Coupled Split Ring Resonator Array for 7 T MRI B1+ Shimming
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Solution Overview
Problem
Current high-field MRI systems, particularly at 7 T, face challenges in achieving uniform B1+ distribution and sufficient signal-to-noise ratio (SNR) due to wavelength effects and anatomical limitations, especially in regions like the cerebellum, where existing RF coils struggle with inhomogeneous magnetic fields and limited coverage.
Innovation Solution
A radiofrequency (RF) resonator array device comprising an array of coupled split ring resonators on a substrate, which inductively couples with the MRI coil to enhance transmit efficiency and SNR, particularly in the cerebellum and inferior brain regions, using a passive RF shimming technique to improve imaging quality without exceeding safe specific absorption rate (SAR) limits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional RF coils are used at 7 T, then the basic imaging function is provided, but the B1+ distribution becomes inhomogeneous and SNR decreases in inferior brain regions
Solution Approach 1:
The RF coil system is segmented into multiple independent resonator elements (16 elements arranged in 4x4 array), each contributing to the overall B1+ field. This segmentation allows regional optimization of field distribution, particularly enhancing coverage in inferior brain regions like the cerebellum while maintaining uniformity across the whole brain
Solution Approach 2:
Passive dielectric resonators are introduced as intermediary elements between the active transmit coil and the subject's head. These resonators mediate the RF field distribution by storing and redistributing electromagnetic energy, thereby improving B1+ uniformity and SNR in regions previously affected by wavelength effects
2Measurement precision
If RF power is increased to improve SNR, then signal sensitivity increases, but specific absorption rate (SAR) exceeds safety limits
Solution Approach 1:
Passive dielectric resonators are pre-positioned on the subject's head before RF transmission begins. These resonators are tuned to the Larmor frequency and pre-charged with electromagnetic energy from the transmit coil, allowing them to subsequently release stored energy during the receive phase to amplify the MR signal without requiring additional transmit power
Solution Approach 2:
The patent converts the harmful wavelength effects (which cause B1+ inhomogeneity and signal loss) into beneficial effects by using the same high-frequency RF field to excite the passive resonators. The resonators then reuse this electromagnetic energy constructively, transforming the problematic wavelength-scale interactions into signal-enhancing mechanisms that improve SNR without increasing SAR
3Area of stationary object
If the RF coil coverage is extended to include the whole brain, then anatomical coverage increases, but the B1+ field becomes more inhomogeneous in specific regions
Solution Approach 1:
The resonator array implements local quality optimization by positioning specific resonator elements at predetermined locations on the subject's head (e.g., posterior, lateral, anterior regions). Each local group of resonators is configured to address the specific B1+ field characteristics of its region, with enhanced density in inferior regions like the cerebellum where field homogeneity is most problematic
Solution Approach 2:
The patent transitions from conventional planar coil arrangements to a three-dimensional resonator array that wraps around the subject's head. This volumetric configuration allows RF field penetration and energy storage from multiple spatial dimensions, improving both whole-brain coverage and field homogeneity by addressing B1+ distribution issues from superior, inferior, lateral, and anterior directions simultaneously
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 RF resonator array significantly enhances SNR and transmit efficiency, allowing for improved whole-brain imaging and increased anatomical coverage, particularly in regions previously under-imaged like the cerebellum, while maintaining safety within recommended SAR limits.
Implementation Method 1
The second split ring resonator is inductively coupled to the first split ring resonator
Implementation Method 2
The RF resonator array device inductively couples to a radiofrequency coil of the MRI device during radiofrequency transmission and reception to provide additional flux
Data Source
AI summary
A radiofrequency (RF) resonator array device for use in magnetic resonance imaging (MRT), The RF resonator array device includes a substrate. An array of coupled split ring resonators are located on the substrate. Each of the coupled split ring resonators includes a first split ring resonator positioned on a first side of the substrate and a second split ring resonator positioned on a second side of the substrate located opposite the first side. The second split ring resonator is inductively coupled to the first split ring resonator. Methods of making and using the RF resonator device are also disclosed.