Hsin-Yu Wei

Three-Dimensional Magnetic Induction Tomography Imaging Using a Matrix Free Krylov Subspace Inversion Algorithm

Magnetic induction tomography (MIT) attempts to image the passive electromagnetic properties (PEP) of an object by measuring the mutual inductances between pairs of coils placed around its periphery. In recent years, there has been an increase in applications of non-contact magnetic induction tomography. When flnite element-based reconstruction methods are used, that rely on the inversion of a derivative operator, the large size of the Jacobian matrix poses a challenge since the explicit formulation and storage of the Jacobian matrix could be in general not feasible. This problem is aggravated further in applications for example when the number of coils is increased and in three-dimension. Krylov subspace methods such as conjugate gradient (CG) methods are suitable for such large scale inverse problems. However, these methods require use of the Jacobian matrix, which can be large scale. This paper presents a matrix-free reconstruction method, that addresses the problems of large scale inversion and reduces the computational cost and memory requirements for the reconstruction. The idea behind the matrix- free method is that information about the Jacobian matrix could be available through matrix times vector products so that the creation and storage of big matrices can be avoided. Furthermore the matrix vector multiplications were performed in multiple core fashion so that the computational time can decrease even further. The method was tested for the simulated and experimental data from lab experiments, and substantial beneflts in computational times and memory requirements have been observed.

Volumetric magnetic induction tomography

Magnetic induction tomography (MIT) is a new and emerging type of tomography technique that is able to map the passive electromagnetic properties (in particular conductivity) of an object. Because of its non-invasive feature, it becomes a suitable technique for many industries, such as metal processing and mining. This paper presents a volumetric MIT (VMIT) system based on an existing measurement setup in our 2D system (MIT Mk-I). By increasing the number of sensors in the axial direction, volumetric imaging can be realized and hence can improve the spatial resolution of the reconstructed images. All of the system control, data acquisition and signal demodulation are accomplished by a commercial data acquisition card and the National Instruments graphical programming language. In this paper, both the system architecture and the forward 3D sensitivity model will be presented. The image reconstruction scheme is modified by introducing a 3D sensitivity map to replace the previous 2D sensitivity map used for the MIT Mk-I system. The iterative Landweber technique was implemented as the inverse solver to reconstruct the images. Several laboratory-based experimental results are demonstrated in this paper, with different shapes of imaging objects. The reconstructed images are satisfactory showing for the first time volumetric conductivity reconstruction using a multi-layer MIT system. The results indicate the high-quality image reconstruction using our novel VMIT system for potential use in industrial applications, such as metal flow imaging.

Electromagnetic Tomography for Medical and Industrial Applications: Challenges and Opportunities [Point of View]

MITs (magnetic induction tomography) low-cost and noninvasive features can offer great excitement and potential to address many challenging problems that exist in the current industrial/medical applications. The foundation development of MIT has been made in the past ten years. Many more advancements can be expected in the next decade, including the first commercialized MIT system for industrial or medical application. It will certainly contribute some impact to the current imaging technology.

Hardware and software design for a National Instrument-based magnetic induction tomography system for prospective biomedical applications

Magnetic induction tomography (MIT) is a new and emerging type of tomography technique that is able to map the passive electromagnetic properties (in particular conductivity) of an object. Excitation coils are used to induce eddy currents in the medium, and the magnetic field produced by the induced eddy current is then sensed by the receiver coils. Because of its non-invasive and contactless feature, it becomes an attractive technique for many applications (especially in biomedical area) compared to traditional contact electrode-based electrical impedance tomography. Due to the low contrast in conductivity between biological tissues, an accurate and stable hardware system is necessary. Most MIT systems in the literature employ external signal generators, power amplifiers and highly stable down-conversion electronics to obtain a satisfactory phase measurement. However, this would increase design complexity substantially. In this paper, a National Instrument-based MIT system is developed at the University of Bath, aiming for biomedical applications. The system utilizes National Instrument products to accomplish all signal driving, switching and data acquisition tasks, which ease the system design whilst providing satisfactory performance. This paper presents a full-scaled medical MIT system, from the sensor and system hardware design, eddy current model verification to the image reconstruction software: the performance of this MIT instrumentation system is characterized in detail, including the system accuracy and system stability. The methods of solving eddy current problem are presented. The reconstructed images of detecting the presence of saline solutions are also included in this paper, which show the capability of national instrument products to be developed into a full-scaled biomedical MIT system, by demonstrating the practical experimental results.

