V. Jazbinsek

Multi-channel atomic magnetometer for magnetoencephalography: A configuration study

Atomic magnetometers are emerging as an alternative to SQUID magnetometers for detection of biological magnetic fields. They have been used to measure both the magnetocardiography (MCG) and magnetoencephalography (MEG) signals. One of the virtues of the atomic magnetometers is their ability to operate as a multi-channel detector while using many common elements. Here we study two configurations of such a multi-channel atomic magnetometer optimized for MEG detection. We describe measurements of auditory evoked fields (AEF) from a human brain as well as localization of dipolar phantoms and auditory evoked fields. A clear N100m peak in AEF was observed with a signal-to-noise ratio of higher than 10 after averaging of 250 stimuli. Currently the intrinsic magnetic noise level is 4fTHz(-1/2) at 10Hz. We compare the performance of the two systems in regards to current source localization and discuss future development of atomic MEG systems.

Assessment of regularization techniques for electrocardiographic imaging

A widely used approach to solving the inverse problem in electrocardiography involves computing potentials on the epicardium from measured electrocardiograms (ECGs) on the torso surface. The main challenge of solving this electrocardiographic imaging (ECGI) problem lies in its intrinsic ill-posedness. While many regularization techniques have been developed to control wild oscillations of the solution, the choice of proper regularization methods for obtaining clinically acceptable solutions is still a subject of ongoing research. However there has been little rigorous comparison across methods proposed by different groups. This study systematically compared various regularization techniques for solving the ECGI problem under a unified simulation framework, consisting of both 1) progressively more complex idealized source models (from single dipole to triplet of dipoles), and 2) an electrolytic human torso tank containing a live canine heart, with the cardiac source being modeled by potentials measured on a cylindrical cage placed around the heart. We tested 13 different regularization techniques to solve the inverse problem of recovering epicardial potentials, and found that non-quadratic methods (total variation algorithms) and first-order and second-order Tikhonov regularizations outperformed other methodologies and resulted in similar average reconstruction errors.

Improved N14 nuclear quadrupole resonance detection of trinitrotoluene using polarization transfer from protons to N14 nuclei

Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) in trinitrotoluene fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for all three N14 NQR lines (the case with η≠0) were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. N14 NQR signals in a 15g trinitrotoluene sample of predominantly monoclinic modification were measured in 15s in different polarization magnetic fields. The conditions for optimal proton-nitrogen level crossing were determined.

Zeeman shift – A tool for assignment of 14N NQR lines of nonequivalent 14N atoms in powder samples

The use of Zeeman perturbed 14N nuclear quadrupole resonance (NQR) to determine the ν+ and ν-14N lines in polycrystalline samples with several nonequivalent nitrogen atoms was investigated. The 14N NQR line shift due to a weak external Zeeman magnetic field was calculated, assuming isotropic distribution of EFG tensor directions. We calculated the broad line distribution of the ν+ and ν- line shifts and experimentally confirmed the calculated Zeeman field dependence of singularities (NQR peaks) in cyclotrimethylenetrinitramine (RDX) and aminotetrazole monohydrate (ATMH). The calculated and measured frequency shifts agreed well. The proposed measurement method enabled determination of which 14N NQR lines in ATMH belong to ν+ and which to ν- transitions.

An advanced phantom study assessing the feasibility of neuronal current imaging by ultra-low-field NMR

In ultra-low-field (ULF) NMR/MRI, a common scheme is to magnetize the sample by a polarizing field of up to hundreds of mT, after which the NMR signal, precessing in a field on the order of several μT, is detected with superconducting quantum interference devices (SQUIDs). In our ULF-NMR system, we polarize with up to 50mT and deploy a single-stage DC-SQUID current sensor with an integrated input coil which is connected to a wire-wound Nb gradiometer. We developed this system (white noise 0.50fT/√Hz) for assessing the feasibility of imaging neuronal currents by detecting their effect on the ULF-NMR signal. Magnetoencephalography investigations of evoked brain activity showed neuronal dipole moments below 50nAm. With our instrumentation, we have studied two different approaches for neuronal current imaging. In the so-called DC effect, long-lived neuronal activity shifts the Larmor frequency of the surrounding protons. An alternative strategy is to exploit fast neuronal activity as a tipping pulse. This so-called AC effect requires the proton Larmor frequency to match the frequency of the neuronal activity, which ranges from near-DC to ∼kHz. We emulated neuronal activity by means of a single dipolar source in a physical phantom, consisting of a hollow sphere filled with an aqueous solution of CuSO4 and NaCl. In these phantom studies, with physiologically relevant dipole depths, we determined resolution limits for our set-up for the AC and the DC effect of ∼10μAm and ∼50nAm, respectively. Hence, the DC effect appears to be detectable in vivo by current ULF-NMR technology.

