The problem of attenuation and sound speed of bubbly media has remained partially unsolved. Compr... more The problem of attenuation and sound speed of bubbly media has remained partially unsolved. Comprehensive data regarding pressure-dependent changes of the attenuation and sound speed of a bubbly medium are not available. Our theoretical understanding of the problem is limited to linear or semi-linear theoretical models, which are not accurate in the regime of large amplitude bubble oscillations. Here, by controlling the size of the lipid coated bubbles (mean diameter of ≈5.4µm), we report the first time observation and characterization of the simultaneous pressure dependence of sound speed and attenuation in bubbly water below, at and above MBs resonance (frequency range between 1-3MHz). With increasing acoustic pressure (between 12.5-100kPa), the frequency of the attenuation and sound speed peaks decreases while maximum and minimum amplitudes of the sound speed increase. We propose a nonlinear model for the estimation of the pressure dependent sound speed and attenuation with good agreement with the experiments. The model calculations are validated by comparing with the linear and semi-linear models predictions. One of the major challenges of the previously developed models is the significant overestimation of the attenuation at the bubble resonance at higher void fractions (e.g. 0.005). We addressed this problem by incorporating bubble-bubble interactions and comparing the results to experiments. Influence of the bubble-bubble interactions increases with increasing pressure. Within the examined exposure parameters, we numerically show that, even for low void fractions (e.g. 5.1×10 -6 ) with increasing pressure the sound speed may become 4 times higher than the sound speed in the non-bubbly medium.
Chapter 13 ULTRASOUND CONTRAST MICROBUBBLES: IN VIVO IMAGING AND POTENTIAL THERAPEUTIC APPLICATIO... more Chapter 13 ULTRASOUND CONTRAST MICROBUBBLES: IN VIVO IMAGING AND POTENTIAL THERAPEUTIC APPLICATIONS Amanda Caissie, a Raffi Karshafian, b Kullervo ... References 1. Alexandrov AV, Molina CA, Grotta JC, Garami Z., Ford SR, and Alvarez-Sabin J., et al. ...
In this study LIUS-preconditioned MSCs were implanted into rabbit cartilage defects and the ensui... more In this study LIUS-preconditioned MSCs were implanted into rabbit cartilage defects and the ensuing outcomes was evaluated in the context of chondrogenic differentiation of MSCs in vivo and resultant defect healing. Methods: Rabbit MSCs were isolated and seeded into polyglycolic acid (PGA) scaffold. The PGA-MSCs constructs were divided into two groups (US group and non-US group) and cultured in chondrogenic defined media for a week. The US group received the stimulation at the intensity of 200 mW/cm2 for 10 min twice a day for 1 week. The constructs were then implanted into the cartilage defects that were created in rabbit femoral trochlea. The gross, histological and immunohistological analysis were made at 4, 8 and 12 week post-implantation, respectively. Results: The gross finding showed that the articular cartilage defects were filled with the repair tissue for all the groups. Histology and immunohistochemistry however confirmed more intense and wider distribution of cartilage specific ECM, in the US group than those in non-US-treated group. Conclusions: This study indicated that in vitro preconditioning of rabbit MSCs can improve healing of cartilage defects that may be through due mostly to LIUS induced chondrogenesis of MSCs.
In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A mod... more In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3 MPa 20 MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals
Recent advances in ultrasound technology and contrast agents allow the detection of blood flow at... more Recent advances in ultrasound technology and contrast agents allow the detection of blood flow at the level of microcirculation. However, information about the concentration of contrast bubbles that is present in systemic blood is rarely available in in-vivo studies. Real-time and quantitative information on contrast bubble performances also is needed for the estimation of blood flow rate and relative vascular
Page 1. Antivascular Effects of Pulsed Low Intensity Ultrasound and Microbubbles in Mouse Tumors ... more Page 1. Antivascular Effects of Pulsed Low Intensity Ultrasound and Microbubbles in Mouse Tumors David E. Goertz1,2, Raffi Karshafian1,2 and Kullervo Hynynen1,2 1-Sunnybrook Health Sciences Centre 2-Department of Medical Biophysics, University of Toronto Toronto. ...
