16 research outputs found

    Parametric evaluation of impedance curve in radiofrequency ablation : a quantitative description of the asymmetry and dynamic variation of impedance in bovine ex vivo model

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    Radiofrequency ablation (RFA) is a treatment for liver tumors with advantages over the traditional treatment of surgical resection. This procedure has the shortest recovery time in early stage tumors. The objective of this study is to parameterize the impedance curve of the RFA procedure in an ex vivo model by defining seven parameters (t1/2, tminimum, tend, Zinitial, Z1/2, Zminimum and Zend). Based on these parameters, three performance indices are defined: one to identify the magnitude of impedance curve asymmetry (δ), one Drop ratio (DR) describing the percentage of impedance decrease until the minimum impedance point is reached, and Ascent Ratio (AR) describing the magnitude of increase in impedance from the minimum impedance point to its maximum point. Fifty ablations were performed in a bovine ex vivo model to measure and evaluate the proposed parameters and performance index. The results show that the groups had an average δ of 29.02%, DR of 22.41%, and AR of 545.33% for RFA without the use of saline or deionized solutions. The saline solution and deionized water-cooled groups indicated the correlation of performance indices δ, DR, and AR with the obtained final ablation volume. Therefore, by controlling these parameters and indices, lower recurrence is achieved

    A Narrow-Band Level Set Method Applied to EIT in Brain for Cryosurgery Monitoring

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    Locating Functionalized Gold Nanoparticles Using Electrical Impedance Tomography

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    Abstract Objective: An imaging device to locate functionalized nanoparticles, whereby therapeutic agents are transported from the site of administration specifically to diseased tissues, remains a challenge for pharmaceutical research. Here, we show a new method based on electrical impedance tomography (EIT) to provide images of the location of gold nanoparticles (GNPs) and the excitation of GNPs with radio frequencies (RF) to change impedance permitting an estimation of their location in cell models Methods: We have created an imaging system using quantum cluster GNPs as a contrast agent, activated with RF fields to heat the functionalized GNPs, which causes a change in impedance in the surrounding region. This change is then identified with EIT. Results: Images of impedance changes of around 804% are obtained for a sample of citrate stabilized GNPs in a solution of phosphate-buffered saline. A second quantification was carried out using colorectal cancer cells incubated with culture media, and the internalization of GNPs into the colorectal cancer cells was undertaken to compare them with the EIT images. When the cells were incubated with functionalized GNPs, the change was more apparent, approximately 402%. This change was reflected in the EIT image as the cell area was more clearly identifiable from the rest of the area. Significance: EIT can be used as a new method to locate functionalized GNPs in human cells and help in the development of GNP-based drugs in humans to improve their efficacy in the future

    Locating functionalized gold nanoparticles using electrical impedance tomography

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    Objective: An imaging device to locate functionalised nanoparticles, whereby therapeutic agents are transported from the site of administration specifically to diseased tissues, remains a challenge for pharmaceutical research. Here, we show a new method based on electrical impedance tomography (EIT) to provide images of the location of gold nanoparticles (GNPs) and the excitation of GNPs with radio frequencies (RF) to change impedance permitting an estimation of their location in cell models Methods: We have created an imaging system using quantum cluster GNPs as contrast agent, activated with RF fields to heat the functionalized GNPs, which causes a change in impedance in the surrounding region. This change is then identified with EIT. Results: Images of impedance changes of around 80±4% are obtained for a sample of citrate stabilized GNPs in a solution of phosphate-buffered saline. A second quantification was carried out using colorectal cancer cells incubated with culture media, and the internalization of GNPs into the colorectal cancer cells was undertaken to compare them with the EIT images. When the cells were incubated with functionalised GNPs, the change was more apparent, approximately 40±2%. This change was reflected in the EIT image as the cell area was more clearly identifiable from the rest of the area. Significance: EIT can be used as a new method to locate functionalized GNPs in human cells and help in the development of GNP-based drugs in humans to improve their efficacy in the future

    The use of charge -charge correlation in impedance measurements: A test of the EPET method

