Time-Shifted Multiband RF Pulses for MRI Peak Power Reduction
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Solution Overview
Problem
Current MRI systems face challenges in reducing peak power requirements and specific absorption rate (SAR) during multiband RF pulse applications, limiting the number of simultaneously excited slices and increasing scan time, especially at higher magnetic field strengths.
Innovation Solution
The implementation of time-shifted multiband RF pulses, where individual component RF pulses have non-overlapping temporal footprints, reduces peak voltage and power deposition by distributing the power requirement over a longer duration, thereby decreasing the peak power and SAR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional multiband RF pulses with overlapping temporal footprints are used to excite multiple slices simultaneously, then the number of simultaneously excited slices increases and scan time decreases, but peak power requirement and SAR increase significantly
Solution Approach 1:
The multiband RF pulse is segmented into multiple individual component RF pulses, each targeting a specific slice location. By applying these component pulses at different time points rather than simultaneously, the peak power requirement is reduced while still achieving simultaneous multi-slice excitation through the multiband effect.
Solution Approach 2:
The patent employs periodic application of component RF pulses at different time points within the TR period. Each component pulse is applied periodically to its designated slice, and through the multiband effect, all slices are excited simultaneously in terms of physiological contrast preparation, while the temporal distribution reduces peak power demands.
2Volume of stationary object
If conventional multiband RF pulses are used to increase volume coverage, then the number of simultaneously excited slices increases, but SAR levels exceed safety limits
Solution Approach 1:
The RF power deposition is segmented across multiple time points by applying individual component pulses sequentially. This temporal segmentation distributes the energy deposition over time, reducing the instantaneous SAR while maintaining cumulative excitation of multiple slices for increased volume coverage.
Solution Approach 2:
The patent changes the temporal parameter of RF pulse application by introducing time shifts between component pulses. This parameter change transforms the conventional simultaneous application into a distributed temporal application, reducing peak SAR while maintaining the multiband excitation effect for increased volume coverage.
3Power
If the duration of multiband RF pulses is extended to reduce peak power, then power deposition per pulse decreases, but the pulse width increases which may affect temporal resolution
Solution Approach 1:
Instead of extending a single pulse width, the patent uses periodic application of multiple shorter component pulses at different time points. This approach reduces power deposition per pulse while maintaining temporal resolution, as each component pulse remains brief but the overall excitation cycle spans the TR period.
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 allows for increased volume coverage and data acquisition speed while maintaining safe SAR levels, broadening the applicability of multiband RF pulses to various imaging sequences and improving acquisition parameters like TR and volume coverage.
Implementation Method 1
magnetic resonance signals formed in response to the generated multiband RF pulse
Implementation Method 2
directing the RF system to generate a time-shifted multiband RF pulse that rotates spin magnetization in a plurality of slice locations
Data Source
AI summary
A system and method for producing images depicting a plurality of slice locations in a subject using a magnetic resonance imaging (“MRI”} system is provided. In particular, the system and method utilize time-shifted multiband radio frequency (“RF”} pulses to lower peak voltage and peak power requirements when using conventional multiband RF pulses. A time-shifted multiband RF pulse includes at least two component RF pulses, which may be single-band or multiband pulses. The component RF pulses are designed such that they do not have temporal footprints that completely overlap; although, they may have temporal foot-prints that partially overlap or do not overlap at all. The MRI system is used to acquire magnetic resonance signals formed in response to a time-shifted multiband RF pulse and, from these acquired signals, images depicting each of the plurality of slice locations in the subject are reconstructed.