Pipeline inspection using magnetic induction tomographybased on a narrowband pass filtering method

Pipelines are the most common apparatus in industries; therefore, the need for inspection during the manufacturing, construction and the operation stage is inevitable and invaluable. Magnetic Induction Tomography (MIT) is a new type of tomography technique that is sensitive to the electrical conductivity of objects. It has been shown that the MIT technique is appropriate for imaging materials with high electrical conductivity contrasts; hence, the majority of the MIT systems were designed for detecting metallic objects. In this paper, MIT technique was proposed for pipeline inspection. Structural damages of the outer surface of the pipe were considered in this study. Nonetheless, it is challenging to use the traditional MIT pixel based reconstruction method (PBRM) as a suitable pipelines inspection tool because of the limited resolution. A narrowband pass flltering method (NPFM) of imaging pipe geometry was developed as a suitable image reconstruction method. The proposed method can overcome the resolution limitations and produce useful information of the pipe structure. This paper shows the comparative results from the novel NPFM and from traditional PBRM. While the PBRM fails to detect damages in outer structure of the pipe the NPFM successfully indentifles these damages. The method has been verifled using experimental data from very challenging test samples. It is well known that using a coil array with an imaging region of 100mm the PBRM based MIT can retrieve information with accuracy of about 10mm (about 10%). With proposed NPFM the information on a resolution of 2mm (which is about 2%) can be detected using the same measurement data.

Design of a Sensor Coil and Measurement Electronics for Magnetic Induction Tomography

Magnetic induction tomography (MIT) is a tomographic imaging technique that is able to map the electromagnetic properties within an object or vessel from magnetic field measurements. Excitation coils are used to induce eddy currents in the medium, and the magnetic field produced by the induced eddy current is then sensed by receiver coils. Because of its noncontact nature, MIT is particularly attractive for biomedical and some industrial applications, such as pipe-flow monitoring, when compared with traditional contact electrode-based electrical impedance tomography. This paper describes the design and performance of an MIT transceiver circuit that can operate from 400 kHz to 12 MHz. The in-phase and quadrature (I/Q) demodulation technique is used to measure the signal perturbation due to the induced conduction eddy currents. The transceiver circuit design employs a single integrated circuit, containing a variable-gain amplifier and an I/Q demodulator. This paper contains characterizations of the transceivers measurement noise, system stability, and sensitivity for detecting saline solutions and metal plates. A novel balanced coaxial screened coil structure with integrated current sensing was also developed to minimize capacitive coupling between coils and to allow measurement of the current in the driving coils. Experiments were carried out at 3 and 10 MHz using bottles of saline solutions (1%-5% concentration) and metal sheets (aluminum and steel) to verify the sensitivity for conductivity imaging.