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    It is well known that biological tissues possess impedance properties that might be useful in medical diagnostics and treatment. Electrical Impedance Tomography (EIT) images internal electrical properties by using numerical methods to solve Laplace\u27s differential equation. The indirect reconstruction method (IRM), a common method in application, predicts internal electrical property distribution by iteratively computing a forward and inverse solution. This approach reduces the non-linear Laplace\u27s equation into a poorly conditioned series of linear equations, which are solved simultaneously. This method suffers from high computational effort and is susceptible to prediction errors that stem from measurement noise.;As an alternative to Laplace\u27s differential equation, this research applies the quasi-static approximation, Dirichlet boundary conditions and a rectangular shaped domain (with corresponding Green\u27s function for Cartesian coordinates) to solve the integral form of Poisson\u27s equation (Green\u27s 2nd identity). The result is the charge-charge correlation method (CCCM), a well-conditioned relationship between static charge build-up at internal structures and induced domain boundary charge build-up (which corresponds to measured boundary current). The CCCM is applied in a reconstruction technique called Electrical Property Enhanced Tomography (EPET). While related to the existing impedance imaging methods, EPET does not attempt to create the image with the electrical data but rather adds electrical property information to an existing conventional imaging modality (CT or MI) and, in fact, requires the data from the other modality to locate the position of internal structures in the object. Predicted electrical properties are then superimposed over the a priori structural image to yield the electrical property distribution.;To test the feasibility of the CCCM, experiments using agar media placed in a saline bath were performed. The position, size and conductivity of the agar were varied and the CCCM applied to predict the conductivities from external boundary current measurements. Predicted conductivities yielded relative errors less than 10%, results that are equal to or better than the IRM. Additionally, CCCM was able to compute these results with a 104 improvement in speed over the IRM

    Stroke type differentiation using spectrally constrained multifrequency EIT: evaluation of feasibility in a realistic head model.

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    We investigate the application of multifrequency electrical impedance tomography (MFEIT) to imaging the brain in stroke patients. The use of MFEIT could enable early diagnosis and thrombolysis of ischaemic stroke, and therefore improve the outcome of treatment. Recent advances in the imaging methodology suggest that the use of spectral constraints could allow for the reconstruction of a one-shot image. We performed a simulation study to investigate the feasibility of imaging stroke in a head model with realistic conductivities. We introduced increasing levels of modelling errors to test the robustness of the method to the most common sources of artefact. We considered the case of errors in the electrode placement, spectral constraints, and contact impedance. The results indicate that errors in the position and shape of the electrodes can affect image quality, although our imaging method was successful in identifying tissues with sufficiently distinct spectra

    Locating functionalized gold nanoparticles using electrical impedance tomography

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    Objective: An imaging device to locate functionalised nanoparticles, whereby therapeutic agents are transported from the site of administration specifically to diseased tissues, remains a challenge for pharmaceutical research. Here, we show a new method based on electrical impedance tomography (EIT) to provide images of the location of gold nanoparticles (GNPs) and the excitation of GNPs with radio frequencies (RF) to change impedance permitting an estimation of their location in cell models Methods: We have created an imaging system using quantum cluster GNPs as contrast agent, activated with RF fields to heat the functionalized GNPs, which causes a change in impedance in the surrounding region. This change is then identified with EIT. Results: Images of impedance changes of around 80±4% are obtained for a sample of citrate stabilized GNPs in a solution of phosphate-buffered saline. A second quantification was carried out using colorectal cancer cells incubated with culture media, and the internalization of GNPs into the colorectal cancer cells was undertaken to compare them with the EIT images. When the cells were incubated with functionalised GNPs, the change was more apparent, approximately 40±2%. This change was reflected in the EIT image as the cell area was more clearly identifiable from the rest of the area. Significance: EIT can be used as a new method to locate functionalized GNPs in human cells and help in the development of GNP-based drugs in humans to improve their efficacy in the future

    Modelagem Bond Graph e controlador discreto de um equipamento médico assistencial de ablação por radiofrequência

