ziya ider - Academia.edu (original) (raw)
Papers by ziya ider
IEEE Transactions on Medical Imaging, 1994
The mathematical basis of a new imaging modality, induced current electrical impedance tomography... more The mathematical basis of a new imaging modality, induced current electrical impedance tomography (EIT), is investigated, The ultimate aim of this technique is the reconstruction of conductivity distribution of the human body, from voltage measurements made between electrodes placed on the surface, when currents are induced inside the body by applied time varying magnetic fields. In this study the two-dimensional problem is analyzed. A specific 9-coil system for generating nine different exciting magnetic fields (50 kHz) and 16 measurement electrodes around the object are assumed, The partial differential equation for the scaler potential function in the conductive medium is derived and finite element method (FEM) is used for its solution. Sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular value decomposition of the sensitivity matrix shows that there are 135 independent measurements. It is found that measurements are less sensitive to changes in conductivity of the object's interior. While in this respect induced current EIT is slightly inferior to the technique of injected current EIT (using Sheffield protocol), its sensitivity matrix is better conditioned. The images obtained are found to be comparable to injected current EIT images In resolution. Design of a coil system for which parameters such as sensitivity to inner regions and condition number of the sensitivity matrix are optimum, remains to be made.
2018 Medical Technologies National Congress (TIPTEKNO), 2018
This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acqui... more This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acquisition system for brain-computer interface (BCI) applications. Available low-cost systems mostly aim at monitoring EEG at low sampling rates whereas higher data rates are needed for advanced processing such as signal averaging. In this study, 4-channel EEG acquisition system was developed, with maximum achievable sampling rate of 2000 samples/s for each channel is achieved. The proposed design consists of Data Acquisition (DAQ) hardware and remote receiver module. DAQ hardware amplifies and digitizes EEG and transmits it wirelessly. Remote receiver receives the data and displays the trend in real-time on computer screen. Whole system is mounted on a 6.6 cm x 4.9 cm PCB and is successfully tested in three different experiments: (i) Electrocardiography, (ECG) recording, (ii) EEG recording, with eyes closed, to observe alpha band (8–12 Hz), and (iii) EEG recording, to observe Steady State Visually Evoked Potential (SSVEP) responses. Further studies will involve addition of visual stimuli hardware and trigger-marking channel.
IEEE Transactions on Medical Imaging, 1997
Electric currents are applied to body in numerous applications in medicine such as electrical imp... more Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single tone broadband frequency modulated (FM) waveform with the ac magnetic field strength determining the modulat...
IEEE Transactions on Medical Imaging, 1994
The mathematical basis of a new imaging modality, induced current electrical impedance tomography... more The mathematical basis of a new imaging modality, induced current electrical impedance tomography (EIT), is investigated, The ultimate aim of this technique is the reconstruction of conductivity distribution of the human body, from voltage measurements made between electrodes placed on the surface, when currents are induced inside the body by applied time varying magnetic fields. In this study the two-dimensional problem is analyzed. A specific 9-coil system for generating nine different exciting magnetic fields (50 kHz) and 16 measurement electrodes around the object are assumed, The partial differential equation for the scaler potential function in the conductive medium is derived and finite element method (FEM) is used for its solution. Sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular value decomposition of the sensitivity matrix shows that there are 135 independent measurements. It is found that measurements are less sensitive to changes in conductivity of the object's interior. While in this respect induced current EIT is slightly inferior to the technique of injected current EIT (using Sheffield protocol), its sensitivity matrix is better conditioned. The images obtained are found to be comparable to injected current EIT images In resolution. Design of a coil system for which parameters such as sensitivity to inner regions and condition number of the sensitivity matrix are optimum, remains to be made.
