Sub-Sm$^{-1}$ electromagnetic induction imaging with an unshielded atomic magnetometer

Progress in electromagnetic induction imaging with atomic magnetometers has brought its domain to the edge of the regime useful for biomedical imaging. However, a demonstration of imaging below the required 1 Sm$^{-1}$ level is still missing. In this Letter, we use an $^{87}$Rb radio-frequency atomic magnetometer operating near room temperature in an unshielded environment to image calibrated solutions mimicking the electric conductivity of live tissues. By combining the recently introduced near-resonant imaging technique with a dual radio-frequency coil excitation scheme, we image 5 mL of solutions down to 0.9 Sm$^{-1}$. We measure a signal-to-noise ratio of 2.7 at 2 MHz for 0.9 Sm$^{-1}$, increased up to 7.2 with offline averaging. Our work is an improvement of 50 times on previous imaging results, and demonstrates the sensitivity and stability in unshielded environments required for imaging biological tissues, in particular for the human heart.