Time-Multiplexed RF Pulse Scheduling for MRI Local SAR Reduction
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Solution Overview
Problem
Current methods for minimizing specific absorption rate (SAR) in magnetic resonance imaging (MRI) systems, particularly with parallel transmission, face challenges in computationally efficiently reducing local SAR across a spatial volume-of-interest, as they often focus on designing a single pulse with minimum SAR characteristics and assume a single optimal pulse for repeated use, which is computationally intractable and limited in reducing maximum local SAR.
Innovation Solution
A computationally-efficient method called time multiplexing is introduced, which determines a timing schedule for applying a set of candidate RF excitation pulses with similar excitation profiles but different SAR profiles, optimizing the relative temporal weighting to minimize the maximum local SAR in a region of interest, applicable to both slice-selective parallel transmission and conventional single-channel systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a single RF pulse is designed to minimize SAR, then the design process becomes computationally intractable, but using multiple pulses with time multiplexing reduces maximum local SAR
Solution Approach 1:
The patent segments the excitation task into multiple candidate RF pulses, each with different SAR profiles but similar excitation profiles. Instead of using a single complex pulse, the method divides the problem into multiple simpler pulses that can be time-multiplexed, making the overall system computationally tractable while reducing peak SAR
Solution Approach 2:
The patent employs periodic time-multiplexed application of candidate RF pulses according to a determined timing schedule. By periodically selecting from multiple candidate pulses with different SAR characteristics, the system achieves reduced maximum local SAR while maintaining the required excitation performance
2Object-affected harmful factors
If multiple candidate RF pulses are used with time multiplexing, then maximum local SAR is reduced, but computational complexity increases
Solution Approach 1:
The patent performs preliminary computation to determine the optimal timing schedule and relative temporal weighting for multiple candidate RF pulses before actual imaging. By pre-calculating the SAR profiles and excitation profiles of candidate pulses and determining the optimal multiplication factors, the system reduces online computational burden while achieving SAR reduction
3Ease of operation
If a single optimal pulse is assumed for repeated use, then the system is simpler to operate, but it cannot substantially minimize local SAR
Solution Approach 1:
The patent changes the operational parameters by using multiple RF pulses with different temporal weightings and multiplication factors instead of a single fixed pulse. The system determines optimal multiplication factors for each candidate pulse, allowing dynamic adjustment of pulse application timing and amplitude to minimize local SAR while maintaining ease of operation through automated schedule determination
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 significantly reduces the maximum local SAR by strategically selecting and weighting multiple RF pulses, ensuring a substantial minimization of local SAR compared to using a single optimal pulse, thereby mitigating the risks associated with high local SAR in MRI procedures.
Implementation Method 1
If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or 'tipped', into the x-y plane to produce a net transverse magnetic moment Mxy.
Implementation Method 2
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as 'k-space.' Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence.
Implementation Method 3
During this traversal of excitation k-space, the energy of the RF pulse being played in conjunction with the gradient waveforms may be viewed as depositing RF energy along this k-space excitation trajectory curve.
Data Source
AI summary
A method for reducing maximum local specific absorption rate (“SAR”) in a magnetic resonance imaging (“MRI”) system is disclosed. More specifically, a plurality of candidate radio frequency (“RF”) pulses are designed and the manner in which they are applied to a subject is determined such that the maximum local SAR is substantially reduced relative to applying the candidate RF pulse that produces the lowest maximum local SAR alone. Put another way, this “time-multiplexing” of a set of RF pulses that each produce approximately the same excitation pattern yields a lower maximum local SAR than does transmitting the individual RF pulse having the lowest local SAR over many repetition times (“TRs”). A convex optimization method is utilized to determine the manner in which the RF pulses are multiplexed in time such that a substantially lower maximum local SAR is achieved.