Cryosurgical monitoring using bioimpedance measurements--a feasibility study for electrical impedance tomography

Arterial Aging Best Reflected in Pulse Wave Velocity Measured from Neck to Lower Limbs: A Whole-Body Multichannel Bioimpedance Study

Pulse wave velocity is a commonly used parameter for evaluating arterial stiffness and the overall condition of the cardiovascular system. The main goal of this study was to establish a methodology to test and validate multichannel bioimpedance as a suitable method for whole-body evaluations of pulse waves. We set the proximal location over the left carotid artery and eight distal locations on both the upper and lower limbs. In this way, it was possible to simultaneously evaluate pulse wave velocity (PWV) in the upper and lower limbs and in the limbs via four extra PWV measurements. Data were acquired from a statistical group of 220 healthy subjects who were divided into three age groups. The data were then analysed. We found a significant dependency of aortic PWV on age in those values measured using the left carotid as the proximal. PWV values in the upper and lower limbs were found to have no significant dependency on age. In addition, the PWV in the left femoral artery shows comparable values to published already carotid-femoral values. Those findings prove the reliability of whole-body multichannel bioimpedance for pulse wave velocity evaluation and provide reference values for whole-body PWV measurement.

A theoretical study on real time monitoring of single cell mitosis with micro electrical impedance tomography

Real time monitoring of cell division, mitosis, at the single cell level, has value for many biomedical applications; such as developing optimal cancer treatments that target the cell division process. The goal of this theoretical study is to explore the feasibility of using Micro Electrical Impedance Tomography (MEIT) for real time monitoring of mitosis in a single cell, through imaging. MEIT employs a micro (single cell) scale electrode cage with electrodes placed around the cell. The electrodes deliver subsensory current and the consequential voltages on the electrodes are measured. An inverse image reconstruction algorithm uses the electric data from the electrodes to generate a map of electrical conductivity distribution in the chamber, which is the image. EIT is a well-known medical imaging technology that is simple to use but lacks good resolution. Therefore, it is not a-priori obvious that EIT has sufficient resolution to monitor single cell mitosis. To accomplish the goal of this study we have developed a mathematical model of MEIT of single cell mitosis, in which an in silico experiment provided the data for the MEIT image reconstruction. This theoretical study shows that MEIT can detect the outlines of the dividing cell during the various stages of mitosis (metaphase, anaphase and telophase) and, therefore, has potential as a technology for real time monitoring of single cell mitosis.

Characteristics of Ice Impedance Recorded From a Ring Electrode Placed at the Anterior Surface of the Cryoballoon: Novel Approach to Define Ice Formation and Pulmonary Vein Isolation

BACKGROUND

The success of cryoablation of the pulmonary vein isolation (PVI) is dependent on transmural and circumferential ice formation. We hypothesize that rising impedance recorded from a ring electrode placed 2 mm from the cryoballoon signifies ice formation covering the balloon surface and indicates ice expansion. The impedance level enables titration of the cryoapplication time to avoid extracardiac damage while ensuring PVI.

METHODS AND RESULTS

In 12 canines, a total of 57 pulmonary veins were targeted for isolation. Two cryoapplications were delivered per vein with a minimum of 90 and maximum of 180-second duration. Cryoapplication was terminated on reaching a 500 Ω change from baseline. Animals recovered 38±6 days post-procedure, and veins were assessed electrically for isolation. Heart tissue was histologically analyzed. Extracardiac structures were examined for damage. PVI was achieved in 100% of the veins if the impedance reached 500 Ω in <90 seconds with freeze time of 90 seconds. When 500 Ω was reached >90 to 180 seconds (142.60±29.3 seconds), 90% PVI was achieved. When the final impedance was between 200 and 500 Ω with 180 seconds of freeze time, PVI was achieved in 86.8%. For impedance of <200 Ω, PVI was achieved in 14%. No extracardiac damage was recorded.

CONCLUSIONS

Impedance rise of 500 Ω at <90 seconds with freeze time of 90 seconds resulted in 100% PVI. Impedance measurements from the nose of the balloon is a direct measure of ice formation on the balloon. It provides real-time feedback on the quality of the ablation and defines the cryoapplication termination time based on ice formation, limiting ice expansion to extracardiac tissues.

Determination of single cryoablation outcome within 30 to 60 seconds of freezing based on ice impedance

Background

A direct indicator of effective pulmonary vein isolation (PVI) based on early ice formation is presently lacking.

