Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment

Abstract

Power-line harmonic interference and fixed-frequency noise peaks may cause stripeartifacts in ultra-low field (ULF) magnetic resonance imaging (MRI) in an unshielded environment and in a conductively shielded room. In this paper we describe an adaptive suppression method to eliminate these artifacts in MRI images. This technique utilizes spatial correlation of the interference from different positions, and is realized by subtracting the outputs of the reference channel(s) from those of the signal channel(s) using wavelet analysis and the least squares method. The adaptive suppression method is first implemented to remove the image artifacts in simulation. We then experimentally demonstrate the feasibility of this technique by adding three orthogonal superconducting quantum interference device (SQUID) magnetometers as reference channels to compensate the output of one 2 nd -order gradiometer. The experimental results show great improvement in the imaging quality in both 1D and 2D MRI images at two common imaging frequencies, 1.3 kHz and 4.8 kHz. At both frequencies, the effective compensation bandwidth is as high as 2 kHz. Furthermore, we examine the longitudinal relaxation times of the same sample before and after compensation, andshow that the MRI properties of the sample did not change after applying adaptive suppression. This technique can effectively increase the imaging bandwidth and be applied to ULF MRI in both an unshielded environment and a shielded room made from aluminum sheets.

High-sensitivity in vivo contrast for ultra-low field magnetic resonance imaging using superparamagnetic iron oxide nanoparticles

“…Recently, the clear market for a low-cost, portable MRI scanner, particularly in the field of emergency medicine, has led to the design and construction of several prototype MRI scanners that operate at low magnetic fields (<0.3 T) (4), where it becomes economically advantageous to generate the main B 0 magnetic field with permanent magnets (5)(6)(7)(8)(9)(10)(11). Further reduction of B 0 into the ultra-low field regime (ULF; B 0 <10 mT) (12) makes possible MRI scanners based on inexpensive, and lightweight, electromagnets (13)(14)(15)(16).…”

Sensor Configuration and Algorithms for Power-Line Interference Suppression in Low Field Nuclear Magnetic Resonance

“…This can help us process the interferences independently, without being affected by the NMR signal. (2) In our case, the homogeneous magnetic field noise components dominate the magnetic field noise [26] and we focus on suppressing these noise components. (3) The interferences from all detectors should be strongly spatially correlated.…”

“…We configure a 4-channel SQUID sensors system which is placed in a commercial liquid helium dewar. The schematic diagram of the system can be found in [26]. The signal sensor, as shown in Figure 1, is composed of a 2nd-order gradiometer connecting to SQUID module (so-called S module in Figure 1) with 22 mm diameter and 50 mm baseline.…”

“…A detailed description of the noise reduction process based on the DWA-LSM can be found in [26]. The effect of the LSM-based noise suppression may be limited by two reasons: (1) some near-field interference could not be suppressed by the method based on spatial correlation, (2) the least squares method finds the globally optimal solution, so the suppression coefficients matrix k can be easily dominated by the interferences with strong intensity, but the weak interferences will be ignored.…”

SQUID-Based Magnetic Resonance Imaging at Ultra-Low Field Using the Backprojection Method

“…Espy et al constructed images from many kinds of samples including the human brain, human hand, liquid explosives, and water by SQUID in microtesla magnetic fields, and the resolution was sufficient to reveal anatomical features [25,26]. Huang et al acquired both 1-D and 2-D MR images with high quality by applying three orthogonal SQUIDs as reference channels to eliminate stripe-artifacts in ULF MRI by measurement of water phantom [27]. Vogel et al have demonstrated an encoding magnet array moving around the sample for the generation of a 3-D image by backprojection [28].…”