Stem compensation of loop sensors for electromagnetic blood flow and vascular diameter measurement

Accuracy of electromagnetic blood flow —and vascular diameter — measurements by means of intravascular loop probes in an extracorporeal alternating magnetic field can be impaired by an error signal induced in the probe stem. The standard method of removing this artifact by twisting the lead wires is difficult to implement in this case because the wires are bifilar and are made of highly resilient metals. The solution described here makes use of a hair pin loop incorporated within the probe stem in which an e.m.f. is induced that can be used to cancel the unwanted e.m.f. induced in the lead wires of the loop of the sensor. This scheme is applicable to simple loop probes as well as to orthogonal double-loop sensors.