Transmit Arrays and Circuitry

For body dimensions at 3T and head dimensions at 7T and above the RF wavelength in dielectric tissue is comparable or smaller than the dimensions of the human anatomy. This in turn leads to RF phase related traveling time differences, prominent wave behavior and the potential for a significant difference between transmit B1 + and receive B1 fields [5-10]. It is possible to correct some of the resulting B1 field in-homogeneities within traditional multi-mode resonant volume transmit coils through individual amplitude and phase control of quadrature feed points [5], coil design modifications[11], as well as individual resonance element adjustments [12, 13]. However even better control of these effects can only be achieved with more degrees of freedom provided by, for example, dedicated transmit array systems that are capable of supporting a higher number of independent channels for RF transmission and that allow for “RF shimming” [14-24]. Ideally transmit arrays can provide for B1 transmit field homogeneity, transmit efficiency and SAR minimization the equivalent of what receive arrays achieve for optimal SNR and parallel imaging performance. Particularly since transmit arrays support parallel excitation pulses across multiple coil elements, as first proposed by Katscher [25] and Zhu [26].The related rapidly emerging parallel transmission methods significantly improve RF excitation homogeneity and they can achieve high spatial selectivity and pulse acceleration by taking full advantage of the ability to influence B1 + fields through temporally and spatially varying RF excitation pulses [21, 27-37].