Browsing by Subject "Magnetic field"

Now showing 1 - 11 of 11

Open Access
Algebraic reconstraction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density
(Institute of Physics and Engineering in Medicine, 2004) Ider, Y. Z.; Onart, S.
Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used.
Open Access
Antibacterial properties and osteoblast interactions of microfluidically synthesized chitosan – SPION composite nanoparticles
(Wiley Periodicals LLC, 2023-05-26) Kafali, M.; Şahinoğlu, O. Berkay; Tufan, Y.; Orsel, Z. C.; Aygun, Elif; Alyuz, Beril; Saritas, Emine Ulku; Erdem, E. Yegan; Ercan, B.
In this research, a multi-step microfluidic reactor was used to fabricate chitosan – superparamagnetic iron oxide composite nanoparticles (Ch – SPIONs), where composite formation using chitosan was aimed to provide antibacterial property and nanoparticle stability for magnetic resonance imaging (MRI). Monodispersed Ch – SPIONs had an average particle size of 8.8 ± 1.2 nm with a magnetization value of 32.0 emu/g. Ch – SPIONs could be used as an MRI contrast agent by shortening T2 relaxation parameter of the surrounding environment, as measured on a 3 T MRI scanner. In addition, Ch – SPIONs with concentrations less than 1 g/L promoted bone cell (osteoblast) viability up to 7 days of culture in vitro in the presence of 0.4 T external static magnetic field. These nanoparticles were also tested against Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa), which are dangerous pathogens that cause infection in tissues and biomedical devices. Upon interaction of Ch – SPIONs with S. aureus and P. aeruginosa at 0.01 g/L concentration, nearly a 2-fold reduction in the number of colonies was observed for both bacteria strains at 48 h of culture. Results cumulatively showed that Ch – SPIONs were potential candidates as a cytocompatible and antibacterial agent that can be targeted to biofilm and imaged using an MRI.
Open Access
Controlling the photoluminescence of quantum emitters in hexagonal boron nitride by external magnetic fields
(IOP Publishing, 2022-10-28) Korkut, Hilal; Sarpkaya, İbrahim
The recent observation of room temperature spin-dependent photoluminescence (PL) emission from hexagonal boron nitride's (h-BN's) defect centers motivates for performing a complementary low-temperature photophysical study of quantum emitters under relatively high magnetic fields. Here, we investigate the PL emission dynamics of h-BN's visible single-photon emitters under an applied out-of-plane magnetic field at cryogenic temperatures. The PL intensity of the emitters in our work strikingly exhibits strong magnetic field dependence and decreases with the increased magnetic field. A substantial decrease in the integrated PL intensity of the emitters by up to one order of magnitude was observed when the applied field is increased from 0 T to 7 T. The observed reversible photodarkening of PL emission due to the applied magnetic field is in very well agreement with the predictions of a recent joint experimental and theoretical study and can happen only if the spin-selective, non-radiative, and asymmetric intersystem crossing transitions proceed from the triplet excited state to the lowest-lying spin-singlet metastable state and from the metastable state to the triplet ground state. Our results not only shed more light on the light emission paths of defect centers in h-BN but also show the use of the magnetic field as an efficient control knob in the development of magneto-optical devices.
Open Access
Experimental results for 2D magnetic resonance electrical impedance tomography (MR-EIT) using magnetic flux density in one direction
(Institute of Physics Publishing, 2003) Birgül, Ö.; Eyüboğlu, B. M.; İder, Y. Z.
Magnetic resonance electrical impedance tomography (MR-EIT) is an emerging imaging technique that reconstructs conductivity images using magnetic flux density measurements acquired employing MRI together with conventional EIT measurements. In this study, experimental MR-EIT images from phantoms with conducting and insulator objects are presented. The technique is implemented using the 0.15 T Middle East Technical University MRI system. The dc current method used in magnetic resonance current density imaging is adopted. A reconstruction algorithm based on the sensitivity matrix relation between conductivity and only one component of magnetic flux distribution is used. Therefore, the requirement for object rotation is eliminated. Once the relative conductivity distribution is found, it is scaled using the peripheral voltage measurements to obtain the absolute conductivity distribution. Images of several insulator and conductor objects in saline filled phantoms are reconstructed. The L2 norm of relative error in conductivity values is found to be 13%, 17% and 14% for three different conductivity distributions.
Open Access
Junction characteristics and magnetic field dependencies of low noise step edge junction Rf-SQUIDs for unshielded applications
(IEEE, 2003-06) Fardmanesh, Mehdi; Schubert, J.; Akram, Rizwan; Bozbey, Ali; Bick, M.; Banzet, M.; Lomparski, D.; Zander, W.; Zhang, Y.; Krause, H-J.
Step edge grain boundary (GB) junctions and rf-SQUIDs have been made using pulsed laser deposited Y-Ba-Cu-O films on crystalline LaAlO3 substrates. The steps were developed using various ion-beam etching processes resulting in sharp and ramp type step structures. Sharp step based GB junctions showed behavior of serial junctions with resistively shunted junction (RSJ)-like I-V characteristics. The ramped type step structures resulted in relatively high critical current, Ic, junctions and noisy SQUIDs. The sharp steps resulted in low noise rf-SQUIDs with a noise level below 140 fT/Hz12/ down to few Hz at 77 K while measured with conventional tank circuits. The Ic of the junctions and hence the operating temperature range of the SQUIDs made using sharp steps was controlled by both the step height and the junction widths. The junction properties of the SQUIDs were also characterized showing RSJ-like characteristics and magnetic field sensitivities correlated to that of the SQUIDs. Two major low and high background magnetic field sensitivities have been observed for our step edge junctions and the SQUIDs made on sharp steps. High quality step edge junctions with low magnetic field sensitivities made on clean sharp steps resulted in low 1/f noise rf-SQUIDs proper for applications in unshielded environment.
