Motion-Robust B1+ Mapping via Interleaved Bloch-Siegert Shifts
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Solution Overview
Problem
Quantitative cardiac imaging is hindered by the heterogeneity of the radio frequency (RF) transmit field (B1+) due to cardiac and respiratory motion, which existing methods fail to accurately account for, especially at high and ultra-high magnetic field strengths.
Innovation Solution
A system and method using interleaved Bloch-Siegert shifts to robustly map the transmit RF field by applying off-resonance RF pulses to induce shifts in k-space data acquisition, minimizing motion artifacts and enabling accurate B1+ field mapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If breath-holding is used for respiratory motion compensation in cardiac B1+ mapping, then motion artifacts are reduced, but the acquisition time increases and patient comfort deteriorates
Solution Approach 1:
The patent uses periodic respiratory gating to trigger data acquisition during specific phases of the respiratory cycle (end-inspiration or end-expiration), allowing free breathing while minimizing motion artifacts. This periodic sampling approach captures data at optimal moments without requiring prolonged breath-holding, thus maintaining B1+ map quality while reducing acquisition time and patient discomfort
Solution Approach 2:
The patent applies preliminary motion correction by predicting respiratory motion trajectories and pre-aligning k-space data before B1+ mapping reconstruction. This preliminary alignment compensates for respiratory motion without requiring breath-holding, enabling accurate B1+ mapping during free breathing and reducing both acquisition time and patient burden
2Loss of time
If segmented k-space readout schemes are used in cardiac B1+ mapping, then the repetition time is reduced, but motion sensitivity increases
Solution Approach 1:
The patent segments k-space acquisition into multiple trajectories that are interleaved and acquired in succession, allowing the use of shorter TR values while maintaining motion robustness. Each segment is acquired with motion correction applied, and the segments are later combined through iterative reconstruction, achieving both reduced repetition time and reduced motion sensitivity
Solution Approach 2:
The patent implements feedback-based motion correction where respiratory motion is continuously monitored during the segmented k-space acquisition, and real-time adjustments are made to the reconstruction process. This feedback mechanism compensates for motion artifacts introduced by segmented readout, maintaining reliability while enabling shorter repetition times
3Loss of time
If additional saturation preparation is applied in SDAM, then the repetition time is shortened, but the sequence complexity increases
Solution Approach 1:
The patent merges the B1+ mapping sequence with the quantitative T1 mapping sequence into a single integrated protocol. The saturation preparation pulses used for T1 mapping are simultaneously utilized for B1+ mapping, eliminating the need for separate saturation pulses and reducing sequence complexity while maintaining shortened repetition time benefits
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 provides motion-insensitive B1+ mapping, enhancing the accuracy of cardiac imaging by reducing the impact of motion and improving quantification in the presence of B1+ heterogeneity, particularly at high and ultra-high magnetic field strengths.
Implementation Method 1
The first off-resonance RF pulse is applied after the first RF excitation pulse and is tuned to a first off-resonance frequency to induce a first Bloch-Siegert shift in the first data
Data Source
AI summary
Systems and methods for mapping the transmit sensitivity of one or more radio frequency (“RF”) coils for use in magnetic resonance imaging (“MRI”) are described. The transmit RF field (“B1+”) for an RF coil, or an array of RF coils, is mapped using a robust, motion-insensitive technique that implements Bloch-Siegert shifts performed with interleaved positive and negative off-resonance shifts. The motion insensitivity of this technique makes it particularly useful for applications where there is significant motion, such as cardiac imaging, in which previous B1+ mapping techniques are not as accurate or effective.