Physics in Medicine and Biology, May 17, 2010
Fourier transform (FT)-based algorithms for magnetic resonance current density imaging (MRCDI) fr... more Fourier transform (FT)-based algorithms for magnetic resonance current density imaging (MRCDI) from one component of magnetic flux density have been developed for 2D and 3D problems. For 2D problems, where current is confined to the xy-plane and z-component of the magnetic flux density is measured also on the xy-plane inside the object, an iterative FT-MRCDI algorithm is developed by which both the current distribution inside the object and the z-component of the magnetic flux density on the xy-plane outside the object are reconstructed. The method is applied to simulated as well as actual data from phantoms. The effect of measurement error on the spatial resolution of the current density reconstruction is also investigated. For 3D objects an iterative FT-based algorithm is developed whereby the projected current is reconstructed on any slice using as data the Laplacian of the z-component of magnetic flux density measured for that slice. In an injected current MRCDI scenario, the current is not divergence free on the boundary of the object. The method developed in this study also handles this situation.
The purpose of this study is to develop a new methodology for designing stimulus sequences for cV... more The purpose of this study is to develop a new methodology for designing stimulus sequences for cVEP BCI based on experimental studies regarding the behavior and the properties of the actual EEG responses of the visual system to coded visual stimuli, such that training time is reduced and the possible number of targets is increased. EEG from 8 occipital sites is recorded with 2000 samples/sec per channel, in response to visual stimuli presented on a computer monitor with 60Hz refresh rate. Onset and offset EEG responses to long visual stimulus pulses are obtained through 160-trial signal averaging. These edge responses are used to predict the EEG responses to arbitrary stimulus sequences using the superposition principle. A BCI speller which utilizes the target templates generated by this principle is also implemented and tested. It is found that certain short stimulus patterns can be accurately predicted by the superposition principle. BCI sequences that are constructed by combinati...
Magnetic resonance–electrical impedance tomography (MR-EIT) is a conductivity imaging method base... more Magnetic resonance–electrical impedance tomography (MR-EIT) is a conductivity imaging method based on injecting currents into the object. In this study, a new MR-EIT method, whereby currents are induced inside the object by using external coils, is proposed. This new method is called induced current magnetic resonance–electrical impedance tomography. In induced current MR-EIT surface electrodes are not used and thereby artifacts due to electrodes are eliminated. The reconstruction algorithm is based on the measurement of only one component of the secondary magnetic flux density. The algorithm is an iterative one, is 3D and is based on the solution of a linear matrix equation at each iteration. For the measurement of secondary magnetic flux density, a pulse sequence to be used in the MRI system is proposed. Numerical simulations are performed to test the algorithm for both noise-free and noisy cases. The singular value behavior of the matrix is monitored and it is observed that at le...
In this study, Ursino and Magosso model that includes respiration effect on cardiovascular system... more In this study, Ursino and Magosso model that includes respiration effect on cardiovascular system is implemented using Matlab. The simulations are performed to investigate the effects of respiration rate, tidal volume and expiration-inspiration time ratio on heart rate variability (HRV) signals. Power spectral density (PSD) of HRV signals that are obtained from model simulation was determined by periodogram and Yule-Walker
2018 Medical Technologies National Congress (TIPTEKNO), 2018
This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acqui... more This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acquisition system for brain-computer interface (BCI) applications. Available low-cost systems mostly aim at monitoring EEG at low sampling rates whereas higher data rates are needed for advanced processing such as signal averaging. In this study, 4-channel EEG acquisition system was developed, with maximum achievable sampling rate of 2000 samples/s for each channel is achieved. The proposed design consists of Data Acquisition (DAQ) hardware and remote receiver module. DAQ hardware amplifies and digitizes EEG and transmits it wirelessly. Remote receiver receives the data and displays the trend in real-time on computer screen. Whole system is mounted on a 6.6 cm x 4.9 cm PCB and is successfully tested in three different experiments: (i) Electrocardiography, (ECG) recording, (ii) EEG recording, with eyes closed, to observe alpha band (8–12 Hz), and (iii) EEG recording, to observe Steady State V...
