Igor Serša

Magnetic resonance electrical impedance tomography for measuring electrical conductivity during electroporation

The electroporation effect on tissue can be assessed by measurement of electrical properties of the tissue undergoing electroporation. The most prominent techniques for measuring electrical properties of electroporated tissues have been voltage-current measurement of applied pulses and electrical impedance tomography (EIT). However, the electrical conductivity of tissue assessed by means of voltage-current measurement was lacking in information on tissue heterogeneity, while EIT requires numerous additional electrodes and produces results with low spatial resolution and high noise. Magnetic resonance EIT (MREIT) is similar to EIT, as it is also used for reconstruction of conductivity images, though voltage and current measurements are not limited to the boundaries in MREIT, hence it yields conductivity images with better spatial resolution. The aim of this study was to investigate and demonstrate the feasibility of the MREIT technique for assessment of conductivity images of tissues undergoing electroporation. Two objects were investigated: agar phantoms and ex vivo liver tissue. As expected, no significant change of electrical conductivity was detected in agar phantoms exposed to pulses of all used amplitudes, while a considerable increase of conductivity was measured in liver tissue exposed to pulses of different amplitudes.

A spectrally composite reconstruction approach for improved resolution of pulsed photothermal temperature profiling in water-based samples

We report on the first experimental evaluation of pulsed photothermal radiometry (PPTR) using a spectrally composite kernel matrix in signal analysis. Numerical studies have indicated that this approach could enable PPTR temperature profiling in watery tissues with better accuracy and stability as compared to the customary monochromatic approximation. By using an optimized experimental set-up and image reconstruction code (involving a projected nu-method and adaptive regularization), we demonstrate accurate localization of thin absorbing layers in agar tissue phantoms with pronounced spectral variation of a mid-infrared absorption coefficient. Moreover, the widths of reconstructed temperature peaks reach 14-17% of their depth, significantly less than in earlier reports on PPTR depth profiling in watery tissues. Experimental results are replicated by a detailed numerical simulation, which enables analysis of the broadening effect as a function of temperature profile amplitude and depth.

Assessing how electroporation affects the effective conductivity tensor of biological tissues

We report calculations of the anisotropy ratio of the electrical conductivity of a simple model of a loose connective biological tissue described as a random assembly of multiscale undeformable core-shell and controlled polydisperse spherical structures. One can estimate a 10% increase in the anisotropy ratio due to the application of electric field (duration 100u2009μm) above the electroporation threshold (40u2009kV m−1) up to 120u2009kV m−1. These findings are consistent with the experimental data on the field-induced anisotropy dependence of the electrical conductivity due to cell membrane electroporation.

Autocorrelation spectra of an air-fluidized granular system measured by NMR

The power spectrum of displacement fluctuation of beads in the air-fluidized granular system is measured by a novel NMR technique of modulated gradient spin-echo. The results of measurement together with the related spectrum of the velocity fluctuation autocorrelation function fit well to an empiric formula based on to the model of bead caging between nearest neighbours; the cage breaks up after a few collisions cite{Menon1}. The fit yields the characteristic collision time, the size of bead caging and the diffusion-like constant for different degrees of system fluidization. The resulting mean squared displacement increases proportionally to the second power of time in the short-time ballistic regime and increases linearly with time in the long-time diffusion regime as already confirmed by other experiments and simulations.

MR microscopy for noninvasive detection of water distribution during soaking and cooking in the common bean

Magnetic resonance microscopy (MRM) was used to study water distribution and mobility in common bean (Phaseolus vulgaris) seed during soaking at room temperature (20°C) and during the cooking of presoaked and dry bean seed in near-boiling water (98°C). Two complementary MRI methods were used to determine the total water uptake into the seed: the T2-weighted 3D RARE method, which yielded an increased signal from regions of highly mobile (bulk) water and a suppressed signal from regions of poorly mobile (bound) water; and the 3D SPI method, which yielded an increased signal from regions of water restricted in motion and a suppressed signal from the bulk water regions owing to the short repetition time of the method. Based on these results, it can be concluded that during soaking water enters the bean through the micropyle, migrating below the seed coat. The raphe and hypocotyl are hydrated first, while the cotyledon tissue is hydrated next. It was also observed that the imbibition rate increases with an increasing soaking temperature.