Objective

The initial impedance rise within 30 to 60 seconds (sec) of single cryoablation relating to ice on the distal surface of the cryoballoon could; predict effective PVI with early termination, the need for prolonging the cryoablation, or failure to achieve effective ablation.

Methods

Impedance measurements were taken between two ring electrodes, at the anterior balloon surface and at the shaft behind the balloon. Ice covering the anterior ring leads to impedance rise. Single cryoablation (eight animals, 37 veins) was applied for 90 to 180 sec. Cryoapplication was terminated if the impedance reached ≥500 Ω. Impedance levels at ≤60 sec of cryoablation were divided into three groups based on the characteristics of the impedance rise. PVI was confirmed acutely and at 45 ± 9 days recovery by electrophysiology mapping and histopathology.

Results

At 60 sec of freezing, an impedance rise of 34.1 ± 15.2 Ω (13‐50 Ω) and slope of the impedance rise (measured during 15‐30 sec of cryoapplication) less than 1 Ω/sec resulted in failed PVI. An impedance rise of 104.4 ± 31.5 Ω (76‐159 Ω) and slope of 2 Ω/sec resulted in 100% PVIs. An impedance rise of 130.9 ± 137.8 Ω (40‐590 Ω) and slope of 10 Ω/sec resulted in 100% PVIs with early termination at 90 sec.

Conclusion

The efficacy of single cryoablation can be defined within 30 to 60 sec based on ice impedance. Three unique impedance profiles described in this investigation are associated with the uniformity and thickness of the ice buildup on the anterior surface of the balloon. One cryoablation with an adequate impedance rise is needed for successful outcomes.

Issues Critical to the Successful Application of Cryosurgical Ablation of the Prostate

The techniques of present-day cryosurgery performed with multiprobe freezing apparatus and advanced imaging techniques yield predictable and encouraging results in the treatment of prostatic and renal cancers. Nevertheless, and not unique to cryosurgical treatment, the rates of persistent disease demonstrate the need for improvement in technique and emphasize the need for proper management of the therapeutic margin. The causes of persistent disease often relate to a range of factors including selection of patients, understanding of the extent of the tumor, limitations of the imaging techniques, and failure to freeze the tumor periphery in an efficacious manner. Of these diverse factors, the one most readily managed, but subject to therapeutic error, is the technique of freezing the tumor and appropriate margin to a lethal temperature [Baust, J. G., Gage, A. A. The Molecular Basis of Cryosurgery. BJU Int 95, 1187–1191 (2005)]. This article describes the recent experiments that examine the molecular basis of cryosurgery, clarifies the actions of the components of the freeze-thaw cycle, and defines the resultant effect on the cryogenic lesion from a clinical perspective. Further, this review addresses the important issue of management of the margin of the tumor through adjunctive therapy. Accordingly, a goal of this review is to identify the technical and future adjunctive therapeutic practices that should improve the efficacy of cryoablative techniques for the treatment of malignant lesions.

Progress toward Optimization of Cryosurgery

Cryosurgery for diverse neoplastic and non-neoplastic diseases has expanded in applicability in recent years, especially since intraoperative ultrasound became available as a method of monitoring the process of tissue freezing. However, persistence of disease after presumably adequate cryosurgical treatment has disclosed deficiencies in the technique, perhaps due to faulty application of the freeze-thaw cycles or due to shortcomings in the imaging method. Clearly cryosurgical technique is less than optimal. The optimal dosimetry for tissue freezing, the recent improvements in imaging techniques, and the need for adjunctive therapy are defined in this review, which assesses the progress toward improving the efficacy of cryosurgery.

The Scanning Cryomacroscope - A Device Prototype for the Study of Cryopreservation

A new cryomacroscope prototype-a visualization device for the in situ analysis of cryopreserved biological samples-is presented in the current study. In order to visualize samples larger than the field of view of the optical setup, a scanning mechanism is integrated into the system, which represents a key improvement over previous cryomacroscope prototypes. Another key feature of the new design is in its compatibility with available top-loading controlled-rate cooling chambers, which eliminates the need for a dedicated cooling mechanism. The objective for the current development is to create means to generate a single digital movie of an experimental investigation, with all relevant data overlaid. The visualization capabilities of the scanning cryomacroscope are demonstrated in the current study on the cryoprotective agent dimethyl sulfoxide and the cryoprotective cocktail DP6. Demonstrated effects include glass formation, various regimes of crystallization, thermal contraction, and fracture formation.

