Reciprocity and sensitivity of opposed-solenoid endovascular MRI probes

Several forms of catheter-mounted "inside-out" probes for endovascular imaging have been proposed in the literature. The "opposed-solenoid" structure has been studied in relatively little detail, although it has some potential advantages over the others. Using a small water sample as a voxel, we measure point by point the spatial variation of the sensitivity and the rf field strength of such a probe, and connect the two by the reciprocity relation. By itself, the corresponding plot provides a nice example of the reciprocity relation at work; and for the characterization of the probe it gives a check on data quality. The results can be understood from simple considerations and agree well with the sensitivity observed in the image of a phantom.