A multichannel continuously selectable multifrequency electrical impedance spectroscopy measurement system.

There is increasing evidence that alterations in the electrical property spectrum of tissues below 10 MHz is diagnostic for tissue pathology and/or pathophysiology. Yet, the complexity associated with constructing a high-fidelity multichannel, multifrequency data acquisition instrument has limited widespread development of spectroscopic electrical impedance imaging concepts. To contribute to the relatively sparse experience with multichannel spectroscopy systems this paper reports on the design, realization and evaluation of a prototype 32-channel instrument.

Model-updated image guidance: initial clinical experiences with gravity-induced brain deformation.

Image-guided neurosurgery relies on accurate registration of the patient, the preoperative image series, and the surgical instruments in the same coordinate space. Recent clinical reports have documented the magnitude of gravity-induced brain deformation in the operating room and suggest these levels of tissue motion may compromise the integrity of such systems. We are investigating a model-based strategy which exploits the wealth of readily-available preoperative information in conjunction with intraoperatively acquired data to construct and drive a three dimensional (3-D) computational model which estimates volumetric displacements in order to update the neuronavigational image set.

Intraoperatively updated neuroimaging using brain modeling and sparse data.

Objective: Image-guided neurosurgery incorporating preoperatively obtained imaging information is subject to spatial error resulting from intraoperative brain displacement and deformation. A strategy to update preoperative imaging using readily available intraoperative information has been developed and implemented.

Methods: Preoperative magnetic resonance imaging is used to generate a patient-specific three-dimensional finite element model of the brain by which deformation resulting from multiple surgical processes may be simulated.

An overlapping subzone technique for MR-based elastic property reconstruction.

A finite element-based nonlinear inversion scheme for magnetic resonance (MR) elastography is detailed. The algorithm operates on small overlapping subzones of the total region of interest, processed in a hierarchical order as determined by progressive error minimization. This zoned approach allows for a high degree of spatial discretization, taking advantage of the data-rich environment afforded by the MR.

Model-Updated Image-Guided Neurosurgery Using the Finite Element Method: Incorporation of the Falx Cerebri.

Surgeons using neuronavigation have realized the value of image guidance for feature recognition as well as for the precise application of surgical instruments. Recently, there has been a growing concern about the extent of intraoperative misregistration due to tissue deformation. Intraoperative imaging is currently under evaluation but limitations related to cost effectiveness and image clarity have made its wide spread adoption uncertain.

Nonactive antenna compensation for fixed-array microwave imaging: Part II--Imaging results.

Model-based imaging techniques utilizing microwave signal illumination rely heavily on the ability to accurately represent the wave propagation with a suitable numerical model. To date, the highest quality images from our prototype system have been achieved utilizing a single transmitter/single receiver measurement system where both antennas are manually repositioned to facilitate multiple illuminations of the imaging region, thus requiring long data acquisition times. In an effort to develop a system that can acquire data in a real time manner, a 32-channel network has been fabricated with all ports capable of being electronically selected for either transmit or receive mode.

Nonactive antenna compensation for fixed-array microwave imaging--Part I: Model development.

Fixed-array microwave imaging with multisensor data acquisition can suffer from nonactive antenna element interactions which cause distortions in the measurements. In Part I of a two-part paper, we develop a nonactive antenna compensation model for incorporation in model-based near-field microwave image reconstruction methods. The model treats the nonactive members of the antenna array as impedance boundary conditions applied over a cylindrical surface of finite radius providing two parameters, the effective antenna radius and impedance factor, which can be determined empirically from measured data.

In vivo electrical impedance spectroscopic monitoring of the progression of radiation-induced tissue injury.

This study evaluates the potential of electrical impedance spectroscopy (EIS) as a noninvasive technique for tracking the progression of radiation-induced damage in normal muscle tissue. Male Sprague-Dawley rats were irradiated locally to the gastrocnemius and biceps femoris muscle. Single doses were administered using a procedure that spares skin and bone.

Application of linear circuit models to impedance spectra in irradiated muscle.