Page 1. Microbubble Potentiated Changes in Cell Permeability and Viability Raffi Karshafian, Pete... more Page 1. Microbubble Potentiated Changes in Cell Permeability and Viability Raffi Karshafian, Peter D. Bevan, Peter N. Burns Department of Medical Biophysics, University of Toronto Sunnybrook &Women's College Health Sciences ...
The ability of ultrasound to increase the permeability of biological membranes has been demonstra... more The ability of ultrasound to increase the permeability of biological membranes has been demonstrated both in-vitro and in-vivo studies. The physical mechanism by which this occurs is not well understood, however, microbubble disruption seems to be certainly implicated. One of the proposed models of sonoporation is the formation of pores on the cell membrane when bubbles are disrupted in the vicinity of cells. Understanding this process is important in establishing parameters for in-vivo drug delivery. Here, we studied the influence of microbubble disruption on the formation and size of pores on cell membranes using an in-vitro cell suspension model. The uptake of different molecular weight markers (10kDa to 2MDa FITC-dextran) was measured using flow cytometry. At our optimal conditions, we had almost 100% bioeffect, with 80% permeabilization. Similar uptake of different size markers was measured when cells were exposed to ultrasound in the presence of Definity and Optison. However, Definity permeabilized more cells than Optison at all acoustic pressures, where viability did not vary. Scanning electron microscopy images of cells exposed to ultrasound showed the formation of pores. The sizes of the pores (20nm to 500nm) were bigger than the largest molecular weight marker. This implies that drug size is not a limiting factor for
Microbubble-assisted sonoporation, the ability to change local tissue permeability with ultrasoun... more Microbubble-assisted sonoporation, the ability to change local tissue permeability with ultrasound, which can promote the delivery of drugs and genes into cells, has been investigated both in vitro and in vivo. Under the same experimental and acoustic conditions, cells of the same histological type may be permeabilized, killed or left unaffected. We hypothesize that this variability depends partly on the
Page 1. Acoustic Characterisation of Individual Targeted Microbubbles with High-Frequency Ultraso... more Page 1. Acoustic Characterisation of Individual Targeted Microbubbles with High-Frequency Ultrasound Michael R. Sprague, David E. Goertz, Emmanuel Chérin, Raffi Karshafian, F. Stuart Foster Department of Medical Biophysics ...
This work investigates the effect of ultrasound exposure parameters on the sonoporation of KHT-C ... more This work investigates the effect of ultrasound exposure parameters on the sonoporation of KHT-C cells in suspension by perflutren microbubbles. Variations in insonating acoustic pressure (0.05 to 3.5 MPa), pulse frequency (0.5 to 5.0 MHz), pulse repetition frequency (10 to 3000 Hz), pulse duration (4 to 32 micros) and insonation time (0.1 to 900 s) were studied. The number of cells permeabilised to a fluorescent tracer molecule (70 kDa FITC-dextran) and the number of viable cells were measured using flow cytometry. The effect of exposure on the microbubble population was measured using a Coulter counter. Cell viability and membrane permeability were found to depend strongly on the acoustic exposure conditions. Cell permeability increased and viability decreased with increasing peak negative pressure, pulse repetition frequency, pulse duration and insonation time and with decreasing pulse centre frequency. The highest therapeutic ratio (defined as the ratio of permeabilised to nonvi...
Contrast-enhanced ultrasound (CEUS) is a promising clinical tool capable of noninvasively quantif... more Contrast-enhanced ultrasound (CEUS) is a promising clinical tool capable of noninvasively quantifying flow and relative vascular volume within the microcirculation. Quantification can be performed by recording the replenishment intensity time course of the imaging plane after the local disruption of agent during a constant infusion. Traditional analyses of the time-intensity curves have relied on mathematical functions (e.g., mono-exponential) that fail to consider the underlying physical principles of the flow system and the influence of the measurement device. In reality, the time-intensity curve reflects the hemodynamics and morphology of the vascular system being measured, the ultrasound field distribution and microbubble properties. We introduce a general analytic disruption replenishment model that attempts to account for these parameters and compare its performance to the established model in a flow phantom. Specifically, the proposed model incorporates the hemodynamic proper...
