Feasibility of Imaging Tissue Electrical Conductivity by Switching Field Gradients with MRI.

Tissue conductivity is a biophysical marker of tissue structure and physiology. Present methods of measuring tissue conductivity are limited. Electrical impedance tomography, and magnetic resonance electrical impedance tomography rely on passing external current through the object being imaged, which prevents its use in most human imaging. Recently, the RF field used for MR excitation has been used to non-invasively measure tissue conductivity. This technique is promising, but conductivity at higher frequencies is less sensitive to tissue structure. Measuring tissue conductivity non-invasively at low frequencies remains elusive. It has been proposed that eddy currents generated during the rise and decay of gradient pulses could act as a current source to map low-frequency conductivity. This work centers on a gradient echo pulse sequence that uses large gradients prior to excitation to create eddy currents. The electric and magnetic fields during a gradient pulse are simulated by a finite-difference time-domain simulation. The sequence is also tested with a phantom and an animal MRI scanner equipped with gradients of high gradient strengths and slew rate. The simulation demonstrates that eddy currents in materials with conductivity similar to biological tissue decay with a half-life on the order of nanoseconds and any eddy currents generated prior to excitation decay completely before influencing the RF signal. Gradient-induced eddy currents can influence phase accumulation after excitation but the effect is too small to image. The animal scanner images show no measurable phase accumulation. Measuring low-frequency conductivity by gradient-induced eddy currents is presently unfeasible

Modelling of pulsed eddy current testing of wall thinning of carbon steel pipes through insulation and cladding

Conventional eddy current techniques have been used to a great extent for detection of surface breaking defects in conductive materials. Pulsed Eddy Current (PEC) techniques excite the probe’s driving coil with a repetitive broadband pulse, usually a square wave, instead of sinusoidal wave. The resulting transient current through the coil induces transient eddy currents in the test piece, these pulses consist of a broad frequency spectrum, and the reflected signal contains important depth information. Surface pancake type pulsed eddy current probes have been used for wall thinning and corrosion detection but these methods can be slow. In order to increase the scanned area, an encircling coil has been proposed, with a view to inspect a complete circumference with a single pulse. The work presented in this paper employs COMSOL Multiphysics finite element (FE) modelling software, to further investigate the behaviour of an encircling probe design as a part of the development work. This work involves modelling of an encircling coil around a steel pipe with insulation and cladding of different materials. Pulsed eddy current testing of wall‐thinning through cladding and insulation was studied for various wall thinning situations. The simulation results show the capability of this system in pipe wall thinning detection

Eddy current generation enhancement using ferrite for electromagnetic acoustic transduction

Eddy currents are generated in an electrically conducting surface as a step in electromagnetic acoustic transduction (EAT). In eddy current testing, wire coils are often wound onto a ferrite core to increase the generated eddy current. With EAT, increased coil inductance is unacceptable as it leads to a reduction in the amplitude of a given frequency of eddy current from a limited voltage source, particularly where the current arises from capacitor discharge. The authors present a method for EAT where ferrite is used to increase the eddy current amplitude without significantly increasing coil inductance or changing the frequency content of the eddy current

Investigation of Infrared Thermography NDE Techniques for Use in Power Station Environments

Three active thermal methods capable of detecting surface breaking cracks in metals are considered in this Thesis. The three thermal methods exploit different means of excitation, each with practical advantages and varying abilities to detect specific types of crack morphology. Thermosonics uses a broadband, high power ultrasonic input to vibrate the test-piece. Defects damp the vibrational energy into heat which is imaged by a thermal camera. Laser-spot thermography uses a short laser pulse to spot heat the surface of the test-piece, and the subsequent radial heat diffusion is then observed. Defects can cause both increased emission of infrared and localised increases in thermal impedance, both effects causing distortion of the radial heat diffusion. Eddy-current induced thermography uses a high power magnetic field to induce a flow of current inside the test-piece. Defects create a localised increase in electrical impedance, diverting the electric field around the defect. This diversion of current flow causes neighbouring regions of high and low current density, the corresponding Joule heating imaged by a thermal camera. In this Thesis the three methods are explored experimentally. For laser-spot thermography and eddy-current induced thermography the physical phenomena are characterised and experimental best-practice for short pulse excitation determined. The effect of crack opening on each of the three methods is found to give insight into which applications the methods are most suited. It was found that the relationship between crack opening and detectability was complex for thermosonics, relatively linear for laser-spot thermography, and that eddy-current induced thermography is largely insensitive to crack opening. The methods are tested for the feasibility of detecting cracks in Inconel buried beneath metallic and ceramic coatings typical of gas turbine blades, with thermosonics and eddy-current induced thermography found to be viable methods. A study of the detectability of a large number of cracks in steel, titanium and Waspaloy by eddy-current induced thermography is detailed, and from this data the probability of detection is established. Eddy-current thermography is shown to be an extremely sensitive method capable of detecting fatigue cracks of approximately 0.25 mm in steel and 0.50-0.75 mm in titanium and Waspaloy. The practicality of the thermal methods is discussed, and the methods put into the context of the wider field of NDE. Based on the works in this Thesis it was found that for most applications eddy-current induced thermography is the most appealing thermal method since it is highly sensitive, rapid, non-contacting and relatively easy to validate. However, both thermosonics and laser-spot thermography remain useful alternative inspections for more niche applications