Two-Phase Low Conductivity Flow Imaging Using Magnetic Induction Tomography

Magnetic Induction Tomography (MIT) is a new and emerging type of tomography technique that is able to map the distribution of all three passive electromagnetic properties, however most of the current interests are focusing on the conductivity and permeability imaging. In an MIT system, coils are used as separate transmitters or sensors, which can generate the background magnetic fleld and detect the perturbed magnetic fleld respectively. Through switching technique the same coil can work as transceiver which can generate fleld at a time and detect the fleld at another time. Because magnetic fleld can easily penetrate through the non-conductive barrier, the sensors do not need direct contact with the imaging object. These non-invasive and contactless features make it an attractive technique for many applications compared to the traditional contact electrode based electrical impedance tomography. Recently, MIT has become a promising monitoring technique in industrial process tomography, for example MIT has been used to determine the distribution of liquidised metal and gas (high conductivity two phase ∞ow monitoring) for metal casting applications. In this paper, a low conductivity two phase ∞ow MIT imaging is proposed so the reconstruction of the low contrast samples (< 6S/m) can be realised, e.g., gas/ionised liquid. An MIT system is developed to test the feasibility. The system utilises 16 coils (8 transmitters and 8 receivers) and an operating frequency of 13MHz. Three difierent experiments were conducted to evaluate all possible situations of two phase ∞ow imaging: 1) conducting objects inside a non-conducting background, 2) conducting objects inside a conducting background (low contrast) and 3) non-conducting objects inside a conducting background. Images are reconstructed using the linearised inverse method with regularisation. An experiment was designed to image the non-conductive samples inside a conducting

Planar magnetic induction tomography for 3D near subsurface imaging

Magnetic induction tomography (MIT) is a tomographic technique utilising inductive coils and eddy currents to map the passive electromagnetic properties of an object. Eddy current methods are widely used for non-destructive testing (NDT) in inspection of metallic structures. Eddy current based NDT uses a single coil or a pair of coils to scan the samples. As an emerging NDT technique, MIT scans the sample with a coil array through an eddy current based tomographic approach. In this paper, a planar array MIT system (PMIT) is proposed for 3D near subsurface imaging. This is of great importance as there are large numbers of potential applications for MIT that allow limited access to the materials under testing. The system development, practical implication, capability and limitations of PMIT are discussed. The fundamental principles are demonstrated through simulations. Experimental data are used to evaluate the capability and detectability this system has as a potential 3D subsurface imaging tool.

Four dimensional reconstruction using magnetic induction tomography:Experimental study

Magnetic Induction Tomography (MIT) is a relatively new and emerging type of tomography techniques that is able to map the distribution of all three passive electrical properties (PEPs). Its non-invasive and contactless features make it an attractive technique for many applications compared to the traditional contact electrode based electrical impedance tomography. Recently, MIT has become a promising monitoring technique in industrial process tomography, and the area of the research interest has moved from 2D to 3D because of the volumetric nature of electromagnetic fleld. Three dimensional MIT images provide more information on the conductivity distribution, especially in the axial direction. However, it has been reported that the reconstructed 3D images can be distorted when the imaging object is located at a less sensitive region. Although this distortion can be compensated by adjusting the regularisation criteria, this is not practical in real life applications as the prior information about the objects location is often unavailable. This paper presents a memory e-cient 4D MIT algorithm which can maintain the image quality under the same regularisation circumstances. Instead of solving each set of measurement individually, the 4D algorithm takes advantage of the correlations between the image and its neighboring data frames to reconstruct 4D of conductivity movements. The 4D algorithm improves the image qualities by increasing the temporal resolution. It also overcomes some sensitivity issues of 3D MIT algorithms and can provide a more stable result in terms of the size consistency of the reconstructed image. Several experimental results using real laboratory data are presented for validating the proposed algorithms.

A Magnetic Induction Tomography System for Prospective Industrial Processing Applications

Abstract Magnetic induction tomography (MIT) is one of the newest industrial process imaging techniques. Main industrial applications of the MIT imaging are in high conductive flow imaging. However, recently it has been shown that the MIT may be useful for low conductive process imaging. This paper presents a cost effective hardware design for MIT in industrial applications, called Bath-MKI industrial MIT system. The system comprises 8 inductor coils and has the possibility of expansion to 16 coils. The excitation signals and the measured voltages are generated and measured using a LabView based system. Two 16 by 1 multiplexers are used to select between the coils. Measurements, excitation and multiplexing are all controlled by a National Instrument (NI) USB based DAQ: USB-6259 and a signal generator. Using the same electronics, the prototype is tested with two different coil arrays; one is a small scale ferrite core coil and one larger scale air cored coil. Experimental image reconstruction results are shown using both small scale and large scale coil arrays.