Analysis of RF transmit performance for a 7T dual row multichannel MRI loop array.

We present a numerical investigation of the RF fields generated inside a human head by single and dual-row loop arrays. For a uniform circular polarization (CP) mode excitation, a dual-row array has no advantage for human brain excitation. Significant improvement of B(1)+ homogeneity with a simultaneous increase of coverage in the axial direction can be obtained by using a dual-row array together with a static RF shim: that is, excitation of both rows separately in CP mode, while providing the upper row elements with a +90° phase shift relative to axially adjacent lower row elements. For this case the excitation efficiency over the entire brain remains practically unaffected, and the improved B(1)+ coverage results in a relatively smaller amount of power delivered to brain. To keep the mean B(1)+ across the brain equal to its value in a uniform CP excitation mode, a larger transmit power level is required. This results in a moderate increase of peak SAR(10 g). The location of peak SAR(10 g) moves from the brain (uniform CP mode location) to the nose skin. The performance of dual-row arrays in transmit SENSE operation will be explored in future investigations.