Physics Publications and Research

Effects of erythrocyte oxygenation on optoacoustic signals

Ratan K. Saha et al. — Thu, 04 Apr 2013 06:13:32 PDT

A theoretical model examining the effects of erythrocyte oxygenation on optoacoustic (OA) signals is presented. Each erythrocyte is considered as a fluid sphere and its optical absorption is defined by its oxygen saturation state. The OA field generated by a cell is computed by solving the wave equation in the frequency domain with appropriate boundary conditions. The resultant field from many cells is simulated by summing the pressure waves emitted by individual cells. A Monte Carlo algorithm generates 2-D spatially random distributions of oxygenated and deoxygenated erythrocytes. Oxygen saturation levels of oxygenated cells a assumed to be 100% and 0% for deoxygenated cells. The OA signal amplitude decreases monotonically for the 700-nm laser source and increases monotonically for 1000 nm optical radiation when blood oxygen saturation varies from 0 to 100%. An approximately sixfold decrease and fivefold increase of the OA signal amplitude were computed at those wavelengths, respectively. The OA spectral power in the low-frequency range (<10 MHz) and in the very high-frequency range (>100 MHz) decreases for 700 nm and increases for 1000 nm with increasing blood oxygen saturation. This model provides a theoretical framework to study the erythrocyte oxygenation-dependent OA signals.

Assessing Mathematical Models of Influenza Infections Using Features of the Immune Response

Hana M. Dobrovolny et al. — Fri, 01 Mar 2013 11:44:31 PST

The role of the host immune response in determining the severity and duration of an influenza infection is still unclear. In order to identify severity factors and more accurately predict the course of an influenza infection within a human host, an understanding of the impact of host factors on the infection process is required. Despite the lack of sufficiently diverse experimental data describing the time course of the various immune response components, published mathematical models were constructed from limited human or animal data using various strategies and simplifying assumptions. To assess the validity of these models, we assemble previously published experimental data of the dynamics and role of cytotoxic T lymphocytes, antibodies, and interferon and determined qualitative key features of their effect that should be captured by mathematical models. We test these existing models by confronting them with experimental data and find that no single model agrees completely with the variety of influenza viral kinetics responses observed experimentally when various immune response components are suppressed. Our analysis highlights the strong and weak points of each mathematical model and highlights areas where additional experimental data could elucidate specific mechanisms, constrain model design, and complete our understanding of the immune response to influenza.

Contrast-enhanced ultrasonography for real-time monitoring of interstitial laser thermal therapy in the focal treatment of prostate cancer

Mostafa Atri et al. — Fri, 05 Nov 2010 07:45:31 PDT

Introduction We report a case study of the application of contrast-enhanced ultrasonography (CEUS) for intraoperative monitoring of thermal ablation of a single focus of prostate cancer. Methods A patient presented with biopsy-proven, solitary-focus, low-risk prostate cancer and was recruited into a clinical trial of interstitial laser thermal focal therapy. Multiparametric magnetic resonance imaging (MRI) was used to locate the single dominant focus, and photothermal ablation was performed at the tumour site under the guidance of transrectal ultrasonography. Transrectal CEUS using systemic bolus injections of the intravascular contrast agent Definity was performed immediately before, several times during and on completion of therapy. Lesions observed on CEUS were compared with treatment effect as measured by tissue devascularization on 1-week gadolinium (Gd)–enhanced MRI. Results Baseline images showed CEUS contrast-agent signal throughout the prostate. During and after treatment, large hypocontrast regions were observed surrounding the treatment fibres, indicating the presence of an avascular lesion resulting from photothermal therapy. Lesion size was found to increase during the delivery of thermal energy. Lesion size measured using CEUS (16 × 11 mm) was similar to the 7-day lesion measured using Gd-enhanced T1-weighted MRI. Conclusion Focal therapy for prostate cancer requires both complete treatment of the dominant tumour focus and minimal morbidity. The application of CEUS during therapy appears to provide an excellent measure of the actual treatment effect. Hence, it can be used to ensure that the therapy encompasses the whole target but does not extend to surrounding critical structures. Future clinical studies are planned with comparisons of intraoperative CEUS to Gd-enhanced MRI at 7 days and whole-mount pathology samples.

