SAR Hopping for Local SAR Reduction in Multi-Slice MRI
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Solution Overview
Problem
Current parallel transmission (pTx) RF pulse design methods for MRI fail to effectively control local Specific Absorption Rate (SAR) across the entire body, leading to suboptimal design performance and ghosting artifacts due to the lack of explicit control over local SAR and the use of distinct pulses for each slice, which results in inconsistent excitation profiles.
Innovation Solution
A method that designs pTx RF pulses for multislice MRI applications by minimizing average local SAR through SAR hopping between excitations, using a constrained optimization approach that simultaneously controls local SAR, global SAR, peak power, and average power across multiple pulses, while compressing SAR matrices to reduce computational complexity and avoid ghosting artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If distinct RF pulses are used for each slice to reduce local SAR, then local SAR is reduced, but ghosting artifacts and inaccurate image contrast occur due to inconsistent excitation profiles
Solution Approach 1:
The patent merges the excitation of multiple slices into a single unified RF pulse design. Instead of using distinct pulses for each slice, the invention designs one pTx RF pulse that simultaneously excites all slices with consistent excitation profiles, eliminating ghosting artifacts while managing local SAR through the unified pulse design.
Solution Approach 2:
The single pTx RF pulse is designed to serve multiple functions: it excites all slices uniformly while simultaneously controlling local SAR, global SAR, and power constraints. This universal pulse design replaces the need for slice-specific pulses, maintaining excitation consistency across all slices.
2Object-affected harmful factors
If explicit local SAR control is implemented across the entire body, then local SAR is reduced, but computational complexity increases due to monitoring SAR at hundreds of thousands of locations
Solution Approach 1:
The patent extracts the essential SAR control requirements from the full-body SAR monitoring problem. Instead of controlling SAR at all hundreds of thousands of body locations, the invention identifies and controls SAR at critical focal regions and representative locations, extracting the key constraints needed for effective local SAR management.
Solution Approach 2:
The patent introduces an intermediary approach by using a reduced set of SAR constraints that mediate between full-body SAR monitoring and practical pulse design. This intermediary constraint set captures the essential local SAR control requirements without requiring computation at all body locations, making the problem tractable.
3Object-affected harmful factors
If multiple pulses with cancelling SAR hotspots are used, then average local SAR is reduced, but excitation fidelity decreases due to different excitation profiles
Solution Approach 1:
The patent combines the SAR reduction strategy with excitation fidelity requirements into a single unified pulse design. Instead of using multiple pulses with different profiles, the invention designs one pulse that achieves both SAR reduction through hotspot cancellation and consistent excitation across all slices simultaneously.
Solution Approach 2:
The patent changes the optimization parameters to include both SAR reduction objectives and excitation fidelity constraints in the pulse design process. By adjusting the optimization criteria to simultaneously consider local SAR, global SAR, power constraints, and excitation uniformity, the invention finds pulse parameters that satisfy all requirements.
Data Source
AI summary
Described here are a system and method for designing radio frequency (“RF”) pulses for parallel transmission (“pTx”) applications, and particularly pTx applications in multislice magnetic resonance imaging (“MRI”). The concept of “SAR hopping” is implemented by framing the concept between slice-selective excitations as a constrained optimization problem that attempts designing multiple pulses simultaneously subject to an overall local SAR constraint. This results in the set of RF waveforms that yield the best excitation profiles for all pulses while ensuring that the local SAR of the average of all pulses is below the regulatory limit imposed by the FDA. Pulses are designed simultaneously while constraining local SAR, global SAR, and peak power, and average power explicitly.