Robert Pikkemaat

Electric impedance tomography for monitoring volume and size of the urinary bladder.

Abstract A novel non-invasive technique for monitoring fluid content in the human bladder is described. Specifically, a precommercial electric impedance tomograph (EIT) was applied to measure and visualize impedance changes in the lower torso due to changes in bladder volume. Preliminary measurements were conducted during routine urodynamic tests of nine male paraplegic patients, in whom a contrast agent was slowly infused into the bladder for diagnostic purposes. In some patients, a good correlation between bladder volume and EIT measurements was found, whereas in others the correlation was still good but inverted, presumably due to a poor electrode positioning. These preliminary results indicate that a sufficiently accurate finite element modeling of the impedance distribution in the abdomen, and proper electrode positioning aids, are important prerequisites to enable this technology to be used for routine measurement of bladder volume.

Recent advances in and limitations of cardiac output monitoring by means of electrical impedance tomography.

BACKGROUND:Currently, the monitoring of cardiac output (CO) and stroke volume (SV) is mainly performed using invasive techniques. Therefore, performing CO monitoring noninvasively by means of electrical impedance tomography (EIT) would be advantageous for intensive care. Our hypothesis was that, by means of EIT, it is possible to assess heart rate (HR) and to quantify changes in SV due to changes in ventilator settings. METHODS:CO (HR and SV) of 14 pigs (32–40 kg body weight) was changed by incremental increases in positive end-expiratory pressure levels (0, 5, 10, 15, and 20 cm·H2O; ramp maneuver). This ramp maneuver was applied 4 times in each animal, yielding 43 evaluable single experiments. At each positive end-expiratory pressure level, SV was assessed by transpulmonary thermodilution using a PiCCO device. EIT data were acquired using a Dräger EIT Evaluation Kit 2. RESULTS:The EIT-based SV-related signal, ZSV (in [AU]), showed only a weak correlation (after excluding 2 measurements) with SVTTD of r = 0.58 (95% confidence interval, 0.43–0.71). If ZSV is calibrated by the reference 1 time for each experiment (defined as SVEIT), the correlation is approximately 0.85 (95% confidence interval, 0.78–0.90). A possible reason for the moderate correlation is the unexpected scaling pattern, leading to amplification of the cardiac impedance signal, found in some animals. The scaling is probably due to the imperfect reconstruction (i.e., a change of sensitivity) of the EIT images or to a change in the position of the heart. CONCLUSIONS:The hypothesis that EIT can be used to monitor CO and SV was confirmed, but further studies are required before this technique can be applied in clinical practice. HR was determined robustly and accurately. For SV monitoring, promising results were obtained in 80% of the experiments. However, unexpected scaling of the cardiac EIT signal causing inaccurate estimation of SV remains an issue. Before robust assessment of SV by EIT is suitable for clinical practice, the cause of and compensation for undesired scaling effects need to be investigated.

Portable Bioimpedance Spectroscopy device and textile electrodes for mobile monitoring applications

A balanced body composition is necessary for a persons health and performance. Therefore, it is important to control the body composition continuously since complications and diseases due to dehydration often appear gradually. Based on these facts a miniaturized mobile Bioimpedance Spectroscopy device was developed that can be integrated into clothing and allows the continuous monitoring of a persons body water. The implemented system has been tested using different body models. The first measurements showed very precise and stable results. Besides the portable measurement system, textile electrodes are needed for continuous long term monitoring. Therefore, special textile electrodes were developed, tested and evaluated. The electrodes are structured in a specific way leading to a rougher surface. Such a surface improves the interface impedance and therefore optimizes the connection between electronic hardware and body. For comparison, five different structured electrodes were manufactured and tested on a special test setup that allows reproducible interface-impedance measurements using a dummy made of agar-agar to simulate the skin. It could be shown that the surface structure significantly influences the interface impedance in a positive way as compared to standard plane textile electrodes. In the future, a combination of the miniaturized BIS electronic and the structured textile electrodes could allow reproducible long term monitoring of a persons body composition.

Electrical Impedance Tomography for hemodynamic monitoring

Electrical Impedance Tomography (EIT) is a known technique to monitor impedance changes in a cross-section of a body segment, which recently gained increasing interest for regional ventilation monitoring. In this paper, we focus on hemodynamic monitoring using EIT. Past and ongoing research activities to obtain cardiac related signals and regional perfusion information from EIT image streams are summarized. Finally, we present some preliminary results on stroke volume estimation using EIT.

A shape-based quality evaluation and reconstruction method for electrical impedance tomography.