Modeling Amantadine Treatment of Inuenza A Virus In Vitro

Catherine AA Beauchemin et al. — Thu, 04 Nov 2010 13:37:24 PDT

We analyzed the dynamics of an influenza A/Albany/1/98 (H3N2) viral infection, using a set of mathematical models highlighting the differences between in vivo and in vitro infection. For example, we found that including virion loss due to cell entry was critical for the in vitro model but not for the in vivo model. Experiments were performed on influenza virus-infected MDCK cells in vitro inside a hollow-fiber (HF) system, which was used to continuously deliver the drug amantadine. The HF system captures the dynamics of an influenza infection, and is a controlled environment for producing experimental data which lend themselves well to mathematical modeling. The parameter estimates obtained from fitting our mathematical models to the HF experimental data are consistent with those obtained earlier for a primary infection in a human model. We found that influenza A/Albany/1/98 (H3N2) virions under normal experimental conditions at 37°C rapidly lose infectivity with a half-life of ~ 6.6 ± 0.2 h, and that the lifespan of productively infected MDCK cells is ~ 13 h. Finally, using our models we estimated that the maximum efficacy of amantadine in blocking viral infection is ~ 74%, and showed that this low maximum efficacy is likely due to the rapid development of drug resistance.

Agent-based modeling of host–pathogen systems: The successes and challenges

Amy L. Bauer et al. — Wed, 27 Oct 2010 08:39:12 PDT

Agent-based models have been employed to describe numerous processes in immunology. Simulations based on these types of models have been used to enhance our understanding of immunology and disease pathology. We review various agent-based models relevant to host–pathogen systems and discuss their contributions to our understanding of biological processes. We then point out some limitations and challenges of agent-based models and encourage efforts towards reproducibility and model validation.

Laser Photothermoacoustic Heterodyned Lock-in Depth Profilometry in Turbid Tissue Phantoms

Ying Fang et al. — Fri, 06 Nov 2009 12:51:00 PST

Frequency-domain correlation and spectral analysis photothermoacoustic FD-PTA imaging is a promising new technique, which is being developed to detect tumor masses in turbid biological tissue. Unlike conventional biomedical photoacoustics which uses time-of-flight acoustic information induced by a pulsed laser to indicate the tumor size and location, in this research, a new FD-PTA instrument featuring frequency sweep chirp and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength is constructed and tested for its depth profilometric capabilities with regard to turbid media imaging. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser-fluence modulation frequencies with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Combining with the depth information carried by the back-propagated acoustic chirp signal at each scanning position, one could rapidly generate subsurface three-dimensional images of the scanning area at optimal signal-to-noise ratios and low laser fluences, a combination of tasks that is difficult or impossible by use of pulsed photoacoustic detection. In this paper, results of PTA scans performed on tissue mimicking control phantoms with various optical, acoustical, and geometrical properties are presented. A mathematical model is developed to study the laser-induced photothermoacoustic waves in turbid media. The model includes both the scattering and absorption properties of the turbid medium. A good agreement is obtained between the experimental and numerical results. It is concluded that frequency domain photothermoacoustics using a linear frequency sweep method and heterodyne lock-in detection has the potential to be a reliable tool for biomedical depthprofilometric imaging.

Ultrasound Backscatter Signal Characterization and Classification Using Autoregressive Modeling and Machine Learning Algorithms

Noushin R. Farnoud et al. — Mon, 13 Jul 2009 09:15:50 PDT

This research explores the possibility of monitoring apoptosis and classifying clusters of apoptotic cells based on the changes in ultrasound backscatter signals from the tissues. The backscatter from normal and apoptotic cells, using a high frequency ultrasound instrument are modeled through an Autoregressive (AR) modeling technique. The proper model order is calculated by tracking the error criteria in the reconstruction of the original signal. The AR model coefficients, which are assumed to contain the main statistical features of the signal, are passed as the input to Linear and Nonlinear machine classifiers (Fisher Linear Discriminant, Conditional Gaussian Classifier, Naive Bayes Classifier and Neural Networks with nonlinear activation functions). In addition, an adaptive signal segmentation method, (Least Squares Lattice Filter) is used to differentiate the data from layers of different cell types into stationary parts ready for modeling and classification.

Spatial Correlation of Flow Induced Temperature Gradients During Tissue Heating with Vascular Geometry using CT Angiography: Implications for Thermal Therapy

Michael C. Kolios et al. — Mon, 13 Jul 2009 09:15:49 PDT

Thermal models are used to predict temperature distributions of heated tissues during thermal therapy. Blood flow plays an important role in tissue heat transfer, yet there is no universally accepted mathematical formulation to model its effects during tissue heating. A better understanding of this process would improve current models of bioheat transfer. The effects of blood flow on the temperature distribution are due to convective heat transfer caused by blood motion and are classified into the effects of thermally significant vessels (vessels with diameter greater than 0.2mm-0.4mm) and the effects of the smaller vessels of the vasculature. Theoretical models predict that large vessels create flow dependent localized temperature gradients. However, there is a paucity of experimental data that examine temperature gradients near large vessels in heated tissues. Furthermore, there are no studies that spatially correlate vessel location with measured steady state and transient temperature profiles. This work examines the flow dependence of temperature profiles recorded in heated tissues near large vessels and correlates spatial fluctuations in the temperature with vascular geometry obtained by volumetric computed tomography (CT) imaging.