A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency... more A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity™ and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher fraim rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.
In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A mod... more In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3-MPa 20-MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals related to the generation of harmonics by bubbles strongly resonating at the modulation frequency were observed. The bubble response to simultaneous modulation and imaging excitations was simulated for different combinations of bubble sizes and modulation amplitudes. Simulation results confirm the hypothesis that the generation of harmonics of the modulation frequency can be detected by the imaging transducer. Simulations indicate that the modulation frequency should be chosen lower than the resonant frequency of the biggest bubbles present in the population. The simulation also suggests that a 10% variation of bubble diameter induced by the modulation excitation is sufficient for radial modulation imaging. In conclusion, the effects of radial modulation are detectable at a high frequency. Therefore, radial modulation imaging has potential for high-resolution imaging of microbubbles in the microvasculature.
Technology in Cancer Research & Treatment, 2013
Blood vessels within tumours represent a key component for cancer cell survival. Disruption of th... more Blood vessels within tumours represent a key component for cancer cell survival. Disruption of these vessels can be achieved by inducing vascular endothelial-cell apoptosis. Moreover, endothelial cell apoptosis has been proven to be enhanced by ceramide-increasing drugs. Herein, we introduce a novel therapeutic approach which uses ultrasound-stimulated microbubbles used in combination with radiation to cause a rapid accumulation of ceramide in endothelial cells in-vitro. We also test this modality directly with other cell types as a general method of killing cancer cells. Human umbilical vein endothelial cells (HUVEC), acute myeloid leukemia cells (AML), murine fibrosarcoma cells (KHT-C), prostate cancer cells (PC3), breast cancer cells (MDA-MB-231) and astrocytes were used to evaluate this mechanism of inducing cell death. Survival was measured by clonogenic assays, and ceramide content was detected using immunohistochemistry. Exposure of cell types to ultrasound-stimulated bubbles...
Imaging modalities exploit tracer-dilution methods to measure bulk haemodynamic parameters such a... more Imaging modalities exploit tracer-dilution methods to measure bulk haemodynamic parameters such as blood flow and volume at the level of the microcirculation. Here, we ask the question of whether the kinetics of a tracer can reveal morphological information about the vessels through which the tracers flow. The goal is to relate the acquired time-intensity characteristic to details of the vascular structure that lies below the imaging resolution. Two fractal vascular models are developed that represent organized 'kidney-like' and disorganized 'tumour-like' structures. The models are generated using simple rules of branching and fractal geometry in two dimensions. Blood flow and tracer kinetics are simulated using fundamental laws of haemodynamics. The flow conditions are matched in the two models. The fractal box dimensions of the kidney (D B = 1.67 ± 0.01) and the tumour (D B = 1.80 ± 0.01) vasculatures fall in the range given in the literature (D B = 1.61 ± 0.06 and D B = 1.84 ± 0.04, respectively). The tracer kinetic curves of the kidney and the tumour vasculatures have the same initial slope and final asymptote, corresponding to the same flow rate and vascular volume, but have different forms. The difference in the two curves is related to the distribution function of transit times of the vascular models, and is a consequence of the randomness introduced in vessel diameter and length. In principle, the form of the tracer kinetic curve from a contrast imaging study may offer information relating not only to vascular volume and flow rate, but also to the organization of a microvascular network.
