RF Transmission Phase Calibration in MRI Systems
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Solution Overview
Problem
In magnetic resonance imaging, multichannel radiofrequency transmission systems face challenges in maintaining phase accuracy between modulators and receivers due to cable lengths and properties, leading to complex synchronization and potential phase shifts during system restarts, which can result in incorrect phase differences and increased SAR exposure.
Innovation Solution
A method involving a first calibration measurement to determine phase differences between internal and external measuring devices, allowing for phase-accurate actuation and correction, and a second calibration measurement to account for phase changes upon system restarts, using a fixed reference to maintain consistent phase settings across transmission paths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If synchronization of modulators is provided to maintain phase accuracy, then phase accuracy between transmission paths is improved, but device complexity increases significantly
Solution Approach 1:
A reference signal is introduced as an intermediary element that mediates between multiple modulators. Each modulator receives the same reference signal and generates its output based on this common reference, thereby maintaining phase accuracy without requiring complex direct synchronization between modulators. The reference signal acts as a common denominator that aligns all transmission paths.
Solution Approach 2:
The system is segmented into independent modulator units, each processing the reference signal independently. This segmentation allows each modulator to operate autonomously while maintaining phase coherence through their shared dependence on the reference signal, reducing the complexity of inter-modulator synchronization.
2Manufacturing precision
If phase differences are corrected for cable length variations, then transmission accuracy is improved, but device complexity and calibration requirements increase
Solution Approach 1:
Cable length compensations and phase corrections are determined in advance during a calibration phase. The system pre-calculates compensation values for each transmission path based on measured cable characteristics, and these pre-determined compensation values are then applied during normal operation. This preliminary action eliminates the need for real-time complex calculations during transmission.
Solution Approach 2:
The system changes the phase parameter of the reference signal dynamically to compensate for cable length variations. By adjusting the phase parameter based on pre-determined compensation values, the system corrects for cable-induced phase shifts without requiring physical modification of the cable connections or real-time measurement during transmission.
3Measurement precision
If external measuring devices are used to verify phase differences at reference plane, then measurement accuracy is improved, but ease of operation deteriorates due to additional connection requirements
Solution Approach 1:
Instead of directly measuring at the reference plane with external devices, the system creates a copy of the reference signal and measures this copy at a more accessible location. The phase characteristics of the copy reflect those at the reference plane, allowing accurate measurement without physical access to the reference plane itself. This copying approach simplifies operation while maintaining measurement accuracy.
Data Source
AI summary
A method for operating a transmission device of a magnetic resonance device is provided. In order to actuate coil elements of a radiofrequency coil with different phases, phase differences in a reference plane are taken into consideration. In a first calibration measurement to be performed once for each transmission path, a first phase of a transmitted radiofrequency signal is measured by an internal measuring device installed permanently in the transmission device spaced apart from the reference plane. A second phase of the transmitted radiofrequency signal is measured by a second, external measuring device to be connected to the reference plane for the first calibration measurement. At least one phase of the first phase and the second phase is taken into consideration in the phase-accurate actuating of the coil elements and/or for correcting further measurements with the internal measuring device.