Reyhan Zengin

Lorentz force electrical impedance tomography using magnetic field measurements.

In this study, magnetic field measurement technique is investigated to image the electrical conductivity properties of biological tissues using Lorentz forces. This technique is based on electrical current induction using ultrasound together with an applied static magnetic field. The magnetic field intensity generated due to induced currents is measured using two coil configurations, namely, a rectangular loop coil and a novel xy coil pair. A time-varying voltage is picked-up and recorded while the acoustic wave propagates along its path. The forward problem of this imaging modality is defined as calculation of the pick-up voltages due to a given acoustic excitation and known body properties. Firstly, the feasibility of the proposed technique is investigated analytically. The basic field equations governing the behaviour of time-varying electromagnetic fields are presented. Secondly, the general formulation of the partial differential equations for the scalar and magnetic vector potentials are derived. To investigate the feasibility of this technique, numerical studies are conducted using a finite element method based software. To sense the pick-up voltages a novel coil configuration (xy coil pairs) is proposed. Two-dimensional numerical geometry with a 16-element linear phased array (LPA) ultrasonic transducer (1 MHz) and a conductive body (breast fat) with five tumorous tissues is modeled. The static magnetic field is assumed to be 4 Tesla. To understand the performance of the imaging system, the sensitivity matrix is analyzed. The sensitivity matrix is obtained for two different locations of LPA transducer with eleven steering angles from [Formula: see text] to [Formula: see text] at intervals of [Formula: see text]. The characteristics of the imaging system are shown with the singular value decomposition (SVD) of the sensitivity matrix. The images are reconstructed with the truncated SVD algorithm. The signal-to-noise ratio in measurements is assumed 80 dB. Simulation studies based on the sensitivity matrix analysis reveal that perturbations with [Formula: see text] mm size can be detected up to a 3.5 cm depth.

Numerical analysis of spinal cord stimulation with a 2-electrode percutaneous lead

Spinal cord stimulation (SCS) is a standard treatment method for drug free treatment for chronic pain. In this approach, an electrical current is applied to the region where the chronic pain signals reach the spinal cord and preventing pain sensation from reaching the brain, the pain sensation is been destroyed. To apply current to source of pain, two types of leads are used: percutaneous lead and paddle lead. In this study, numerical studies of current application are done using percutaneous lead with 2-electrode model. Spinal canal is modeled as 3D with epidural fat, CSF, white matter and gray matter tissues. The lead with 2-electrode is placed in epidural fat. One of the electrode is chosen as anode, the other one is as cathode. The surface current density, 70 A/m2, is applied to each electrode. At the contact surface of electrodes and epidural fat, obtained maximum current density is 68.02 mA/cm2 and this value is in the safety limits. The maximum electrical voltage occurred in the spinal canal is 1.14 V and this value is in the interval of perception value for patients.

A numerical analysis of Magneto-Acousto Electrical Tomography with a simplified breast model

In this study, Magneto-Acousto Electrical Tomography with magnetic measurements technique is investigated using two dimensional (2D) simplified numerical breast model. This technique comprises of the electrical current induction and ultrasound in the static magnetic field. 2D numerical simplified breast geometry is modeled with an ultrasonic transducer (linear phased array (LPA)) placed on the top side of the simplified breast model. A rectangular loop coil is encircled the breast. The sensitivity matrix is obtained for this transducer-coil configuration. LPA transducer is steered with eleven angles (−25 ° to 25 ° at intervals of 5 °). The characteristics of this modality is shown by the singular value decomposition (SVD) method. The reconstruction of the images is performed by truncated SVD approach. This study shows that perturbations (5 mm × 5 mm) up to a depth of 3 cm can be detected.

Numerical implementation of magneto-acousto-electrical tomography (MAET) using a linear phased array transducer

