Parallel transmit has shown to be advantageous in improving B1 field homogeneity¹, slice selectivity and reduction of SAR especially in high fields. In this work the opportunity of using Class-E amplifier for on coil excitation is exploited. Designed for high efficiency, this amplifier can have good performance in heating, size reduction and output power while minimizing the overall cost of the system. We have introduced a single coil amplifier for 1.5 T earlier before², but no MR experiment was presented. Also, in multichannel transmit the coupling between channels has to be considered. Coupling and simultaneous operation of on-coil modified dual Class-E amplifiers was investigated in a 3 T MR scanner, as a step toward implementing a 36 channel transmit array coil.

The amplifier consists of a wideband, 3 stage wideband digital driver similar to the one presented in the previous work. Driver is controlled by FPGA (ML509 XUP) through LVDS signaling. As input to FPGA 125 MHz, 10 MHz clocks and trigger signals from the scanner are used. A 6-cm diameter coil is connected directly to the amplifier board. It is tuned with 4 distributed capacitors and used for transmit purpose as well as the tuning network of the Class E amplifier. Instead of a choke inductor, a short coaxial transmission line was used in order to avoid high current flow in large inductor. Adapting the amplifier from 64 MHz (1.5T) to 123 MHz (3T) operation requires only tuning the coil capacitors and a single capacitor in the amplifier board.

Near the transmit coil a 2 cm diameter 50 ohm matched pickup coil is embedded for signal observation and output power measurement. By calibrating the coupling between pickup coil with the main coil and measuring the impedance of the loaded coil, the output power was calculated.

In multichannel transmit coupling is considered to be an issue. Our design is not matched to 50 ohm and the coils are tuned without considering coupling to the other channels because this design is intrinsically decoupled from the other coils. In order to prove this by experiment, we measured the coupling between the coils and observe the interaction of the amplifiers when both of the amplifiers are transmitting: First amplifier was transmitting at 123 MHz whereas the second was at 123.4 MHz. The signals frequencies were kept close so that filtering by the pickup coils is minimized. After that, the level of each signal in the pickup coils was measured and compared. This procedure was repeated by increasing the distance between coils from 6 cm to 12cm with 2 cm steps.

An experiment was conducted to test operation of 2 amplifiers inside the scanner. The test was done in a 3T Siemens Tim Trio MR scanner. A 2.56 ms rectangular envelope pulse was sent from the designed amplifiers at the same frequency while the scanners transmit system was off. The amplifiers were placed such that coils are 5 cm away. The excitation profile is expected to be of Sinc shape. For receive, a spine coil was used. As phantom, two small cylindrical phantoms were attached at each coil. The amplifiers were tested by transmitting from each of coils one at a time.

The amplifier was tested with output power levels up to 79 W and efficiency measured at minimum level 78% as the data illustrated in Table-1 a). Since the driver was designed to be wideband, with only tuning capacitors, the amplifier was adapted for both 1.5T and 3T. The decoupling values of 2 adjacent coils with respect to coil distance are shown in Table-1 b). Starting from a distance between coils of 6 cm the coupling is low and the amplifiers are not affected from each other. The presence of the amplifier near the coil did not bring any observable artifact in the image. Note that since the coils are not matched to 50 ohms, the conventional S₁₂ measurements are not possible.Class E amplifier is a good candidate for being used as an on coil amplifier for transmit array. It was shown that it can be implemented as on coil amplifier. Accurate tuning for high efficiency removes the need for heat sink and reduces the overall needed power. Being an important issue in array systems, coupling between coils was observed to be at levels that do not compromise the operation of nearby amplifiers. However it is highly related to distance between coils. As future work the target is to design a 36 channel transmit array with the presented on coil amplifiers.