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K D Paulsen

Publications and source records attributed to K D Paulsen.

At least 19 recordsLinked to original sources
PubMed1996

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. Reconstruction of scattering coefficient only and simultaneous reconstruction of both scattering and absorption profiles in tissue-like phantoms are obtained from measured and simulated data. Images with different contrast levels between the heterogeneity and the background are also reported and the results show that although it is possible to obtain qualitative visual information on the location and size of a heterogeneity, it may not be possible to quantitatively resolve contrast levels or optical properties using reconstructions from DC data only. Sensitivity of image reconstruction to noise in the measurement data is investigated through simulations. The application of boundary constraints has also been addressed.

Absorption↗
PubMed1996

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. Quantitative assessments based on full-width half-height criteria reveal that errors in diameter estimates of reconstructed targets are less than 10 mm in all cases, whereas, positioning errors are less than 1 mm in single object experiments but degrade to 4-10 mm when multiple targets are present. Recovery of actual electrical properties is found to be frequency dependent for the real and imaginary components with background values being typically within 10-20% of their correct size and embedded object having similar accuracies as a percentage of the electrical contrast, although errors as high as 50% can occur. The quantitative evaluation of imaging performance has revealed potential advantages in a two-tiered receiver antenna configuration whose measured field values are more sensitive to target region changes than the typical tomographic type of approach which uses reception sites around the full target region perimeter. This measurement strategy has important implications for both the image reconstruction algorithm where there is a premium on minimizing problem size without sacrificing image quality and the hardware system design which seeks to economize on the amount of measured data required for quantitative image reconstruction while maximizing its sensitivity to target perturbations.

Algorithms↗
PubMed1995

Initial assessment of a simple system for frequency domain diffuse optical tomography.

Diffuse optical tomography is an imaging technique whereby spatial maps of absorption and scattering coefficients are derived from the characteristics of multiply scattered light transmitted through the object. The system described here used four intensity-modulated light sources and measurements of the intensity and phase (relative to each source) at 16 or 20 detectors on the surface of a 10 cm diameter cylinder. An iterative Newton-Raphson algorithm was used to estimate the absorption and scattering coefficients at each pixel in a 17 x 17 array minimizing the difference between measured and calculated values of the intensity and phase at the measurement sites. Forward calculations of the intensity and phase were based on a multigrid finite-difference solution of the frequency domain diffusion equation. Numerical simulations were used to examine the resolution, contrast, and accuracy of the reconstructions as well as the effects of measurement noise, systematic uncertainties in source-detector location, and accuracy of the initial estimates for the optical properties. Experimental tests also confirmed that the system could identify and locate both scattering and absorbing inhomogeneities in a tissue-simulating phantom.

Algorithms↗
PubMed1995

An active microwave imaging system for reconstruction of 2-D electrical property distributions.

The goal of this work is to develop a microwave-based imaging system for hyperthermia treatment monitoring and assessment. Toward this end, a four transmit channel and four receive channel hardware device and concomitant image reconstruction algorithm have been realized. The hardware is designed to measure electric fields (i.e., amplitude and phase) at various locations in a phantom tank with and without the presence of various heterogeneities using standard heterodyning principles. Particular attention has been paid to designing a receiver with better than 115 dB of linear dynamic range which is necessary for imaging biological tissue which often has very high conductivity, especially for tissues with high water content. A calibration procedure has been developed to compensate for signal loss due to three-dimensional radiation in the measured data, since the reconstruction process is only two-dimensional at the present time. Results are shown which demonstrate the stability and accuracy of the measurement system, the extent to which the forward computational model agrees with the measured field distribution when the electrical properties are known, and image reconstructions of electrically unknown targets of varying diameter. In the latter case, images of both the reactive and resistive component of the electrical property distribution have been recoverable. Quantitative information on object location, size, and electrical properties results when the target is approximately one-half wavelength in size. Images of smaller objects lack the same level of quantitative information, but remain qualitatively correct.

Algorithms↗
PubMed1995

Spatially varying optical property reconstruction using a finite element diffusion equation approximation.

A finite element reconstruction algorithm for optical data based on a diffusion equation approximation is presented. A frequency domain approach is adopted and a unified formulation for three combinations of boundary observables and conditions is described. A multidetector, multisource measurement and excitation strategy is simulated, which includes a distributed model of the light source that illustrates the flexibility of the methodology to modeling adaptations. Simultaneous reconstruction of both absorption and scattering coefficients for a tissue-like medium is achieved for all three boundary data types. The algorithm is found to be computationally practical, and can be implemented without major difficulties in a workstation computing environment. Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels. Absorption images appear to more closely identify the true size of the heterogeneity; however, both the absorption and scattering reconstructions have difficulty with sharp transitions at increasing depth. Quantitatively, the reconstructions are not accurate, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scattering profiles in multicentimeter tissues geometries may prove to be extremely difficult.