Signature of effective mass in crackling noise asymmetry

Crackling noise is a common feature in many dynamic systems [1-9], the most familiar instance of which is the sound made by a sheet of paper when crumpled into a ball. Although seemingly random, this noise contains fundamental information about the properties of the system in which it occurs. One potential source of such information lies in the asymmetric shape of noise pulses emitted by a diverse range of noisy systems [8-12], but the cause of this asymmetry has lacked explanation [1]. Here we show that the leftward asymmetry observed in the Barkhausen effect [2] - the noise generated by the jerky motion of domain walls as they interact with impurities in a soft magnet - is a direct consequence of a magnetic domain wall's negative effective mass. As well as providing a means of determining domain wall effective mass from a magnet's Barkhausen noise our work suggests an inertial explanation for the origin of avalanche asymmetries in crackling noise phenomena more generally.Comment: 13 pages, 4 figures, to appear in Nature Physic

Effects of residual magnetism due to minor loop on magnetic property of permanent magnet type of MRI

Summary form only given. The flux distribution of a permanent magnet type of MRI shown in Fig.1 is affected by the hysteresis (minor loop) and eddy currents in the pole piece and yoke due to the pulse current (Fig.2) of the gradient coil. In this paper, the effects of the hysteresis and the eddy current in the yoke on the residual flux density of the probe coil are investigated. It can be assumed that the eddy current does not flow in the pole piece because it is divided into pieces. The eddy current flows in the yoke. Fig.3 shows the change of residual flux density /spl Delta/B/sub z/ at the point S(0,0) in Fig.1. /spl Delta/B/sub z/ is given by /spl Delta/B/sub z/=B/sub z1/-B/sub z0/ (1), where B/sub z0/ is the flux density at the instant t=0(I=0A). B/sub z1/ is the flux density at the instant t=i(I=0A). The instant of 1,2,3,... in Fig.2 corresponds to 1,2,3,... in Fig.3. Fig.3 shows that the hysteresis in the pole piece and yoke should be taken into account. The effect of eddy current in the yoke on the residual flux density /spl Delta/B/sub z/ is not negligible. These results suggests that the reduction of the amplitudes of minor loop and eddy current is important in order to improve the operating characteristics of the permanent magnet type of MRI.</p

A pulsed eddy current system for flaw detection using an encircling coil on a steel pipe

Conventional eddy current techniques have been used to a great extent for detection of surface breaking defects in conductive materials. However, detection of sub-surface defects is limited due to skin effect phenomena and material properties. Pulsed Eddy Current (PEC) techniques excite the probe’s driving coil with a repetitive broadband pulse, usually a square wave. The resulting transient current through the coil induces transient eddy currents in the test piece, these pulses consist of a broad frequency spectrum, and the reflected signal contains important depth information. The work in this paper employs COMSOL Multiphysics, the finite element (FE) modelling software, to investigate the behaviour of a new encircling probe design. This work involves modelling of an encircling coil around a steel pipe with high lift-off to simulate insulation. The 3D modelling of the coil wrapped around a steel pipe was employed and surface breaking discontinuities were modelled. The simulation of these scenarios provided essential information about the behaviour of this probe design

Motor run-up system

A starting system is described for bringing a large synchronous motor up to speed to prevent large power line disturbances at the moment the motor is connected to the power line. The system includes (1) a digital counter which generates a count determined by the difference in frequency between the power line and a small current generated by the synchronous motor; (2) a latch which stores the count; and (3) a comparator which compares the stored count with a newly generated count to determine whether the synchronous motor is accelerating or decelerating. Signals generated by the counter and comparator control the current to a clutch that couples a starting motor to the large synchronous motor

Simulated design strategies for SPECT collimators to reduce the eddy currents induced by MRI gradient fields

Combining single photon emission computed tomography (SPECT) with magnetic resonance imaging (MRI) requires the insertion of highly conductive SPECT collimators inside the MRI scanner, resulting in an induced eddy current disturbing the combined system. We reduced the eddy currents due to the insert of a novel tungsten collimator inside transverse and longitudinal gradient coils. The collimator was produced with metal additive manufacturing, that is part of a microSPECT insert for a preclinical SPECT/MRI scanner. We characterized the induced magnetic field due to the gradient field and adapted the collimators to reduce the induced eddy currents. We modeled the x-, y-, and z-gradient coil and the different collimator designs and simulated them with FEKO, a three-dimensional method of moments / finite element methods (MoM/FEM) full-wave simulation tool. We used a time analysis approach to generate the pulsed magnetic field gradient. Simulation results show that the maximum induced field can be reduced by 50.82% in the final design bringing the maximum induced magnetic field to less than 2% of the applied gradient for all the gradient coils. The numerical model was validated with measurements and was proposed as a tool for studying the effect of a SPECT collimator within the MRI gradient coils