High Frequency Ultrasound Imaging of Changes in Cell Structure Including Apoptosis

RE Baddour et al. — Mon, 13 Jul 2009 09:15:48 PDT

It has been previously shown that high frequency ultrasound (20 - 100 MHz) can be used to detect cellular structure changes in tissues and cell ensembles. Using spectral analysis methods to analyze radio-frequency data collected from in vitro and in vivo models, the changes seen during apoptotic cell death are very striking. Imaging changes in cell structure has implications in a broad range of fields, from cancer treatment monitoring to organ transplantation. However, the changes seen in the backscattered ultrasound intensity and frequency spectrum are not fully understood. In this paper we propose and explore a model for studying how the changes in the sizes, spatial distribution, and acoustic impedance of the scattering sources within the cells are related to the resulting backscattered ultrasound signal.

Analysis of Ultrasound Backscatter from Ensembles of Cells and Isolated Nuclei

Michael C. Kolios et al. — Mon, 13 Jul 2009 09:15:47 PDT

We have previously shown that the intensity of the ultrasound backscatter from cells ensembles undergoing apoptosis increases and shifts in their normalized power spectra are detected when compared to the backscatter from non-apoptotic cells. The etiology of these changes is unknown. During apoptosis many cellular changes occur, perhaps the most striking being the condensation and subsequent fragmentation of the cell nucleus. In this set of experiments have exposed either whole Acute Myeloid Leukemia (AML) cells or nuclei isolated from AML cells to different ionic strengths known to induce specific and reproducible cellular and nuclear changes. Ultrasound images and rf backscatter data were collected and analyzed at the different ionic strengths, and electron micrographs were made. Exposing cells to higher ionic strengths increased the ultrasound backscatter by 12 dB, but exposing the nuclei to the same experimental conditions decreased the backscatter by 23 dB. Furthermore, while the spectral slopes of the rf backscatter were similar for cells and nuclei at physiological saline, at increased concentrations the slope increased for the nuclei but decreased for the cells. The paper discusses the implications and significance of the findings. In conclusion, disruptions in cell and nuclear structure induced by exposure to strong ionic environments can greatly alter the ultrasound backscatter signal characteristics

New Acoustic Beams Designed for Rapid Lesion Formation: Limitations Near the Skin During Multiple Lesion Treatments

JW Hunt et al. — Mon, 13 Jul 2009 09:15:45 PDT

Minimally invasive surgery by intense focused ultrasound beams producing defined lesions is being studied extensively by different groups. Lesion formation from a single pulse, depending on treatment time, tissue temperature, and pulse repetition of about 1 minute, should produce little damage near the skin. However, this scheme results in unacceptably long treatment times when used on larger tumors. A possible solution is to generate more rapid treatment times, or larger lesion volumes per pulse. However, hyperthermic temperatures in the overlying normal tissues including the skin may limit these treatments. In a previous presentation, simulations using an “ideal” transducer, pulses as short as 4 s and rapid stirring of the coupling bolus would reduce the temperature rise near the skin. Thus pulse repetitions as short of 10 s would be acceptable. However, real transducer beams show large aberrations which can greatly increase the near-field intensities, and make them unacceptable for hyperthermia therapy. Some artifacts are be caused by clamping of the transducer, others are related to thickness variations of the transducers which generate heterogenous phase shifts from different parts of the transducer which produce unwanted spreads at beam's focus. The authors present detailed amplitude and phase scans near different transducers demonstrating the artifacts, and confirm them using novel ultrasound/magnetic-resonance phantoms showing the measured temperatures at the focus, and at 1 cm depth from the “skin” where the heating is considerably larger than that predicted by theory. Finally, we will discuss solutions for problems in the near field by improving the transducer mounting and reducing the unwanted phase shifts

Correlation of Steady State and Transient Temperature Profiles in Perfused Fixed Kidneys: Implications for Thermal Models

Michael C. Kolios et al. — Mon, 13 Jul 2009 09:15:44 PDT

Thermal models are used in hyperthermia to predict temperature distributions for treatment and applicator optimization. It is known that blood flow can significantly influence temperature profiles but an accurate description of this effect is unknown. Two models that have been used to model microvascular effects are the Pennes Bioheat Transfer Equation (BHTE) and the Effective Thermal Conductivity Equation (ETCE) [1], while an advection term is used in combination with the above to model large vessel effects [2]. The purpose of this work is to compare model predictions in an experimental system and to critically examine the effects of thermally significant vessels.

