Raffi Karshafian

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents.

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 frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.

Antivascular effects of pulsed low intensity ultrasound and microbubbles in mouse tumors

In this study we investigate the feasibility of inducing transient and sustained blood flow changes in murine tumors with pulsed low intensity ultrasound. Subcutaneous xenograft melanoma tumors were situated in the hind limbs of mice and exposed to 1 MHz (740 kPa) pulsed ultrasound following a bolus injection of Definitytrade microbubbles. Flow within the tumors was monitored using a 9 MHz array probe in contrast imaging mode following the bolus injection of agent during treatment, and at 2 and 24 hours post treatment. The contrast enhancement was quantified in ROIs encompassing either the periphery or center of the tumors. The results demonstrate the feasibility of producing transient reductions of flow in tumors lasting for less than 15 minutes and, with repeated exposures, the ability to induce sustained reductions in contrast enhancement in lasting for at least 24 hours. Both short and long term flow modifications have implications for the guidance and enhancement of microbubble potentiated local drug enhancement.

Ultrasound-induced uptake of different size markers in mammalian cells

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

The Effect of Acoustic Exposure Parameters on Cell Membrane Permeabilisation by Ultrasound and Microbubbles

This work investigates the mechanism of sonoporation through the effect of ultrasound exposure parameters with microbubbles in a suspension of KHT‐C cells. We investigated the effect of acoustic pressure, pulse repetition frequency, pulse duration, insonation time and frequency. Flow cytometry and fluorescence microscopy were used to quantify permeability and viability of treated cells. Scanning electron microscopy images showed pores of up to 500nm after treatment, indicating that the mechanism of permeabilisation was the formation of pores on the surface of cell membranes. Cell permeability increases and viability decreases with peak negative pressure, duty cycle and insonation time. Higher pulse centre frequencies are more effective at permeabilising cells, but also at killing them. In this study, the highest Therapeutic Ratio is achieved at 570kPa peak negative pressure, 8μs pulse duration, 3kHz pulse repetition frequency, 500kHz centre frequency and 12 seconds of insonation time; Definity at 3.5% vol...

High frequency subharmonic pulsed-wave doppler and color flow imaging of microbubble contrast agents

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 frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters. (E-mail: aneedles@visualsonics.com)

10C-3 Radial Modulation Imaging of Microbubbles at High Frequency

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. Simulations 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 10-20% 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.