Improving UHF Transmit Efficiency with Voltage Baluns

Debra Strick Rivera¹, Carsten Koegler¹, Robert Turner¹

Cable radiation reduces effective transmit power. Radiated power increases with the fourth power of frequency, and can therefore be detrimental to UHF transmit arrays. Baluns can improve voltage balance across transmit coils, and thereby reduce cable radiation. Amplitude balance, phase balance, and common-mode rejection ratio were introduced as simple quantitative methods. Wide-band and narrow-band 180 baluns were analyzed. Performance of narrow-band baluns was dramatically improved through amplitude and phase matching measurements. Transmit efficiency was characterized in the scanner from flip-angle maps. The addition of 180 baluns improved transmit power efficiency between 10% (on average) and over 50% at 7T.