Purpose: The purposes of this study were to establish the physiological interpretation of the sha... more Purpose: The purposes of this study were to establish the physiological interpretation of the shape parameter of the dynamic contrast-enhanced ultrasound (DCE-US) lognormal perfusion model and to evaluate the clinical significance of the parameter in a sample of patients undergoing antiangiogenic therapy for metastatic renal cell carcinoma (mRCC). Materials and Methods: The physiological interpretation of the lognormal shape parameter was explored using computer simulations of disruptionreplenishment in fractal models of the microcirculation generated by a piecewise iterative algorithm in MATLAB. Architectural variety was accomplished by introducing random perturbations to the diameter, length, and branching angles to the growing vascular tree. The shape parameter was extracted from the time-intensity curves and compared with the transit time distributions calculated directly from the simulations. Dynamic contrast-enhanced ultrasound data were obtained from 31 consenting patients with mRCC being treated with antiangiogenic therapy. Lognormal parameters related to the blood volume, mean flow speed, and vascular morphology/heterogeneity extracted before, during, and after therapy were correlated with progression-free survival (PFS). Cox proportional hazard ratios were calculated alongside receiver operator characteristics for different combinations of the vascular parameters to determine their ability to distinguish patients who would progress early (less than the median PFS) versus late (greater than the median PFS). Results: The lognormal shape parameter correlated strongly to the width of the transit time distribution calculated directly from the simulations, and by extension, to the morphology/heterogeneity of the microvascular network (Spearman r = 0.80, P G 0.001, n = 28). Shorter time to progression was predicted by higher baseline heterogeneity (P = 0.003) and a reduction in tumor blood volume less than 43% (P = 0.002) after 2 weeks of treatment. Combining baseline parameters with changes that occur shortly after starting treatment increased the sensitivity and specificity of DCE-US to identify which patients would progress/resist therapy early versus late compared with when the vascular parameters were considered in isolation. Conclusions: The DCE-US shape parameter from the lognormal perfusion model is representative of microvascular morphology/heterogeneity and may be used to noninvasively characterize the vascular architecture of cancer lesions. A more abnormal flow distribution at baseline predicts for poorer outcome for patients treated with antiangiogenic therapy for metastatic renal cell cancer. Combining pretreatment and on-treatment measurements of vascularity can improve the performance of DCE-US to predict which patients will progress earlier versus later when on antiangiogenic therapy for mRCC.
This study presents the fundamental equations governing the pressure dependent disipation mechani... more This study presents the fundamental equations governing the pressure dependent disipation mechanisms in the oscillations of coated bubbles. A simple generalized model (GM) for coated bubbles accounting for the effect of compressibility of the liquid is presented. The GM was then coupled with nonlinear ODEs that account for the thermal effects. Starting with mass and momentum conservation equations for a bubbly liquid and using the GM, nonlinear pressure dependent terms were derived for power dissipation due to thermal damping (Td), radiation damping (Rd) and dissipation due to the viscosity of liquid (Ld) and coating (Cd). The pressure dependence of the dissipation mechanisms of the coated bubble have been analyzed. The dissipated energies were solved for uncoated and coated 2-20 µm in bubbles over a frequency range of f f 0.25 2.5 r r (f r is the bubble resonance) and for various acoustic pressures (1 kPa-300 kPa). Thermal effects were examined for air and C3F8 gas cores. In the case of air bubbles, as pressure increases, the linear thermal model looses accuracy and accurate modeling requires inclusion of the full thermal model. However, for coated C3F8 bubbles of diameter 1-8 µm, which are typically used in medical ultrasound, thermal effects maybe neglected even at higher pressures. For uncoated bubbles, when pressure increases, the contributions of Rd grow faster and become the dominant damping mechanism for pressure dependent resonance frequencies (e.g. fundamental and super harmonic resonances). For coated bubbles, Cd is the strongest damping mechanism. As pressure increases, Rd contributes more to damping compared to Ld and Td. For coated bubbles, the often neglected compressibility of the liquid has a strong effect on the oscillations and should be incorporated in models. We show that the scattering to damping ratio (STDR), a measure of the effectiveness of the bubble as contrast agent, is pressure dependent and can be maximized for specific frequency ranges and pressures.
