A liquid nitrogen-cooled cryostat for multichannel HTS magnetoencephalography

Magnetoencephalography (MEG) is a functional neuroimaging technology used in neuroscience as well as in the diagnosis and treatment of brain disorders like epilepsy and for surgical planning. In MEG neural activity is measured by sensing the weak magnetic fields (~10s to 100s of femtotesla) that are generated by neural currents in the brain. Highly sensitive magnetic field sensors are required for MEG. Today’s commercial systems therefore employ liquid helium-cooled low-Tc SQUIDs. With advances in high-temperature superconducting (HTS) technology high-Tc SQUIDs have become a serious alternative. Due to the higher operating temperature they can be placed closer to the head and therefore measure higher magnitude signals than low-Tc SQUIDs that must record from further away – thus enabling them to overcome the typically higher noise compared to their commercial counterparts [1]. We have developed a liquid nitrogen-cooled cryostat for a 7-channel HTS MEG system. The cryostat is designed for minimum distance between the cold (~77 K) SQUIDs and the head of a subject at room temperature. With superinsulation around the nitrogen vessel and a small vacuum space between the sensors and a thin window, we can achieve minimum sensor-to-head distances of less than 3 mm. ... mehrThe magnetometers are arranged in a dense hexagonal pattern for high spatial sampling of a small area of the head (≈ 15 cm2). The outer sensors are tilted towards the middle to align them to the average adult head’s curvature. The sensor tilt combined with a thin, curved window ensures minimal distance to the head for all sensors. The system employs 10 mm × 10 mm bicrystal dc SQUIDs made from YBa2Cu3O7-x with direct injection feedback (to minimize crosstalk [2]). The SQUIDs are thermally connected to liquid nitrogen via a sapphire fixture. To control temperature the nitrogen reservoir can be pumped on. The cryostat achieves high temperature stability (<0.1K) over long measurement times (>18h with a single filling). We will present the design and performance of the cryostat and show results from measurements on a head phantom.

[1] Schneiderman, J. Neurosci. Methods 222, 42-46 (2014)
[2] Ruffieux, Supercond. Sci. Technol. 30, 054006 (2017)