Characterization and Classification Using Autoregressive Modeling and Machine Learning Algorithms

Noushin R. Farnoud et al. — Fri, 10 Jul 2009 12:08:52 PDT

This research explores the possibility of monitoring apoptosis and classifying clusters of apoptotic cells based on the changes in ultrasound backscatter signals from the tissues. The backscatter from normal and apoptotic cells, using a high frequency ultrasound instrument are modeled through an Autoregressive (AR) modeling technique. The proper model order is calculated by tracking the error criteria in the reconstruction of the original signal. The AR model coefficients, which are assumed to contain the main statistical features of the signal, are passed as the input to Linear and Nonlinear machine classifiers (Fisher Linear Discriminant, Conditional Gaussian Classifier, Naive Bayes Classifier and Neural Networks with nonlinear activation functions). In addition, an adaptive signal segmentation method ,(Least Squares Lattice Filter) is used to differentiate the data from layers of different cell types into stationary parts ready for modeling and classification.

High Frequency Ultrasound in Monitoring Suitability for Transplantation

RM Vlad et al. — Fri, 10 Jul 2009 12:08:51 PDT

Currently there are no validated clinical methods to assess liver preservation injury. In this work we use high frequency ultrasound integrated backscatter (HFUIB) to assess liver damage in different experimental models of liver ischemia. The ultimate goal of this work is to provide a non-invasive tool to assess organ suitability for transplantation. To examine the effects of liver ischemia at different temperatures, livers from Wistar rats are surgically excised, immersed in phosphate buffer saline (PBS) and stored at 4 and 20°C for 24h. To mimic organ preservation, livers are excised, flushed with University of Wisconsin (UW) solution and stored at 4°C for 24h. Preservation injury is simulated by not flushing livers with UW solution. Ultrasound images and corresponding radio frequency data are collected over the ischemic periods. No significant increase in HFUIB is measured for the livers prepared using standard preservation conditions. For all other ischemia models, the HFUIB increases by 4-9 dBr demonstrating kinetics dependent on storage conditions. HFUIB increase is associated with liver tissue injury. The results provide a possible framework for using high frequency imaging to non-invasively assess liver preservation injury.

Attenuation Mapping for Monitoring Thermal Therapy Using Ultrasound Transmission Imaging,

N Parmar et al. — Fri, 10 Jul 2009 12:08:19 PDT

The use of an ultrasound (US) transmission imaging system to monitor attenuation changes during tissue heating was investigated. This paper presents preliminary results of images obtained from an acoustic camera before, during and after heating tissue phantoms using a heated needle. Two types of tissue-mimicking phantoms were used, agar and polyacrylamide-based. Regions of interests were chosen in images obtained from the real-time imaging system, and the pixel intensity values before, during and after heating were compared. In both phantoms, a decrease in image intensities was observed during heating, indicating an increase in tissue attenuation. Additionally, an irreversible change in image intensity was observed in regions close to the heat source. The reversibility of the intensity change was shown to be a function of the distance from the heating needle to the selected region. Initial results indicate that US transmission imaging can be used to monitor thermal therapy.

An Investigation of Backscatter Power Spectra from Cells, Cell Pellets and Microspheres

Michael C. Kolios et al. — Fri, 10 Jul 2009 12:08:18 PDT

It has been previously shown that high frequency ultrasound (20 - 100 MHz) can be used to detect cellular structure changes in tissues and cell ensembles. However, the changes seen in the backscattered ultrasound intensity and frequency spectrum are not fully understood. In this paper we attempt to better understand the nature of these changes by examination of the backscatter power spectra from cell ensembles (in pellet form) that have undergone two different types of cell death: by exposure to the chemotherapeutic cisplatin and by withdrawal of nutrients (decay). Three different ultrasound transducers were used, centered at 20MHz and 40MHz. In both death pathways, an increase of the midband fit of 10-12dB was measured, and there were significant changes in the spectral slopes. Furthermore, our initial analysis of the backscatter from single cells and polystyrene microspheres demonstrates the potential of the technique to assess scatterer size.

