Active electro-optical probe system for B1-field polarization mapping in magnetic resonance imaging systems

We present a medical resonance imaging (MRI) B1-field polarisation mapping system based on active electro-optical H-field probes. The active electro-optical field probes have a bandwidth ranging from 10 MHz to 6 GHz with a sensitivity of <; -110dB(A/m)/√Hz at 128MHz. The miniature size of the probe elements (2×2 mm²) provides a high spatial resolution. The electrical isolation using fiber-optics enables the system to be used in the harsh electro-magnetic environment of an MRI system and eliminates any feedback on the RF transmit coil under test. The developed measurement system has been tailored to the B1-field measurement application with a specific control and postprocessing software and allows for traceable B-field calibration with an absolute measurement uncertainty of better than ±1 dB. The relative measurement uncertainties in terms of the axial ratio and the relative phase of the B1x and B1y (polarisation ellipse parameter) are better than 0.4 dB and 3%, respectively.