Magnetic Resonance in Medicine, 2016
Purpose: To develop a fast, practically applicable, and boundary artifact free electrical conduct... more Purpose: To develop a fast, practically applicable, and boundary artifact free electrical conductivity imaging method that does not use transceive phase assumption, and that is more robust against the noise. Theory: Starting from the Maxwell's equations, a new electrical conductivity imaging method that is based solely on the MR transceive phase has been proposed. Different from the previous phase based electrical properties tomography (EPT) method, a new formulation was derived by including the gradients of the conductivity into the equations. Methods: The governing partial differential equation, which is in the form of a convection-reaction-diffusion equation, was solved using a three-dimensional finite-difference scheme. To evaluate the performance of the proposed method numerical simulations, phantom and in vivo human experiments have been conducted at 3T. Results: Simulation and experimental results of the proposed method and the conventional phase-based EPT method were illustrated to show the superiority of the proposed method over the conventional method, especially in the transition regions and under noisy data. Conclusion: With the contributions of the proposed method to the phase-based EPT approach, a fast and reliable electrical conductivity imaging appears to be feasible, which is promising for clinical diagnoses and local SAR estimation. Magn Reson Med 77:137-150, 2017. V
Proceedings of 18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 1996
Electric currents are applied to body in numerous applications in medicine, such as electrical im... more Electric currents are applied to body in numerous applications in medicine, such as electrical impedance tomography, cardiac defibrillators, electrocautery and some treatment methods in physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated by AC currents injected into a phantom are measured using MRI. A pulse sequence that is originally designed for mapping static magnetic field is used. AC currents in the form of burst sine waves are applied synchronously with the pulse sequence. Results show that this method can be used in applications where the frequency of the currents is in the audio range and the amplitude is a few milliamperes or larger depending on SNR
The theory behind induced current EIT is summarized. A prototype data acquisition system is descr... more The theory behind induced current EIT is summarized. A prototype data acquisition system is described which is realized to verify the theoretical studies. Data collected from a two-dimensional (2D) object distribution is compared with the calculated data obtained by the finite element method. It is shown that the data acquisition system is capable of collecting real data which closely follows the theoretically expected perturbation in boundary potential differences. Images reconstructed by real data sets are indicatives of the sizes and locations of the inhomogeneities in the saline solution.<<ETX>>
Magnetic resonance-electrical impedance tomography (MR-EIT) was first proposed in 1992. Since the... more Magnetic resonance-electrical impedance tomography (MR-EIT) was first proposed in 1992. Since then various reconstruction algorithms have been suggested and applied. These algorithms use peripheral voltage measurements and internal current density measurements in different combinations. In this study the problem of MR-EIT is treated as a hyperbolic system of first-order partial differential equations, and three numerical methods are proposed for its solution. This approach is not utilized in any of the algorithms proposed earlier. The numerical solution methods are integration along equipotential surfaces (method of characteristics), integration on a Cartesian grid, and inversion of a system matrix derived by a finite difference formulation. It is shown that if some uniqueness conditions are satisfied, then using at least two injected current patterns, resistivity can be reconstructed apart from a multiplicative constant. This constant can then be identified using a single voltage m...
ABSTRACT In order to provide noiseless transmission of speech in wireless communication systems a... more ABSTRACT In order to provide noiseless transmission of speech in wireless communication systems a real-time implementable noise cancellation algorithm is developed. Speech and noise sources are not known but only their mixtures are observed. That system is modeled with instantaneous mixture model. Combination of independent component analysis (ICA) and particle swarm optimization (PSO) algorithms is used to separate speech and noise. However, ICA has an ambiguity such that it is not possible to know which one of the separated signals is speech or noise. As a result, the transmitted signal can be noise, instead of speech. To overcome this ambiguity problem, a pitch extraction (PE) algorithm is developed and combined with ICA-PSO. ICAPSO-PE algorithm is implemented in MATLAB. Contributions of this work are modifying objective functions of ICA algorithm to make them more robust, combining ICA with PSO to make it work fast and robust, and overcoming the ambiguity problem using PE algorithm.
Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on rec... more Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on reconstructing the internal conductivity distribution of a conductive object from the Laplacian of only one component of the magnetic flux density (∇²B(z)) generated by the internal current distribution. In this study, a new algorithm is proposed to solve this ∇²B(z)-based MREIT problem which is mathematically formulated as the steady-state scalar pure convection equation. Numerical methods developed for the solution of the more general convection-diffusion equation are utilized. It is known that the solution of the pure convection equation is numerically unstable if sharp variations of the field variable (in this case conductivity) exist or if there are inconsistent boundary conditions. Various stabilization techniques, based on introducing artificial diffusion, are developed to handle such cases and in this study the streamline upwind Petrov-Galerkin (SUPG) stabilization method is incorpo...
Biomedical Physics & Engineering Express
Biomedical Physics & Engineering Express
Objective. This study aims at investigating the potential mechanism of period doubling (PD) (subh... more Objective. This study aims at investigating the potential mechanism of period doubling (PD) (subharmonic generation) in human steady-state visual evoked potentials (SSVEPs) by using a mathematical model. Approach. Robinson's Corticothalamic Model, which includes three main neuronal populations (cortical, thalamic reticular, and thalamic relay neurons) was employed. SSVEP experiments were simulated using this model and dependence of PD behavior in relation to the values of model parameters was investigated. The feedback loop in the model that is responsible for the generation of subharmonic components was thus identified, and this loop was isolated from the rest of the model and analyzed with a describing function approach. Main Results. It has been found in general, for a wide range of parameter values, that if the excitation frequency or half of it is close to the native oscillation frequency of the system, the native oscillation ceases to exist and oscillations at either the excitation frequency or half of it are observed. This observation is in line with the experimental findings except for some discrepancies which are also discussed. The intrathalamic feedback loop is identified to be the potential source of subharmonic oscillations. When isolated from the rest of the model and simulated by itself, it has been found that this feedback loop can show a resonance phenomenon at the subharmonic frequency. By deriving a set of equations based on the necessary conditions for a resonance phenomenon, a semi-analytical method was developed by which one can predict the existence of subharmonic generation for a given set of parameters and stimulus frequency. Significance. This study is the first model-based investigation of the mechanism of subharmonic oscillations. The proposed semianalytical method can replace extensive time and memory consuming parameter sweep studies.
Biomedical Physics & Engineering Express
Objective. Previous human steady state visual evoked potential (SSVEP) experiments have yielded d... more Objective. Previous human steady state visual evoked potential (SSVEP) experiments have yielded different results regarding the range of stimulus frequencies in which period doubling (PD) behavior is observed. This study aims at obtaining experimental and statistical data regarding the frequency range of PD generation and also investigates other characteristics of PD. Approach. In two sets of experiments, seven subjects were presented a sinusoidal flickering light stimulus with frequencies varying from 15 to 42 Hz. To observe the short term variations in PD generation, another set of 5 successive experiments were performed on five subjects with 10 min breaks in between. To obtain the SSVEP responses, filtering, signal averaging and power spectral density (PSD) estimation were applied to the recorded electroencephalogram. From the PSD estimates, subharmonic occurrence rates were calculated for each experiment and were used along with ANOVA for interpreting the outcomes of the short term repeatability experiments. Main results. Although fundamental (excitation frequency) and second harmonic components appear in almost all SSVEP spectra, there is considerable inter-subject and intra-subject variability regarding PD occurrence. PD occurs for all stimulus frequencies from 15 to 42 Hz when all subjects are considered together. Furthermore, the statistical analyses of short term repeatability experiments suggest that in the short term, PD generation is consistent when all frequencies are considered together but for a single frequency significant short term differences occur. There also is considerable variation in the ratio of subharmonic amplitude to fundamental amplitude across different frequencies for a given subject. Significance. Important results and statistical data are obtained regarding PD generation. Our results indicate that modeling studies should attempt to generate PD for a broader range of stimulus frequencies. It is argued that SSVEP based braincomputer interface applications would likely benefit from the utilization of subharmonics in classification.
