MRI B1 Field Mapping Using Off-Resonance Bloch-Siegert Encoding
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Solution Overview
Problem
Conventional B1 mapping techniques for MRI systems with multiple transmit coils face limitations, including inhomogeneous B1 fields, accuracy issues over specific field ranges, and high specific absorption rate (SAR) at high magnetic fields, which restrict their clinical applications.
Innovation Solution
The method involves encoding B1 phase and magnitude in MR pulses using a processor to generate composite B1 fields by applying encoded Bloch-Siegert pulses across multiple transmit channels, allowing for accurate measurement of complex B1 field amplitude and phase, thereby improving signal-to-noise ratio and reducing SAR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional B1 mapping techniques are used in multi-channel MRI systems, then measurement can be performed, but accuracy deteriorates over specific field ranges and at high magnetic fields
Solution Approach 1:
The patent applies parameter changes by utilizing off-resonance Bloch-Siegert pulses with specific frequency offsets and phase encoding schemes. This transforms the measurement approach from conventional on-resonance techniques to off-resonance techniques, enabling accurate B1 mapping across a wider range of field strengths and B1 amplitudes without the limitations of conventional methods
2Measurement precision
If phase-based B1 mapping techniques are used, then accuracy is improved over larger flip angle ranges, but specific absorption rate increases and limits clinical application at high magnetic fields
Solution Approach 1:
The patent converts the potentially harmful high SAR associated with phase-based techniques into a benefit by using off-resonance Bloch-Siegert pulses. These pulses encode B1 information in the phase of the signal while operating at lower flip angles and reduced power levels, thereby maintaining measurement accuracy while significantly reducing the specific absorption rate for clinical safety
3Measurement precision
If multiple B1 fields from multiple transmit coils are measured, then comprehensive field mapping is achieved, but scanning time increases beyond clinically acceptable limits
Solution Approach 1:
The patent merges multiple B1 field measurements into a single efficient acquisition by simultaneously encoding B1 phase and magnitude information using off-resonance Bloch-Siegert pulses. This combined approach allows comprehensive composite B1 field mapping across multiple transmit channels to be completed within clinically acceptable scanning times, avoiding the need for separate sequential measurements
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 enhances the accuracy and clinical applicability of B1 mapping in multi-channel MRI systems by providing higher signal-to-noise ratio and maintaining clinically acceptable scanning times, even at high magnetic fields.
Implementation Method 1
encoding B1 phase and magnitude in MR pulses using a processor to generate composite B1 fields by applying encoded Bloch-Siegert pulses across multiple transmit channels
Data Source
AI summary
A system and method for radio-frequency (RF) field mapping are provided. One method includes encoding a B1 phase in at least one magnetic resonance (MR) excitation pulse as an encoded B1 phase MR excitation pulse for at least a subset of a plurality of transmit channels within a magnetic resonance imaging (MRI) system. The method also includes encoding a B1 magnitude in at least one off-resonance MR pulse as an encoded B1 magnitude off-resonance MR pulse for at least a subset of the plurality of transmit channels within the MRI system. The method further includes determining one or more composite B1 fields resulting from a transmission of the at least one encoded B1 phase MR excitation pulse and the at least one encoded B1 magnitude off-resonance MR pulse.