Volumetric electromagnetic phase-shift spectroscopy of brain edema and hematoma

Motivated by the need of poor and rural Mexico, where the population has limited access to advanced medical technology and services, we have developed a new paradigm for medical diagnostic based on the technology of “Volumetric Electromagnetic Phase Shift Spectroscopy” (VEPS), as an inexpensive partial substitute to medical imaging. VEPS, can detect changes in tissue properties inside the body through non-contact, multi-frequency electromagnetic measurements from the exterior of the body, and thereby provide rapid and inexpensive diagnostics in a way that is amenable for use in economically disadvantaged parts of the world. We describe the technology and report results from a limited pilot study with 46 healthy volunteers and eight patients with CT radiology confirmed brain edema and brain hematoma. Data analysis with a non-parametric statistical Mann-Whitney U test, shows that in the frequency range of from 26 MHz to 39 MHz, VEPS can distinguish non-invasively and without contact, with a statistical significance of p<0.05, between healthy subjects and those with a medical conditions in the brain. In the frequency range of between 153 MHz to 166 MHz it can distinguish with a statistical significance of p<0.05 between subjects with brain edema and those with a hematoma in the brain. A classifier build from measurements in these two frequency ranges can provide instantaneous diagnostic of the medical condition of the brain of a patient, from a single set of measurements. While this is a small-scale pilot study, it illustrates the potential of VEPS to change the paradigm of medical diagnostic of brain injury through a VEPS classifier-based technology. Obviously substantially larger-scale studies are needed to verify and expand on the findings in this small pilot study.

Linear superposition electrical impedance tomography imaging with multiple electrical/biopsy probes

In medical diagnostics, tissue is often examined with multiple discrete biopsies taken under ultrasound placement. In a previous theoretical study, we have suggested that the linear nature of the equations used in electrical impedance tomography (EIT) can be employed with the conventional practice of biopsy sampling to produce an image of the tissue between the biopsy samplings. Specifically, the biopsy probes can be used to record EIT-type electrical data during the discrete tissue sampling. The location of the discrete biopsy needle insertions available from the ultrasound placement of the probes can be combined with the electrical measurement data and used with linear superposition to produce a complete EIT image of the tissue between the sampled sites. In this study, we explore the concept experimentally using gel phantoms to simulate tissue and heterogeneities in the tissue. The experiments are performed in 2-D and 3-D configurations, and data are taken discretely, one at a time, through single electrical probe insertions. In the 2-D configuration, we were able to produce images of reasonable quality for heterogeneities with a diameter larger than 3 mm (conductivity ratio 1:5) and with relative conductivity differences above 50% (diameter 5 mm).

Imaging cryosurgery with EIT: tracking the ice front and post-thaw tissue viability

Cryosurgery employs freezing for targeted destruction of undesirable tissues such as cancer. Ice front imaging has made controlled treatment of deep body tumors possible. One promising method, recently explored for this task, is EIT, which recovers images of electrical impedance from measurements made at boundary electrodes. However, since frozen tissue near the ice front survives, ice front imaging is insufficient. Monitoring treatment effect would enable iterative cryosurgery, where extents of ablation and need for further treatment are assessed upon thawing. Since lipid bilayers are strong barriers to low frequency electrical current and cell destruction implies impaired membranes, EIT should be able to detect the desired effect of cryosurgery: cell death. Previous work has tested EIT for ice front imaging with tank studies while others have simulated EIT in detecting cryoablation, but in vivo tests have not been reported in either case. To address this, we report 3D images of differential conductivity throughout the freeze-thaw cycle in a rat liver model in vivo with histological validation, first testing our system for ice front imaging in a gel and for viability imaging post-thaw in a raw potato slice.

