Byung H. Han

Multichannel magnetic stimulation system design considering mutual couplings among the stimulation coils

We introduce some simulation and experiment results of the multichannel magnetic stimulator development that has been carried out as an initial attempt to realize a multichannel functional magnetic stimulator. For efficient functional magnetic stimulations, precise spatial localization of stimulation sites without any movements of the stimulation coils is very important. We have found that the mutual coupling effect among the adjacent stimulation coils in the coil array has to be considered in the determination of the charge voltages in some coil array configurations. Experimental results obtained with a 4-channel magnetic stimulator are presented.

Gradient waveform synthesis for magnetic propulsion using MRI gradient coils

Navigating an untethered micro device in a living subject is of great interest for both diagnostic and therapeutic applications. Magnetic propulsion of an untethered device carrying a magnetic core in it is one of the promising methods to navigate the device. MRI gradients coils are thought to be suitable for navigating the device since they are capable of magnetic propulsion in any direction while providing magnetic resonance images. For precise navigation of the device, especially in the peripheral region of the gradient coils, the concomitant gradient fields, as well as the linear gradient fields in the main magnetic field direction, should be considered in driving the gradient coils. For simple gradient coil configurations, the Maxwell coil in the z-direction and the Golay coil in the x- and y-directions, we have calculated the magnetic force fields, which are not necessarily the same as the conventional linear gradient fields of MRI. Using the calculated magnetic force fields, we have synthesized gradient waveforms to navigate the device along a desired path.

Magnetic propulsion of a magnetic device using three square-Helmholtz coils and a square-Maxwell coil.

We introduce a square coil system for remote magnetic navigation of a magnetic device without any physical movements of the coils. We used three square-Helmholtz coils and a square-Maxwell coil for magnetic propulsion of a small magnet along the desired path. All the square coils are mountable on a cubic frame that has an opening to accommodate a living subject. The square-Helmholtz coils control the magnetic propulsion direction by generating uniform magnetic field along the desired direction while the square-Maxwell coil controls the propulsion force by generating magnetic gradient field. We performed magnetic propulsion experiments with a downscaled coil set and a three-channel coil driver. Experimental results demonstrate that we can use the square coil set for magnetic navigation of a magnetic device without any physical movements of the coils.

Some technical aspects of magnetic stimulation coil design with the ferromagnetic effect

The performance of the stimulation coil in a magnetic nerve stimulator can be improved by attaching a ferromagnetic structure to the coil. This reduces heat generation at the coil and increases magnetic field strength for a given unit of current. Some technical aspects of the design of a stimulation coil with a ferromagnetic structure have been studied. Finite element method analysis results are presented for the effect of size, depth and magnetic saturation of the ferromagnetic structure on the stimulation coil performance. The experimental results show that the stimulation coil performance is improved by up to 40% by the attaching of a ferromagnetic structure on the coil.

Strain measurement from 3D micro-CT images of a breast-mimicking phantom

Strain distribution in compressed tissues gives information about elasticity of the tissues. We have measured strain from two sets of 3D micro-CT images of a breast-mimicking phantom; one obtained without compressing the phantom and the other with compressing it. To measure strain, we first calculated compression-induced displacements of high-intensity feature patterns in the image. In measuring displacement of a pixel of interest, we searched the pixel in the compressed-phantom image, whose surrounding resembles the uncompressed-phantom image most closely, using the image correlation technique. From the displacement data, we calculated average strain at a region of interest. With the calculated average strains, we could distinguish the hard inclusion in the phantom which was not distinguishable from the background body of the phantom in the ordinary micro-CT images. The calculated strains account for stiffness of the tissue of interest, one of the important parameters for diagnosing malignant tissues. We present experimental results of the displacement and strain measurement along with FEM analysis results.

Iterative image reconstruction in spectral-CT

Scan time of spectral-CTs is much longer than conventional CTs due to limited number of x-ray photons detectable by photon-counting detectors. However, the spectral pixel information in spectral-CT has much richer information on physiological and pathological status of the tissues than the CT-number in conventional CT, which makes the spectral- CT one of the promising future imaging modalities. One simple way to reduce the scan time in spectral-CT imaging is to reduce the number of views in the acquisition of projection data. But, this may result in poorer SNR and strong streak artifacts which can severely compromise the image quality. In this work, spectral-CT projection data were obtained from a lab-built spectral-CT consisting of a single CdTe photon counting detector, a micro-focus x-ray tube and scan mechanics. For the image reconstruction, we used two iterative image reconstruction methods, the simultaneous iterative reconstruction technique (SIRT) and the total variation minimization based on conjugate gradient method (CG-TV), along with the filtered back-projection (FBP) to compare the image quality. From the imaging of the iodine containing phantoms, we have observed that SIRT and CG-TV are superior to the FBP method in terms of SNR and streak artifacts.

X-ray elastography: A feasibility study

We propose a new elastography method based on x-ray imaging. After taking two x-ray tomographic images of the breast-mimicking phantom with applying different compressing pressure to it, we calculated displacement and strain maps from the two images using a non-rigid body image registration. The strain maps showed elasticity characteristics of the phantom medium. We expect that the proposed elastography method can be incorporated into breast tomosynthesis or breast CT systems to detect early stage breast cancers.

A preliminary study on temperature change monitoring using the MR current density imaging technique

Based on the fact that the electrical impedance of biological tissues is very sensitive to temperature, we have proposed a method to monitor local temperature changes inside the tissues. Using an analytic model and a finite element method model, we have analysed the effect of the local temperature change on the phase image obtained by the magnetic resonance current density imaging technique. We show preliminary experimental results of the temperature change monitoring performed with a 0.3 T magnetic resonance imaging system. We expect that the proposed method can be utilized for the development of non-invasive temperature imaging techniques.

Comparison of positioning methods of a ferromagnetic sphere for magnetic steering using 3 T MRI

Magnetic steering of an untethered ferromagnetic device in a living body has many advantages in the clinical fields. In this paper, the positioning and tracking methods of a ferromagnetic sphere have been compared with magnetic resonance phantom images obtained with three different imaging sequences, spin echo, gradient echo, and selected positive contrast sequences. The position of the ferromagnetic sphere has been calculated from the MRI images which have susceptibility artifacts caused by the magnetic sphere. We have compared the positioning errors among the magnetic sphere images.

Registration of Sheep Brain MR Images for Cell Tracking Using Ferrite-Composite Micro-Beads as Markers

In the recent molecular imaging studies of the magnetic resonance imaging field, in-vivo cell tracking is becoming an issue for the observation of cell therapy and disease behavior. In order to perform cell tracking, high resolution images and long-term studies are required, because those images make better performance than low resolution images in the recognition of cells and cells are traced at least two weeks. Image registration is an essential image processing technique for long-term imaging. In this study, we proposed the image registration technique using ferrite-composite micro- beads which could be used as substitutes for the cells labeling MR contrast agent such as superparamagnetic iron oxide (SPIO) nanoparticles. Registration of sheep brain images tagging micro-beads was performed with the sufficient accuracy for cell tracking.