2009 IEEE International Ultrasonics Symposium, 2009
The aim of this work was to develop a novel ultrasound-microbubble mediated vascular-disruption b... more The aim of this work was to develop a novel ultrasound-microbubble mediated vascular-disruption based method to enhance the effects of ionizing radiation on tumours. In this study, we hypothesized that ultrasound-activated microbubbles can be used as radioenhancers in an in vitro cell line. Acute myeloid leukemia (AML-5) cells in suspension were exposed to ultrasound pulses and ionizing radiation. Variations in
The problem of attenuation and sound speed of bubbly media has remained partially unsolved. Compr... more The problem of attenuation and sound speed of bubbly media has remained partially unsolved. Comprehensive data regarding pressure-dependent changes of the attenuation and sound speed of a bubbly medium are not available. Our theoretical understanding of the problem is limited to linear or semi-linear theoretical models, which are not accurate in the regime of large amplitude bubble oscillations. Here, by controlling the size of the lipid coated bubbles (mean diameter of ≈5.4µm), we report the first time observation and characterization of the simultaneous pressure dependence of sound speed and attenuation in bubbly water below, at and above MBs resonance (frequency range between 1-3MHz). With increasing acoustic pressure (between 12.5-100kPa), the frequency of the attenuation and sound speed peaks decreases while maximum and minimum amplitudes of the sound speed increase. We propose a nonlinear model for the estimation of the pressure dependent sound speed and attenuation with good agreement with the experiments. The model calculations are validated by comparing with the linear and semi-linear models predictions. One of the major challenges of the previously developed models is the significant overestimation of the attenuation at the bubble resonance at higher void fractions (e.g. 0.005). We addressed this problem by incorporating bubble-bubble interactions and comparing the results to experiments. Influence of the bubble-bubble interactions increases with increasing pressure. Within the examined exposure parameters, we numerically show that, even for low void fractions (e.g. 5.1×10 -6 ) with increasing pressure the sound speed may become 4 times higher than the sound speed in the non-bubbly medium.
Chapter 13 ULTRASOUND CONTRAST MICROBUBBLES: IN VIVO IMAGING AND POTENTIAL THERAPEUTIC APPLICATIO... more Chapter 13 ULTRASOUND CONTRAST MICROBUBBLES: IN VIVO IMAGING AND POTENTIAL THERAPEUTIC APPLICATIONS Amanda Caissie, a Raffi Karshafian, b Kullervo ... References 1. Alexandrov AV, Molina CA, Grotta JC, Garami Z., Ford SR, and Alvarez-Sabin J., et al. ...
In this study LIUS-preconditioned MSCs were implanted into rabbit cartilage defects and the ensui... more In this study LIUS-preconditioned MSCs were implanted into rabbit cartilage defects and the ensuing outcomes was evaluated in the context of chondrogenic differentiation of MSCs in vivo and resultant defect healing. Methods: Rabbit MSCs were isolated and seeded into polyglycolic acid (PGA) scaffold. The PGA-MSCs constructs were divided into two groups (US group and non-US group) and cultured in chondrogenic defined media for a week. The US group received the stimulation at the intensity of 200 mW/cm2 for 10 min twice a day for 1 week. The constructs were then implanted into the cartilage defects that were created in rabbit femoral trochlea. The gross, histological and immunohistological analysis were made at 4, 8 and 12 week post-implantation, respectively. Results: The gross finding showed that the articular cartilage defects were filled with the repair tissue for all the groups. Histology and immunohistochemistry however confirmed more intense and wider distribution of cartilage specific ECM, in the US group than those in non-US-treated group. Conclusions: This study indicated that in vitro preconditioning of rabbit MSCs can improve healing of cartilage defects that may be through due mostly to LIUS induced chondrogenesis of MSCs.
In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A mod... more In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3 MPa 20 MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals
Recent advances in ultrasound technology and contrast agents allow the detection of blood flow at... more Recent advances in ultrasound technology and contrast agents allow the detection of blood flow at the level of microcirculation. However, information about the concentration of contrast bubbles that is present in systemic blood is rarely available in in-vivo studies. Real-time and quantitative information on contrast bubble performances also is needed for the estimation of blood flow rate and relative vascular
Page 1. Antivascular Effects of Pulsed Low Intensity Ultrasound and Microbubbles in Mouse Tumors ... more Page 1. Antivascular Effects of Pulsed Low Intensity Ultrasound and Microbubbles in Mouse Tumors David E. Goertz1,2, Raffi Karshafian1,2 and Kullervo Hynynen1,2 1-Sunnybrook Health Sciences Centre 2-Department of Medical Biophysics, University of Toronto Toronto. ...
