Figure 2
(a) To-scale Solidworks drawing™ of our arbitrary waveform relaxometer (AWR) accurately representing the actual physical device. It comprises a miniature drive coil (Tx) with a low net inductance of 2.5 μH, two receive coils in gradiometric configuration (Rx1, Rx2), and a biasing coil to extend the applied field range. The gradiometer-shimming mechanism shifts Rx2 relative to Tx2 in increments of 22 μm to fine-tune the Tx2-Rx2 coupling to match that of Tx1-Rx1, minimizing net Tx-Rx coupling. The concept of cancellation amplitude adjustment for inductive decoupling is not new46,48, but our novel mechanical implementation allows for in-bore facile precision “spatial-shimming” allowing for simultaneous feedback of gradiometer performance during adjustment. The in-bore adjustment is important because removing, adjusting then re-inserting the receive coil into the MPS setup may incur placement error and we have shown in Fig. 3a that even tens of microns can affect gradiometer performance. Prior hardware using inductive decoupling46,47,48 do not allow in-bore adjustment. (b) Prior sinusoidal MPI spectrometers/relaxometers requires capacitors in the transmit chain to reduce reactive power and/or a band-stop filter (BSF) in the receive chain to reduce feedthrough. However, arbitrary drive waveforms precludes the use of tuned circuit elements. Instead, the AWR’s novel untuned design relies on a very low coil inductance of 2.5 μH coupled with a high coil efficiency of 1.06 mT/ampere for transmit power handling. An improved gradiometer is used for broadband feedthrough attenuation on the receive.
