Juho Luomahaara

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.

Electrical Transport and Field-Effect Transistors Using Inkjet-Printed SWCNT Films Having Different Functional Side Groups

The electrical properties of random networks of single-wall carbon nanotubes (SWNTs) obtained by inkjet printing are studied. Water-based stable inks of functionalized SWNTs (carboxylic acid, amide, poly(ethylene glycol), and polyaminobenzene sulfonic acid) were prepared and applied to inkjet deposit microscopic patterns of nanotube films on lithographically defined silicon chips with a back-side gate arrangement. Source-drain transfer characteristics and gate-effect measurements confirm the important role of the chemical functional groups in the electrical behavior of carbon nanotube networks. Considerable nonlinear transport in conjunction with a high channel current on/off ratio of approximately 70 was observed with poly(ethylene glycol)-functionalized nanotubes. The positive temperature coefficient of channel resistance shows the nonmetallic behavior of the inkjet-printed films. Other inkjet-printed field-effect transistors using carboxyl-functionalized nanotubes as source, drain, and gate electrodes, poly(ethylene glycol)-functionalized nanotubes as the channel, and poly(ethylene glycol) as the gate dielectric were also tested and characterized.

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.

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.

Kinetic inductance magnetometer

Sensing ultra-low magnetic fields has various applications in the fields of science, medicine and industry. There is a growing need for a sensor that can be operated in ambient environments where magnetic shielding is limited or magnetic field manipulation is involved. To this end, here we demonstrate a new magnetometer with high sensitivity and wide dynamic range. The device is based on the current nonlinearity of superconducting material stemming from kinetic inductance. A further benefit of our approach is of extreme simplicity: the device is fabricated from a single layer of niobium nitride. Moreover, radio frequency multiplexing techniques can be applied, enabling the simultaneous readout of multiple sensors, for example, in biomagnetic measurements requiring data from large sensor arrays.

Optical and Electrical Characterization of a Large Kinetic Inductance Bolometer Focal Plane Array

Sub-THz imaging techniques are currently emerging with applications especially in security screening requiring higher throughput in mass transit and public areas. In the context of person imagers, the field of view and the spatial resolution set the requirement for the number of image pixels. We perform an experimental feasibility study on a fully staring radiometric camera with one detector per image pixel. The aim is to avoid the shortcomings characteristic of optomechanical scanners with a limited number of detectors. Our approach is based on superconducting kinetic inductance bolometer arrays. We demonstrate successful fabrication and operation of a focal plane array with 2500 nanomembrane-integrated bolometers, and a compatible optical system enabling standoff imaging at the distance of 5 m. We characterize the system in terms of radiometric contrast and spatial resolution.

Influence of Substrate on Plasmon-Induced Absorption Enhancements

A set of periodic plasmonic nanostructures is designed and fabricated as a means to investigate light absorption in single-crystal silicon thin-film structures with silicon-on-insulator (SOI) wafers as a model system. It is shown both computationally and experimentally that plasmon-induced absorption enhancement is remarkably higher for such devices than for thick or semi-infinite structures or for the thin-film amorphous silicon solar cells reported in the literature. Experimental photocurrent enhancements of the orders of 12 and 20 are demonstrated for non-optimized 2200-nm-thick photoconductive and 300-nm-thick photovoltaic test structures, respectively. Theoretical absorption enhancements as high as 80 are predicted to be achievable for the similar structures. The features of the spectral enhancements observed are attributed to several interacting resonance phenomena: not just to the favourable scattering of light by the periodic plasmonic nanoparticle arrays into the SOI device layer and coupling to the waveguide modes interacting with the plasmonic array but also to the Fabry-Pérot type interferences in the layered structure. We show that the latter effect gives a significant contribution to the spectral features of the enhancements, although frequently ignored in the discussions of previous reports.

Characterization of sub-range-bin spatial features with frequency modulated continuous wave radar operated at 650 GHz

In this paper we present imaging results obtained from frequency modulated continuous wave radar setup operated at 650 GHz and optimized for imaging of objects with lateral dimensions between 1-20 cm. We consider in particular the ability and the benefits of using such system for resolving features corresponding to depth variations smaller than the standard depth resolution of FMCW systems. This is accomplished by observing the lateral variation of the amplitude and the phase of the reflection coefficient within one range bin. The presented imaging results from 3D printed plastic phantom objects resolve features with depth profiles down to 0.1 mm in the system with a range bin of 5 mm.

Combination of MEG and MRI in one setup

R.J. Ilmoniemi, J. Dabek, F.-H. Lin, J.O. Nieminen, L.T. Parkkonen, P.T. Vesanen, K.C.J. Zevenhoven, A.V. Zhdanov, J. Luomahaara, J. Hassel, J. Penttila, J. Simola, A.I. Ahonen, and J.P. Makela Dept. Biomedical Engineering and Computational Science, Aalto University School of Science, Espoo, Finland; Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan; Martinos Center, Massachusetts General Hospital, Charlestown, MA, United States; Elekta Oy, Helsinki, Finland; BioMag Laboratory, HUSLAB, Helsinki University Central Hospital, Helsinki, Finland; VTT Technical Research Centre of Finland, Espoo, Finland; Aivon Oy, Espoo, Finland

Side-wall spacer passivated sub-μm Josephson junction fabrication process

We present a structure and a fabrication method for superconducting tunnel junctions down to the dimensions of 200 nm using i-line UV lithography. The key element is a sidewall-passivating spacer structure (SWAPS) which is shaped for smooth crossline contacting and low parasitic capacitance. The SWAPS structure enables formation of junctions with dimensions at or below the lithography-limited linewidth. An additional benefit is avoiding the excessive use of amorphous dielectric materials which is favorable in sub-Kelvin microwave applications often plagued by nonlinear and lossy dielectrics. We apply the structure to niobium trilayer junctions, and provide characterization results yielding evidence on wafer-scale scalability, and critical current density tuning in the range of 0.1–3.0 kA cm−2. We discuss the applicability of the junction process in the context of different applications, such as SQUID magnetometers and Josephson parametric amplifiers.