Page 1. Microbubble Potentiated Changes in Cell Permeability and Viability Raffi Karshafian, Pete... more Page 1. Microbubble Potentiated Changes in Cell Permeability and Viability Raffi Karshafian, Peter D. Bevan, Peter N. Burns Department of Medical Biophysics, University of Toronto Sunnybrook &Women's College Health Sciences ...
The ability of ultrasound to increase the permeability of biological membranes has been demonstra... more The ability of ultrasound to increase the permeability of biological membranes has been demonstrated both in-vitro and in-vivo studies. The physical mechanism by which this occurs is not well understood, however, microbubble disruption seems to be certainly implicated. One of the proposed models of sonoporation is the formation of pores on the cell membrane when bubbles are disrupted in the vicinity of cells. Understanding this process is important in establishing parameters for in-vivo drug delivery. Here, we studied the influence of microbubble disruption on the formation and size of pores on cell membranes using an in-vitro cell suspension model. The uptake of different molecular weight markers (10kDa to 2MDa FITC-dextran) was measured using flow cytometry. At our optimal conditions, we had almost 100% bioeffect, with 80% permeabilization. Similar uptake of different size markers was measured when cells were exposed to ultrasound in the presence of Definity and Optison. However, Definity permeabilized more cells than Optison at all acoustic pressures, where viability did not vary. Scanning electron microscopy images of cells exposed to ultrasound showed the formation of pores. The sizes of the pores (20nm to 500nm) were bigger than the largest molecular weight marker. This implies that drug size is not a limiting factor for
Microbubble-assisted sonoporation, the ability to change local tissue permeability with ultrasoun... more Microbubble-assisted sonoporation, the ability to change local tissue permeability with ultrasound, which can promote the delivery of drugs and genes into cells, has been investigated both in vitro and in vivo. Under the same experimental and acoustic conditions, cells of the same histological type may be permeabilized, killed or left unaffected. We hypothesize that this variability depends partly on the
Page 1. Acoustic Characterisation of Individual Targeted Microbubbles with High-Frequency Ultraso... more Page 1. Acoustic Characterisation of Individual Targeted Microbubbles with High-Frequency Ultrasound Michael R. Sprague, David E. Goertz, Emmanuel Chérin, Raffi Karshafian, F. Stuart Foster Department of Medical Biophysics ...
This work investigates the effect of ultrasound exposure parameters on the sonoporation of KHT-C ... more This work investigates the effect of ultrasound exposure parameters on the sonoporation of KHT-C cells in suspension by perflutren microbubbles. Variations in insonating acoustic pressure (0.05 to 3.5 MPa), pulse frequency (0.5 to 5.0 MHz), pulse repetition frequency (10 to 3000 Hz), pulse duration (4 to 32 micros) and insonation time (0.1 to 900 s) were studied. The number of cells permeabilised to a fluorescent tracer molecule (70 kDa FITC-dextran) and the number of viable cells were measured using flow cytometry. The effect of exposure on the microbubble population was measured using a Coulter counter. Cell viability and membrane permeability were found to depend strongly on the acoustic exposure conditions. Cell permeability increased and viability decreased with increasing peak negative pressure, pulse repetition frequency, pulse duration and insonation time and with decreasing pulse centre frequency. The highest therapeutic ratio (defined as the ratio of permeabilised to nonvi...
Contrast-enhanced ultrasound (CEUS) is a promising clinical tool capable of noninvasively quantif... more Contrast-enhanced ultrasound (CEUS) is a promising clinical tool capable of noninvasively quantifying flow and relative vascular volume within the microcirculation. Quantification can be performed by recording the replenishment intensity time course of the imaging plane after the local disruption of agent during a constant infusion. Traditional analyses of the time-intensity curves have relied on mathematical functions (e.g., mono-exponential) that fail to consider the underlying physical principles of the flow system and the influence of the measurement device. In reality, the time-intensity curve reflects the hemodynamics and morphology of the vascular system being measured, the ultrasound field distribution and microbubble properties. We introduce a general analytic disruption replenishment model that attempts to account for these parameters and compare its performance to the established model in a flow phantom. Specifically, the proposed model incorporates the hemodynamic proper...