Algorithms↗
PubMed1994

Finite-element solution of Maxwell's equations with Helmholtz forms.

Interest in scattering and/or absorption involving three-dimensional penetrable bodies has driven numerous efforts to develop computational methods for such problems. When the object is geometrically and electrically complex, the finite-element method is a logical numerical choice. Helmholtz weak forms have recently been advocated, and computational successes have been achieved with the approach. An overview of the Helmholtz formulation, with particular emphasis on its spurious-mode-resistant properties, some efficient and reliable solution procedures for the algebra that it generates, and an approach to unstructured mesh generation, is presented. As a whole these procedures provide the basis for a methodology for realizing three-dimensional finite-element solutions of Maxwell's equations in a workstation computing environment Examples of calculations that demonstrate several important properties of the Helmholtz technique and illustrate the extent to which practical three-dimensional calculations can be accomplished with readily available computing power are shown.

Computer Simulation↗
PubMed1993

Finite element computations of specific absorption rates in anatomically conforming full-body models for hyperthermia treatment analysis.

Finite element techniques for three-dimensional specific absorption rate (SAR) computation in anatomically based human models are presented. The formulations center on Helmholtz weak forms which have been shown to be numerically robust and to afford additional sparsity in the resulting system of algebraic equations. Practical solution of these equations depends critically on the realization of an effective sparse matrix solver. Experience with several conjugate gradient-type methods is reported. The findings show that convergence rate (and even convergence in some cases) degrades significantly with increasing matrix rank and decreasing electrical loss for mesh spacings which adequately resolve the physical wavelengths of the electromagnetic wave propagation. However, with proper choice of algorithm and preconditioning, reliable convergence has been achieved for matrix ranks exceeding 2 x 10(5) on domains having sizeable volumes of electrically lossless regions. An automatic grid generation scheme for constructing meshes which consist of variable element sizes that conform to a predefined set of boundaries is discussed. Example meshes of homogeneous and heterogeneous human anatomies, the boundaries of which have been derived from CT-scan information, are shown. These results highlight the fact that 3D finite element mesh generation remains a difficult problem, but usable meshes with this level of complexity can be generated. Integration of the finite element formulation, the sparse matrix solver, and the mesh generation scheme is shown to lead to algorithms that can be implemented on inexpensive reduced instruction set computer (RISC) workstations with run times on the order of hours. An example of hyperthermia device simulation is presented which suggests that the finite element method is a practical alternative that rivals the impressive finite-difference time-domain (FDTD) computations that have appeared.

Algorithms↗
PubMed1989

Optimization of the absorbed power distribution for an annular phased array hyperthermia system.

One of the systems under investigation for producing hyperthermia noninvasively for treating deep-seated tumors is the annular phased array. This device consists of two rings of eight electromagnetic apertures that are placed concentrically about the long axis of the patient and radiate energy toward the center. Previous theoretical and clinical studies have concentrated primarily on systems where the amplitude and phase of the signal applied to each aperture were the same, and these studies have shown that the system is capable of depositing power deep within the patient. Nevertheless, in many situations the system was not capable of producing desirable temperature distributions in the tumor and normal tissue. In this paper we report on a 2-dimensional theoretical investigation where an optimization routine was used to select the amplitude and phases of each of eight apertures. The optimization procedure and resulting calculations were based on CT scans of patients with tumors. The electrical and thermal properties of the different organs and tissues were taken into account. The optimization routine tried to achieve uniform absorbed power in the tumor region with zero absorbed power outside. Using the optimized amplitudes and phases, the SAR (specific absorption rate, W/kg) was calculated for the array. The results show that in general the optimization procedure was successful in that the power deposited within the tumor volume was increased with less power deposited into normal tissue when compared to the equal amplitude and phase case. This SAR data was then used as the input to a program based on the bioheat transfer equation, which calculated the temperature distribution in the patient model for an assumed set of blood perfusion rates. Depending on the location, size of the tumor, and blood perfusion rates, the improvement in the percentage of the tumor brought to therapeutic temperature varied from 0% to as much as 80%.

Humans↗
PubMed1989

Theoretical quantification of the effects of plastic wall thickness on phantom measurements in electromagnetic hyperthermia.

