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Zijun Meng,김형중,Saurav ZK Sajib,Munish Chauhan,Woo Chul Jeong,Young Tae Kim,우응제 대한의용생체공학회 2012 Biomedical Engineering Letters (BMEL) Vol.2 No.1
Purpose In performing human MREIT imaging experiments,we addressed two technical issues of the chemical shift artifact and measurement noise. In this study, we present improved conductivity images of in vivo human lower extremity using the chemical shift artifact correction and multi-echo methods. Methods To remove the chemical shift artifact in both MR magnitude and phase images, the three-point Dixon’s waterfat separation technique was modified for MREIT. Since the knee is more sensitive to injection currents, we limited the current amplitude in knee experiments to 3 mA to avoid painful sensation. We implemented this technique by incorporating a lately developed multi-echo based MREIT pulse sequence to enhance MR signals themselves and also by prolonging the total current injection time. Results Experimental results clearly show that the correction method effectively eliminates artifacts related with the chemical shift phenomenon in reconstructed conductivity images. The multi-echo method is advantageous in terms of SNR of MR magnitude, noise level of Bz compared with single-echo. The chemical shift artifact correction using multi-echo method allowed conductivity image reconstruction of the knee with 3 mA injection currents. Conclusions We expect that MREIT conductivity imaging incorporating both the chemical shift artifact correction and multi-echo pulse sequence would accelerate further experimental MREIT studies.
Three-dimensional MREIT Simulator of Static Bioelectromagnetism and MRI
우응제,Atul S. Minhas,Zijun Meng,Young Tae Kim,김형중,김혜현 대한의용생체공학회 2011 Biomedical Engineering Letters (BMEL) Vol.1 No.2
Purpose Magnetic resonance electrical impedance tomography (MREIT) aims to produce high-resolution cross-sectional images of a conductivity distribution inside the human body. We perform conductivity image reconstructions based on a relation between the conductivity distribution and induced magnetic flux density distributions subject to externally injected currents. This induced magnetic flux density is measured in MREIT using an MRI scanner. To facilitate MREIT research, we need a numerical simulator including static bioelectromagnetism and MRI data collection process. In this paper, we describe the development of a threedimensional MREIT simulator (MREITSim). Methods We describe various features of MREITSim including geometry modeling, meshing, finite element modeling and numerical computations of magnetic flux density and k-space MR data. We demonstrate the underlying bioelectromagnetic phenomena and MR data collection process using phantom models of without and with anomaly. We illustrate effects of noise in MR data and echo time on magnetic flux density computations. Results We demonstrate numerical computations of current density and magnetic flux density distributions for current injections orthogonal to z-direction, the direction of the main magnetic field of an MRI scanner. The k-space MREIT data generation procedure is illustrated using a phantom model with an insulating anomaly. Conclusions The simulator functions as a virtual MREIT scanner and provides quantitative numerical results of intended experimental studies. We suggest the simulator as a basic research tool for future MREIT studies of its theory, algorithm,experimental techniques and pulse sequence design.