We have applied a number of modeling schemes to previously reported in vivo electrical impedance measurements on irradiated and normal muscle in the hind legs of rats. Specifically, seven-parameter parallel pathways and embedded membrane circuit models have been fit to group averages of impedance spectra measured at different doses and time points. Correlations between histologically scored tissue sections and model parameters have also been determined.

Optimization of pelvic heating rate distributions with electromagnetic phased arrays.

Deep heating of pelvic tumours with electromagnetic phased arrays has recently been reported to improve local tumour control when combined with radiotherapy in a randomized clinical trial despite the fact that rather modest elevations in tumour temperatures were achieved. It is reasonable to surmise that improvements in temperature elevation could lead to even better tumour response rates, motivating studies which attempt to explore the parameter space associated with heating rate delivery in the pelvis. Computational models which are based on detailed three-dimensional patient anatomy are readily available and lend themselves to this type of investigation.

Comparison of imaging geometries for diffuse optical tomography of tissue.

Images produced in six different geometries with diffuse optical tomography simulations of tissue have been compared using a finite element-based algorithm with iterative refinement provided by the Newton-Raphson approach. The source-detector arrangements studied include (i) fan-beam tomography, (ii) full reflectance and transmittance tomography, as well as (iii) sub-surface imaging, where each of these three were examined in a circular and a flat slab geometry. The algorithm can provide quantitatively accurate results for all of the tomographic geometries investigated under certain circumstances.

A computational model for tracking subsurface tissue deformation during stereotactic neurosurgery.

Recent advances in the field of stereotactic neurosurgery have made it possible to coregister preoperative computed tomography (CT) and magnetic resonance (MR) images with instrument locations in the operating field. However, accounting for intraoperative movement of brain tissue remains a challenging problem. While intraoperative CT and MR scanners record concurrent tissue motion, there is motivation to develop methodologies which would be significantly lower in cost and more widely available.

Non-invasive thermal assessment of tissue phantoms using an active near field microwave imaging technique.

An active microwave imaging system for non-invasive temperature sensing has been developed and evaluated. The system is designed to assess biological tissues undergoing thermal therapy. This paper presents results that demonstrate the imaging capabilities of the microwave method using simulated and experimental phantom materials.

Intraoperative brain shift and deformation: a quantitative analysis of cortical displacement in 28 cases.

Objective: A quantitative analysis of intraoperative cortical shift and deformation was performed to gain a better understanding of the nature and extent of this problem and the resultant loss of spatial accuracy in surgical procedures coregistered to preoperative imaging studies.

Methods: Three-dimensional feature tracking and two-dimensional image analysis of the cortical surface were used to quantify the observed motion. Data acquisition was facilitated by a ceiling-mounted robotic platform, which provided a number of precision tracking capabilities.

Initial In-Vivo Analysis of 3D Heterogeneous Brain Computations for Model-Updated Image-Guided Neurosurgery.

Registration error resulting from intraoperative brain shift due to applied surgical loads has long been recognized as one of the most challenging problems in the field of frameless stereotactic neurosurgery. To address this problem, we have developed a 3-dimensional finite element model of the brain and have begun to quantify its predictive capability in an porcine model. Previous studies have shown that we can predict the average total displacement within 15% and 6.

Improved continuous light diffusion imaging in single- and multi-target tissue-like phantoms.

The image reconstruction enhancement schemes of total variation minimization, dual meshing and iterative spatial filtering have been applied to laboratory data collected from continuous light illumination of tissue-like phantoms. Experiments include both single- and multi-target cases where variations in object size (4 mm to 20 mm), position (centred to near boundary) and contrast with the background (2:1 to 8:1) have been explored. The results show that dramatic improvements in image quality have been obtained in terms of geometric and spatial resolution measures relative to those previously reported for continuous light, but quantitative information on the actual optical properties of embedded heterogeneities is still lacking.

Frequency-domain near-infrared photo diffusion imaging: initial evaluation in multitarget tissuelike phantoms.