Image monitoring for an intraperitoneal bleeding model of pigs using electrical impedance tomography

Current medical imaging techniques are not effective for timely detection of internal hemorrhage when the bleeding is slow and in small quantities. In this study, electrical impedance tomography (EIT) was applied to monitor the intraperitoneal bleeding of an animal model. Five healthy pigs three months old were used. The process of intraperitoneal bleeding was simulated with the injection of anticoagulated blood which was controlled by an electronic syringe pump. The injected rate was no more than 100 ml h(-1) and the total injection volumes ranged from 300 ml to 500 ml. Sixteen electrodes were attached to the abdomen and used for electrical current excitation and surface voltage measurement. Dynamic changes in impedance distribution within the abdomen were calculated by the back-projection algorithm and a series of EIT images were displayed in a unified range. The monitoring was performed with EIT at a rate of one frame per second and continued for at least 4 h. Intraperitoneal blood volume changes could be identified by inspection of consecutive EIT images during the progression of blood injection. 30 ml of blood in the peritoneum could be detected. EIT was shown to be a promising technique for continuous monitoring of intraperitoneal bleeding over periods of time.

Finite-element analysis of ex vivo and in vivo hepatic cryoablation

Cryoablation is a widely used method for the treatment of nonresectable primary and metastatic liver tumors. A model that can accurately predict the size of a cryolesion may allow more effective treatment of tumor, while sparing normal liver tissue. We generated a computer model of tissue cryoablation using the finite-element method (FEM). In our model, we considered the heat transfer mechanism inside the cryoprobe and also cryoprobe surfaces so our model could incorporate the effect of heat transfer along the cryoprobe from the environment at room temperature. The modeling of the phase shift from liquid to solid was a key factor in the accurate development of this model. The model was verified initially in an ex vivo liver model. Temperature history at three locations around one cryoprobe and between two cryoprobes was measured. The comparison between the ex vivo result and the FEM modeling result at each location showed a good match, where the maximum difference was within the error range acquired in the experiment (< 5 degrees C). The FEM model prediction of the lesion size was within 0.7 mm of experimental results. We then validated our FEM in an in vivo experimental porcine model. We considered blood perfusion in conjunction with blood viscosity depending on temperature. The in vivo iceball size was smaller than the ex vivo iceball size due to blood perfusion as predicted in our model. The FEM results predicted this size within 0.1-mm error. The FEM model we report can accurately predict the extent of cryoablation in the liver.

A narrow-band level set method applied to EIT in brain for cryosurgery monitoring

In this paper, we investigate the feasibility of applying a novel level set reconstruction technique to electrical imaging of the human brain. We focus particularly on the potential application of electrical impedance tomography (EIT) to cryosurgery monitoring. In this application, cancerous tissue is treated by a local freezing technique using a small needle-like cryosurgery probe. The interface between frozen and nonfrozen tissue can be expected to have a relatively high contrast in conductivity and we treat the inverse problem of locating and monitoring this interface during the treatment. A level set method is used as a powerful and flexible tool for tracking the propagating interfaces during the monitoring process. For calculating sensitivities and the Jacobian when deforming the interfaces we employ an adjoint formula rather than a direct differentiation technique. In particular, we are using a narrow-band technique for this procedure. This combination of an adjoint technique and a narrow-band technique for calculating Jacobians results in a computationally efficient and extremely fast method for solving the inverse problem. Moreover, due to the reduced number of unknowns in each step of the narrow-band approach compared to a pixel- or voxel-based technique, our reconstruction scheme tends to be much more stable. We demonstrate that our new method also outperforms its pixel-/voxel-based counterparts in terms of image quality in this application.

Use of X-ray Tomography to Map Crystalline and Amorphous Phases in Frozen Biomaterials

