A comparison of iterative minimum norm estimation and current dipole estimation for magnetic field measurements from small animals

We developed a multi-channel high-resolution SQUID magnetometer for measuring biomagnetic fields produced by small animals. The diameter of the pickup coils in our system is 5 mm, i.e., smaller than typical systems, 12 magnetic fields on a 7.5 mm grid were measured with better spatial resolution than typical systems. We measured the magnetic field response to auditory stimuli in rat brains in a magnetically shielded room. Eight male rats were anesthetized intraperitoneally with Urethane (1 g/1 kg) and stimuli consisting of 3 Hz tone burst sounds were introduced to the left ears. Averaging of the magnetic fields was performed for over 3000 auditory stimuli. The localization of current sources was calculated by two methods, the iterative minimum norm estimation (MNE) and the current dipole model (CDM). Current sources were localized in the auditory area of the right hemisphere in the rat´s cortex by iterative MNE. Estimated sources by CDM were localized more deeply than estimated current sources by iterative MNE. The correlation coefficient between the measured magnetic fields and the reconstructed magnetic fields in current sources was higher for iterative MNE than CDM. Iterative MNE is useful for source localization of auditory cortical activities in rats. Our system is a useful and noninvasive tool for brain research in small animals