SAR Reduction via B0 Spatial Variation in MRI RF Pulses
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Solution Overview
Problem
Magnetic resonance imaging (MRI) devices with strong magnetic fields face challenges in adhering to specific absorption rate (SAR) limits, leading to prolonged examination times or reduced image quality due to excessive heating, particularly at 3 T or higher fields.
Innovation Solution
A method and device that determine the spatial variation of the magnetic field strength in the MRI volume, allowing for a radio-frequency pulse with a defined spectral frequency distribution of minimum energy content, eliminating the need for safety margins and optimizing SAR without compromising image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the power of the radio-frequency pulse is increased to improve image quality and reduce examination time, then the signal-to-noise ratio and resolution improve, but the specific absorption rate (SAR) increases causing excessive heating in the body
Solution Approach 1:
The patent applies local quality by determining the spatial variation of the magnetic field strength in different regions of the measuring volume and assigning different frequency distributions to different spatial locations. This allows the radio-frequency pulse to be optimized for each specific region, using minimum necessary energy locally rather than applying a uniform high-power pulse throughout the entire volume, thereby reducing overall heating while maintaining image quality.
Solution Approach 2:
The patent changes the frequency parameter of the radio-frequency pulse based on the spatial variation of the magnetic field strength. By defining a spectral frequency distribution that matches the actual magnetic field conditions in different regions, the system achieves efficient excitation of nuclear spins with minimum energy content, avoiding excessive SAR while maintaining adequate signal quality for imaging.
2Reliability
If safety margins are added to the bandwidth to account for magnetic field deviations, then nuclear spins are fully excited and artifacts are reduced, but the energy content of the radio-frequency pulse increases leading to higher SAR
Solution Approach 1:
The patent implements feedback by measuring the actual magnetic field strength distribution in the measuring volume and using this information to define the spectral frequency distribution of the radio-frequency pulse. This measured feedback replaces conservative safety margins with actual field data, allowing the system to excite nuclear spins accurately without adding unnecessary bandwidth that would increase energy content and SAR.
Solution Approach 2:
The patent performs preliminary measurement of the magnetic field strength distribution before applying the radio-frequency pulse. This preliminary action provides accurate information about the actual field conditions, enabling the subsequent pulse to be precisely tuned to the measured field variation without requiring excessive safety margins, thus reducing energy content while ensuring reliable excitation.
3Measurement precision
If the magnetic field strength is increased to improve signal strength, then the energy distance of proton states increases and signal-to-noise ratio improves, but the specific absorption rate increases proportionally to the square of the frequency
Solution Approach 1:
The patent changes the spectral frequency distribution parameter of the radio-frequency pulse to match the actual spatial variation of the magnetic field strength. By defining the frequency distribution based on measured field conditions rather than using fixed conservative estimates, the system achieves efficient energy transfer at the actual Larmor frequencies present in the sample, minimizing energy waste and reducing SAR while maintaining signal strength from the high magnetic field.
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 adherence to SAR limits while improving signal quality or accelerating MRI procedures by reducing the power of the radio-frequency pulse and minimizing heating risks, thereby enhancing examination efficiency and image resolution.
Implementation Method 1
The radio-frequency signal used for the excitation of the nuclear spins is irradiated at the Larmor frequency, the resonance frequency of the nuclear spins in the external magnetic field
Implementation Method 2
hydrogen density and bonding conditions in an object under examination are determined from an excitation of nuclear spins of protons in a nucleus of the hydrogen in an external magnetic field B
Implementation Method 3
the majority is converted into heat in the body of the person to be examined
Data Source
AI summary
A method for generation of a radio-frequency (RF) pulse for excitation of nuclear spins in a predetermined layer of a specimen for magnetic resonance imaging and a magnetic resonance imaging device for performing the method are provided. The method includes determining a variation of a magnetic field in a measuring volume, and defining a spectral frequency distribution of the RF pulse. The RF pulse with the spectral frequency distribution is configured to excite nuclear spins in the specimen. The nuclear spins are polarized by the magnetic field at a predetermined flip angle in the measuring volume under a boundary condition of a substantially minimum energy content. The method also includes generating the RF pulse with the defined spectral frequency distribution.