Detection of body noise with an ultra-sensitive SQUID system

Using an ultra-sensitive single-channel SQUID system an upper limit of 80 aT Hz(-1/2) for the body noise contribution of the human head in ultra-low-field SQUID-based MRI or MEG is determined. We discuss in detail the various noise contributions which need to be taken into account. Simulations and measurements of conducting phantoms show that presumably residual radio frequency interference cause an increase in the sensor noise at the aT Hz(-1/2)-level and need to be considered. Using a phenomenological approach, the body noise contribution of the human head is determined to 55 aT Hz(-1/2) for our setup. We also provide simulations of the expected body noise for other sensor geometries.