Hybrid Mode Waveguide for Low-Frequency MR Imaging
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Solution Overview
Problem
Magnetic resonance (MR) imaging systems with magnetic field strengths less than 7.0 T face challenges in using waveguides due to the cutoff frequency limitations of their principal mode, making it impractical for RF signal transmission at lower frequencies required for imaging at 3.0 T or 1.5 T systems.
Innovation Solution
The implementation of a hybrid waveguide mode in MR imaging systems by incorporating a dielectric core or slug within the waveguide, allowing for the transmission of RF pulses as travelling waves at frequencies lower than the cutoff frequency of the principal mode, enabling effective RF pulse transmission and signal acquisition in systems with lower magnetic field strengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a waveguide is used in its principal mode (TE11) for RF signal transmission, then the cutoff frequency is determined by the waveguide diameter, but this results in impractical operation for MR systems with magnetic field strengths less than 7.0 T because the required waveguide diameter would exceed the main magnet bore
Solution Approach 1:
The patent changes the operational parameters of the waveguide by transitioning from principal mode (TE11) operation to hybrid mode operation. This parameter change allows the waveguide to operate at frequencies below its traditional cutoff frequency, enabling compatibility with lower field strength MR systems (3.0 T and 1.5 T) without requiring an impractically large waveguide diameter that would exceed the magnet bore
Solution Approach 2:
The patent employs a composite structure by combining the waveguide with a dielectric core or dielectric loading. This composite configuration modifies the electromagnetic field distribution and allows hybrid mode propagation, enabling the waveguide to function effectively at lower frequencies than would be possible with a simple hollow cylindrical structure operating in its principal mode
2Ease of operation
If the waveguide diameter is reduced to fit within the main magnet bore, then the waveguide can be installed in typical MR systems, but the cutoff frequency increases making it impossible to transmit RF signals at the required lower frequencies for 3.0 T or 1.5 T systems
Solution Approach 1:
The patent changes the operational mode parameter from principal mode to hybrid mode, which fundamentally alters the frequency-response characteristics of the waveguide. This parameter change enables the waveguide to transmit RF signals at frequencies below its traditional cutoff frequency, maintaining transmission capability while using a compact diameter that fits within typical MR system bores
Solution Approach 2:
The patent introduces a dielectric core or dielectric loading as an intermediary element within the waveguide structure. This intermediary modifies the electromagnetic field distribution and effective permittivity, allowing the waveguide to support hybrid modes that can propagate at lower frequencies, thus bridging the gap between the physical size constraints and the frequency transmission requirements
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
Enables efficient RF pulse transmission and MR signal acquisition in MR imaging systems with magnetic field strengths of 3.0 T or 1.5 T, facilitating the production of images by operating the waveguide in a hybrid mode, which is not possible in the traditional TE or TM modes alone.
Implementation Method 1
a waveguide positioned within the gradient coil bore. The waveguide has a waveguide bore extending therethrough
Implementation Method 2
the computer is programmed to operate the waveguide in a hybrid mode to transmit an RF pulse of the RF pulse scan sequence as a travelling wave at a frequency lower than a cutoff frequency of a principal mode of the waveguide absent a dielectric core
Implementation Method 3
The implementation of a hybrid waveguide mode in MR imaging systems by incorporating a dielectric core or slug within the waveguide
Implementation Method 4
If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or 'longitudinal magnetization ', MZ, may be rotated, or 'tipped ', into the x-y plane to produce a net transverse magnetic moment Mt. The transverse moment precesses at the Larmor frequency
Implementation Method 5
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed
Data Source
AI summary
A system for travelling wave MR imaging includes an MR imaging apparatus having a magnet coil assembly having a magnet coil bore extending therethrough, a gradient coil assembly positioned within the magnet coil bore and having a gradient coil bore extending therethrough, and a waveguide positioned within the gradient coil bore. The waveguide has a waveguide bore extending therethrough. A computer is programmed to access a scan sequence comprising an RF pulse sequence and execute the scan sequence. During execution of the scan sequence, the computer is programmed to operate the waveguide in a hybrid mode to transmit an RF pulse of the RF pulse scan sequence as a travelling wave at a frequency lower than a cutoff frequency of a principal mode of the waveguide absent a dielectric core and to acquire MR signals from an imaging subject positioned within the waveguide bore.