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    Tese (doutorado) — Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2022.A Ablação por Radiofrequência (ARF) é uma técnica cirúrgica moderna utilizada como primeira opção de tratamento em pacientes com câncer hepático em estágio inicial de desenvolvimento. A redução do tempo de internação e a redução de complicações pós-cirúrgicas são vantagens que se destacam nessa técnica, entre outras. O procedimento possui limitações, dentre as quais elencamos três: 1) quando aplicada em tumores maiores do que 3 cm ela não responde adequadamente, sendo o tamanho do tumor o maior fator de recorrência da técnica; 2) apresenta protocolos de aplicação de energia genéricos que nem sempre levam em conta a subjetividade de cada paciente e 3) não há uma padronização na descrição do comportamento dinâmico da resposta do tecido submetido a ablação, por exemplo, qual a impedância inicial do procedimento, qual impedância mínima, critério de finalização do procedimento, entre outros. Estas lacunas são os principais pontos abordados neste trabalho. Esta tese tem como objetivo obter indicadores paramétricos do comportamento da curva de impedância e a proposição de um controlador discreto para um equipamento de ARF. Isso permitirá aperfeiçoar o protocolo de aplicação e, consequentemente, ampliar o volume de dano térmico e diminuir a irregularidade na formação do volume de coagulação, o qual é o volume de células que realmente sofrem morte celular. Este trabalho contribui com os benefícios em se evidenciar a zona de carbonização na divulgação de dados experimentais com fins na expansão do dano térmico. A curva de impedância foi obtida para 5 cenários ex-vivo e, a partir deles, 7 parâmetros foram identificados na curva. Destes parâmetros, 3 índices de desempenho são derivados: um parâmetro que avalia a assimetria da curva; um segundo para avaliar o quanto a impedância decai com o tempo e um terceiro que avalia o quanto a impedância eleva-se a partir do ponto mínimo. O processo de desenvolvimento do controlador digital foi realizado por meio da obtenção de um modelo contínuo da planta do equipamento de ARF pela técnica Bond Graph e, em seguida, desenvolveu-se um controlador contínuo e seu equivalente discreto para o controle otimizado da potência do gerador. Os parâmetros e índices propostos apresentaram similaridades entre os grupos experimentais avaliados e fornecem uma forma alternativa para a descrição do comportamento da curva de impedância em diversos cenários de aplicação de RFA. O desenvolvimento de um controlador digital proporcionou melhoras no desempenho do equipamento com uma redução significativa do sobressinal e rastreio do sinal de referência. Esta melhoria propicia um controle adequado da homogeneidade do volume e alcance da margem de segurança do processo, tornando-o mais seguro. Com a obtenção do controlador discreto e a parametrização da curva de impedância pode-se minimizar as lacunas existentes no procedimento de RFA.Radiofrequency Ablation (RFA) is a modern surgical technique used as the first treatment option in patients with liver cancer at an early stage of development. Among others, the reduction in hospital stay and postoperative complications are the main advantages of this technique. The procedure has its limitations, among which we list three: 1) it does not respond adequately when applied to tumors larger than 3 cm, with tumor size being the greatest factor for recurrence of the technique; 2) it presents generic energy application protocols that do not always take into account the subjectivity of each patient and 3) there is no standardization in the description of the dynamic behavior of the tissue submitted to ablation, such as, for example, the initial impedance of the procedure, minimum impedance, procedure completion criterion, among others. These research gaps are the main points addressed in this work. This thesis aims at determining parametric indicators of the impedance curve behavior and proposing a discrete controller for an RFA equipment. This will allow to improve the application protocol and consequently, increase the volume of thermal damage and decrease the irregularity in the formation of the clotting volume, which is the volume that actually undergo cell death. This work contributes to the identification of the carbonization zone in the dissemination of experimental data on thermal damage. The impedance curve was obtained for five ex-vivo scenarios and, from them, seven parameters were identified in the curve. From these, three performance indices are derived: one that evaluates the asymmetry of the curve; a second that evaluates how much the impedance decays with time, and a third that evaluates how much the impedance rises from the minimum point. The digital controller was generated by obtaining a continuous model of the RFA equipment plant through the Bond Graph technique and subsequently, a continuous controller and its discrete equivalent were developed for the optimized control of the generator power. The parameters and indices proposed present similarities between the experimental groups evaluated and provide an alternative way to describe the behavior of the impedance curve in different RFA application scenarios. The development of a digital controller contributed to improve the equipment performance with a significant reduction in the overshoot and tracking of the reference signal. This improvement provides adequate control of volume homogeneity and facilitates attainment of the process’ safety margin, making it safer. Through the discrete controller and the parameterization of the impedance curve, the gaps in the RFA procedure can be minimized

    Techniques for imaging small impedance changes in the human head due to neuronal depolarisation

