Xucai Chen

The measurement of backscatter coefficient from a broadband pulse-echo system: a new formulation

A new formulation for obtaining the absolute backscatter coefficient from pulse-echo measurements is presented. Using this formulation, performing the diffraction correction and system calibration is straightforward. The diffraction correction function for the measurement of backscatter coefficient and the acoustic coupling function for a pulse-echo system are defined. Details of these functions for two very useful cases are presented: a flat disk transducer and a spherically focused transducer. Approximations of these functions are also provided. For a flat disk transducer, the final formulation appears as a modification to the established Sigelmann-Reid formulation. For a focused transducer, the final correction is a weak function of frequency when the scattering volume is near the focal area, rather than the frequency squared dependence proposed by earlier investigators.

Ultrasound enhancement of thrombolysis and reperfusion in vitro.

OBJECTIVES The aims of this study were 1) to develop an in vitro flow system in which reperfusion mediated by ultrasound-accelerated thrombolysis could be studied, and 2) to test whether ultrasound-accelerated thrombolysis could hasten reperfusion in this system. BACKGROUND Ultrasound has been shown to increase tissue plasminogen activator (t-PA)-induced thrombolysis in vitro as assessed by radioactive fibrinogen release from labeled clots and in an animal in vivo model. METHODS To test whether reperfusion is accelerated, we created obstructive whole blood clots in an in vitro flow system. Four control clots were exposed to ultrasound only without any thrombolytic agent (group 1). Sixteen clots were exposed to continuous infusion of recombinant tissue-type plasminogen activator rt-PA and randomized to either continuous wave ultrasound exposure at a frequency of 0.5 MHz and an intensity of 8 W/cm2 (group 2) or to no ultrasound (group 3). Flow distal to the clot and the rate of release of radiolabeled fibrin products were used as an index of reperfusion and thrombolysis, respectively. Samples were obtained for measurements of lytic variables such as plasminogen, fibrinogen and rt-PA concentrations. RESULTS Flow was significantly higher in the rt-PA-treated clots within 10 min of exposure to ultrasound than in those without such exposure (9.4 +/- 9.9% of maximal flow in group 2 vs. 0.5 +/- 1.5% in group 3, p < 0.05). The maximal difference in flow between groups 2 and 3 was achieved at 25 min (61.0 +/- 30.4% vs. 14.2 +/- 14.7%, p = 0.03). Thrombolysis was significantly higher after 15 min of ultrasound exposure (12.8 +/- 9.1% in the ultrasound-treated group 2 vs. 4.0 +/- 3.9% in group 3, p < 0.05). The maximal difference between groups 2 and 3 occurred at 25 min (26.7 +/- 13.1% vs. 7.24 +/- 5.7%, p < 0.004). Neither flow nor clot lysis occurred in group 1. Plasminogen and fibrinogen concentrations and rt-PA antigen concentrations were consistent with those observed during fibrinolytic therapy in vivo. CONCLUSIONS Continuous wave ultrasound at 0.5 MHz and an intensity of 8 W/cm2 accelerates rt-PA-induced thrombolysis and reperfusion in vitro.

Radiation force on a spherical object in an axisymmetric wave field and its application to the calibration of high‐frequency transducers

The general formulation for the radiation force on a spherical object in an axisymmetric acoustic field is provided. The sphere is described in general by three parameters: its density, compressional wave speed, and shear wave speed. Other types of spheres, including the rigid and immovable sphere and the infinitely soft sphere (void), are treated as limiting cases. Specialized formulations of the radiation force function are provided for several types of incident waves of common interest. A low-frequency expansion for each case is provided for comparison with results from the literature. Among the solutions provided are the scattering function of an elastic sphere in a focused acoustic field and the radiation force on the sphere. The radiation force function is used to calibrate high-frequency transducers. Experimental data are provided for a focused transducer for frequencies up to 10 MHz, where the size of the elastic sphere is comparable to or longer than the (-3)-dB beamwidth of the sound field.

