Measurement of AC magnetic field distribution using magnetic resonance imaging

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 modulation index, An algorithm is developed to calculate the ac magnetic field intensity at each voxel using the frequency spectrum of the voxel signal, Experimental results show that the proposed algorithm can be used to calculate ac magnetic field distribution within a conducting sample that is placed in an MRI system

Measuring AC magnetic field distribution using MRI

Electric currents Pre applied to body in numerous applications in medicine, such as Electrical Impedance Tomography (EIT), cardiac defibrillators, electrocautery and some treatment methods in physiotherapy. IT 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 is 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 wave is applied sychronous 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

Development of a compression algorithm suitable for exercise ECG data

Huge amount of data recorded during exercise electrocardiography may be stored for further analysis or be transferred to a remote physician through telephone lines. Due to channel limitations, data must be compressed prior to the transfer. In this study, an algorithm suitable for compression of exercise electrocardiography data is proposed. 2-D Discrete Cosine Transformation is applied in the algorithm to make use of the pseudo periodic behavior of the data. To increase the correlation, data is aligned from the R peaks. QRS detection is performed using Fast Dyadic Wavelet Transform. The success rate of the detection algorithm is found to be 99.78%. Uniform scalar quantization is used with zonal coding method in the coding of 2-D Discrete Cosine Transform coefficients. The performance of the compression algorithm is evaluated in terms of compression ratio, reconstruction error and by comparing the reports of the ST segment depression test applied before the compression and after the reconstruction to inspect whether vital information is preserved

Electrical impedance tomography using the magnetic field generated by injected currents

In 2D EIT imaging, the internal distribution of the injected currents generate a magnetic field in the imaging region which can be measured by magnetic resonance imaging techniques. This magnetic field is perpendicular to the imaging region on the imaging region and it can be used in reconstructing the conductivity distribution inside the imaging region. For this purpose, internal current distribution is found using the finite element method. The magnetic fields due to this current is found using Biot-Savart law. Sensitivity of magnetic field distribution to inner conductivity perturbations for different current injection profiles is studied. it is found that, to achieve a uniform spatial resolution, a current profile which generates uniform current inside the imaging region is to be applied. The condition number of the sensitivity matrix obtained for this case is found to be very low. Several images are obtained using simulation data

LINE INTERFERENCE SUBTRACTION FILTER FOR SIGNAL-AVERAGED ELECTROCARDIOGRAPHY

Use of notch filters in signal averaged ECG applications is avoided because they introduce ringing following the QRS. The authors therefore studied the use of a line interference subtraction (LIS) filter by which a scaled and shifted version of a signal at line frequency (line reference signal, LRS) is subtracted from the ECG signal in order to reduce the level of line interference. The frequency of the LRS must be the same as that of the line interference. For this, hardware is developed such that line frequency common mode interference is recorded simultaneously with the ECG. When applied to late potential detection it is seen that line interference is completely removed and no additional noise is introduced so that Simson's criteria derived from the vector magnitude are not changed. Similarly it is shown that despite high level of line interference notchy details of signal averaged P waves are completely recovered

ANALYSIS OF 3-DIMENSIONAL SOFTWARE EIT PHANTOMS BY THE FINITE-ELEMENT METHOD

In electrical impedance tomography, two-dimensional (2D) finite element solutions are used in the imaging algorithms. It is assumed that a major part of the current flowing through the object is restricted to the measurement plane (i.e. the plane determined by the electrodes which are used for measuring voltage differences) and the current flowing elsewhere is negligible. However, there is usually a three-dimensional (3D) variation of the conductivity distribution and if there are regions of high contrast close to the measurement plane, the measured voltage values may be considerably affected. In this work a 3D finite element analysis is utilised to demonstrate the previously mentioned effects. Examples are given to show the measured voltage differences for conductivity distributions which are identical on the measurement plane but different elsewhere

A COMPARATIVE-STUDY OF SEVERAL EXCITING MAGNETIC-FIELDS FOR INDUCED CURRENT EIT

In this study, the selection of coil configuration parameters (coil radius and coil centre shift) for induced current Err using circular coils is investigated . An alternative coil configuration is suggested, which produces approximately linear (spatially) magnetic fields in order to strengthen the currents in the central region. Injected current EIT, with Sheffield data collection protocol, and induced current EIT, With two different coil configurations, are compared with respect to singular-value patterns, sensitivity distributions and imaging performances. It is observed that for the proposed alternative coil configuration the measurements are more sensitive to inner region conductivity perturbations when compared to injected current EIT and induced current EIT using circular coils. The images obtained by induced current EIT are comparable to that obtained by injected current EIT

ELECTRICAL-IMPEDANCE TOMOGRAPHY USING INDUCED CURRENTS - AN EXPERIMENTAL-STUDY

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