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    A new imaging modality is being developed, which may be capable of imaging small impedance changes in the human head due to neuronal depolarization. One way to do this would be by imaging the impedance changes associated with ion channels opening in neuronal membranes in the brain during activity. The results of previous modelling and experimental studies indicated that impedance changes between 0.6%and 1.7% locally in brain grey matter when recorded at DC. This reduces by a further of 10% if measured at the surface of the head, due to distance and the effect of the resistive skull. In principle, this could be measured using Electrical Impedance Tomography (ElT) but it is close to its threshold of detectability. With the inherent limitation in the use of electrodes, this work proposed two new schemes. The first is a magnetic measurement scheme based on recording the magnetic field with Superconducting Quantum Interference Devices (SQUIDs), used in Magnetoencephalography (MEG) as a result of a non-invasive injection of current into the head. This scheme assumes that the skull does not attenuate the magnetic field. The second scheme takes into consideration that the human skull is irregular in shape, with less and varying conductivity as compared to other head tissues. Therefore, a key issue is to know through which electrodes current can be injected in order to obtain high percentage changes in surface potential when there is local conductivity change in the head. This model will enable the prediction of the current density distribution at specific regions in the brain with respect to the varying skull and local conductivities. In the magnetic study, the head was modelled as concentric spheres, and realistic head shapes to mimic the scalp, skull, Cerebrospinal Auid (CSF) and brain using the Finite Element Method (FEM). An impedance change of 1 % in a 2cm-radius spherical volume depicting the physiological change in the brain was modelled as the region of depolarisation. The magnetic field, 1 cm away from the scalp, was estimated on injecting a constant current of 100 µA into the head from diametrically opposed electrodes. However, in the second scheme, only the realistic FEM of the head was used, which included a specific region of interest; the primary visual cortex (V1). The simulated physiological change was the variation in conductivity of V1 when neurons were assumed to be firing during a visual evoked response. A near DC current of 100 µA was driven through possible pairs of 31 electrodes using ElT techniques. For a fixed skull conductivity, the resulting surface potentials were calculated when the whole head remained unperturbed, or when the conductivity of V1 changed by 0.6%, 1 %, and 1.6%. The results of the magnetic measurement predicted that standing magnetic field was about 10pT and the field changed by about 3fT (0.03%) on depolarization. For the second scheme, the greatest mean current density through V1 was 0.020 ± 0.005 µAmm-2, and occurred with injection through two electrodes positioned near the occipital cortex. The corresponding maximum change in potential from baseline was 0.02%. Saline tank experiments confirmed the accuracy of the estimated standing potentials. As the noise density in a typical MEG system in the frequency band is about 7fT/√Hz, it places the change at the limit of detectability due to low signal to noise ratio. This is therefore similar to electrical recording, as in conventional ElT systems, but there may be advantages to MEG in that the magnetic field direcdy traverses the skull and instrumentation errors from the electrode-skin interface will be obviated. This has enabled the estimation of electrode positions most likely to permit recording of changes in human experiments and suggests that the changes, although tiny, may just be discernible from noise

    Multifrequency methods for Electrical Impedance Tomography

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    Multifrequency Electrical Impedance Tomography (MFEIT) is an emerging imaging modality which exploits the dependence of tissue impedance on frequency to recover images of conductivity. Given the low cost and portability of EIT scanners, MFEIT could provide emergency diagnosis of pathologies such as acute stroke, brain injury and breast cancer. Whereas time-difference, or dynamic, EIT is an established technique for monitoring lung ventilation, MFEIT has received less attention in the literature, and the imaging methodology is at an early stage of development. MFEIT holds the unique potential to form images from static data, but high sensitivity to noise and modelling errors must be overcome. The subject of this doctoral thesis is the investigation of novel techniques for including spectral information in the image reconstruction process. The aim is to improve the ill-posedness of the inverse problem and deliver the first imaging methodology with sufficient robustness for clinical application. First, a simple linear model for the conductivity is defined and a simultaneous multifrequency method is developed. Second, the method is applied to a realistic numerical model of a human head, and the robustness to modelling errors is investigated. Third, a combined image reconstruction and classification method is developed, which allows for the simultaneous recovery of the conductivity and the spectral information by introducing a Gaussian-mixture model for the conductivity. Finally, a graph-cut image segmentation technique is integrated in the imaging method. In conclusion, this work identifies spectral information as a key resource for producing MFEIT images and points to a new direction for the development of MFEIT algorithms
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