Phantom experiments are a staple of research and development in electromagnetic hyperthermia. Phantom containers and compartments are typically constructed from plastics which are readily available in a wide variety of thicknesses and material compositions. The perturbation effects of these plastics on the electric fields to be measured may be important, especially if one is trying to obtain quantitative results such as when comparing with a numerical model. This communication presents a theoretical investigation into the effects of plastic wall thickness on the computed electric field. Design curves are reported which aid in the selecting of an acceptable wall thickness given a maximum degree of wall perturbation that can be tolerated. Many other materials such as rubbers and polystyrenes also have electrical properties within the ranges considered herein; hence, the results should apply to a variety of commonly used phantom construction materials.

Electric Conductivity↗
PubMed1985

Comparative theoretical performance for two types of regional hyperthermia systems.

Regional hyperthermia systems have drawn attention because of their potential for depositing power noninvasively in deep-seated tumors. Two such systems that have received clinical attention because of their ability to deposit significant amounts of power in tissue are magnetic induction devices and annular phased array applicators. In this paper, theoretical calculations for the specific absorption rate (SAR) and the resulting temperature distributions for these systems are compared. The finite element method is used in the formulation of both the electromagnetic and thermal boundary value problems. Six detailed patient models based on CT-scan data from the pelvic, visceral, and thoracic regions are generated to simulate a variety of tumor locations. In general, the annular phased array deposited more power within the tumor and produced better temperature distributions than the magnetic induction device. However, the ratio of the maximum power absorbed by the tumor to the maximum power absorbed in normal tissue does not appear to be high enough for either device to heat significant portions of perfused tumors to therapeutic temperatures under a wide range of physiological conditions. The results contained herein should aid the physician in comparative treatment planning with existing regional hyperthermia systems.

Electromagnetic Fields↗
PubMed1984

Theoretical temperature profiles for concentric coil induction heating devices in a two-dimensional, axi-asymmetric, inhomogeneous patient model.

In this paper we report on theoretical calculations for the temperature distributions produced by an rf magnetic induction device that is placed concentrically about the long axis of the patient. A two-dimensional, axi-asymmetric, inhomogeneous patient model was used in conjunction with a numerical moment method for calculating the electric fields in the tissues of the model and a numerical finite element method for calculating the resulting temperature distributions. The electric fields and the absorbed power per unit volume of tissue were calculated for both a thorax and viscera model, each of which included a tumor volume. The absorbed power values were input into the bioheat transfer equation and the temperature distributions were calculated for a wide range of blood flow rates. Based on the steady-state and transient results, our computer simulations predict poor therapeutic temperature profiles for tumors embedded deeply in the thorax and viscera. This heating technique appears to produce significant therapeutic volumes in superficial tumors located not greater than 7 cm in depth. These theoretical calculations should aid the clinician in the evaluation of induction heating devices for their effectiveness in heating deep-seated and superficial tumors.

Humans↗
PubMed1984

Theoretical temperature distributions produced by an annular phased array-type system in CT-based patient models.

Theoretical calculations for the specific absorption rate (SAR) and the resulting temperature distributions produced by an annular phased array (APA)-type system are made. The finite element numerical method is used in the formulation of both the electromagnetic (EM) and thermal boundary value problems. A number of detailed two-dimensional patient models based on CT-scan data from the pelvic, visceral, and thoracic regions are generated to simulate a variety of tumor locations and surrounding normal tissues. The SAR values from the EM solution are put into the bioheat transfer equation, and steady-state temperature distributions are calculated for a wide range of blood flow rates. Based on our theoretical modeling, the APA shows no preferential heating of superficial over deep-seated tumors. However, in most cases for all three regions of the human trunk only fair thermal profiles (therapeutic area near 60%) are obtained in tumors with little or no blood flow and poor temperature patterns (therapeutic area less than 50%) are found in tumors with moderate to high perfusion rates. These theoretical calculations should aid the clinician in the evaluation of the effectiveness of APA-type devices in heating tumors located in the trunk region.

Humans↗
PubMedDate not supplied

Comparison of numerical calculations with phantom experiments and clinical measurements.

Three-dimensional models, while fundamentally desirable in hyperthermia treatment simulation, are only beginning to emerge and may take a number of years to perfect for routine clinical use. Two dimensional calculations, on the other hand, can be efficiently performed on today's inexpensive computer workstations; however, the accuracy of two-dimensional models in the pretreatment planning context is questionable. This paper investigates the ability of a general two-dimensional finite element model to predict power deposition patterns in phantoms and temperature distributions during actual clinical treatments. The experiments and simulations have been performed for an annular phased array (APA) operating at 70 MHz. Comparisons between model predictions and measurements show that quantitative agreement occurs in phantoms containing moderate complexities in heterogeneity, but that only qualitative agreement appears possible in clinical treatments. However, the results suggest that the lack of blood flow information may contribute as much, if not more, to the uncertainties in the clinical predictions than the two-dimensional nature of the model itself.

