Design and Performance Evaluation of Superconducting Wire-Wound Gradiometers for SQUID-Based Ultralow-Field MRI

In recent years, ultralow-field (ULF) magnetic resonance imaging (MRI) has shown its potential in portable MRI System. As the superconducting magnet is replaced by permanent magnet or electronic magnet, the weight and the cost of the system could be effectively reduced. However, the low main field strength leads to low signal-to-noise ratio (SNR). Superconducting quantum interference devices (SQUIDs), as the most sensitive magnetic flux sensors known to date, have been employed in the system to measure the weak magnetic signal. Since a bare SQUID chip has limited magnetic field resolution and insufficient adaptability to the practical environment, a flux transformer, which consists of a pickup coil and input coil, is typically introduced and coupled with the SQUID. For example, in our Al shielded ULF MRI system, second-order axial gradiometers are applied as pickup coils to suppress the external magnetic field noise, thus, enhance the SNR. However, the geometrical parameters of gradiometers significantly affect their performance, which includes the ability to suppress the ambient noise and the sensitivity to the signal. In this article, we first present geometrical requirements of the second-order gradiometers to ensure that the designed configurations have equal noise suppression ability. Then, we acquire a general expression of the sensitivity pattern of a second-order gradiometer through mathematical calculation. Through the above two steps, we propose a design and evaluation method for second-order SQUID-based gradiometers for ULF MRI. Based on this method, an asymmetric configuration wire-wound gradiometer with optimized loop numbers is proposed and applied in our ULF MRI system. Compared to the conventional symmetric configuration gradiometer, the proposed one achieved approximately a 1.5-fold improvement in sensitivity to the signal for a shallow imaging slice while maintaining similar noise suppression capabilities. This improvement offers a broader imaging area and higher measurement accuracy for the ULF MRI system.