Magnetic Resonance in Medicine
IEEE Transactions on Medical Imaging, 1994
The mathematical basis of a new imaging modality, induced current electrical impedance tomography... more The mathematical basis of a new imaging modality, induced current electrical impedance tomography (EIT), is investigated, The ultimate aim of this technique is the reconstruction of conductivity distribution of the human body, from voltage measurements made between electrodes placed on the surface, when currents are induced inside the body by applied time varying magnetic fields. In this study the two-dimensional problem is analyzed. A specific 9-coil system for generating nine different exciting magnetic fields (50 kHz) and 16 measurement electrodes around the object are assumed, The partial differential equation for the scaler potential function in the conductive medium is derived and finite element method (FEM) is used for its solution. Sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular value decomposition of the sensitivity matrix shows that there are 135 independent measurements. It is found that measurements are less sensitive to changes in conductivity of the object's interior. While in this respect induced current EIT is slightly inferior to the technique of injected current EIT (using Sheffield protocol), its sensitivity matrix is better conditioned. The images obtained are found to be comparable to injected current EIT images In resolution. Design of a coil system for which parameters such as sensitivity to inner regions and condition number of the sensitivity matrix are optimum, remains to be made.
2018 Medical Technologies National Congress (TIPTEKNO), 2018
This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acqui... more This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acquisition system for brain-computer interface (BCI) applications. Available low-cost systems mostly aim at monitoring EEG at low sampling rates whereas higher data rates are needed for advanced processing such as signal averaging. In this study, 4-channel EEG acquisition system was developed, with maximum achievable sampling rate of 2000 samples/s for each channel is achieved. The proposed design consists of Data Acquisition (DAQ) hardware and remote receiver module. DAQ hardware amplifies and digitizes EEG and transmits it wirelessly. Remote receiver receives the data and displays the trend in real-time on computer screen. Whole system is mounted on a 6.6 cm x 4.9 cm PCB and is successfully tested in three different experiments: (i) Electrocardiography, (ECG) recording, (ii) EEG recording, with eyes closed, to observe alpha band (8–12 Hz), and (iii) EEG recording, to observe Steady State Visually Evoked Potential (SSVEP) responses. Further studies will involve addition of visual stimuli hardware and trigger-marking channel.
IEEE Transactions on Medical Imaging, 1997
Electric currents are applied to body in numerous applications in medicine such as electrical imp... more Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single tone broadband frequency modulated (FM) waveform with the ac magnetic field strength determining the modulat...
IEEE Transactions on Medical Imaging, 1994
The mathematical basis of a new imaging modality, induced current electrical impedance tomography... more The mathematical basis of a new imaging modality, induced current electrical impedance tomography (EIT), is investigated, The ultimate aim of this technique is the reconstruction of conductivity distribution of the human body, from voltage measurements made between electrodes placed on the surface, when currents are induced inside the body by applied time varying magnetic fields. In this study the two-dimensional problem is analyzed. A specific 9-coil system for generating nine different exciting magnetic fields (50 kHz) and 16 measurement electrodes around the object are assumed, The partial differential equation for the scaler potential function in the conductive medium is derived and finite element method (FEM) is used for its solution. Sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular value decomposition of the sensitivity matrix shows that there are 135 independent measurements. It is found that measurements are less sensitive to changes in conductivity of the object's interior. While in this respect induced current EIT is slightly inferior to the technique of injected current EIT (using Sheffield protocol), its sensitivity matrix is better conditioned. The images obtained are found to be comparable to injected current EIT images In resolution. Design of a coil system for which parameters such as sensitivity to inner regions and condition number of the sensitivity matrix are optimum, remains to be made.