Ultrasound-Targeted Microbubble Destruction to Deliver siRNA Cancer Therapy

Microbubble contrast agents can specifically deliver nucleic acids to target tissues when exposed to ultrasound treatment parameters that mediate microbubble destruction. In this study, we evaluated whether microbubbles and ultrasound-targeted microbubble destruction (UTMD) could be used to enhance delivery of EGF receptor (EGFR)-directed siRNA to murine squamous cell carcinomas. Custom-designed microbubbles efficiently bound siRNA and mediated RNAse protection. UTMD-mediated delivery of microbubbles loaded with EGFR-directed siRNA to murine squamous carcinoma cells in vitro reduced EGFR expression and EGF-dependent growth, relative to delivery of control siRNA. Similarly, serial UTMD-mediated delivery of EGFR siRNA to squamous cell carcinoma in vivo decreased EGFR expression and increased tumor doubling time, relative to controls receiving EGFR siRNA-loaded microbubbles but not ultrasound or control siRNA-loaded microbubbles and UTMD. Taken together, our results offer a preclinical proof-of-concept for customized microbubbles and UTMD to deliver gene-targeted siRNA for cancer therapy.

Quantitative echo contrast concentration measurement by Doppler sonography

The hypothesis investigated in this study is that Doppler ultrasound can be used to make quantitative echo contrast concentration measurements in flow systems. Our motivation was to demonstrate the utility and some of the pitfalls of using scattered ultrasound intensity to quantify echo contrast in chambers and vessels. Doppler ultrasound was used rather than conventional imaging techniques because of its natural association with the assessment of flow in chambers and vessels. We compared the intensity of audio Doppler to various steady-state concentrations echo contrast in a carefully controlled in vitro flow system. A total of 62 paired audio Doppler intensity and echo contrast concentration measurements were made. A weak positive correlation was found between the absolute echo contrast concentration and audio Doppler intensity (r = 0.510, p = 0.001). The correlation was weak because of the many unknowns and effervescent nature of microbubble echo contrast agents. However, audio Doppler intensity was shown to correlate strongly with the relative concentration of echo contrast over short time periods (r = 0.958, p = 0.0001). The results show that Doppler intensity can be used to quantitatively measure the relative, but not the absolute concentration of echo contrast in in vitro flow systems.

Ultrasound accelerates urokinase-induced thrombolysis and reperfusion

We have shown that ultrasound accelerates TPA-induced thrombolysis in vitro as assessed by release of labeled fibrinogen from radioactive labeled clots. Others have shown that ultrasound shortens the time to recanalization of TPA treated thrombi in animal models. The aim of this study was to test the hypothesis that ultrasound enhances thrombolysis and reperfusion by using urokinase in an in vitro flow system. An in vitro flow system of a branching tubing circuit was developed. Flow in one branch was obstructed by a thrombus. Five control clots were exposed to continuous wave ultrasound at a frequency of 1 MHz and intensity of 2.5 W/cm2 only without any thrombolytic agent (group 1). Twenty clots were exposed to a bolus of 80,000 U of urokinase and randomized to either ultrasound exposure (group 2) or to urokinase only without ultrasound (group 3). Flow distal to the clot and the rate of release of radiolabeled fibrin were used as indexes of reperfusion and thrombolysis, respectively. Exposure to ultrasound significantly accelerated urokinase-mediated reperfusion, with 40.6% +/- 11.8% of maximal flow in group 2 versus 1.3% +/- 0.7% in group 3, p < 0.0015 after 25 min. The maximal difference in flow between groups 2 and 3 was achieved at 40 minutes (67.4% +/- 11.1% vs 13.1% +/- 5.6%, p < 0.0009). Thrombolysis was significantly higher after 25 minutes of ultrasound exposure (24.1% +/- 4.6% in the ultrasound-treated group vs 9.7% +/- 3.5% in group 3, p < 0.013). The maximal difference in thrombolysis between groups 2 and 3 was 60 minutes. (52.5% +/- 5.1% vs 18.7% +/- 6.2%, p < 0.00015).(ABSTRACT TRUNCATED AT 250 WORDS)

