A magnetocardiogram is recorded with a SQUID array placed over a pregnant volunteer's…
PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Biophysics & Medical Physics · Bioelectricity, the senses and the physics of the body
Forces, Machines and Signals, Ch Muscle, heart and the fields outside the body
Problem
A magnetocardiogram is recorded with a SQUID array placed over a pregnant volunteer's abdomen. Treat both the mother's heart and the fetal heart as point current dipoles in a uniform conductor, and use the order-of-magnitude Biot-Savart estimate B∼μ0p/4πr2 for each. The mother's heart has the adult dipole moment estimated from a typical skin potential, pm=2.5×10−5Am, and lies about rm=12cm from the sensor; the fetal heart, smaller and beating faster, has a dipole moment about one tenth as large, pf=2.5×10−6Am, and lies closer, about rf=6.0cm from the sensor.
((a)) Estimate the peak field the sensor would see from the maternal heart alone and from the fetal heart alone.
((b)) Compare the two fields with each other and with typical urban magnetic noise of 10nT; state what this implies about the apparatus needed to see either signal at all.
((c)) The two fields are within a factor of a few of each other, so amplitude alone will not separate mother from fetus. The maternal heart beats at about 70 beats per minute and the fetal heart at about 140 beats per minute. Explain briefly, without further calculation, how the two periodic signals could still be told apart once recorded.