An assessment of a non-destructive magneto-optical imaging technique for the recovery of laser engraved marks from steel plates and firearm components

The International Tracing Instrument (ITI) is a document adopted by United Nations Member States in 2005, which outlines challenges faced in the tracing of illicit weapons and offers suggestions to increase the success of tracing operations. A key provision of the ITI states that serial numbers must be recoverable if obliterated. This research, therefore, investigates two methods of recovering laser engraved marks on steel, due to the increase in firearms manufacturing relying on lasers to apply critical markings. This work uses 20 steel plates (CR4 grade), 6 AK-pattern rifle top covers, and a steel pistol slide to investigate the potential for visualising laser engraved marks. All samples had marks applied using a BWM-30F Fibre laser marking machine, in addition to any original manufacture's stamped marks on the pistol. All marks were removed using a grinding belt (plates) and a Dremel® rotary tool (slide and top covers) to average depths of 1200 µm (stamped) and 240 µm (laser engraved). Recovery of the marks was attempted using two techniques to compare the two techniques in their ability to recover laser engraved marks; Regula Forensics® Eddy Current Recovery kit, followed by traditional Fry's reagent. Both recovery techniques recovered 100 % (n = 6) of the stamped characters on the pistol slide, and 0 % (n = 509) of the laser engraved characters on the pistol, top covers and steel plates. Recommendations are made for forensic firearms examiners to avoid attempting the recovery of laser engraved marks using Fry's reagent to preserve the integrity of evidence. It is also suggested that manufacturers avoid laser engraving unique identifying marks on firearms, and to use more permanent methods, such as stamping, to remain compliant with politically binding documents such as the ITI.

The impacts of Persian Gulf water and ocean-atmosphere interactions on tropical cyclone intensification in the Arabian Sea

During the last two decades, the number of tropical cyclone (TC) events in the Arabian Sea has increased dramatically. These events have led to severe human and economic damage in Oman, Iran and Pakistan. Within this context, Gonu, Phet and Shaheen were the Arabian Sea's most destructive TCs on record, leading to a total of 6.07 billion USD in damages and 159 fatalities. Previous studies have mainly focused on atmospheric, sea surface temperature (SST) and anthropogenic impacts of TC generation and intensification. By contrast, oceanographic currents, Persian Gulf water outflow and the role of ocean-atmospheric interactions on the distribution of outflow water into the Arabian Sea and their impacts on TC intensification, are poorly understood. In order to address this issue, we use historical TC records, satellite data, atmospheric and reanalyzed oceanographic data to shed new light on the relationship between large-scale atmospheric forcing and ocean currents on TC intensification in the Arabian Sea. The results demonstrate that pre-monsoon TCs mainly occurred during co-existing La Niña, cold Indian Ocean Basin Model (IOBM) and anomalous northern hemisphere circulations over the Persian Gulf. By contrast, post-monsoon TCs were generally generated during warming acceleration period. Poleward movement of the monsoon belt provided the required humidity and energy for TC generation and increased upwelling events. Water salinity and temperature have increased in the north and northwestern parts of the Arabian Sea following rising upwelling events and a decrease in Persian Gulf outflow water depth. Rapid TC intensification has increased noticeably since 2007 and >72 % of cyclones have reached category 3 or more. We find that the rate of SST rise in the Arabian Sea is higher than the other parts of the northern Indian Ocean since 1998. SST and salinity in the Arabian Sea have been controlled by Persian Gulf outflow water and oceanographic currents. TC intensity is controlled by warm and saline (>36.6 PSU) water distribution patterns, mediated by eddy and jet currents. Rapid intensification of pre-monsoon TCs occurred by tracking to the north and northwest, with most landfalls occurring during this period. Post-monsoon TCs generally affect the center and the southwest of the Arabian Sea. The risk of intensive TCs manifests an increasing trend since 2007, therefore education programs via international platforms such as the International Ocean Institute (IOI) and UNESCO are required for the countries most at risk.

RF shielding designs for birdcage coils for preclinical MRI at 9.4 T

Birdcage coils are widely used in preclinical MRI as they perform well, allow for quadrature drive, and can provide a homogeneous transmit field. Unlike in larger bore scanners, an RF shield is essential to avoid strong cross-talk with gradient coils that are in close proximity. However, gradient switching induces eddy currents that heat the shield and coil and impair the temporal signal-to-noise ratio (tSNR). The motivation of this study is to investigate the performance of different designs of RF shields on a birdcage coil used for high resolution functional MRI of small primates at 9.4 T. We found the choice of materials for RF shields significantly affected ghosting and tSNR in fMRI scans. Both ultrathin foils and a slotted pattern reduce eddy currents and improve imaging quality. Our results also demonstrate that a 9-um-thick copper foil is sufficiently thin to reduce the eddy current effects for high-resolution fMRI scans and there is no need for high-cost 4-um-thick foil. For slotted shields, our results demonstrate that the number of slots should be carefully considered, and an excessive number of slots can lead to a lower SNR and tSNR. We believe the results from this study can be used as a reference to design future RF coil shields selection for preclinical scanners.