A miniaturized NQR spectrometer for a multi-channel NQR-based detection device

A low frequency (0.5-5 MHz) battery operated sensitive pulsed NQR spectrometer with a transmitter power up to 5 W and a total mass of about 3 kg aimed at detecting (14)N NQR signals, predominantly of illicit materials, was designed and assembled. This spectrometer uses a standard software defined radio (SDR) platform for the data acquisition unit. Signal processing is done with the LabView Virtual instrument on a personal computer. We successfully tested the spectrometer by measuring (14)N NQR signals from aminotetrazole monohydrate (ATMH), potassium nitrate (PN), paracetamol (PCM) and trinitrotoluene (TNT). Such a spectrometer is a feasible component of a portable single or multichannel (14)N NQR based detection device.

Influence of different presentations of oscillometric data on automatic determination of systolic and diastolic pressures.

Most non-invasive blood pressure measurements are based on either the auscultatory or the oscillometric technique. In this study, we performed an extensive analysis of the signals, i.e., responses of a microphone implanted in the cuff and pressure changes in the cuff, which can be recorded during such measurements. We applied several methods to separate the cuff deflation from the arterial pressure pulses, as well as to separate the microphone data into an audible part (Korotkoff sounds) and a low frequency part. The oscillometric technique is based on some empirically derived criteria applied to the oscillometric index, which is defined as a certain characteristic physical property of pressure pulses. In addition to the pressure pulses, which are a typical physical property used for the oscillometric index, we also used in this study other properties such as a time derivative and an audible part of data measured by a microphone implanted in the cuff (Korotkoff sounds). We performed a case study of 23 healthy subjects to evaluate the influence of different presentations of the oscillometric index on known height-based and slope-based empirical algorithms for the automatic determination of the systolic and diastolic blood pressures.

14N Nuclear Quadrupole Resonance Study of Polymorphism in Famotidine

(14)N nuclear quadrupole resonance (NQR) in two known polymorphs of famotidine was measured. At room temperature, seven quadrupolar sets of transition frequencies (ν(+), ν(-), and ν(0)) corresponding to seven different nitrogen sites in the crystal structure of each of the two polymorphs were found. This confirms the expected ability of NQR to distinguish polymorph B from its analog A. NQR can also measure their ratio in a solid mixture and in the final dosage form, that is, a tablet. The NQR frequencies, line shapes, and tentative assignation to all seven molecular (14)N atoms were obtained. Unravelment of these two entangled NQR spectra presents a valuable contribution to the NQR database and enables studies of some possible correlations therein. Moreover, nondestructive (14)N NQR studies of commercial famotidine tablets can reveal some details of the drug fabrication process connected with compression.

Enhancement of oscillometric index in non-invasive blood pressure measurements

Most non-invasive blood pressure measurements are based on either the auscultatory or the oscillometric technique. The oscillometric technique is based on some empirically derived criteria applied to the oscillometric index, which is defined as a certain characteristic physical property of pressure pulses. In this study, we introduced a new type of enhanced oscillometric index obtained by a powered short time variance of the pressure data. Such a presentation shows significant activity only below the systolic and above the diastolic blood pressure, which simplifies the criteria for automatic determination of blood pressure values, i.e., as in the case of the auscultatory technique, where only the presence and absence of pulses has to be detected.

Polarization Enhanced Nqr Detection at Low Frequencies

In this contribution we present our current research on polarization enhanced nuclear quadrupole resonance (NQR) detection at low frequencies with the emphasis on 14 N NQR trinitrotoluene (TNT) detection at room temperature. Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes 14 N NQR in TNT fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for 14 N NQR lines in TNT were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. 14 N NQR signals in a 15 g trinitrotoluene sample of predominantly monoclinic modification were measured in 15 s in different polarization magnetic fields. The conditions for optimal polarization enhancement were determined. Introduction Nuclear Quadrupole Resonance (NQR), with its ability to identify specific molecules, is potentially a powerful method in solid state physics, chemistry and pharmacy. In the last 10 to 15 years, several attempts have been made to improve the detection of military explosives, improvised explosive devices (IED) and other illicit materials – mainly narcotics by 14 N NQR [1-9]. Unfortunately, many of these substances have 14 N NQR frequencies in the low frequency domain between 100 and 1000 kHz, hence a rather low signal to noise (s/n) ratio. Therefore, the measuring times for the required signal averaging can be hours and they are thus too long for practical applications. With a combination of proton polarization transfer to nitrogen nuclei and multi-pulse spin-locking sequences the measuring time can be significantly reduced to an acceptable level of the order of 10 s, provided the proton and the nitrogen spin-lattice relaxation times (T1) are suitable. There are two ways to increase the s/n ratio by proton-nitrogen level crossing polarization transfer: a) proton-nitrogen nuclear double resonance techniques [10-13] using changes in the proton NMR signal as an indirect indication of the 14 N NQR transitions; and b) direct 14 N NQR detection where the signal is enhanced by proton polarization transfer via proton-nitrogen level crossing in a time variable magnetic field [14-19]. The first technique requires a homogeneous applied external magnetic field and is therefore not convenient for work in the