GENE THERAPY OF CARCINOMA USING ULTRASOUND-TARGETED MICROBUBBLE DESTRUCTION

When microbubble contrast agents are loaded with genes and systemically injected, ultrasound-targeted microbubble destruction (UTMD) facilitates focused delivery of genes to target tissues. A mouse model of squamous cell carcinoma was used to test the hypothesis that UTMD would specifically transduce tumor tissue and slow tumor growth when treated with herpes simplex virus thymidine kinase (TK) and ganciclovir. UTMD-mediated delivery of reporter genes resulted in tumor expression of luciferase and green fluorescent protein (GFP) in perivascular areas and individual tumor cells that exceeded expression in control tumors (p=0.02). The doubling time of TK-treated tumors was longer than GFP-treated tumors (p=0.02), and TK-treated tumors displayed increased apoptosis (p=0.04) and more areas of cellular drop-out (p=0.03). These data indicate that UTMD gene therapy can transduce solid tumors and mediate a therapeutic effect. UTMD is a promising nonviral method for targeting gene therapy that may be useful in a spectrum of tumors.

Harmonic imaging with levovist

Our purpose was to test the hypothesis that second harmonic imaging preferentially detects backscatter from microbubbles compared with tissue structural components. A prototype second harmonic scanner was used to image a flow channel in a tissue-mimicking rubber phantom (liver density). Video time-intensity curves were calculated from repeated bolus injections of microbubble echocardiographic contrast material under the same fluid dynamic conditions but with three different imaging modes: (1) fundamental imaging at 2.5 MHz (transmit and receive at 2.5 MHz), (2) fun damental imaging at 5.0 MHz (transmit and receive at 5.0 MHz), and (3) second harmonic imaging (transmit at 2.5 MHz and receive at 5.0 MHz). Each video time-intensity curve was calibrated-such that quantitative backscatter intensity was measured relative to the tissue phantom (0 dB). The peak increase in backscatter from the contrast material in the channel relative to the tissue phantom and the intensity in the channel before the contrast effect (the noise floor) was measured along with the area under the calibrated time-intensity curve relative to the phantom. When referenced to the noise floor in the flow channel, all imaging modes produced approximately 25 dB of enhancement. However, when referenced to the tissue phantom, second harmonic imaging produced a 22.3 +/- 1.8 dB peak enhancement, which was greater than either fundamental imaging at 2.5 MHz (15.5 +/- 0.8 dB; p < 0.001) or fundamental imaging at 5.0 MHz (15.3 +/- 1.5 dB; p < 0.001). The area under the time-intensity curves confirmed that harmonic imaging has approximately 7 dB of relative enhancement to the phantom compared with fundamental imaging at either frequency. Second harmonic imaging specifically enhances backscatter from microbubbles compared with a tissue-mimicking phantom. This specificity for microbubbles is due to a decrease in backscatter for the tissue phantom, rather than an increase in backscatter for the microbubbles. These data support the hypothesis that second harmonic imaging may be able to detect microbubbles in the tissue vascular space by preferentially decreasing the backscatter from tissue structural components.

The Doppler kinetics of microbubble echo contrast

The right and left heart kinetics of a saccharide-based microbubble echo contrast agent were measured in 11 anesthetized dogs using Doppler intensity as a measure of microbubble concentration while controlling for the dose administered, weight of the subject and cardiac output. A two-phase Doppler time-intensity curve was noted in all vascular regions. A brief first pass effect (phase 1) was found to depend on the contrast dose, cardiac output and subject size. This was followed by a much longer nearly steady-state elevation in the Doppler intensity compared with baseline (phase 2). The kinetics of phase 2 were found to be the same in all vascular distributions and independent of cardiac output. The phase 2 kinetics depend on the contrast dose, subject size and elimination characteristics of the contrast agent. The clinically important conclusions are: (1) the magnitude of Doppler enhancement and duration of the contrast effect can be predicted using the simple formulas presented; (2) the flow-dependent portion of the arterial contrast effect is effectively over only a few seconds after intravenous injection; and (3) the kinetics of phase 2 are the same throughout the body.

Radiation pattern of a focused transducer: A numerically convergent solution

The radiation pattern of a focused transducer is reexamined. The radiation field is divided into an illuminated zone and a shadow zone. A numerically convergent solution of the pressure distribution in terms summations of Bessel functions is provided. This solution is computationally more advantageous than earlier results where a double or single integral in the complex plane is required. The pressure amplitude differs from earlier reports slightly for off-axis locations at low frequency. This difference may have significance for backscatter coefficient determination where scatterers are assumed present over a time-gated volume. The solution for a flat disk radiator is obtained as a limiting case.