Human In-vivo Brain MR Current Density Imaging (MRCDI) based on Steady-state Free Precession Free Induction Decay (SSFP-FID)

MRCDI is a novel technique for non-invasive measurement of weak currents in the human head, which is important in several neuroscience applications. Here, we present reliable in-vivo MRCDI measurements in the human brain based on SSFP-FID, yielding an unprecedented accuracy. We demonstrate the destructive influences of stray magnetic fields caused by the current passing through feeding cables, and propose a correction method. Also, we show inter-individual differences in MRCDI measurements for two different current profiles, and compare the measurements with simulations based on individualized head models. The simulations of the current-induced magnetic fields show good agreement with in-vivo brain measurements

Human In-vivo MR Current Density Imaging (MRCDI) Based on Optimized Multi-echo Spin Echo (MESE)

MRCDI aims at imaging an externally injected current flow in the human body, and might be useful for many biomedical applications. However, the method requires very sensitive measurement of the current-induced magnetic field component ?Bz,c parallel to main field. We systematically optimized MESE to determine its most efficient parameters. In one of the first human in-vivo applications of MRCDI, the optimized sequence was successfully used to image the ?Bz,c distribution in the brain caused by a two-electrode montage, as confirmed by finite-element calculations of ?Bz,c. Further improvements will be performed to increase its robustness to field drifts

Comparison of two alternative sequences for human in-vivo brain MR Current Density Imaging (MRCDI)

MRCDI is a novel technique, utilizing different phase-sensitive MR methods for non-invasive measurements of weak currents in the human body, which is important in several neuroscience applications. Here, we compare the in-vivo performance of two different MR methods, multi-echo spin echo (MESE) and steady-state free precession free induction decay (SSFP-FID), with single- vs. multi-gradient-echo readouts. We demonstrate that multi-gradient-echo readouts improve both methods. We validate the linear dependence of the measured current-induced magnetic field on the injected current strength for both methods, and propose the more efficient SSFP-FID method as being well suited for highly sensitive single-slice human in-vivo MRCDI

Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)

Accurate mapping of current flows in the human brain is important for many neuroscientific applications. MRCDI is an emerging method, which combines MRI with externally applied alternating currents to derive current flow distributions based on measurements of the current-induced magnetic fields. However, inaccurate and inconsistent measurements occur unless the stray magnetic fields ca used by the currents flowing in the feeding cables are corrected [1] . Here, we explore the influences of the stray magnetic fields due to the cable - currents in realistic experimental MRCDI set - ups

Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)

Accurate mapping of current flows in the human brain is important for many neuroscientific applications. MRCDI is an emerging method, which combines MRI with externally applied alternating currents to derive current flow distributions based on measurements of the current-induced magnetic fields. However, inaccurate and inconsistent measurements occur unless the stray magnetic fields ca used by the currents flowing in the feeding cables are corrected [1] . Here, we explore the influences of the stray magnetic fields due to the cable - currents in realistic experimental MRCDI set - ups

Optimization of magnetic flux density for fast MREIT conductivity imaging using multi-echo interleaved partial fourier acquisitions

BACKGROUND: Magnetic resonance electrical impedance tomography (MREIT) has been introduced as a non-invasive method for visualizing the internal conductivity and/or current density of an electrically conductive object by externally injected currents. The injected current through a pair of surface electrodes induces a magnetic flux density distribution inside the imaging object, which results in additional magnetic flux density. To measure the magnetic flux density signal in MREIT, the phase difference approach in an interleaved encoding scheme cancels out the systematic artifacts accumulated in phase signals and also reduces the random noise effect by doubling the measured magnetic flux density signal. For practical applications of in vivo MREIT, it is essential to reduce the scan duration maintaining spatial-resolution and sufficient contrast. In this paper, we optimize the magnetic flux density by using a fast gradient multi-echo MR pulse sequence. To recover the one component of magnetic flux density B(z), we use a coupled partial Fourier acquisitions in the interleaved sense. METHODS: To prove the proposed algorithm, we performed numerical simulations using a two-dimensional finite-element model. For a real experiment, we designed a phantom filled with a calibrated saline solution and located a rubber balloon inside the phantom. The rubber balloon was inflated by injecting the same saline solution during the MREIT imaging. We used the multi-echo fast low angle shot (FLASH) MR pulse sequence for MRI scan, which allows the reduction of measuring time without a substantial loss in image quality. RESULTS: Under the assumption of a priori phase artifact map from a reference scan, we rigorously investigated the convergence ratio of the proposed method, which was closely related with the number of measured phase encode set and the frequency range of the background field inhomogeneity. In the phantom experiment with a partial Fourier acquisition, the total scan time was less than 6 seconds to measure the magnetic flux density B(z) data with 128×128 spacial matrix size, where it required 10.24 seconds to fill the complete k-space region. CONCLUSION: Numerical simulation and experimental results demonstrated that the proposed method reduces the scanning time and provides the recovered B(z) data comparable to what we obtained by measuring complete k-space data