The outcome of both cryopreservation and cryosurgical freezing applications is influenced by the concentration and type of the cryoprotective agent (CPA) or the cryodestructive agent (i.e., the chemical adjuvants referred to here as CDA) added prior to freezing. It also depends on the amount and type of crystalline, amorphous and/or eutectic phases formed during freezing which can differentially affect viability. This work describes the use of X-ray computer tomography (CT) for non-invasive, indirect determination of the phase, solute concentration and temperature within biomaterials (CPA, CDA loaded solutions and tissues) by X-ray attenuation before and after freezing. Specifically, this work focuses on establishing the feasibility of CT (100-420 kV acceleration voltage) to accurately measure the concentration of glycerol or salt as model CPA and CDAs in unfrozen solutions and tissues at 20 degrees C, or the phase in frozen solutions and tissue systems at -78.5 and -196 degrees C. The solutions are composed of water with physiological concentrations of NaCl (0.88% wt/wt) and DMEM (Dulbecco's Modified Eagle's Medium) with added glycerol (0-8 M). The tissue system is chosen as 3 mm thick porcine liver slices as well as 2 cm diameter cores which were either imaged fresh (3-4 h cold ischemia) or after loading with DMEM based glycerol solutions (0-8 M) for times ranging from hours to 7 days at 4 degrees C. The X-ray attenuation is reported in Hounsfield units (HU), a clinical measurement which normalizes X-ray attenuation values by the difference between those of water and air. NaCl solutions from 0 to 23.3% wt/wt (i.e. water to eutectic concentration) were found to linearly correspond to HU in a range from 0 to 155. At -196 degrees C the variation was from -80 to 95 HU while at -78.5 degrees C all readings were roughly 10 HU lower. At 20 degrees C NaCl and DMEM solutions with 0-8 M glycerol loading show a linear variation from 0 to 145 HU. After freezing to -78.5 degrees C the variation of the NaCl and DMEM solutions is more than twice as large between -90 and +190 HU and was distinctly non-linear above 6 M. After freezing to -196 degrees C the variation of the NaCl and DMEM solutions increased even further to -80 to +225 HU and was distinctly non-linear above 4 M, which after modeling the phase change and crystallization process is shown to correlate with an amorphous phase. In all tissue systems the HU readings were similar to solutions but higher by roughly 30 HU, as well as showing some deviations at 0 M after storage, probably due to tissue swelling. The standard deviations in all measurements were roughly 5 HU or below in all samples. In addition, two practical examples for CT use were demonstrated including: (1) glycerol loading and freezing of tissue cores and, (2) a mock cryosurgical procedure. In the loading experiment CT was able to measure the permeation of the glycerol into the sample at 20 degrees C, as well as the evolution of distinct amorphous vs. crystalline phases after freezing to -196 degrees C. In the mock cryosurgery example, the iceball edge was clearly visualized, and attempts to determine the temperature within the iceball are discussed. An added benefit of this work is that the density of these frozen samples, an essential property in measurement and modeling of thermal processes, was obtained in comparison to ice.

Detecting cryoablation with EIT and the benefit of including ice front imaging data

Imaging has made cryosurgery, the destruction of unwanted tissue through freezing, valuable. Electrical impedance tomography (EIT) has been explored as a method to determine the volume of tissue that is frozen during the procedure. However, studies have shown that tissue near the edge of the frozen zone often survives since in this region it may only be the extra-cellular space that is frozen. This threatens the usefulness of cryosurgery for cancer therapy since inaccurate ablation either allows the cancer to survive or increases the chances of complications. Since low-frequency conductivity of tissue increases due to cell membrane impairment, and ablated tissue implies impaired membranes, EIT has the capability to recover images of tissue viability. Cryosurgery is a technique that can benefit from this: EIT scans before freezing and after thawing can show changes in conductivity and hence viability due to treatment. Assuming unfrozen tissue will survive treatment, we explore the use of differential EIT in combination with intra-operative ice front imaging modes that are currently in clinical practice to recover enhanced-resolution images of cryosurgical treatment efficacy in a set of simulated experiments. We also investigate the sensitivity to violation of this assumption and predict tolerable levels of measurement noise.

Electrical impedance tomography imaging using a priori ultrasound data

Background

Different imaging systems (e.g. electrical, magnetic, and ultrasound) rely on a wide variety of physical properties, and the datasets obtained from such systems provide only partial information about the unknown true state. One approach is to choose complementary imaging systems, and to combine the information to achieve a better representation.

Methods

This paper discusses the combination of ultrasound and electrical impedance tomography (EIT) information. Ultrasound reflection signals are good at locating sharp acoustic density changes associated with the boundaries of objects. Some boundaries, however, may be indeterminable due to masking from intermediate boundaries or because they are outside the ultrasound beam. Conversely, the EIT data contains relatively low-quality information, but it includes the whole region enclosed by the electrodes.

Results

Results are shown from a narrowband level-set method applied to 2D and 3D EIT incorporating limited angle ultrasound time of flight data.

Conclusion

The EIT reconstruction is shown to be faster and more accurate using the additional edge information from both one and four transducer ultrasound systems.