In this study, the performance and implementation of magneto-acousto-electric tomography (MAET) is investigated using a linear phased array (LPA) transducer. The goal of MAET is to image the conductivity distribution in biological bodies. It uses the interaction between ultrasound and a static magnetic field to generate velocity current density distribution inside the body. The resultant voltage due to velocity current density is sensed by surface electrodes attached on the body. In this study, the theory of MAET is reviewed. A 16-element LPA transducer is used to provide beam directivity and steerability of acoustic waves. Different two-dimensional (2D) numerical models of breast and tumour are formed to analyze the multiphysics problem coupled with acoustics and electromagnetic fields. In these models, velocity current density distributions are obtained for pulse type ultrasound excitations. The static magnetic field is assumed as 1 Tesla. To sense the resultant voltage caused by the velocity current density, it is assumed that two electrodes are attached on the surface of the body. The performance of MAET is shown through sensitivity matrix analysis. The sensitivity matrix is obtained for two transducer positions with 13 steering angles between -30° to 30° with 5° angular intervals. For the reconstruction of the images, truncated singular value decomposition (SVD) method is used with different signal-to-noise ratio (SNR) values (20 dB, 40 dB, 60 dB and 80 dB). The resultant images show that the perturbation (5 mm × 5 mm) placed 35 mm depth can be detected even if the SNR is 20 dB.In this study, the performance and implementation of magneto-acousto-electrical tomography (MAET) is investigated using a linear phased array (LPA) transducer. The goal of MAET is to image the conductivity distribution in biological bodies. It uses the interaction between ultrasound and a static magnetic field to generate velocity current density distribution inside the body. The resultant voltage due to velocity current density is sensed by surface electrodes attached on the body. In this study, the theory of MAET is reviewed. A 16-element LPA transducer with 1 MHz excitation frequency is used to provide beam directivity and steerability of acoustic waves. Different two-dimensional numerical models of breast and tumour are formed to analyze the multiphysics problem coupled with acoustics and electromagnetic fields. In these models, velocity current density distributions are obtained for pulse type ultrasound excitations. The static magnetic field is assumed as 1 T. To sense the resultant voltage caused by the velocity current density, it is assumed that two electrodes are attached on the surface of the body. The performance of MAET is shown through sensitivity matrix analysis. The sensitivity matrix is obtained for two transducer positions with 13 steering angles between [Formula: see text] to [Formula: see text] with [Formula: see text] angular intervals. For the reconstruction of the images, truncated singular value decomposition method is used with different signal-to-noise ratio (SNR) values (20 dB, 40 dB, 60 dB and 80 dB). The resultant images show that the perturbation (5 mm  ×  5 mm) placed 35 mm depth can be detected even if the SNR is 20 dB.

Numerical analysis of spinal cord stimulation with triple leads with guarded cathode

Spinal cord stimulation (SCS) is a alternative treatment method for drug free treatment for chronic pain. Electrodes are placed to Spinal columns epidural tissue to blocked transfer of pain to brain and to stimulate dorsally column of pain. Today there are two two designs of electrode which are used in Spinal cord stimulation (SCS): a percutaneous (PERC) design, consisting of annular electrodes distributed along a flexible cylindrical shaft, which is inserted with a hypodermic needle; and a laminectomy/paddle design (LAM) implanted during a surgical laminectomy, consisting of a grid of rectangular electrodes on a flexible planar substrate. Also, only controlled percutaneous cathode electrode is used for stimulation a certain area in spinal cord. In this study, numerical studies of direct current application are done using percutaneous lead with 3-electrode model. Spinal canal is modeled in four layers which are epidural fat, CSF, white matter and gray matter tissues as 3D with using Finite Element Method (FEM). The lead with 3-electrode is placed in epidural fat. Anode and cathode electrodes are stimulated so as to flow current respectively 0.25mA and -2mA. At the contact surface of electrodes and epidural fat, obtained maximum current density is 128.161 mA/cm2 and this value is in the safety limits. The maximum electrical voltage occurred in the spinal canal is 2.133 V and this value is in the interval of perception value for patients.

Numerical studies for Hall Effect Imaging using linear phased array transducer

Hall Effect Imaging (HEI) is a method used to image conductivity distribution of a biological body. HEI benefits from the interaction of ultrasound with a static magnetic field to generate velocity current density distribution inside the body. Due to the velocity current density, a resultant voltage is sensed by electrodes attached on surface of the body and used for reconstructing the conductivity distribution. In this study, the feasibility of HEI technique using linear phased array transducer is investigated by numerical studies. A model of 16-element linear phased array transducer is used to benefit from electronic properties of directivity and steerability of linear phased array transducer. In addition, a numerical model of breast and tumor medium is formed to analyze the multiphysics solution which couples acoustics and electromagnetic fields. For image reconstruction, Truncated Singular Value Decomposition method is used with different signal-to-noise ratio values.

Sensitivity matrix analysis for contactless electrical conductivity imaging

In this study, the sensitivity matrix for contactless electrical conductivity imaging was analyzed. Images of conductivity distribution were obtained using the reconstruction algorithms. To analyze the sensitivity matrix, a homogeneous volume conductor with 0.2 S/m conductivity and a 7×7 grid of coil pairs were chosen. Sensitivity distributions for all receiver- transmitter coils were imaged. To image conductivity distribution, forward problem with different conductivity distributions was solved. These results were used as data for inverse problem solution. Images of conductivity distributions were obtained by using Steepest Descent Method, Conjugate Gradient Method and Tikhonov Regularization Method. The best image was obtained using Steepest Descent Method with 2.5% error.

Forward problem solution for contactless electrical conductivity imaging with realistic head model

In this study, forward problem of contactless electrical conductivity imaging is solved by using realistic-head model. Realistic-head model is modelled by a single volume with the ANSYS software and divided into hexahedral elements. Conductivity information for each element is obtained from MR gray-scale images obtained as a result of segmentation.For realistic-head model, a coil of radius 1 cm, 1 cm above the scalp and carrying a sinusoidal (50 kHz) current of 1 A is used as a source. Realistic-head modelis composed of the scalp, skull, brain fluid and brain (gray matter and white matter) tissues. For each tissue magnetic flux densities and current densities have been obtained.