Open Access
Metal nanoring and tube formation on carbon nanotubes
(American Physical Society, 2002) Bagci, V. M. K.; Gülseren, O.; Yildirim, T.; Gedik, Z.; Ciracil, S.
The structural and electronic properties of aluminum-covered single-wall carbon nanotubes (SWNT's) are studied from first principles for a large number of coverages. Aluminum-aluminum interaction, that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum ring and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT's and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT.
Open Access
Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field
(Springer, 2006) El-Sharkawy, A. M.; Schär, M.; Bottomley, P. A.; Atalar, Ergin
The homogeneity and stability of the static magnetic field are of paramount importance to the accuracy of MR procedures that are sensitive to phase errors and magnetic field inhomogeneity. It is shown that intense gradient utilization in clinical horizontal-bore superconducting MR scanners of three different vendors results in main magnetic fields that vary on a long time scale both spatially and temporally by amounts of order 0.8-2.5 ppm. The observed spatial changes have linear and quadratic variations that are strongest along the z direction. It is shown that the effect of such variations is of sufficient magnitude to completely obfuscate thermal phase shifts measured by proton-resonance frequency-shift MR thermometry and certainly affect accuracy. In addition, field variations cause signal loss and line-broadening in MR spectroscopy, as exemplified by a fourfold line-broadening of metabolites over the course of a 45 min human brain study. The field variations are consistent with resistive heating of the magnet structures. It is concluded that correction strategies are required to compensate for these spatial and temporal field drifts for phase-sensitive MR protocols. It is demonstrated that serial field mapping and phased difference imaging correction protocols can substantially compensate for the drift effects observed in the MR thermometry and spectroscopy experiments.
Open Access
Noise, junction characteristics, and magnetic field dependencies of bicrystal grain boundary junction Rf-SQUIDs
(IEEE, 2003) Fardmanesh, Mehdi; Schubert, J.; Akram, Rizwan; Bick, M.; Banzet, M.; Zander, W.; Zhang, Y.; Krause, H-J.
Bicrystal grain boundary (GB) Josephson junctions and rf-SQUID's were made of 200 nm thick PLD YBCO films on bi-crystal SrTiO3 substrates. The junction characteristics were studied to investigate optimal parameters in the rf-SQUID) layout designs and the limits imposed by the technology. The I c of 3 to 8 μm wide test junctions scaled with the junction widths, showing clear linear RSJ-like I-V characteristics at 77 K. All the junctions showed hysteretic RCSJ-like behavior at very low temperatures. Classical Josephson flux motion type (long junction) nonlinearity in I-V curves of all the junctions was also observed at lower temperatures with systematic dependence on the junction widths. Measurements of the magnetic field dependence of the Ic of the junctions resulted in junction width dependent well-defined Fraunhofer-pattern like characteristics. The obtained characteristics of the junctions led to feasible criteria for the rf-SQUID layouts with desired device characteristics. Rf-SQUID's were made using designs for optimal performance at 77 K while avoiding large superconducting weak links across the substrate GB. Devices with low noise characteristics and junction field sensitivities proper for operation in environmental background magnetic fields were obtained. A nonsystematic spread of optimal working temperature of the SQUID's were also observed which is associated to the spread of the junction parameters caused by the defects at the GB of substrates.
Open Access
Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots
(The American Physical Society, 2005) Moldoveanu, V.; Ţolea, M.; Aldea, A.; Tanatar, Bilal
A detailed description of the tunneling processes within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Büttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot. ©2005 The American Physical Society.
Open Access
A simple analytical expression for the gradient induced potential on active implants during MRI
(2012) Turk, E.A.; Kopanoglu, E.; Guney, S.; Bugdayci, K.E.; Ider, Y. Z.; Erturk, V. B.; Atalar, Ergin
During magnetic resonance imaging, there is an interaction between the time-varying magnetic fields and the active implantable medical devices (AIMD). In this study, in order to express the nature of this interaction, simplified analytical expressions for the electric fields induced by time-varying magnetic fields are derived inside a homogeneous cylindrical volume. With these analytical expressions, the gradient induced potential on the electrodes of the AIMD can be approximately calculated if the position of the lead inside the body is known. By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method was tested with isolated frog leg stimulation experiments. As a result, these simplified expressions may help in assessing the gradient-induced stimulation risk to the patients with implants.
Open Access
Tunneling properties of quantum dot arrays in a strong magnetic field
(The American Physical Society, 2004) Moldoveanu, V.; Aldea, A.; Tanatar, Bilal
We study the transport properties of coherently coupled quantum dots in the quantum Hall regime within the Landauer-Büttiker formalism which captures and explains the experimentally observed features in terms of the spectral properties of the coupled dot system. The subpeak structure of the transmittance spectrum and the charging stability diagrams are obtained and discussed. The role of the intradot and interdot Coulomb interaction are pointed out. We show the subpeak evolution with the magnetic field and predict a specific oscillatory behavior of the Hall resistance in strong magnetic field which can be experimentally tested.