In all MRI scanners B₁ field strongly varies with coil resistance (patient load) and reactance (offresonance), and necessitates time consuming MRI RF transmitter power adjustments and tuning. Multislice and offcenter imaging issues due to offresonance cannot be solved that way. We propose fully digital RF coil current sources, which completely remove these problems. Additionally, in difference to all known linear or switch mode RF amplifiers our proposal maintains high efficiency under modulation, and thus provides sufficent average RF power even at low flip angles, e.g. in FLASH sequences without the need for significant, safety critical transmitter oversizing.

The detrimental dependence of RF coil current on Q and detuning is due to finite RF transmitter output impedance and load matching. Any mismatch breaks control over B₁ field and flip angle. Only RF current sources solve this issue [1,2].

Secondly, MRI scanners should be able to reach the SAR limit with arbitrary reduced RF power level and correspondingly high duty cycle, e.g. in FLASH sequences. Unfortunately the efficiency of all known linear analog and switch mode RF amplifiers [3,4] severely degrades under modulation. Commercial scanners thus oversize the transmitter by e.g. 7-fold (!) beyond the SAR power limit, at associated costs and safety risks.

We herewith propose direct digital single sideband RF current sources with high efficiency at any modulation and duty cycle. No oversizing is needed. Our approach is to invert the very low output Thevenin impedance R_out of switch mode AC generators - effectively AC voltage sources with load independent high efficiency - to (near) infinite. Impedance transformer can be a transmission line segment or a lumped element Pi or T section.

RF switcher output voltage, and thus RF coil current and transmit B₁ are under digital computer control. RF power is generated in amplitude and phase directly at baseband without any analog small signal modulators, amplifiers, circulators and dummy loads, current sensor or feedback control. Baseband RF synthesis and the wideband properties of the inverter circuit allow to place the idle AM sideband far from the base band close to an impedance pole. This also avoids harmonics within the base band and at circuit series resonances.

Ultrasound pulser ICs are cheap, readily available, tiny RF voltage sources with high clock rate synchronous digital three level digital control.

We used Maxim MAX14809 pulser ICs featuring eight ±100V_pk, ±2.5A_pk output channels in a 10x10mm² package. 63.6MHz band center frequency and 80pF channel output capacitance suggested individual Z₀=32Ohm lumped Pi lowpass lambda/4 transformers (see schematic). Dual ±30V_dc supply voltage resulted in 40V_pk AC, or 1.25A_pk coil current, and 50Ohm maximum load per current source. Our L=320nH RF transmit loop coil had 7Ohm max series impedance including offresonance reactance, and thus accepted up to 8 parallel current sources, i.e. a single full IC. 10A_pk combined output current (320W peak RF power) was thus available over more than ±1MHz bandwidth. We augmented the series resonant RF coil with low inductance resistors to cover a range Q~90 to 15. The B₁ field amplitude varied by only about 3% under such Q and ±1MHz frequency variations.

The pulser load circuit had series resonances at about 57MHz and 71MHz and a parallel resonance at 90MHz. 180MHz bit clock avoided aliasing into the base band or series resonances up to the 23^th harmonic, and minimized power at the 180-63.6=116.4MHz AM mirror sideband. Single channel full power digital noise SNR was 40dB per channel, and dynamic range was about 60dB. No dynamic range is lost to adjust RF transmit amplitudes. RF current sources can be combined by parallelization, which increases RF coil current proportionally and improves SNR and dynamic range by square-root depending on digital control.

Three-level modulation allows sparse switching with correspondingly low power dissipation and high efficiency at any instantaneous power level. 10W average output power kept the 4W IC thermal limit independent of instantaneous burst amplitudes.

Digital RF switch mode current sources eliminate image artifacts due to B₁ field amplitude errors, time consuming transmitter calibration and adjustment, and costly and safety critical RF transmitter oversizing to compensate low power efficiency under modulation. No known analog control linear or switch mode amplifier can provide that.

Our 320W current source modules occupy only a few cm² PCB space. 40kW peak power for a whole body resonator amounts to about 512 channels or 64 ICs, which are now scheduled to be integrated inside or close to the RF birdcage. Optimal array control bit pattern design is an ongoing research topic.