Time-Division Multiplexing for Parallel Transmission at Ultra-High Field With Limited RF Channels

Investigating time-division multiplexing for parallel transmission in ultra high-field imaging, striving for homogeneous whole brain excitation with a limited number of RF channels.

A fast RF switch was built to alternately route 8 transmit channels to each row of a double-row 16-element transmit coil array at a 9.4 T human MRI system. Methods for SAR monitoring and pulse design for this temporal degree of freedom were developed and investigated in electromagnetic simulations and in vivo measurements, employing parallel transmission kT points pulses aiming for homogeneous whole-brain excitation. The achievable trade-off between local SAR and excitation homogeneity was compared for multiplexed and simultaneous transmission.

Using time-division multiplexing, similar excitation fidelity as with 16 transmit channels can be achieved with only 8 channels. For instance, multiplexing reduces the flip angle inhomogeneity by 2.22-fold compared to exciting only a single row of the array, and by 1.85-fold compared to statically splitting and routing 8 channels to 16 transmit coil elements. As a trade-off, compared to simultaneous excitation, multiplexing requires either increased pulse duration or amplitudes, the latter causing increased SAR. However, with appropriate SAR-aware pulse design, the multiplexing-induced local SAR increase can be controlled. This allows for viable pulse design solutions for the considered low-flip-angle imaging scenarios.

Time-division multiplexing allows driving a larger number of transmit elements with a smaller number of RF channels, resulting in improved parallel transmission performance. This opens up new possibilities for using advanced multi-row transmit coil arrays in sites with only 8 RF channels available.