Linear methods of reconstruction play an important role in medical electrical impedance tomography (EIT) and there is a wide variety of algorithms based on several assumptions. With the Graz consensus reconstruction algorithm for EIT (GREIT), a novel linear reconstruction algorithm as well as a standardized framework for evaluating and comparing methods of reconstruction were introduced that found widespread acceptance in the community. In this paper, we propose a two-sided extension of this concept by first introducing a novel method of evaluation. Instead of being based on point-shaped resistivity distributions, we use 2759 pairs of real lung shapes for evaluation that were automatically segmented from human CT data. Necessarily, the figures of merit defined in GREIT were adjusted. Second, a linear method of reconstruction that uses orthonormal eigenimages as training data and a tunable desired point spread function are proposed. Using our novel method of evaluation, this approach is compared to the classical point-shaped approach. Results show that most figures of merit improve with the use of eigenimages as training data. Moreover, the possibility of tuning the reconstruction by modifying the desired point spread function is shown. Finally, the reconstruction of real EIT data shows that higher contrasts and fewer artifacts can be achieved in ventilation- and perfusion-related images.

Monitoring of lobectomy in cystic fibrosis with electrical impedance tomography – a new diagnostic tool

Abstract Electrical impedance tomography (EIT) is a radiation-free technique generating cross-sectional images of the lung. EIT visualizes global and regional ventilation by illustrating the distribution of electrical bioimpedance. With an electrode belt around the patient’s thorax, rotating injection-couples of a harmless alternating current allow voltage measurement of the remaining electrodes. This enables the reconstruction of a tomogram with highly dynamic changes within ventilation. We report on a female six-year-old patient with cystic fibrosis and complete destruction of the upper and middle lobe of the right lung. Lobectomy, a rare therapeutic option in patients with cystic fibrosis that needs to be considered in cases of severe localized destruction, was performed. We show a pre- and postoperative documentation of static (radiology) and dynamic investigation tools (spirometry) in correlation with EIT as a new non-invasive and radiation-free diagnostic tool for this patient group.

A GREIT-type linear reconstruction algorithm for EIT using eigenimages

Reconstruction in electrical impedance tomography (EIT) is a nonlinear, ill-posed inverse problem. Based on point-shaped training and evaluation data, the Graz consensus Reconstruction algorithm for EIT (GREIT) constitutes a universal, homogenous method. While this is a very reasonable approach to the general problem, we ask the question if an optimized reconstruction method for a specific application of EIT, i.e. thoracic imaging, can be found. Instead of point-shaped training data we propose to use spatially extended training data consisting of eigenimages. To evaluate the quality of reconstruction of the proposed approach, figures of merit (FOMs) derived from the ones used in GREIT are developed. For the application of thoracic imaging, lung-shapes were segmented from a publicly available CT-database (www.dir-lab.com) and used to calculate the novel FOMs. With those, the general feasibility of using eigenimages is demonstrated and compared to the standard approach. In addition, it is shown that by using different sets of training data, the creation of an individually optimized linear method of reconstruction is possible.

Regional Pulmonary Time-Constant Maps based on EIT-Measurements

This article introduces a promising approach of assessing a patient’s lung status non-invasively by analyzing regional time constants from electrical impedance tomography (EIT) sequences. The main idea of this approach depends on the fact that the lung can be modeled using gas-flow resistivity Rrs and lung compliance Crs, which describes the elasticity of the lung tissue. Thus, global lung mechanics may be approximated by a first order system in which the global time constant τrs = Rrs x Crs is a very important parameter. Based on EIT images, a noninvasive method that provides images of impedance distribution in the thorax allowing to quantify regional ventilation, we extend the concept of a first order mechanical lung model to regional time constant maps. We expect monitoring of regional time constants to be an important tool to assess lung status and to evaluate protective ventilation schemes.

Automation of Protective Ventilation in Acute Lung Injury

Mechanical ventilation with positive pressure is the supportive therapy for patients with acute lung failure. To minimize pulmonary stress by prevention of end-expiratory alveolar collapse and over-distension of pulmonary areas, lung protective ventilation strategy has become standard therapy. Low tidal volume ventilation (VT ≤ 6ml, per kg predicated body weight) proved to reduce mortality rates in patients with lung failure notably. Recent surveys on intensive care units showed that the transfer of this evidence-based knowledge to ventilation therapy has not been realized in the current care of ventilated patients. Automated execution of protective ventilation protocols would help to optimize the individual setting in mechanical ventilated patients. To test the ability to automate protective ventilation protocols, the adjustment of positive pressure ventilation was realized in a saline lavage induced lung injury study in pigs. The implemented controllers were programmed to meet the therapeutic goals (tidal volume, oxygenation, plateau pressure, pH, inspiratory to expiratory ratio) of the ARDSnet-protocol. During the trial, all measurements were made using an online blood gas monitor (TrendCare Satellite, Diametrics Medical Inc., England), a monitor for hemodynamic parameters (Sirecust 1281, Siemens, Germany), a capnograph (CO2SMO+, Respironics, Inc., USA), and an electrical impedance tomography (EIT) prototype system (EIT evaluation Kit, Draeger Medical, Germany). After successful automated therapy, PaCO2 and FiO2 levels could be significantly reduced. Thus, the execution of automated protective ventilation protocols with an electronically controlled ventilator was possible and led to pulmonary stabilization in saline lavage induced acute lung injury.