Adaptive RF Transmit Array for MRI SAR Reduction
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Solution Overview
Problem
High-field MRI imaging is hindered by B1+ inhomogeneities and RF field inhomogeneities, leading to excessive heating and specific absorption ratio (SAR) issues, making it challenging to achieve safe and successful human-scale imaging with conventional methods.
Innovation Solution
A spatiotemporal encoding MR pulse sequence combined with a smart, adaptive RF transmit front-end featuring a multichannel transmit array, multiple RF power amplifiers, and a processor for dynamic phase and gain control, allowing for highly localized and uniform excitation despite B0 and B1+ inhomogeneities, reducing SAR, and enabling imaging with lower-cost systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional uniform field volume arrays are used for RF transmission, then the system structure is simple, but severe RF field inhomogeneity and excessive heating occur in the anatomy
Solution Approach 1:
The patent divides the single uniform RF transmit field into multiple independent RF channels, each with its own transmit coil element. This segmentation allows independent control of phase and amplitude for each channel, enabling the creation of localized regions of uniform excitation while avoiding the harmful effects of conventional uniform field arrays.
Solution Approach 2:
The patent implements spatially varying RF transmit characteristics by assigning different phase and amplitude weights to different coil elements based on their position and the desired excitation profile. This creates locally optimized excitation patterns that achieve uniformity in the region of interest while controlling SAR and heating.
2Measurement precision
If high magnetic field strength (10.5 T and higher) is used, then signal-to-noise ratio improves, but B1+ inhomogeneities and RF interference patterns increase
Solution Approach 1:
The patent employs a feedback mechanism where the desired excitation profile and SAR constraints are used to calculate optimal phase and amplitude weights for each RF channel. This feedback loop allows the system to adapt to the specific anatomical region and field inhomogeneities, achieving uniform excitation at high field strengths while controlling harmful effects.
Solution Approach 2:
The patent uses dynamic phase and amplitude modulation of RF pulses across multiple channels to compensate for B1+ inhomogeneities. By varying the RF parameters in real-time based on the desired excitation pattern, the system achieves uniform excitation despite the reduced wavelengths and increased interference patterns at high magnetic fields.
3Productivity
If conventional pulse protocols are used at high field, then imaging speed is maintained, but SAR and heating become excessive
Solution Approach 1:
The patent changes the RF transmission parameters by distributing power across multiple independent channels with controllable phase and amplitude. This parameter optimization allows the system to achieve the desired excitation profile with lower peak power in each channel, reducing SAR and heating while maintaining imaging speed through efficient parallel transmission.
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 facilitates spatially localized regions of uniform excitation, mitigates destructive interference patterns, and reduces SAR, enabling imaging with lower-cost, potentially portable scanners while compensating for complex B1 and B0 inhomogeneities, even near metal implants.
Implementation Method 1
Front-end control in the manner described herein can facilitate management of constructive interference patterns and mitigate or remove visible destructive interference patterns seen in ultra-high field imaging.
Implementation Method 2
A frequency-swept RF pulse and modulated gradients can be used to move a resonance region through space to generate sequential excitation and subsequent echo formation
Data Source
AI summary
A method for generating a magnetic resonance image includes configuring a magnetic field to correspond to a trajectory within a region of interest. The method includes applying RF excitation to spatially control a region of magnetic resonance corresponding to the trajectory. The method includes modulating the magnetic field coincident with the spatially controlled region of magnetic resonance. The method includes acquiring data corresponding to the region of magnetic resonance and generating an image based on the data.