The Effect of Volume Fraction on the Backscatter from Nucleated Cells at High Frequencies

RE Baddour et al. — Fri, 10 Jul 2009 08:47:44 PDT

Small variations in scatterer volume fraction, which can result from changes in tissue microstructure due to cancer therapies or organ preservation, may have a significant impact on ultrasound backscatter. Although the effect of volume fraction has been studied for non-biological scatterers and red blood cells, this study addresses the case of nucleated cells. Suspensions with volume fractions up to 70% of acute myeloid leukemia cells were insonified with broadband 20 MHz and 40 MHz pulses. The resultant average normalized backscatter intensities plotted as a function of volume fraction demonstrated a better agreement with the Yagi-Nakayama continuum scattering theory rather than the Mo-Cobbold particle scattering model (using hard sphere packing). Normalized backscatter increased with cell volume fraction up to a maximum value, occurring between 20 and 30% volume fraction, varying with frequency and then decreased with further increases in volume fraction. This result may have implications in the development of new quantitative, ultrasound-based tissue characterization techniques.

Visualization of Apoptotic Cells using Scanning Acoustic Microscopy and High Frequency Ultrasound

S Brand et al. — Fri, 10 Jul 2009 08:47:42 PDT

The goal of this project is to investigate changes in the acoustical properties of cells undergoing cell death for the development of a method for tissue apoptosis detection using high frequency ultrasound (10-60 MHz). A scanning acoustic microscope (SAM) was used for visualization of individual cells undergoing apoptosis (SASAM, Fraunhofer IBMT, Germany). The use of the SAM offers high resolution (1 μm spot size) and therefore enables the exploration of acoustical properties of the cell nucleus. Cells were labeled with H33342 and DIOC 3(5) for visualizing condensed chromatin and membranes in fluorescence microscopy. In addition the same cell lines interrogated microscopically were investigated using high frequency ultrasound. Recorded radio frequency (rf) data were analyzed using ultrasound spectroscopy. Integrated backscatter coefficients and attenuation values were computed for two cell lines: HeLa and MDCK. Both cell lines responded to the applied chemotherapeutic agent by apoptosis, assessed by fluorescence microscopy. Acoustical and optical microscopy using the SASAM system clearly enabled a differentiation between apoptotic cells and cells not responding to the treatment. Apoptotic cells displayed a higher contrast in the acoustic images and were less regular in shape. Optical images of the same cells showed nuclear condensation and membrane disruption. Spectral parameters estimated from rf ultrasound showed a 100% increase in the integrated backscatter coefficients for HeLa and MDCK. Attenuation values were increased by 50% to 70% for both cell lines as a function of treatment time. The results of this investigation provide a better understanding of changes in the acoustical properties of cells with cell death and thus to the development of a non-invasive method for measuring the treatment response of tumors using acoustic waves.

Using High Frequency Ultrasound Envelope Statistics to Determine Scatterer Number Density in Dilute Cell Solutions

AS Tunis et al. — Fri, 10 Jul 2009 08:47:41 PDT

It has previously been demonstrated in tissuemimicking phantoms and in tissue that envelope statistics of US backscatter are affected by changes in the scatterer properties [1–4, 32, 37]. At higher frequencies the wavelength of the US begins to approach the size of cells and cellular components and at this scale the envelope statistics of HFUS backscatter become more sensitive to structural changes within cells. To investigate the relation between the envelope statistics and cell structure, experiments were performed in vitro. The physical meaning of the fit parameters was evaluated by investigating HFUS backscatter from suspensions of various concentrations of two different cell lines of different sizes.

High Frequency Ultrasound Signal Statistics From Mouse Mammary Tissue During Involution

AS Tunis et al. — Fri, 10 Jul 2009 08:47:40 PDT

We investigate the use of signal envelope statistics to monitor the tissue restructuring process during mouse mammary tissue involution. Using a f/3 transducer operating at a centre frequency of 20MHz, ultrasound backscatter data were collected from mouse mammary tissue following removal of the litter. The signal envelope statistics were examined by fitting the Rayleigh and generalized gamma distributions. The goodness of fit of the distributions was evaluated by the Kolmogorov-Smirnov test. The goodness of fit of the Rayleigh distribution increased to a maximum at day 3 of involution, roughly corresponding to the peak of apoptosis in the tissue. Similarly, the fit parameters of the generalized gamma distribution indicate that at day 3 the distribution is most Rayleigh like. It is believed that the condensation of the nucleus and reorganization of the tissue structure produced these changes in the statistics of the signal envelope. The results demonstrate that high frequency ultrasound signal envelope statistics can be applied to monitor changes to tissue structure in vivo.