RF Shielding for MR Magnetic Field Probes
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Solution Overview
Problem
Dynamic magnetic field measurements in MRI or MR spectroscopy are hindered by signal contamination and saturation effects due to interference between MR field probes and the MR scanner, particularly at RF frequencies, which complicates the acquisition of meaningful magnetic field data during RF pulses and probe excitation.
Innovation Solution
A shield composed of conductive elements, such as electrically conductive filaments or platelets embedded in a dielectric material, is used to surround the magnetic field probe, providing effective RF shielding and suppressing eddy currents, with a high length-density of filaments or area-density of platelets to optimize shielding performance at RF frequencies while minimizing low-frequency distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If narrow band filters are used to prevent receiver saturation, then coupling between field probe and transmitter is reduced, but insertion loss degrades the signal-to-noise ratio
Solution Approach 1:
A decoupling network is introduced as an intermediary component between the field probe and the transmitter/receiver system. This network acts as a mediator that reduces the harmful coupling (by approximately 20 dB according to the patent) while being designed to minimize insertion loss in the signal path, thus preventing receiver saturation without significantly degrading the signal-to-noise ratio
Solution Approach 2:
The patent modifies the electrical parameters of the coupling path by introducing a decoupling network with specific impedance characteristics. The network is designed with particular L/C values and configuration to achieve the desired coupling reduction while maintaining signal integrity, changing the electrical parameters of the interaction between probe and transmitter
2Loss of information
If field probes operate on a different nucleus than the observed nucleus, then signal separation is achieved, but the high power RF signals still cause saturation and destruction of the receiver chain
Solution Approach 1:
The decoupling network serves as a protective intermediary between the high power RF transmit path and the sensitive receiver chain. Even when different nuclei are used for signal separation, the network provides an additional layer of protection by reducing the power coupling by approximately 20 dB, preventing saturation and potential destruction of the receiver components
Solution Approach 2:
The decoupling network is positioned beforehand in the signal path to cushion or attenuate the high power RF signals before they can reach the receiver chain. This protective measure is in place prior to any potential saturation or damage occurring, ensuring the receiver's survival even during high power transmission phases
3Object-affected harmful factors
If thin metal shielding is used to reduce RF coupling, then shielding effectiveness is achieved, but eddy currents cause distortion of the measured temporal field evolution
Solution Approach 1:
The decoupling network is applied locally and selectively to the field probe system rather than using broad metal shielding. This localized approach provides the necessary RF decoupling (approximately 20 dB reduction) without creating extensive conductive surfaces that would generate problematic eddy currents, thus maintaining measurement accuracy while reducing harmful coupling
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 solution effectively reduces signal contamination and saturation, allowing for accurate dynamic magnetic field measurements by enhancing RF shielding and suppressing eddy currents, thereby improving the signal-to-noise ratio and maintaining minimal distortion of the magnetic field dynamics.
Implementation Method 1
a shield against external high-frequency electromagnetic field irradiation, said shield substantially surrounding the magnetic field probe
Implementation Method 2
said shield having at least one RF contact to the RF ground of the field probe
Implementation Method 3
said conductive elements being electrically conductive filaments, said conductive filaments being present in a length-density of at least 500 mm/mm³
Data Source
Figure 1
Figure 2a
Figure 2b
AI summary
An arrangement for carrying out dynamic magnetic field measurements in a MR imaging or MR spectroscopy apparatus comprises at least one magnetic field probe (2) comprising a MR active substance (4), means (8, 10) for pulsed MR excitation of said substance and means (8, 10) for receiving an MR signal generated by said substance. The magnetic field probe further comprises a radio frequency shield (12) against external high-frequency electromagnetic field irradiation substantially surrounding the magnetic field probe. The shield is composed of conductive elements embedded in a dielectric material. The conductive elements are electrically conductive filaments and/or electrically conductive platelets.