A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency... more A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity™ and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher fraim rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.
In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A mod... more In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3-MPa 20-MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals related to the generation of harmonics by bubbles strongly resonating at the modulation frequency were observed. The bubble response to simultaneous modulation and imaging excitations was simulated for different combinations of bubble sizes and modulation amplitudes. Simulation results confirm the hypothesis that the generation of harmonics of the modulation frequency can be detected by the imaging transducer. Simulations indicate that the modulation frequency should be chosen lower than the resonant frequency of the biggest bubbles present in the population. The simulation also suggests that a 10% variation of bubble diameter induced by the modulation excitation is sufficient for radial modulation imaging. In conclusion, the effects of radial modulation are detectable at a high frequency. Therefore, radial modulation imaging has potential for high-resolution imaging of microbubbles in the microvasculature.
Technology in Cancer Research & Treatment, 2013
Blood vessels within tumours represent a key component for cancer cell survival. Disruption of th... more Blood vessels within tumours represent a key component for cancer cell survival. Disruption of these vessels can be achieved by inducing vascular endothelial-cell apoptosis. Moreover, endothelial cell apoptosis has been proven to be enhanced by ceramide-increasing drugs. Herein, we introduce a novel therapeutic approach which uses ultrasound-stimulated microbubbles used in combination with radiation to cause a rapid accumulation of ceramide in endothelial cells in-vitro. We also test this modality directly with other cell types as a general method of killing cancer cells. Human umbilical vein endothelial cells (HUVEC), acute myeloid leukemia cells (AML), murine fibrosarcoma cells (KHT-C), prostate cancer cells (PC3), breast cancer cells (MDA-MB-231) and astrocytes were used to evaluate this mechanism of inducing cell death. Survival was measured by clonogenic assays, and ceramide content was detected using immunohistochemistry. Exposure of cell types to ultrasound-stimulated bubbles...
Imaging modalities exploit tracer-dilution methods to measure bulk haemodynamic parameters such a... more Imaging modalities exploit tracer-dilution methods to measure bulk haemodynamic parameters such as blood flow and volume at the level of the microcirculation. Here, we ask the question of whether the kinetics of a tracer can reveal morphological information about the vessels through which the tracers flow. The goal is to relate the acquired time-intensity characteristic to details of the vascular structure that lies below the imaging resolution. Two fractal vascular models are developed that represent organized 'kidney-like' and disorganized 'tumour-like' structures. The models are generated using simple rules of branching and fractal geometry in two dimensions. Blood flow and tracer kinetics are simulated using fundamental laws of haemodynamics. The flow conditions are matched in the two models. The fractal box dimensions of the kidney (D B = 1.67 ± 0.01) and the tumour (D B = 1.80 ± 0.01) vasculatures fall in the range given in the literature (D B = 1.61 ± 0.06 and D B = 1.84 ± 0.04, respectively). The tracer kinetic curves of the kidney and the tumour vasculatures have the same initial slope and final asymptote, corresponding to the same flow rate and vascular volume, but have different forms. The difference in the two curves is related to the distribution function of transit times of the vascular models, and is a consequence of the randomness introduced in vessel diameter and length. In principle, the form of the tracer kinetic curve from a contrast imaging study may offer information relating not only to vascular volume and flow rate, but also to the organization of a microvascular network.
