Nonlinear Gradient Fields for MRI SAR Reduction
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Solution Overview
Problem
Conventional magnetic resonance imaging (MRI) techniques face challenges in reducing the specific absorption rate (SAR) of radio-frequency (RF) energy while maintaining optimal slice profiles, leading to inefficiencies and potential tissue damage due to the linear dependence of gradient fields on spatial coordinates.
Innovation Solution
The use of nonlinear magnetic fields for slice selection allows for the alteration of the RF pulse and slice profile relationship, enabling SAR reduction without compromising the slice profile, and enabling curved slice imaging and reduced scan times by modifying the RF envelope and slice profile simultaneously.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If sinc-type RF pulse envelopes are used to achieve rectangular slice profiles with linear gradient fields, then slice profile quality is improved, but specific absorption rate (SAR) increases
Solution Approach 1:
The patent changes the gradient field parameter from linear to nonlinear spatial dependence. This allows using simpler RF pulse envelopes while achieving the desired slice profile, thereby reducing SAR without compromising slice quality. The nonlinear gradient field transforms the relationship between RF envelope and slice profile, breaking the direct Fourier transform connection that necessitates high-SAR sinc pulses.
2Loss of energy
If variable-rate selective excitation (VERSE) is used to reduce SAR by modifying gradient currents as a function of time, then specific absorption rate decreases, but off-resonance effects cause blurring
Solution Approach 1:
Instead of changing gradient currents as a function of time (VERSE approach), the patent changes the spatial parameter of the gradient field from linear to nonlinear. This static nonlinear transformation achieves SAR reduction without introducing time-varying off-resonance effects, thereby maintaining slice profile precision while reducing energy loss.
3Ease of operation
If conventional linear gradient fields are used for slice selection, then the relationship between RF envelope and slice profile is straightforward, but SAR reduction is limited
Solution Approach 1:
The patent modifies the gradient field parameter from linear to nonlinear spatial dependence. This change decouples the direct Fourier transform relationship between RF envelope and slice profile, enabling the use of lower-SAR RF envelopes while maintaining slice selection effectiveness. The nonlinear transformation provides new degrees of freedom in pulse sequence design.
4Loss of energy
If RF pulse duration is extended to reduce peak power and SAR, then specific absorption rate decreases, but total scan time increases
Solution Approach 1:
By changing the gradient field from linear to nonlinear spatial dependence, the patent enables the use of shorter, lower-SAR RF envelopes that maintain effective slice selection. The nonlinear gradient provides enhanced spatial encoding efficiency, allowing reduced RF pulse duration without compromising slice profile quality, thereby reducing both energy loss and time loss simultaneously.
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 effectively reduces SAR, improves slice profile selectivity, and decreases total scan time by allowing for the design of both RF envelope and slice profile simultaneously, while maintaining efficient excitation and minimizing tissue exposure to RF energy.
Implementation Method 1
an additional gradient coil is used to produce a nonlinear gradient field in the slice selection direction
Implementation Method 2
a radio-frequency (RF) signal is transmitted to the sample (examination subject) in order to excite proton spins inside the sample
Data Source
AI summary
In the acquisition of magnetic resonance data from an examination subject according to a pulse sequence that causes radiation into the examination subject of a radiated radio frequency (RF) pulse having a frequency spectrum and a slice profile with a relationship therebetween, the examination subject is substantially simultaneously, with the radiated RF pulse, to a non-linear magnetic field that alters the relationship in the radiated RF pulse between the frequency spectrum and the slice profile. The alteration of this relationship can be used, for example, to reduce the specific absorption rate (SAR) of the examination subject during the acquisition of the magnetic resonance data.