MR Exciter Predistortion for RF Amplifier Nonlinearity
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Solution Overview
Problem
In magnetic resonance (MR) systems, the nonlinearity of RF amplifiers leads to distortion in the B1 field, resulting in a poor slice profile due to the RF amplifier's complex distortion, which existing methods have not adequately addressed at a system level.
Innovation Solution
A method and device for pre-distorting the exciter by obtaining and applying gain amplitude and phase correction values based on the RF amplifier's characteristic curve, using equations such as Cor(Pin)+Gx[Pin+Cor(Pin)]=0 and CPS(Pin)+PX[Pin+Cor(Pin)]=0 to correct the input power, stored in the form of a lookup table for predistortion, thereby linearizing the exciter and amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If predistortion is applied at carrier frequency rather than at baseband, then the RF amplifier distortion can be compensated, but the system-level application and slice profile quality remain inadequate
Solution Approach 1:
The patent applies predistortion at the baseband stage before RF modulation and amplification, rather than at carrier frequency. This preliminary correction of amplitude and phase characteristics at baseband enables the RF amplifier to operate in a more linear region, thereby improving slice profile quality while maintaining system-level simplicity through a unified predistortion approach
2Power
If the RF amplifier operates at high power, then the excitation strength is sufficient, but nonlinearity causes signal distortion and poor slice profile
Solution Approach 1:
The patent introduces a predistortion module that applies opposite distortion characteristics to the baseband signal before RF amplification. By pre-correcting amplitude and phase deviations in reverse of the expected amplifier nonlinearity, the system enables high-power operation while maintaining linear output characteristics and accurate slice profiles
Solution Approach 2:
The system employs feedback mechanisms to measure actual RF amplifier output characteristics and uses this information to dynamically adjust predistortion parameters. This closed-loop approach ensures that the predistortion compensation remains accurate even as amplifier characteristics vary with operating conditions, maintaining both high power output and slice profile precision
Data Source
AI summary
A method for pre-distorting an exciter includes obtaining a characteristic curve of a radio frequency (RF) amplifier, obtaining a first relationship between a gain amplitude correction value and an input power and a second relationship between a gain phase correction value and the input power based on the characteristic curve, obtaining a transmission gain for use in a scan from a magnetic resonance pre-scan, and obtaining the gain amplitude correction value and the gain phase correction value of an input of the exciter based on the input of the exciter, the transmission gain, the first relationship, and the second relationship. An input of the exciter is pre-distorted using the gain amplitude correction value and the gain phase correction value.