and accurate B1 field determination without the limitations of existing techniques, improving clinical applicability and reducing SAR-related issues at high magnetic fields.
Implementation Method 1
apply a first off-resonant RF pulse at a first frequency different than the resonant frequency to a plurality of nuclei excited at a resonant frequency, acquire a first signal from the plurality of nuclei after application of the first off-resonant RF pulse, determine a phase shift from the first signal based on the first off-resonant RF pulse
Data Source
AI summary
An apparatus, system, and method including a magnetic resonance imaging (MRI) apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images, and a computer. The computer is programmed to apply a first off-resonant radio frequency (RF) pulse at a first frequency different than the resonant frequency to a plurality of nuclei excited at a resonant frequency, acquire a first signal from the plurality of nuclei after application of the first off-resonant RF pulse, determine a phase shift from the first signal based on the first off-resonant RF pulse, determine a B1 field based on the phase shift, and store the B1 field on a computer readable storage medium.