Dynamical cancellation of pulse-induced transients in a metallic shielded room for ultra-low-field magnetic resonance imaging

Abstract

Pulse-induced transients such as eddy currents can cause problems in measurement techniques where a signal is acquired after an applied preparatory pulse. In ultra-low-field magnetic resonance imaging, performed in magnetic fields typically of the order of 100 μT, the signal-to-noise ratio is enhanced in part by prepolarizing the proton spins with a pulse of much larger magnetic field and in part by detecting the signal with a Superconducting QUantum Interference Device (SQUID). The pulse turn-off, however, can induce large eddy currents in the shielded room, producing an inhomogeneous magnetic-field transient that both seriously distorts the spin dynamics and exceeds the range of the SQUID readout. It is essential to reduce this transient substantially before image acquisition. We introduce dynamical cancellation (DynaCan), a technique in which a precisely designed current waveform is applied to a separate coil during the later part and turn off of the polarizing pulse. This waveform, which bears no resemblance to the polarizing pulse, is designed to drive the eddy currents to zero at the precise moment that the polarizing field becomes zero. We present the theory used to optimize the waveform using a detailed computational model with corrections from measured magnetic-field transients. SQUID-based measurements with DynaCan demonstrate a cancellation of 99%. Dynamical cancellation has the great advantage that, for a given system, the cancellation accuracy can be optimized in software. This technique can be applied to both metal and high-permeability alloy shielded rooms, and even to transients other than eddy currents.

Evaluating the Performance of Ultra-Low-Field MRI for in-vivo 3D Current Density Imaging of the Human Head

“…However, approaching such high fields will cause flux trapping in the sensor [18,29,30] and the superconducting filaments of the coil [17,31], which has to be dealt with. Also larger currents required for the compensation of the field transient [32] cause excessive heating in the compensation coils, requiring more sophisticated techniques [33]. In addition, it should be mentioned that most tDCS devices do not support the application of 4.5-mA current.…”

Accessible magnetic resonance imaging: A review

“…The results indicate that it is possible to perform ULF‐MRI in unshielded environments through incorporation of spatially correlated active compensation. The effect of and compensation for eddy currents at ULF‐MRI in MSR have also been studied through the application of precisely designed currents applied on separate coils …”

“…The effect of and compensation for eddy currents at ULF-MRI in MSR have also been studied through the application of precisely designed currents applied on separate coils. 58 Very Low Field (VLF) MRI (10-100 mT) MR images generated at this field retain some of the T 1 contrast advantages seen in ULF-MRI. 10 In addition, the use of rare earth materials like neodymium iron boron (NdFeB) produce magnetic fields in the VLF range and rapidly decay as a function of distance from the magnet.…”

Automatic Spatial Calibration of Ultra-Low-Field MRI for High-Accuracy Hybrid MEG–MRI

“…However, as the overall aim is to maximize the image quality while solving also these issues, we have left modeling of their effects out of this work. For example, field distortions related to pulsing MRI coils can be reduced by coil design and Dynamical Coupling for Additional dimeNsions (DynaCAN) [23], [24], and the MRI electronics should have high precision in order not to affect the image quality [25].…”