Transmit array systems have applications ranging from improved excitation homogeneity at high field to controlled RF decoupling for implants and interventional devices at 1.5T and 3T. In conventional deployments, the power amplifiers are located in equipment rooms, can be prohibitive in cost, and require complex monitoring. These factors create difficulties in performing feasibility applications of PTx (parallel transmit) technology by most research groups. Our goal is to develop locally deployable PTx arrays leveraging existing low-cost broadcast amplifier pallets. We demonstrate the conversion of a conventional slow-gated, non-linear FM band (88-108MHz) 1kW pallet to a fast-gated, linear non-magnetic amplifier targeting transmit array deployment in both 1.5T and 3T B₀ fields. Besides cost, this approach provides advantages such as reduced coaxial line loss between the amplifiers and transmit coils, and provides versatility in the hardware layout.

Virtually all FM RF power pallet amplifiers employ transmission line transformer construction techniques for the input and output impedance matching [1]. Since as a rule of thumb, these provide an approximate octave in matching, the suspicion was that FM band (88-108MHz) pallets could support a wider output power-bandwidth match (Fig. 1 left). FM pallets are typically biased at 50-100mA (deep class B) for efficiency but not linearity, while millisecond off-gating suffices for high VSWR detection. For MRI, the amplifier must be biased for linearity (class AB), have microsecond scale fast gating and non-ferrous matching elements. We selected a Broadcast Concepts model P1000FM-188XR FM pallet amplifier to demonstrate conversion techniques to class AB linear operation at both 64 and 128MHz.

Input match: The first step is to replace the input ferrous transformer with a non-ferrous 9:1 impedance air-core transmission line transformer (TUI-9 Communication Concepts). Figure 1 (right) shows the S11 match of the amplifier after this conversion. This transformer provides an adequate match at 128MHz, but not at 64MHz. To address this, an L-matching network using a shunt-series topology, shown in Figure 2(a), was added. The series branch is a series LC, and the shunt branch a shunt LC with both double tuned for 64MHz transformation but resonant at 128MHz to be transparent for 3T operation. Magnet proximity tests for 64MHz show full power is delivered with the air-core input modification even inside the bore (Fig. 3).

Biasing: Virtually all commercial pallets employ an LM723 voltage regulator with gate bias typically set through a somewhat high resistance potentiometer. One simply needs to adjust the bias from 50-100mA to approximately 3A for class AB operation as shown in Figure 4. An I_Dq of 3A provided a good compromise between linearity, efficiency and gain. This should be done with a power supply with low supply current limit to prevent accidental destruction of the LDMOS transistor.

Gating: The schematic in Figure 2(b) shows a high current op-amp LM7321MF [1] that is unconditionally stable driving capacitive loads used in conjunction with an analog switch, Vishay DG4599 [2], to quickly gate the amplifier on and off. The op-amp was inserted as a buffer on the gate bias potentiometer, and drove the SPDT analog switch. The switch rapidly grounds the gate or connects to the op-amp output. Gating time was reduced to about 30us, as shown in Figure 5.

Driver: The input drive for the pallet requires on the order of 2-4 Watts to attain output power in excess of 900W. We used Mitsubishi RA07H0608M (7W) and RA08H1317M (8W) power modules. The input gate drive was biased for linear modulation. At 64MHz and 128MHz respectively, these modules are just outside their specified power band, and have been verified also to deliver full power inside the magnet bore at 1.5T.

Broadcast pallets offer a low-cost approach for creating MR compatible non-magnetic transmit array amplifiers capable of operating in both 1.5T and 3T B₀ fields. Fine-tuning of power bandwidth for dedicated use at 1.5T or 3T will, however, require a length adjustment of the output transformers, at least. While the core functions of fast gating, power and linearity have been demonstrated, memory effects, and bias drift will require more complex controls. Even so, these techniques provide a means to rapidly prototype a basic PTx amplifier chain for research applications.