Numerical modelling of magnetic characteristics of ferrite core taking account of both eddy current and displacement current

In the magnetic field analysis of magnetic devices using a ferrite core, such as a pulse transformer, the frequency-domain analysis is often carried out using the measured complex permeability under different frequency range. However, the nonlinear magnetic characteristics cannot be considered in the frequency-domain analysis because of the harmonics caused by it cannot be represented. The nonlinear magnetic characteristics can be considered in the time-domain analysis, but suitable constant conductivity and permittivity taking account of the microstructure of ferrite core, which can represent the measured complex permeability under different frequencies, needs to be investigated for the time-domain analysis. In this paper, the effective permeability of a toroidal ferrite core is tried to be demonstrated by using the linear ac steady state magnetic field analysis taking account eddy currents and displacement currents. It is shown that the measured permeability can be realized roughly by using the modified constant conductivity and permittivity. The nonlinear time-domain magnetic field analysis can be carried out using the modified constant conductivity and permittivity obtained from this paper.

Automated quality control for within and between studies diffusion MRI data using a non-parametric framework for movement and distortion correction

Diffusion MRI data can be affected by hardware and subject-related artefacts that can adversely affect downstream analyses. Therefore, automated quality control (QC) is of great importance, especially in large population studies where visual QC is not practical. In this work, we introduce an automated diffusion MRI QC framework for single subject and group studies. The QC is based on a comprehensive, non-parametric approach for movement and distortion correction: FSL EDDY, which allows us to extract a rich set of QC metrics that are both sensitive and specific to different types of artefacts. Two different tools are presented: QUAD (QUality Assessment for DMRI), for single subject QC and SQUAD (Study-wise QUality Assessment for DMRI), which is designed to enable group QC and facilitate cross-studies harmonisation efforts.

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Two tools to automatically perform QC of diffusion MRI data.

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Automated generation of single subject reports for visual inspection and database.

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Group databases and reports allow to compare subjects within and between studies.

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Categorical and continuous variables can be used to update the reports.

Development of Focused Transcranial Magnetic Stimulation for Rodents by Copper-Array Shields

Transcranial magnetic stimulation (TMS) is one of the most widely used noninvasive brain stimulation method. It has been utilized for both treatment and diagnosis of many neural diseases, such as neuropathic pain and loss of function caused by stroke. Existing TMS tools cannot deliver focused electric field to targeted penetration depth even though many important neurological disorders are originated from there. A breakthrough is needed to achieve noninvasive, focused brain stimulation. We demonstrated using magnetic shield to achieve magnetic focusing without sacrificing significant amount of throughput. The shield is composed of multiple layers of copper ring arrays, which utilize induced current to generate counter magnetic fields. We experimentally set up a two-pole stimulator system to verify device simulation. A transient magnetic field probe was used for field measurements. The focusing effect highly depends on the geometric design of shield. A tight focal spot with a diameter of smaller than 5mm (plotted in Matlab contour map) can be achieved by using copper ring arrays. With properly designed array structures and rings locations, the combined original and induced counter fields can produce a tightly focused field distribution with enhanced field strength at a depth 7.5mm beyond the shield plane, which is sufficient to reach many deep and critical parts of a mouse brain.

Spatiotemporal magnetic fields enhance cytosolic Ca2+ levels and induce actin polymerization via activation of voltage-gated sodium channels in skeletal muscle cells

Cellular function is modulated by the electric membrane potential controlling intracellular physiology and signal propagation from a motor neuron to a muscle fiber resulting in muscle contraction. Unlike electric fields, magnetic fields are not attenuated by biological materials and penetrate deep into the tissue. We used complex spatiotemporal magnetic fields (17-70 mT) to control intracellular signaling in skeletal muscle cells. By changing different parameters of the alternating magnetic field (amplitude, inversion time, rotation frequency), we induced transient depolarization of cellular membranes leading to i) Na⁺ influx through voltage-gated sodium channels (VGSC), ii) cytosolic calcium increase, and iii) VGSC- and ryanodine receptor-dependent increase of actin polymerization. The ion fluxes occurred only, when the field was applied and returned to baseline after the field was turned off. The 30-s-activation-cycle could be repeated without any loss of signal intensity. By contrast, static magnetic fields of the same strength exhibited no effect on myotube Ca²⁺ levels. Mathematical modeling suggested a role for the alternating magnetic field-induced eddy current, which mediates a local change in the membrane potential triggering the activation of VGSC. These findings might pave the way for the use of complex magnetic fields to improve function of skeletal muscles in myopathies.