Contactless bio-impedance monitoring technique for brain cryosurgery in a 3D head model

A contactless induced-current bio-impedance system for monitoring brain cryosurgery procedure was modeled and numerically simulated, where the excitation coil was also performing as the measuring, or pick-up coil. A segmented three-dimensional (3D) MRI database was used for building the volume conductor geometry, and the numerical finite-volume method was employed for solving the forward problem for calculating the scalar potential distribution and the second-order voltage change on the pick-up coil. Several coil configurations were considered, varying in their relative positioning to the 3D head model. For each case, the sensitivity of the measured voltage change on the excitation coil to the volume of a frozen lesion was calculated. The highest sensitivity (1.1 x 10(-5) relative voltage change per mm3 of frozen tissue) was obtained for a coil arrangement where its closest segment to the volume conductor is at the maximum distance away from the frozen region position. The simulated system signal-to-carrier ratio was O(10(-8)).

Front-tracking image reconstruction algorithm for EIT-monitored cryosurgery using the boundary element method

The effectiveness of cryosurgery, treatment of tumors by freezing, is highly dependent on knowledge of transient freezing extent, and therefore relies heavily on real-time imaging techniques for monitoring. Electrical impedance tomography (EIT) holds much promise for this application. In cryosurgery there is a three order of magnitude change in impedance across the freezing boundary and there is a priori knowledge of the freezing origin. Furthermore, an EIT image of the tissue can be done prior to the cryosurgery. In this study, we have developed an EIT front tracking reconstruction algorithm which takes advantage of these particular attributes of cryosurgery. The method tracks the freezing interface rather than the impedance distribution in the freezing tissue. In addition to drastically reducing the number of parameters needed to define the image, the computational complexity is further reduced by using the more appropriate boundary element method (BEM) for solution to the forward problem. The front-tracking method was found to converge rapidly and accurately to a variety of simulated phantom images.

Improving the forward solver for the complete electrode model in EIT using algebraic multigrid

Image reconstruction in electrical impedance tomography is an ill-posed nonlinear inverse problem. Linearization techniques are widely used and require the repeated solution of a linear forward problem. To account correctly for the presence of electrodes and contact impedances, the so-called complete electrode model is applied. Implementing a standard finite element method for this particular forward problem yields a linear system that is symmetric and positive definite and solvable via the conjugate gradient method. However, preconditioners are essential for efficient convergence. Preconditioners based on incomplete factorization methods are commonly used but their performance depends on user-tuned parameters. To avoid this deficiency, we apply black-box algebraic multigrid, using standard commercial and freely available software. The suggested solution scheme dramatically reduces the time cost of solving the forward problem. Numerical results are presented using an anatomically detailed model of the human head.

Temperature Dependence of Tissue Impedivity in Electrical Impedance Tomography of Cryosurgery

The temperature-dependent impedivity of rat liver, transverse abdominal muscle and full skin was determined in vitro as a function of frequency across the temperature range 5 degrees C to 37 degrees C and from 100 Hz to 10 kHz. This study was motivated by an increasing interest in using electrical impedance tomography (EIT) for imaging of cryosurgery and a lack of applicable data in the hypothermic range. Using a controlled-temperature impedance analyzer, it was found that as the temperature is reduced the resulting increase in tissue impedivity is more pronounced at low frequencies and that the beta dispersion, resulting from cell membrane polarization, shifts to lower frequencies. With these new data a simple case study of EIT of liver cryosurgery was examined, using a finite-element model incorporating the Pennes bio-heat equation, to determine the impact of this behavior on imaging accuracy. Overestimation of the ice-front position was found to occur if the EIT system ignored the effects of the low-temperature zone surrounding the frozen tissue. This error decreases with increasing blood perfusion and with higher measurement frequencies.

Distributed network imaging and electrical impedance tomography of minimally invasive surgery