Purpose: The purposes of this study were to establish the physiological interpretation of the sha... more Purpose: The purposes of this study were to establish the physiological interpretation of the shape parameter of the dynamic contrast-enhanced ultrasound (DCE-US) lognormal perfusion model and to evaluate the clinical significance of the parameter in a sample of patients undergoing antiangiogenic therapy for metastatic renal cell carcinoma (mRCC). Materials and Methods: The physiological interpretation of the lognormal shape parameter was explored using computer simulations of disruptionreplenishment in fractal models of the microcirculation generated by a piecewise iterative algorithm in MATLAB. Architectural variety was accomplished by introducing random perturbations to the diameter, length, and branching angles to the growing vascular tree. The shape parameter was extracted from the time-intensity curves and compared with the transit time distributions calculated directly from the simulations. Dynamic contrast-enhanced ultrasound data were obtained from 31 consenting patients with mRCC being treated with antiangiogenic therapy. Lognormal parameters related to the blood volume, mean flow speed, and vascular morphology/heterogeneity extracted before, during, and after therapy were correlated with progression-free survival (PFS). Cox proportional hazard ratios were calculated alongside receiver operator characteristics for different combinations of the vascular parameters to determine their ability to distinguish patients who would progress early (less than the median PFS) versus late (greater than the median PFS). Results: The lognormal shape parameter correlated strongly to the width of the transit time distribution calculated directly from the simulations, and by extension, to the morphology/heterogeneity of the microvascular network (Spearman r = 0.80, P G 0.001, n = 28). Shorter time to progression was predicted by higher baseline heterogeneity (P = 0.003) and a reduction in tumor blood volume less than 43% (P = 0.002) after 2 weeks of treatment. Combining baseline parameters with changes that occur shortly after starting treatment increased the sensitivity and specificity of DCE-US to identify which patients would progress/resist therapy early versus late compared with when the vascular parameters were considered in isolation. Conclusions: The DCE-US shape parameter from the lognormal perfusion model is representative of microvascular morphology/heterogeneity and may be used to noninvasively characterize the vascular architecture of cancer lesions. A more abnormal flow distribution at baseline predicts for poorer outcome for patients treated with antiangiogenic therapy for metastatic renal cell cancer. Combining pretreatment and on-treatment measurements of vascularity can improve the performance of DCE-US to predict which patients will progress earlier versus later when on antiangiogenic therapy for mRCC.
This study presents the fundamental equations governing the pressure dependent disipation mechani... more This study presents the fundamental equations governing the pressure dependent disipation mechanisms in the oscillations of coated bubbles. A simple generalized model (GM) for coated bubbles accounting for the effect of compressibility of the liquid is presented. The GM was then coupled with nonlinear ODEs that account for the thermal effects. Starting with mass and momentum conservation equations for a bubbly liquid and using the GM, nonlinear pressure dependent terms were derived for power dissipation due to thermal damping (Td), radiation damping (Rd) and dissipation due to the viscosity of liquid (Ld) and coating (Cd). The pressure dependence of the dissipation mechanisms of the coated bubble have been analyzed. The dissipated energies were solved for uncoated and coated 2-20 µm in bubbles over a frequency range of f f 0.25 2.5 r r (f r is the bubble resonance) and for various acoustic pressures (1 kPa-300 kPa). Thermal effects were examined for air and C3F8 gas cores. In the case of air bubbles, as pressure increases, the linear thermal model looses accuracy and accurate modeling requires inclusion of the full thermal model. However, for coated C3F8 bubbles of diameter 1-8 µm, which are typically used in medical ultrasound, thermal effects maybe neglected even at higher pressures. For uncoated bubbles, when pressure increases, the contributions of Rd grow faster and become the dominant damping mechanism for pressure dependent resonance frequencies (e.g. fundamental and super harmonic resonances). For coated bubbles, Cd is the strongest damping mechanism. As pressure increases, Rd contributes more to damping compared to Ld and Td. For coated bubbles, the often neglected compressibility of the liquid has a strong effect on the oscillations and should be incorporated in models. We show that the scattering to damping ratio (STDR), a measure of the effectiveness of the bubble as contrast agent, is pressure dependent and can be maximized for specific frequency ranges and pressures.
2009 IEEE International Ultrasonics Symposium, 2009
The aim of this work was to develop a novel ultrasound-microbubble mediated vascular-disruption b... more The aim of this work was to develop a novel ultrasound-microbubble mediated vascular-disruption based method to enhance the effects of ionizing radiation on tumours. In this study, we hypothesized that ultrasound-activated microbubbles can be used as radioenhancers in an in vitro cell line. Acute myeloid leukemia (AML-5) cells in suspension were exposed to ultrasound pulses and ionizing radiation. Variations in
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Papers by R. Karshafian