In this paper, an initial evaluation of our finite element based frequency-domain image reconstruction algorithm is performed for experiments where multiple millimeter-sized heterogeneities are embedded within a tissue-equivalent (optically) background medium having multicentimeter dimensions. The cases considered consist of several interesting geometry and optical property contrast combinations including (i) two different-sized targets with the same contrast at three different separation distances; (ii) two different-sized targets with different contrasts at two different separation distances; and (iii) three targets with the same and different sizes and contrasts, respectively. The reconstruction algorithm that has been used is an enhanced version of our originally developed regularized least squares approach that now includes total variation minimization, dual meshing, and spatial low-pass filtering.

Instrumentation and design of a frequency-domain diffuse optical tomography imager for breast cancer detection.

The instrument development and design of a prototype frequency-domain optical imaging device for breast cancer detection is described in detail. This device employs radio-frequency intensity modulated near-infrared light to image quantitatively both the scattering and absorption coefficients of tissue. The functioning components of the system include a laser diode and a photomultiplier tube, which are multiplexed automatically through 32 large core fiber optic bundles using high precision linear translation stages.

An enhanced electrical impedance imaging algorithm for hyperthermia applications.

Electrical impedance imaging is a technique which is under investigation as a noninvasive method of tracking subsurface temperature distributions and/or associated cellular response during hyperthermia. In previous work, a finite element image reconstruction algorithm for converting surface potential distributions recorded at discrete electrode positions into spatial maps of conductivity values was developed. This paper reports on a series of significant improvements in the basic image reconstruction approach.

Roles of the methane monooxygenase reductase component in the regulation of catalysis.

The reductase component (MMOR) of the soluble methane monooxygenase isolated from Methylosinus trichosporium OB3b catalyzes transfer of 2e- from NADH to the hydroxylase component (MMOH) where oxygen activation and substrate oxidation occur. It is shown here that MMOR can also exert regulatory effects on catalysis by binding to MMOH or to the binary complex of MMOH and component B (MMOB), another regulatory protein. MMOR alters the oxidation-reduction potentials of the dinuclear iron cluster at the active site of MMOH.

Frequency-domain optical image reconstruction in turbid media: an experimental study of single-target detectability.

An experimental study of the detectability of an object embedded in optically tissue-equivalent media by frequency-domain image reconstruction is presented. The experiments were performed in an 86-mm-diameter cylindrical phantom containing an optically homogeneous cylindrical target whose absorption and scattering properties presented a 2:1 contrast with the background medium. The parameter space explored during experimentation involved object size (15-, 8-, and 4-mm targets) and location (centered, 20-mm off-centered, and 35-mm off-centered) variations.

Microwave imaging for tissue assessment: initial evaluation in multitarget tissue-equivalent phantoms.

A prototype microwave imaging system is evaluated for its ability to recover two-dimensional (2-D) electrical property distributions under transverse magnetic (TM) illumination using multitarget tissue equivalent phantoms. Experiments conducted in a surrounding lossy saline tank demonstrate that simultaneous recovery of both the real and imaginary components of the electrical property distribution is possible using absolute imaging procedures over a frequency range of 300-700 MHz. Further, image reconstructions of embedded tissue-equivalent targets are found to be quantitative not only with respect to geometrical factors such as object size and location but also electrical composition.

Initial in vivo experience with EIT as a thermal estimator during hyperthermia.

Thermal imaging experiments using electrical impedance tomography (EIT) have been conducted during hyperthermia treatments delivered to two human patients and one animal subject. Coplanar and circumferential arrays of 16 and 32 tin-plated copper electrodes etched on a 0.005" polyimide sheet were used to inject 12.

Optical image reconstruction using DC data: simulations and experiments.

In this paper, we explore optical image formation using a diffusion approximation of light propagation in tissue which is modelled with a finite-element method for optically heterogeneous media. We demonstrate successful image reconstruction based on absolute experimental DC data obtained with a continuous wave 633 nm He-Ne laser system and a 751 nm diode laser system in laboratory phantoms having two optically distinct regions. The experimental systems used exploit a tomographic type of data collection scheme that provides information from which a spatially variable optical property map is deduced.