Jaakko O. Nieminen

Hybrid ultra-low-field MRI and magnetoencephalography system based on a commercial whole-head neuromagnetometer

Ultra‐low‐field MRI uses microtesla fields for signal encoding and sensitive superconducting quantum interference devices for signal detection. Similarly, modern magnetoencephalography (MEG) systems use arrays comprising hundreds of superconducting quantum interference device channels to measure the magnetic field generated by neuronal activity. In this article, hybrid MEG‐MRI instrumentation based on a commercial whole‐head MEG device is described. The combination of ultra‐low‐field MRI and MEG in a single device is expected to significantly reduce coregistration errors between the two modalities, to simplify MEG analysis, and to improve MEG localization accuracy. The sensor solutions, MRI coils (including a superconducting polarizing coil), an optimized pulse sequence, and a reconstruction method suitable for hybrid MEG‐MRI measurements are described. The performance of the device is demonstrated by presenting ultra‐low‐field‐MR images and MEG recordings that are compared with data obtained with a 3T scanner and a commercial MEG device. Magn Reson Med, 2013.

Avoiding eddy-current problems in ultra-low-field MRI with self-shielded polarizing coils.

In ultra-low-field magnetic resonance imaging (ULF MRI), superconductive sensors are used to detect MRI signals typically in fields on the order of 10-100 μT. Despite the highly sensitive detectors, it is necessary to prepolarize the sample in a stronger magnetic field on the order of 10-100 mT, which has to be switched off rapidly in a few milliseconds before signal acquisition. In addition, external magnetic interference is commonly reduced by situating the ULF-MRI system inside a magnetically shielded room (MSR). With typical dipolar polarizing coil designs, the stray field induces strong eddy currents in the conductive layers of the MSR. These eddy currents cause significant secondary magnetic fields that may distort the spin dynamics of the sample, exceed the dynamic range of the sensors, and prevent simultaneous magnetoencephalography and MRI acquisitions. In this paper, we describe a method to design self-shielded polarizing coils for ULF MRI. The experimental results show that with a simple self-shielded polarizing coil, the magnetic fields caused by the eddy currents are largely reduced. With the presented shielding technique, ULF-MRI devices can utilize stronger and spatially broader polarizing fields than achievable with unshielded polarizing coils.

Solving the problem of concomitant gradients in ultra-low-field MRI.

In ultra-low-field magnetic resonance imaging (ULF MRI), spin precession is detected typically in magnetic fields of the order of 10-100 μT. As in conventional high-field MRI, the spatial origin of the signals can be encoded by superposing gradient fields on a homogeneous main field. However, because the main field is weak, gradient field amplitudes become comparable to it. In this case, the concomitant gradients forced by Maxwells equations cause the assumption of linearly varying field gradients to fail. Thus, image reconstruction with Fourier transformation would produce severe image artifacts. We propose a direct linear inversion (DLI) method to reconstruct images without limiting assumptions about the gradient fields. We compare the quality of the images obtained using the proposed reconstruction method and the Fourier reconstruction. With simulations, we show how the reconstruction errors of the methods depend on the strengths of the concomitant gradients. The proposed approach produces nearly distortion-free images even when the main field reaches zero.

Experimental Characterization of the Electric Field Distribution Induced by TMS Devices

BACKGROUND In transcranial magnetic stimulation (TMS) a strong, brief current pulse driven through a coil is used for non-invasively stimulating the cortex. Properties of the electric field (E-field) induced by the pulse together with physiological parameters determine the outcome of the stimulation. In research and clinical use, TMS is delivered using a wide range of different coils and stimulator units, all having their own characteristics; however, the parameters of the induced E-field are often inadequately known by the user. OBJECTIVE To better understand how the use of a specific TMS device may affect the resulting cortical stimulation, our objective was to develop an instrument for automated measurement of the E-fields induced by TMS coils in spherically symmetric conductors approximating the head. METHODS We built a saline-free, robotized measurement tool based on the triangle construction. The 5-mm-wide measurement probe allows complete sampling of the induced E-field at the studied depth. We used the instrument to characterize TMS coils and stimulators made by two companies. RESULTS The measurements revealed that all tested stimulators performed as expected, but we also found significant differences between the different stimulators. Measurements of different coil specimens of the same stimulator models agreed with each other. CONCLUSION The presented TMS calibrator allows a straightforward characterization of the E-fields induced by TMS coils. By performing measurements using this kind of a tool helps in ensuring that an investigator knows the properties of the E-field.

All-planar SQUIDs and pickup coils for combined MEG and MRI

Flux trapping and random flux movement are common problems in superconducting thin-film devices. Ultrasensitive magnetic field sensors based on superconducting quantum interference devices (SQUIDs) coupled to large pickup coils are especially vulnerable to strong external fields. The issue has become particularly relevant with the introduction of SQUID-based ultra-low-field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques. In this paper, we study the constraints of thin-film-based magnetometers and gradiometers as exposed to magnetic field sequences of ULF MRI. In particular, we address issues such as response recovery, transient noise, magnetization and behaviour under shielded room conditions after prepolarization. As a result, we demonstrate sensors that are suitable for a combined multi-channel magnetoencephalography (MEG) and MRI imaging system.

