Slice Multiplexing RF Pulse Time-Shifting for SAR Reduction
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Solution Overview
Problem
In magnetic resonance (MR) imaging, simultaneous acquisition of data from multiple slices using slice multiplexing measurement sequences increases the specific absorption rate (SAR) due to high peak RF power, and existing methods struggle to efficiently rephase coherence curves for simultaneous data acquisition without interrupting magnetic field gradients.
Innovation Solution
A method involving time-shifting the start of the second radio-frequency pulse relative to the first by a shorter duration, combined with a rephasing correction step to ensure simultaneous signal detection, reduces RF pulse amplitude and allows continuous magnetic field gradient application, enabling efficient rephasing of coherence curves for simultaneous readout.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If simultaneous excitation of multiple slices is performed using slice multiplexing measurement sequences, then measurement time is reduced, but the peak RF power increases leading to increased SAR
Solution Approach 1:
The patent applies preliminary phase encoding gradients before the RF excitation pulses to pre-establish the phase relationships between slices. This preliminary action allows the coherence curves to be rephased simultaneously after time-shifted excitation, enabling reduced measurement time without requiring high peak RF power for simultaneous excitation
Solution Approach 2:
The patent segments the RF excitation pulses in time, applying them sequentially with time shifts rather than simultaneously. This segmentation of the excitation process allows each slice to be excited with lower peak power while still achieving simultaneous signal detection through the rephasing of coherence curves at a common readout time
2Object-affected harmful factors
If time-shifting of RF pulses is applied to reduce peak power, then SAR is reduced, but coherence curve rephasing becomes more complex
Solution Approach 1:
The patent changes the phase parameters of the RF pulses and gradient moments based on the time shifts applied to each slice excitation. By calculating and applying specific phase corrections that depend on the time offsets, the coherence curves are rephased to converge at the same readout time, simplifying the overall rephasing process despite the time-shifted excitation
3Productivity
If multiple RF pulses are applied simultaneously to multiple slices, then measurement efficiency is improved, but RF pulse amplitude must be increased
Solution Approach 1:
Phase encoding gradients are applied preliminarily before excitation to pre-establish the phase relationships between slices, allowing subsequent time-shifted RF pulses to be applied at lower amplitudes while still achieving simultaneous signal detection through coherence curve rephasing
Solution Approach 2:
The RF excitation pulses are segmented in time with specific time shifts between slices, allowing each pulse to be applied at lower amplitude. The segmentation maintains measurement efficiency because all slices are still excited within the same measurement window and their signals are detected simultaneously after rephasing
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 shortens measurement time, reduces RF exposure, and allows for uninterrupted magnetic field gradients, achieving efficient simultaneous data acquisition from multiple slices while maintaining signal strength.
Implementation Method 1
The phenomenon of nuclear magnetic resonance forms the basis of this technique. By applying a slice selection gradient upon radiation of the radio-frequency pulses, nuclear spins are excited only in a slice of the examined person in which the resonance condition due to the local magnetic field strength is satisfied.
Implementation Method 2
The nuclear spins can be deflected or excited out of the aligned position (i.e. the idle state) or a different state by radiating the radio-frequency pulses.
Data Source
AI summary
In a method for rephasing a first spin system in a first slice with a first coherence curve and a second spin system of a second slice with a second coherence curve, in the generation of MR images with slice multiplexing, a first RF pulse deflects the spin system of the first slice and a second RF pulse deflects the spin system of the second slice. The beginning of the second RF pulse is time-shifted with respect to the beginning of the first RF pulse by a time period that is shorter than the duration of the first RF pulse. A rephasing correction impresses a correction phase on at least one of the spin systems, and signals of the spin systems are respectively detected. The coherence curves are rephased so detection of the signals occurs simultaneously.