An integrated approach to correction for off-resonance effects and subject movement in diffusion MR imaging

In this paper we describe a method for retrospective estimation and correction of eddy current (EC)-induced distortions and subject movement in diffusion imaging. In addition a susceptibility-induced field can be supplied and will be incorporated into the calculations in a way that accurately reflects that the two fields (susceptibility- and EC-induced) behave differently in the presence of subject movement. The method is based on registering the individual volumes to a model free prediction of what each volume should look like, thereby enabling its use on high b-value data where the contrast is vastly different in different volumes. In addition we show that the linear EC-model commonly used is insufficient for the data used in the present paper (high spatial and angular resolution data acquired with Stejskal–Tanner gradients on a 3 T Siemens Verio, a 3 T Siemens Connectome Skyra or a 7 T Siemens Magnetome scanner) and that a higher order model performs significantly better.

The method is already in extensive practical use and is used by four major projects (the WU-UMinn HCP, the MGH HCP, the UK Biobank and the Whitehall studies) to correct for distortions and subject movement.

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We present a new method for correction of eddy current-induced distortions and subject movement in diffusion data.

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It is based on alignment to predictions based on a Gaussian process

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The results indicate that one can achieve reliable corrections even for high b-value data (b = 7000).

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A comparison to correlation ratio based registration (eddy_correct) indicates that the new method is vastly superior.

Coupled circuit numerical analysis of eddy currents in an open MRI system

We performed a new coupled circuit numerical simulation of eddy currents in an open compact magnetic resonance imaging (MRI) system. Following the coupled circuit approach, the conducting structures were divided into subdomains along the length (or width) and the thickness, and by implementing coupled circuit concepts we have simulated transient responses of eddy currents for subdomains in different locations. We implemented the Eigen matrix technique to solve the network of coupled differential equations to speed up our simulation program. On the other hand, to compute the coupling relations between the biplanar gradient coil and any other conducting structure, we implemented the solid angle form of Ampere's law. We have also calculated the solid angle for three dimensions to compute inductive couplings in any subdomain of the conducting structures. Details of the temporal and spatial distribution of the eddy currents were then implemented in the secondary magnetic field calculation by the Biot-Savart law. In a desktop computer (Programming platform: Wolfram Mathematica 8.0®, Processor: Intel(R) Core(TM)2 Duo E7500 @ 2.93GHz; OS: Windows 7 Professional; Memory (RAM): 4.00GB), it took less than 3min to simulate the entire calculation of eddy currents and fields, and approximately 6min for X-gradient coil. The results are given in the time-space domain for both the direct and the cross-terms of the eddy current magnetic fields generated by the Z-gradient coil. We have also conducted free induction decay (FID) experiments of eddy fields using a nuclear magnetic resonance (NMR) probe to verify our simulation results. The simulation results were found to be in good agreement with the experimental results. In this study we have also conducted simulations for transient and spatial responses of secondary magnetic field induced by X-gradient coil. Our approach is fast and has much less computational complexity than the conventional electromagnetic numerical simulation methods.

Skin and proximity effects in the conductors of split gradient coils for a hybrid Linac-MRI scanner

In magnetic resonance imaging (MRI), rapidly changing gradient fields are applied to encode the magnetic resonance signal with spatial position; however eddy currents are induced in the surrounding conducting structures depending on the geometry of the conductor and the excitation waveform. These alternating fields change the spatial profile of the current density within the coil track with the applied frequencies of the input waveform and by their proximity to other conductors. In this paper, the impact of the conductor width and the excited frequency over the parameters that characterise the performance of split transverse and longitudinal gradient coils are studied. Thirty x-gradient coils were designed using a "free-surface" coil design method and the track width was varied from 1mm to 30mm with an increment value of 1mm; a frequency sweep analysis in the range of 100Hz to 10kHz was performed using the multi-layer integral method (MIM) and parameters such as power loss produced by the coil and generated in the cryostat, inductance, coil efficiency (field strength/operating current), magnetic field profile produced by the coil and the eddy currents were studied. An experimental validation of the theoretical model was performed on an example coil. Coils with filamentary conductor segments were also studied to compare the simulated parameters with those produced by coils with a finite track. There was found to be a significant difference between the parameters calculated using filamentary coils and those obtained when the coil is simulated using finite size tracks. A wider track width produces coil with superior efficiency and low resistance; however, due to the skin effect, the power loss increases faster in wider tracks than in those generated in coils with narrow tracks. It was demonstrated that rapidly changing current paths must be avoided in order to mitigate the power loss and the spatial asymmetry in the current density profile. The decision of using narrow or wider tracks in split coils should be carefully investigated using a temperature analysis which includes skin and proximity effects.