The Spatial and Temporal Distortion of Magnetic Fields Applied Inside a Magnetically Shielded Room

In the context of biomagnetism, a magnetically shielded room (MSR) is designed for shielding against external magnetic fields. Recently, several applications, such as combined structural magnetic resonance imaging (MRI) and functional magnetoencephalography (MEG), have emerged that require applying relatively strong magnetic fields inside the MSR. These magnetic fields induce eddy currents and magnetize the MSR walls that are made of materials with high permeability and conductivity. These eddy currents and magnetization generate secondary magnetic fields inside the room that disturb, e.g., combined MEG-MRI by affecting sample spins and by exceeding the available dynamic range of the magnetic field sensors. In this work, static and dynamic magnetic fields applied inside an MSR are studied both theoretically and experimentally. Using a spherical model, analytical expressions are derived for the amplitudes and time constants of the various secondary magnetic field modes. These predictions are validated by comparison with experimental measurements in a rectangular MSR. The results of this study facilitate, e.g., the design of coils compatible with an MSR; a self-shielded coil is presented that decreases the secondary magnetic fields to a small fraction.

Temperature dependence of relaxation times and temperature mapping in ultra-low-field MRI

Ultra-low-field MRI is an emerging technology that allows MRI and NMR measurements in microtesla-range fields. In this work, the possibilities of relaxation-based temperature measurements with ultra-low-field MRI were investigated by measuring T1 and T2 relaxation times of agarose gel at 50 μT-52 mT and at temperatures 5-45°C. Measurements with a 3T scanner were made for comparison. The Bloembergen-Purcell-Pound relaxation theory was combined with a two-state model to explain the field-strength and temperature dependence of the data. The results show that the temperature dependencies of agarose gel T1 and T2 in the microtesla range differ drastically from those at 3T; the effect of temperature on T1 is reversed at approximately 5 mT. The obtained results were used to reconstruct temperature maps from ultra-low-field scans. These time-dependent temperature maps measured from an agarose gel phantom at 50 μT reproduced the temperature gradient with good contrast.

Polarization encoding as a novel approach to MRI

In magnetic resonance imaging (MRI), there have been three basic techniques to encode the spatial origin of the signals, i.e. Fourier and radio frequency encoding and the use of sensitivity information of sensor arrays. In this paper, we introduce a new encoding method, which we call polarization encoding. The method utilizes sets of polarizing fields with various spatial profiles; it is tailored for MRI at ultra-low fields (ULF MRI). In ULF MRI, signals from a prepolarized sample are typically detected using an array of SQUID (superconducting quantum interference device) sensors at microtesla fields. The prepolarization is achieved with a field of the order of 10-100mT preceding the signal acquisition. In polarization encoding, the prepolarizing field is varied in a way to gain additional information about the sample. The method may also prove useful for modalities that in the absence of any precession aim to image the DC magnetization profile of a sample.

Consciousness and cortical responsiveness: a within-state study during non-rapid eye movement sleep

When subjects become unconscious, there is a characteristic change in the way the cerebral cortex responds to perturbations, as can be assessed using transcranial magnetic stimulation and electroencephalography (TMS–EEG). For instance, compared to wakefulness, during non-rapid eye movement (NREM) sleep TMS elicits a larger positive–negative wave, fewer phase-locked oscillations, and an overall simpler response. However, many physiological variables also change when subjects go from wake to sleep, anesthesia, or coma. To avoid these confounding factors, we focused on NREM sleep only and measured TMS-evoked EEG responses before awakening the subjects and asking them if they had been conscious (dreaming) or not. As shown here, when subjects reported no conscious experience upon awakening, TMS evoked a larger negative deflection and a shorter phase-locked response compared to when they reported a dream. Moreover, the amplitude of the negative deflection—a hallmark of neuronal bistability according to intracranial studies—was inversely correlated with the length of the dream report (i.e., total word count). These findings suggest that variations in the level of consciousness within the same physiological state are associated with changes in the underlying bistability in cortical circuits.

Suppressing Multi-Channel Ultra-Low-Field MRI Measurement Noise Using Data Consistency and Image Sparsity

Ultra-low-field (ULF) MRI (B 0 = 10–100 µT) typically suffers from a low signal-to-noise ratio (SNR). While SNR can be improved by pre-polarization and signal detection using highly sensitive superconducting quantum interference device (SQUID) sensors, we propose to use the inter-dependency of the k-space data from highly parallel detection with up to tens of sensors readily available in the ULF MRI in order to suppress the noise. Furthermore, the prior information that an image can be sparsely represented can be integrated with this data consistency constraint to further improve the SNR. Simulations and experimental data using 47 SQUID sensors demonstrate the effectiveness of this data consistency constraint and sparsity prior in ULF-MRI reconstruction.