Frequency-Dependent Conductivity Contrast for Tissue Characterization Using a Dual-Frequency Range Conductivity Mapping Magnetic Resonance Method

Electrical conductivities of biological tissues show frequency-dependent behaviors, and these values at different frequencies may provide clinically useful diagnostic information. MR-based tissue property mapping techniques such as magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance electrical property tomography (MREPT) are widely used and provide unique conductivity contrast information over different frequency ranges. Recently, a new method for data acquisition and reconstruction for low- and high-frequency conductivity images from a single MR scan was proposed. In this study, we applied this simultaneous dual-frequency range conductivity mapping MR method to evaluate its utility in a designed phantom and two in vivo animal disease models. Magnetic flux density and B1+ phase map for dual-frequency conductivity images were acquired using a modified spin-echo pulse sequence. Low-frequency conductivity was reconstructed from MREIT data by the projected current density method, while high-frequency conductivity was reconstructed from MREPT data by B1+ mapping. Two different conductivity phantoms comprising varying ion concentrations separated by insulating films with or without holes were used to study the contrast mechanism of the frequency-dependent conductivities related to ion concentration and mobility. Canine brain abscess and ischemia were used as in vivo models to evaluate the capability of the proposed method to identify new electrical properties-based contrast at two different frequencies. The simultaneous dual-frequency range conductivity mapping MR method provides unique contrast information related to the concentration and mobility of ions inside tissues. This method has potential to monitor dynamic changes of the state of disease.

Velocity autocorrelation spectra in molten polymers measured by NMR modulated gradient spin-echo

The segmental dynamics in molten linear polymers is studied by the NMR method of modulated gradient spin-echo, which directly probes a spectrum of molecular velocity autocorrelation function. Diffusion spectra of mono-disperse poly(isoprene-1.4) with different molecular masses, measured in the frequency range 0.1–10 kHz at a temperature of , have a form similar to the spectrum of Rouse chain dynamics, which implicates the tube-Rouse motion as the dominant dynamic process in this frequency range. The scaling of the center-of-mass diffusion coefficient, given from the fitting parameters, changes from into at around Kuhn steps, which is less than predicted by theory and simulations, while the correlation times of the tube-Rouse mode do not follow the anticipated scaling.

Optimization of magnetic flux density measurement using multiple RF receiver coils and multi-echo in MREIT.

Magnetic Resonance Electrical Impedance Tomography (MREIT) is an MRI method that enables mapping of internal conductivity and/or current density via measurements of magnetic flux density signals. The MREIT measures only the z-component of the induced magnetic flux density Bxa0=xa0(Bx, By, Bz) by external current injection. The measured noise of Bz complicates recovery of magnetic flux density maps, resulting in lower quality conductivity and current-density maps. We present a new method for more accurate measurement of the spatial gradient of the magnetic flux density gradient (∇ Bz). The method relies on the use of multiple radio-frequency receiver coils and an interleaved multi-echo pulse sequence that acquires multiple sampling points within each repetition time. The noise level of the measured magnetic flux density Bz depends on the decay rate of the signal magnitude, the injection current duration, and the coil sensitivity map. The proposed method uses three key steps. The first step is to determine a representative magnetic flux density gradient from multiple receiver coils by using a weighted combination and by denoising the measured noisy data. The second step is to optimize the magnetic flux density gradient by using multi-echo magnetic flux densities at each pixel in order to reduce the noise level of ∇ Bz and the third step is to remove a random noise component from the recovered ∇ Bz by solving an elliptic partial differential equationxa0in a region of interest. Numerical simulation experiments using a cylindrical phantom model with included regions of low MRI signal to noise (defects) verified the proposed method. Experimental results using a real phantom experiment, that included three different kinds of anomalies, demonstrated that the proposed method reduced the noise level of the measured magnetic flux density. The quality of the recovered conductivity maps using denoised ∇ Bz data showed that the proposed method reduced the conductivity noise level up to 3-4 times at each anomaly region in comparison to the conventional method.

Monitoring of Electric Field Distribution during Electroporation by Means of MREIT: From Concept to Experiments

Monitoring of electroporation process presents one of the most important aspects towards safe and efficient use of electroporation in clinical procedures such as electrochemotherapy and non-thermal irreversible electroporation. Various methods of monitoring electroporation process were already suggested, particularly of irreversible electroporation where immediate changes of tissue properties can be detected. However, monitoring of reversible electroporation is more demanding task since there are almost no immediate visible physical changes in treated tissue. As accurate coverage of the tissue with a sufficiently large electric field presents one of the most important conditions for successful electroporation, we proposed a method for determining electric field distribution during electroporation based on magnetic resonance electrical impedance tomography (MREIT). We demonstrated that MREIT can be used to determine electric field distribution during electroporation in agar phantoms, ex vivo tissues and in silico. In this study we briefly explain the concept of monitoring electric field during electroporation using MREIT and present key evaluation results of its feasibility.

Magnetic Resonance Current Density Imaging: Applications to Electrochemotherapy and Irreversible Electroporation

Electrochemotherapy (ECT) and Irreversible Electroporation (IRE) are two advanced methods in medical treatment that both critically depend on the magnitude and duration of the applied electric field over the targeted tissue (tumor or other tissue abnormality). Therefore a method that would enable a reliable monitoring of the electric field during application of ECT or IRE electric pulses is of a high priority. Recent advances in magnetic resonance imaging (MRI) and electrical impedance tomography (EIT) indicate that a combination of the two methods is a good candidate for the task. In this study, theory and results on a model system demonstrate current capability of MRI and EIT for monitoring ECT and IRE treatment.