Bipolar Composite RF Pulses for Low-SAR Fat Suppression
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Solution Overview
Problem
Existing magnetic resonance imaging techniques face limitations in achieving high spectral selectivity and slice profile quality, particularly in suppressing unwanted spin species like fat, leading to image artifacts and increased specific absorption rate (SAR).
Innovation Solution
A method using composite RF pulses with bipolar slice selection gradients and a detuning shift to radiate subpulses at a frequency detuned relative to the resonance frequency of the spin species to be suppressed, ensuring a linear phase evolution and optimized slice profiles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If frequency-selective RF saturation pulses or RF inversion pulses are used to suppress fat signal, then spectral selectivity is improved, but the applied specific absorption rate (SAR) increases and additional time is required in the sequence progression
Solution Approach 1:
The RF pulse is divided into multiple subpulses with different polarities and phases. By segmenting the pulse into bipolar components, the patent achieves spectral selectivity without requiring additional saturation pulses, thereby reducing SAR while maintaining fat suppression capability.
Solution Approach 2:
The patent modifies the frequency parameters of the RF pulse by applying a detuning shift relative to the resonance frequency of the spin species to be suppressed. This parameter change enables the pulse to selectively affect fat protons while reducing the overall energy deposition and SAR.
2Measurement precision
If frequency-selective RF saturation pulses are used to suppress fat signal, then spectral selectivity is improved, but the sequence progression time increases
Solution Approach 1:
The patent combines spectral selection and slice selection functions into a single RF pulse sequence with bipolar subpulses. This merging eliminates the need for separate saturation pulses, reducing the overall sequence time while maintaining spectral selectivity for fat suppression.
3Device complexity
If conventional RF pulses are used for excitation, then the sequence is simple, but slice profile quality and spectral selectivity are limited
Solution Approach 1:
The RF pulse is segmented into multiple subpulses with alternating polarities. This segmentation enables precise control over the excitation profile, improving slice profile quality and spectral selectivity while maintaining a relatively simple overall sequence structure.
Solution Approach 2:
The patent employs periodic bipolar subpulses with alternating polarities and phases. This periodic action creates a refined slice profile and enhances spectral selectivity through constructive and destructive interference patterns, achieving high precision without excessive complexity.
4Object-affected harmful factors
If spectral selection is applied to suppress spin species with different chemical shifts, then image artifacts are reduced, but the pulse sequence complexity increases
Solution Approach 1:
The patent applies a detuning shift to the RF pulse frequency relative to the resonance frequency of the spin species to be suppressed. This parameter change enables spectral selection to eliminate image artifacts from chemical shift while maintaining a relatively simple pulse sequence through unified bipolar RF excitation.
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 enhances spectral selectivity and slice profile quality, reducing image artifacts and SAR while allowing thinner slices, improving image clarity and diagnostic accuracy.
Implementation Method 1
high frequency excitation pulses (RF pulses) are radiated into the examination object and the nuclear spin resonances produced are measured as so-called k-space data
Implementation Method 2
Different environments of protons shield the B0 field to different extents so that a different magnetic field arises at a nucleus that leads to different resonance frequencies. What is referred to here is a chemical shift between the different spin species.
Implementation Method 3
the subpulses are radiated in at a frequency that is detuned by a detuning shift relative to a resonance frequency of a spin species that is to be represented, such that by way of the detuning shift a linear evolution of the phase over the temporal progression of the composite RF pulse results
Implementation Method 4
switching bipolar slice selection gradients so that successive subpulses of the composite RF pulse are encoded with differently polarized slice selection gradients
Implementation Method 5
A targeted excitation and/or suppression of signal contributions from spin species with a particular chemical shift has a high level of relevance in MR imaging
Data Source
AI summary
A method for recording scan data of an examination object which includes spins of at least two different spin species by means of a magnetic resonance system. The method includes: radiating in a composite RF pulse, for example, a binomial pulse comprising at least two subpulses; switching bipolar slice selection gradients so that successive subpulses of the composite RF pulse are encoded with differently polarized slice selection gradients; recording as scan data magnetic resonance signals triggered by the composite RF pulse; and storing and/or further processing the recorded scan data, wherein the subpulses are radiated in at a frequency that is detuned by a detuning shift relative to a resonance frequency of a spin species that is to be represented, such that by way of the detuning shift a linear evolution of the phase over the temporal progression of the composite RF pulse results.