Noise Shielding And Suppression For Ultra-low-field MRI Based On SQUID

Superconducting Quantum Interference Device?SQUID?is an ultra-sensitive magnetic flux detector,whose flux resolution can reach few ??₀/?Hz at liquid helium?4.2 K?.With application of SQUID to nuclear magnetic resonance?NMR?and magnetic resonance imaging?MRI?,researches on ultra-low-field?ULF?NMR and MRI have arose great attentions.Unlike the convertional high field MRI,the static magnetic field of ULF MRI is in the order of tens to hundreds of micro-tesla??T?.So,the major challenge of the ULF NMR and MRI is to improve the signal-to-noise?SNR?.In order to improve SNR,we took some methods to upgrade our system.We used the B_p coil instead of using permanent magnet?PM?to produce the polarizing field B_p.After upgrading ULF MRI system,the strong RF noise brought by B_p coil and the electric apparatus noise affected the operation of the system.In order to slove this problem,we decided to constructe an aluminum shielded room to shield the system and improve the SNR.First,we measured the noise in free space and in3-mm-thick shielded room.The shielding factor of 3-mm-thick shielded room may not meet the requirement.So,we need to design and built a new shielded room which may attenuate the white noise by half.Then we calculated and simulated the shielding effectiveness for different thickness of aluminum sheets and chose the6-mm-thick sheets to construct our new shielded room.After constructing the shielded room,the performance of the new shielded room was measured.The result meets our requirement.Besides,the NMR signals of water at 5.9 kHz were measured in free space and in the 6-mm-thick shielded room,and the SNR was improved from 3 to 20.Due to the working mode of the polarizing coil,we had to cut off the polarizing power in very short time.The transient magnetic fields produced by the induced eddy current in aluminum plates would interfere with the measurements.In order to solve this problem,we can design an eddy current cancellation system by simulation.Then,we can construct the eddy current cancellation system according to the simulation.Currently,we have simulated the coil system,which can decrease the system dead time and reduce eddy current influence on the system.The application of shielded room can attenuate the system noise effectively,but power line harmonic interference may still affect the system and in an unshielded room the power line harmonic cause stripe-artifacts in ULF MRI.In order to eliminate these artfacts in MRI images,we describe an adaptive suppression method.This technique utilizes spatial correlation of the interference from different positions and is realized by substrating the outputs of the reference channels from the signal channel.This method can effectively eliminate artifacts and improve the quality of images.At the end of the paper,the work of this paper is summarized and the next step is prospected.