Minimally invasive surgery has become highly dependent on imaging. For instance, the effectiveness of cryosurgery in treating cancer is dependent on knowledge of freezing extent, and relies on real-time imaging techniques for monitoring. However, medical imaging is often very expensive and therefore not available to most of the world population. Here we propose the concept of distributed network imaging (DNI) which could make medical imaging and minimally invasive surgery available to all who need these advanced medical modalities. We demonstrate the concept through electrical impedance tomography (EIT) of cryosurgery. The central idea is to develop an inexpensive measurend (data collection hardware) at a remote site and then to connect the measurend apparatus to an advanced image reconstruction server, which can serve a large number of distributed measurends at remote sites, using existing communication conduits (Ethernet, telephone, satellite, etc.). These conduits transfer the raw data from the measurend to the server and the reconstructed image from the server to the measurend. Electrical impedance tomography (EIT) is an imaging modality which utilizes tissue impedance variation to construct an image. The EIT measurend which consists of electrodes, a power supply, and means to measure voltage is inexpensive, and therefore suitable for DNI. EIT is also very well-suited to imaging cryosurgery since frozen tissue impedance is much higher than that of unfrozen tissue. In this study, we first develop numerical models to illustrate the theoretical ability of EIT to image cryosurgery. We begin with a simplified two dimensional model, and then extend the study to the more appropriate three dimensional model. Our simulated finite element phantoms and pixel-based Newton-Raphson reconstruction algorithms were able to produce easily identifiable images of frozen regions within tissue. Then, we demonstrate the feasibility of the DNI concept though a case study using EIT to image an in vitro liver cryosurgery procedure through a modem. We find that the acquired raw data packets are less than 5KB per image and the images, using compression, do not exceed 50KB per image.

A new method for noninvasive measurement of multilayer tissue conductivity and structure using divided electrodes

This paper outlines a new method for measuring multilayer tissue conductivity and structure by using divided electrodes, in which current electrodes are divided into several parts. Our purpose is to estimate the multilayer tissue structure and the conductivity distribution in a cross section of the local tissue by using bioresistance data measured noninvasively. The effect of the new method is assessed by computer simulations using a typical two-dimensional (2-D) model. In this paper, the conductivity distribution in the model is analyzed based on a finite difference method (FDM) and a steepest descent method (SDM). Simulation results show that the conductivity values of skin, fat, and muscle layers can be estimated with an error of less than 0.1%. When random noise at various levels is added to the measured resistance values, estimates of the conductivity values for skin, fat, and muscle layers are still reasonably precise: their root mean square errors are about 1.06%, 1.39%, and 1.61% for 10% noise. In a 2-D model, increasing the number of divided electrodes permits simultaneous estimates of tissue structure and conductivity distribution. Optimal configuration for divided electrodes is examined in terms of dividing pattern.

Basic study of new diagnostic modality according to non-invasive measurement of the electrical conductivity of tissues

The purposes of this study were to estimate the electrical conductivity of tissues by non-invasively measuring the electrical bio-impedance, to develop a new method for tissue diagnosis, i.e., electrical impedance tomography (EIT). Tissue models were first designed taking into consideration the distribution of the fat tissue, muscle and bone in the human forearm, and then the intra-tissue distributions of electrical potential and field, and the electrical impedance in the models was theoretically analyzed by the three-dimensional finite element method. The electrical impedance of both forearms was measured in healthy human subjects, and estimated the electrical conductivity of individual local tissues. The results of the analysis showed that the distributions of electrical potential and field were affected by the presence of fat tissue but not by the presence or absence of bone. In addition, as a result of calculation of the electrical resistance of the extracellular fluid (Re) in each model, it was found that the value of bio-impedance was influenced by the presence of fat tissue, and the value of bio-impedance was increased by the intervention of a fat layer. The electrical conductivity estimated by fitting the observed values to the values obtained by finite element analysis was 0.40 S/m and 0.15 S/m for male muscle and fat tissue, and 0.35 S/m and 0.11 S/m for female muscle and fat tissue, respectively. The sex difference in the slope of linear approximation in the estimation of electrical conductivity of the males and females was thought to be due to sex differences in the properties and structure of fat tissue. These results suggest that local tissues can be diagnosed differentially and electrically by percutaneous measurement of local bio-impedance and subsequent estimation of the electrical conductivity of each tissue.

Using multiple-electrode impedance measurements to monitor cryosurgery

We outfitted cryoprobes with electrodes and used them in conjunction with a multiple channel electrical impedance tomography (EIT) system to record data during freezing experiments in a shallow saline tank. We made measurements using electrodes mounted on the probes and the tank's periphery. Reconstructed images based on both sets of electrodes indicate a significant improvement in the appearance of the ice ball over using tank electrodes alone. The size of the ice balls was varied by deliberately altering the cooling rate. We found a positive correlation between the measured size of the ice ball and the sizes of isocontour lines in the reconstructed impedance maps. Similarly, the shape of the ice balls was altered by circulating the saline about the probe. Two-dimensional reconstructed impedance contours indicated a deformation in agreement with the shape of the ice ball during the experiments. These findings suggest that using multielectrode impedance sensing may constitute a means for monitoring cryosurgery.