Body Temperature↗
PubMedDate not supplied

Current sheet applicator arrays for superficial hyperthermia of chestwall lesions.

The current sheet applicator is an electromagnetic heating device whose size may be chosen virtually independent of frequency even though practical limitations may restrict it to VHF and UHF bands. In this paper we investigate absorbed power distributions in muscle tissue from current sheet applicators when used as elements of a planar array intended for superficial hyperthermic treatment of tumours. Advantages offered by current sheet applicators for tissue heating include compact size, a linear polarization of the induced electric field and relatively large heating area. It is shown that the effective field produced by a pair of these elements is continuous regardless of whether the common edges of the elements are perpendicular or parallel to the direction of impressed current. The feasibility of customizing the shape and size of the field is also illustrated. The absorbed power distribution patterns due to a coherently driven array operating around 434 MHz is relatively insensitive to phase variations of about 20 degrees but is sensitive to relative power level variations as low as 10%. Mutual coupling between array elements may be reduced to acceptable levels by incorporating suitable spacing between them. It is also demonstrated that there is good agreement between measurements of absorbed power distributions and predictions using the Gaussian beam model.

Biomedical Engineering↗
PubMedDate not supplied

Temperature field estimation using electrical impedance profiling methods. I. Reconstruction algorithm and simulated results.

Algorithmic methods for estimating complete temperature fields during hyperthermia treatments based on surface and internal electrical measurements are presented. The techniques utilized draw upon impedance imaging concepts, but rather than limit the measurements to positions on the body surface, internal impedance recording sites are allowed. Theoretical simulations show that this strategy improves the reconstructed image in the target region when either internal measurement locations are added to a given number of external recording sites or some external measurement locations are replaced by internal recording positions. The algorithms developed are tested on a set of problems with increasing levels of complexity. The culmination of these investigations is a complete simulation of a hyperthermia treatment and reconstruction of a thermal image for a body cross-section of an actual cancer patient. The results of this work suggest that the surface plus internal measurement approach holds some promise as a method for estimating temperature distributions during hyperthermia treatments. However, the simulations while promising are idealizations in that they are two-dimensional with modest levels of additive noise. In a companion paper, we explore the viability of this approach in several laboratory phantom experiments which include both static and heat-induced transient electrical property profiles.

Algorithms↗
PubMedDate not supplied

Temperature field estimation using electrical impedance profiling methods. II. Experimental system description and phantom results.

An electrical impedance tomography system has been developed and tested for the purpose of thermal imaging. Since impedance changes with temperature, images of impedance subtracted from normothermic baselines will provide a map of temperature data. A system was designed to be operational at 10-50 kHz and to utilize 16 external electrodes around the periphery of a tissue-equivalent phantom encompassing the region of interest. These electrodes serve as current sources for the 5 mA constant-current inputs and are also used for reading differential voltages. Hyperthermia treatments for cancer require that internal thermometry probes be inserted into the tumour volume. Linear arrays of electrodes with thermometry tracks for micro-dimension thermometry serve this function, as well as providing localized voltage measurements in the region of interest. The embedded temperature sensors provide a quality assurance and calibration standard for the linear arrays in reconstruction of impedance profiles. Results of transient heating experiments with conductive and ultrasound heating are shown where image reconstruction is performed using a finite element model. Temperature predictions in these studies were accurate to better than 1 degree C on average when using information from surface electrodes combined with internal linear arrays. Maximum temperature errors, however, was found to be > 5 degrees C which suggests that further noise reduction during data acquisition and improvements in the reconstructions algorithms are needed.

Electric Impedance↗
PubMedDate not supplied

Finite element simulation of Sigma 60 heating in the Utah phantom: computed and measured data compared.

An initial series of comparisons are made between finite element computations and laboratory measurements obtained during heterogeneous phantom heating with the Sigma 60 applicator. The phantom is a relatively complex, though still idealized, rendering of the pelvic area which has been used to study the deep heating characteristics of the Sigma 60 in this anatomy. Direct electric field measurements as well as inferred SAR through transient temperature analysis are plotted against computed results along 11 one-dimensional tracks through the phantom. Quantitative comparisons provided through the track-by-track analysis show generally good agreement between computation and measurement. The finite element method is found to predict well the jumps in the electric field when polarized perpendicularly to a muscle/fat interface. Visualizations of the complete three-dimensional distributions are also highlighted and correlate well with physical reasoning about the expected behaviour of the fields produced. Some discrepancies in the data persist and are discussed and analysed in depth. They underscore the difficulties that can arise in performing comparisons between measured and computed results and stress the need for careful and thorough investigations when attempting these types of model validation studies.

Biophysical Phenomena↗