MRI RF Envelope Correction for Amplifier Nonlinearity
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Solution Overview
Problem
MRI apparatuses face challenges in accurately transmitting RF signals due to amplifier nonlinearity, which affects Specific Absorption Ratio (SAR) and image quality, as the nonlinearity causes distortion in the output RF signals, leading to inconsistent energy absorption and temperature increases in the object being scanned.
Innovation Solution
The MRI apparatus includes processing circuitry that performs correction processing on the envelope of RF pulses to compensate for the nonlinear input-output characteristics of the amplifier, using a correction table or function generated from prescan measurements to ensure the output RF signals match the intended envelope shape and power levels, thereby enhancing SAR accuracy and image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If amplifier is used to amplify RF pulse, then output power is increased, but nonlinearity causes distortion of RF signal envelope
Solution Approach 1:
The patent applies preliminary action by measuring the amplifier's nonlinear characteristics in advance through prescan and storing correction data in a lookup table. The envelope correction unit then retrieves and applies appropriate correction values before RF pulse transmission, compensating for expected nonlinear distortion without requiring real-time complex calculations during actual imaging.
Solution Approach 2:
The patent implements feedback by measuring the actual output envelope of the amplifier during prescan and using this measurement to generate correction data. The correction process continuously adapts to the amplifier's actual performance characteristics, creating a closed-loop system that compensates for nonlinearities based on real measured data.
2Productivity
If high frequency RF signals are transmitted to object, then MR signals are acquired, but object temperature increases
Solution Approach 1:
The patent replaces direct mechanical control of RF power with an informational correction system. Instead of physically adjusting amplifier components to achieve linear output, the system uses digital envelope correction based on measured characteristics, substituting a computational approach for physical adjustment mechanisms.
3Measurement precision
If envelope correction is performed for entire RF pulse, then SAR accuracy is improved, but processing complexity increases
Solution Approach 1:
The patent reduces processing complexity during actual imaging by performing all complex measurements and correction table generation in advance during prescan. The envelope correction unit simply retrieves pre-calculated correction values from the lookup table based on desired output power levels, avoiding complex real-time calculations while maintaining high SAR accuracy.
Solution Approach 2:
The patent uses a lookup table structure that stores correction data for various power levels. This approach trades increased memory usage for simplified processing logic, allowing the system to quickly retrieve correction values without performing complex envelope calculations during each RF pulse transmission.
Data Source
AI summary
According to one embodiment, an MRI apparatus includes an amplifier and processing circuitry. The amplifier amplifies an RF pulse and outputs the amplified RF pulse to an RF coil. The processing circuitry performs correction processing on an envelope of an RF pulse to be inputted to the amplifier so as to compensate nonlinear input-output characteristics of the amplifier. As to this correction processing, the processing circuitry selects a correction information item out of a plurality of correction information items prepared for a corresponding plurality of imaging conditions and performs the correction processing by using the selected information item.