Physics in Medicine and Biology, May 17, 2010
Fourier transform (FT)-based algorithms for magnetic resonance current density imaging (MRCDI) fr... more Fourier transform (FT)-based algorithms for magnetic resonance current density imaging (MRCDI) from one component of magnetic flux density have been developed for 2D and 3D problems. For 2D problems, where current is confined to the xy-plane and z-component of the magnetic flux density is measured also on the xy-plane inside the object, an iterative FT-MRCDI algorithm is developed by which both the current distribution inside the object and the z-component of the magnetic flux density on the xy-plane outside the object are reconstructed. The method is applied to simulated as well as actual data from phantoms. The effect of measurement error on the spatial resolution of the current density reconstruction is also investigated. For 3D objects an iterative FT-based algorithm is developed whereby the projected current is reconstructed on any slice using as data the Laplacian of the z-component of magnetic flux density measured for that slice. In an injected current MRCDI scenario, the current is not divergence free on the boundary of the object. The method developed in this study also handles this situation.
The purpose of this study is to develop a new methodology for designing stimulus sequences for cV... more The purpose of this study is to develop a new methodology for designing stimulus sequences for cVEP BCI based on experimental studies regarding the behavior and the properties of the actual EEG responses of the visual system to coded visual stimuli, such that training time is reduced and the possible number of targets is increased. EEG from 8 occipital sites is recorded with 2000 samples/sec per channel, in response to visual stimuli presented on a computer monitor with 60Hz refresh rate. Onset and offset EEG responses to long visual stimulus pulses are obtained through 160-trial signal averaging. These edge responses are used to predict the EEG responses to arbitrary stimulus sequences using the superposition principle. A BCI speller which utilizes the target templates generated by this principle is also implemented and tested. It is found that certain short stimulus patterns can be accurately predicted by the superposition principle. BCI sequences that are constructed by combinati...
Magnetic resonance–electrical impedance tomography (MR-EIT) is a conductivity imaging method base... more Magnetic resonance–electrical impedance tomography (MR-EIT) is a conductivity imaging method based on injecting currents into the object. In this study, a new MR-EIT method, whereby currents are induced inside the object by using external coils, is proposed. This new method is called induced current magnetic resonance–electrical impedance tomography. In induced current MR-EIT surface electrodes are not used and thereby artifacts due to electrodes are eliminated. The reconstruction algorithm is based on the measurement of only one component of the secondary magnetic flux density. The algorithm is an iterative one, is 3D and is based on the solution of a linear matrix equation at each iteration. For the measurement of secondary magnetic flux density, a pulse sequence to be used in the MRI system is proposed. Numerical simulations are performed to test the algorithm for both noise-free and noisy cases. The singular value behavior of the matrix is monitored and it is observed that at le...
In this study, Ursino and Magosso model that includes respiration effect on cardiovascular system... more In this study, Ursino and Magosso model that includes respiration effect on cardiovascular system is implemented using Matlab. The simulations are performed to investigate the effects of respiration rate, tidal volume and expiration-inspiration time ratio on heart rate variability (HRV) signals. Power spectral density (PSD) of HRV signals that are obtained from model simulation was determined by periodogram and Yule-Walker
2018 Medical Technologies National Congress (TIPTEKNO), 2018
This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acqui... more This paper puts forward a low-cost portable 4-channel wireless Electroencephalography (EEG) Acquisition system for brain-computer interface (BCI) applications. Available low-cost systems mostly aim at monitoring EEG at low sampling rates whereas higher data rates are needed for advanced processing such as signal averaging. In this study, 4-channel EEG acquisition system was developed, with maximum achievable sampling rate of 2000 samples/s for each channel is achieved. The proposed design consists of Data Acquisition (DAQ) hardware and remote receiver module. DAQ hardware amplifies and digitizes EEG and transmits it wirelessly. Remote receiver receives the data and displays the trend in real-time on computer screen. Whole system is mounted on a 6.6 cm x 4.9 cm PCB and is successfully tested in three different experiments: (i) Electrocardiography, (ECG) recording, (ii) EEG recording, with eyes closed, to observe alpha band (8–12 Hz), and (iii) EEG recording, to observe Steady State V...
