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

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

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

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

3 Tesla’da manyetik rezonans akım yoğunluğu görüntüleme (MRAYG) için programlanabilir akım kaynağı.

Electrical properties of biological tissues are distinctive between various types of the tissues, and significantly related with the pathological conditions of the tissues. For instance, conductivity images can be used for tumor identification. Besides, current density distribution may provide useful information in research and development of electrical stimulation, electro-surgery, defibrillation, and cardiac pacing devices. Magnetic resonance current density imaging (MRCDI) and magnetic resonance electrical impedance tomography (MREIT) are two imaging modalities based on the current density distribution and the conductivity variation in biological tissues. In this thesis, a four-channel programmable current source to be used in MRCDI and MREIT applications, is designed and implemented. The current source is composed of a microcontroller unit (MCU), a digital to analog converter (DAC), a step-up DC-DC converter, and a voltage to current (V-I) converter with a current steering topology. Two different MRCDI experiments are performed in this study. In the first experiment, the current density distribution inside a uniform phantom is investigated. In the second experiment, a current steering insulating layer is inserted inside the uniform phantom, and the current density distribution is investigated. In order to check the accuracy of the experiments, the total injected current is compared with the total integrated current, which is calculated from reconstructed current density images. It is concluded that, the experiments are performed with 7.4% error. Keywords: Magnetic resonance, current density imaging, programmable current source, microcontroller unit, step-up DC-DC converter.M.S. - Master of Scienc