Sensitivity analysis of magnetic field measurements for magnetic resonance electrical impedance tomography (MREIT)

Purpose Clinical use of magnetic resonance electrical impedance tomography (MREIT) still requires significant sensitivity improvements. Here, the measurement of the current-induced magnetic field (ΔBz,c) is improved using systematic efficiency analyses and optimization of multi-echo spin echo (MESE) and steady-state free precession free induction decay (SSFP-FID) sequences. Theory and Methods Considering T1, T2, and math formula relaxation in the signal-to-noise ratios (SNRs) of the MR magnitude images, the efficiency of MESE and SSFP-FID MREIT experiments, and its dependence on the sequence parameters, are analytically analyzed and simulated. The theoretical results are experimentally validated in a saline-filled homogenous spherical phantom with relaxation parameters similar to brain tissue. Measurement of ΔBz,c is also performed in a cylindrical phantom with saline and chicken meat. Results The efficiency simulations and experimental results are in good agreement. When using optimal parameters, ΔBz,c can be reliably measured in the phantom even at injected current strengths of 1 mA or lower for both sequence types. The importance of using proper crusher gradient selection on the phase evolution in a MESE experiment is also demonstrated. Conclusion The efficiencies observed with the optimized sequence parameters will likely render in-vivo human brain MREIT feasible

Magnetic resonance electrical impedance tomography based on the solution of the convection equation and 3D Fourier transform-magnetic resonance current density imaging

Ankara : The Department of Electrical and Electronics Engineering and the Graduate School of Engineering and Sciences of Bilkent University, 2011.Thesis (Master's) -- Bilkent University, 2011.Includes bibliographical references leaves 80-87.Oran, Ömer FarukM.S

Anisotropy in Diffusion and Electrical Conductivity Distributions of TX-151 Phantoms

abstract: Among electrical properties of living tissues, the differentiation of tissues or organs provided by electrical conductivity is superior. The pathological condition of living tissues is inferred from the spatial distribution of conductivity. Magnetic Resonance Electrical Impedance Tomography (MREIT) is a relatively new non-invasive conductivity imaging technique. The majority of conductivity reconstruction algorithms are suitable for isotropic conductivity distributions. However, tissues such as cardiac muscle and white matter in the brain are highly anisotropic. Until recently, the conductivity distributions of anisotropic samples were solved using isotropic conductivity reconstruction algorithms. First and second spatial derivatives of conductivity (∇σ and ∇2σ ) are integrated to obtain the conductivity distribution. Existing algorithms estimate a scalar conductivity instead of a tensor in anisotropic samples. Accurate determination of the spatial distribution of a conductivity tensor in an anisotropic sample necessitates the development of anisotropic conductivity tensor image reconstruction techniques. Therefore, experimental studies investigating the effect of ∇2σ on degree of anisotropy is necessary. The purpose of the thesis is to compare the influence of ∇2σ on the degree of anisotropy under two different orthogonal current injection pairs. The anisotropic property of tissues such as white matter is investigated by constructing stable TX-151 gel layer phantoms with varying degrees of anisotropy. MREIT and Diffusion Magnetic Resonance Imaging (DWI) experiments were conducted to probe the conductivity and diffusion properties of phantoms. MREIT involved current injection synchronized to a spin-echo pulse sequence. Similarities and differences in the divergence of the vector field of ∇σ (∇2σ) among anisotropic samples subjected to two different current injection pairs were studied. DWI of anisotropic phantoms involved the application of diffusion-weighted magnetic field gradients with a spin-echo pulse sequence. Eigenvalues and eigenvectors of diffusion tensors were compared to characterize diffusion properties of anisotropic phantoms. The orientation of current injection electrode pair and degree of anisotropy influence the spatial distribution of ∇2σ. Anisotropy in conductivity is preserved in ∇2σ subjected to non-symmetric electric fields. Non-symmetry in electric field is observed in current injections parallel and perpendicular to the orientation of gel layers. The principal eigenvalue and eigenvector in the phantom with maximum anisotropy display diffusion anisotropy.Dissertation/ThesisMasters Thesis Bioengineering 201