Magnetic Resonance in Medicine, 2016
Purpose: To develop a fast, practically applicable, and boundary artifact free electrical conduct... more Purpose: To develop a fast, practically applicable, and boundary artifact free electrical conductivity imaging method that does not use transceive phase assumption, and that is more robust against the noise. Theory: Starting from the Maxwell's equations, a new electrical conductivity imaging method that is based solely on the MR transceive phase has been proposed. Different from the previous phase based electrical properties tomography (EPT) method, a new formulation was derived by including the gradients of the conductivity into the equations. Methods: The governing partial differential equation, which is in the form of a convection-reaction-diffusion equation, was solved using a three-dimensional finite-difference scheme. To evaluate the performance of the proposed method numerical simulations, phantom and in vivo human experiments have been conducted at 3T. Results: Simulation and experimental results of the proposed method and the conventional phase-based EPT method were illustrated to show the superiority of the proposed method over the conventional method, especially in the transition regions and under noisy data. Conclusion: With the contributions of the proposed method to the phase-based EPT approach, a fast and reliable electrical conductivity imaging appears to be feasible, which is promising for clinical diagnoses and local SAR estimation. Magn Reson Med 77:137-150, 2017. V
Proceedings of 18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 1996
Electric currents are applied to body in numerous applications in medicine, such as electrical im... more Electric currents are applied to body in numerous applications in medicine, such as electrical impedance tomography, cardiac defibrillators, electrocautery and some treatment methods in physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated by AC currents injected into a phantom are measured using MRI. A pulse sequence that is originally designed for mapping static magnetic field is used. AC currents in the form of burst sine waves are applied synchronously with the pulse sequence. Results show that this method can be used in applications where the frequency of the currents is in the audio range and the amplitude is a few milliamperes or larger depending on SNR
The theory behind induced current EIT is summarized. A prototype data acquisition system is descr... more The theory behind induced current EIT is summarized. A prototype data acquisition system is described which is realized to verify the theoretical studies. Data collected from a two-dimensional (2D) object distribution is compared with the calculated data obtained by the finite element method. It is shown that the data acquisition system is capable of collecting real data which closely follows the theoretically expected perturbation in boundary potential differences. Images reconstructed by real data sets are indicatives of the sizes and locations of the inhomogeneities in the saline solution.<<ETX>>
Magnetic resonance-electrical impedance tomography (MR-EIT) was first proposed in 1992. Since the... more Magnetic resonance-electrical impedance tomography (MR-EIT) was first proposed in 1992. Since then various reconstruction algorithms have been suggested and applied. These algorithms use peripheral voltage measurements and internal current density measurements in different combinations. In this study the problem of MR-EIT is treated as a hyperbolic system of first-order partial differential equations, and three numerical methods are proposed for its solution. This approach is not utilized in any of the algorithms proposed earlier. The numerical solution methods are integration along equipotential surfaces (method of characteristics), integration on a Cartesian grid, and inversion of a system matrix derived by a finite difference formulation. It is shown that if some uniqueness conditions are satisfied, then using at least two injected current patterns, resistivity can be reconstructed apart from a multiplicative constant. This constant can then be identified using a single voltage m...
ABSTRACT In order to provide noiseless transmission of speech in wireless communication systems a... more ABSTRACT In order to provide noiseless transmission of speech in wireless communication systems a real-time implementable noise cancellation algorithm is developed. Speech and noise sources are not known but only their mixtures are observed. That system is modeled with instantaneous mixture model. Combination of independent component analysis (ICA) and particle swarm optimization (PSO) algorithms is used to separate speech and noise. However, ICA has an ambiguity such that it is not possible to know which one of the separated signals is speech or noise. As a result, the transmitted signal can be noise, instead of speech. To overcome this ambiguity problem, a pitch extraction (PE) algorithm is developed and combined with ICA-PSO. ICAPSO-PE algorithm is implemented in MATLAB. Contributions of this work are modifying objective functions of ICA algorithm to make them more robust, combining ICA with PSO to make it work fast and robust, and overcoming the ambiguity problem using PE algorithm.
Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on rec... more Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on reconstructing the internal conductivity distribution of a conductive object from the Laplacian of only one component of the magnetic flux density (∇²B(z)) generated by the internal current distribution. In this study, a new algorithm is proposed to solve this ∇²B(z)-based MREIT problem which is mathematically formulated as the steady-state scalar pure convection equation. Numerical methods developed for the solution of the more general convection-diffusion equation are utilized. It is known that the solution of the pure convection equation is numerically unstable if sharp variations of the field variable (in this case conductivity) exist or if there are inconsistent boundary conditions. Various stabilization techniques, based on introducing artificial diffusion, are developed to handle such cases and in this study the streamline upwind Petrov-Galerkin (SUPG) stabilization method is incorpo...
Biomedical Physics & Engineering Express
Biomedical Physics & Engineering Express
Objective. This study aims at investigating the potential mechanism of period doubling (PD) (subh... more Objective. This study aims at investigating the potential mechanism of period doubling (PD) (subharmonic generation) in human steady-state visual evoked potentials (SSVEPs) by using a mathematical model. Approach. Robinson's Corticothalamic Model, which includes three main neuronal populations (cortical, thalamic reticular, and thalamic relay neurons) was employed. SSVEP experiments were simulated using this model and dependence of PD behavior in relation to the values of model parameters was investigated. The feedback loop in the model that is responsible for the generation of subharmonic components was thus identified, and this loop was isolated from the rest of the model and analyzed with a describing function approach. Main Results. It has been found in general, for a wide range of parameter values, that if the excitation frequency or half of it is close to the native oscillation frequency of the system, the native oscillation ceases to exist and oscillations at either the excitation frequency or half of it are observed. This observation is in line with the experimental findings except for some discrepancies which are also discussed. The intrathalamic feedback loop is identified to be the potential source of subharmonic oscillations. When isolated from the rest of the model and simulated by itself, it has been found that this feedback loop can show a resonance phenomenon at the subharmonic frequency. By deriving a set of equations based on the necessary conditions for a resonance phenomenon, a semi-analytical method was developed by which one can predict the existence of subharmonic generation for a given set of parameters and stimulus frequency. Significance. This study is the first model-based investigation of the mechanism of subharmonic oscillations. The proposed semianalytical method can replace extensive time and memory consuming parameter sweep studies.
Biomedical Physics & Engineering Express
Objective. Previous human steady state visual evoked potential (SSVEP) experiments have yielded d... more Objective. Previous human steady state visual evoked potential (SSVEP) experiments have yielded different results regarding the range of stimulus frequencies in which period doubling (PD) behavior is observed. This study aims at obtaining experimental and statistical data regarding the frequency range of PD generation and also investigates other characteristics of PD. Approach. In two sets of experiments, seven subjects were presented a sinusoidal flickering light stimulus with frequencies varying from 15 to 42 Hz. To observe the short term variations in PD generation, another set of 5 successive experiments were performed on five subjects with 10 min breaks in between. To obtain the SSVEP responses, filtering, signal averaging and power spectral density (PSD) estimation were applied to the recorded electroencephalogram. From the PSD estimates, subharmonic occurrence rates were calculated for each experiment and were used along with ANOVA for interpreting the outcomes of the short term repeatability experiments. Main results. Although fundamental (excitation frequency) and second harmonic components appear in almost all SSVEP spectra, there is considerable inter-subject and intra-subject variability regarding PD occurrence. PD occurs for all stimulus frequencies from 15 to 42 Hz when all subjects are considered together. Furthermore, the statistical analyses of short term repeatability experiments suggest that in the short term, PD generation is consistent when all frequencies are considered together but for a single frequency significant short term differences occur. There also is considerable variation in the ratio of subharmonic amplitude to fundamental amplitude across different frequencies for a given subject. Significance. Important results and statistical data are obtained regarding PD generation. Our results indicate that modeling studies should attempt to generate PD for a broader range of stimulus frequencies. It is argued that SSVEP based braincomputer interface applications would likely benefit from the utilization of subharmonics in classification.
Magnetic Resonance in Medicine