Publications

This research project tested the hypothesis that cold-equilibrated (∼0°C) human erythrocytes in vitro in the presence of an ultrasound contrast agent (Albunex^®) will undergo greater ultrasound-induced hemolysis than physiologically equilibrated (37°C) human erythrocytes in vitro because of a temperature-related transition in membrane fluidity leading to increased fragility. First, it was shown that cold-equilibrated erythrocytes are more susceptible to mechanically induced hemolysis than physiologically equilibrated erythrocytes. Second, when adjustments were made for (1) temperature-dependent efficiencies of a 1-MHz transducer (200 μs pulse length, 20 ms interpulse interval, 30 s exposure duration) such that when cold or physiological temperatures were employed, there were equivalent acoustic outputs in terms of peak negative pressure (MPa P⁻) and (2) comparable viscosities of the 0 and 37°C blood plasmas, the cold (∼0°C) erythrocytes displayed substantially greater amounts of ultrasound-induced hemolysis than the physiological (37°C) erythrocytes. The data supported the hypothesis.

Wormlike micellar (WM) fluids, which flow when subjected to long term stresses, are mechanically viscoelastic over shorter durations. These fluids are birefringent under shear, allowing the study of shear wave propagation using both optical and acoustic modalities. In this work, the temperature and frequency dependent behavior of ultrasonically generated shear waves in a WM fluid is studied. A fluid consisting of hexadecyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSAL) combined in a 5:3 ratio is used in a 200 mM concentration. A high intensity focused ultrasound (HIFU) beam generates radiation pressure in the fluid and can induce shear waves of sufficient amplitude to be visualized optically when the beam is modulated. By pulsing the HIFU beam, a train of shear waves are generated which propagate laterally from the focal region. The temperature and frequency dependent behavior of the HIFU generated shear waves are correlated with the rheological and microstructural properties of the fluid. Discontinuities in slope in the temperature dependence of the shear modulus and viscosity provide evidence to suggest that there are phase transitions of the 200 mM CTAB/NaSal WM fluid at about 28°C and 38°C.

The mechanical index (MI) quantifies the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a nonthermal mechanism. The current formulation of the MI is based on inertial cavitation thresholds in two liquids, water and blood, as calculated by a formalism assuming very short pulse durations. Although tissue contains a high proportion of water, it is not a liquid but a viscoelastic solid. Further, acoustic radiation force impulse imaging employs high-intensity pulses up to several hundred acoustic periods long. The effect of these differences was studied in water, blood and five representative tissues.

In biomedical ultrasound, experimental work may be performed at various temperatures whereas hydrophone calibration data are typically specified at only one temperature. It is necessary to know the temperature dependence, if any, of hydrophone sensitivity. Comparison of the results of a three-transducer reciprocity calibration of an Imotec type 80-0.5-4.0 passive PVDF probe at 3/spl deg/C, 15/spl deg/C and 37/spl deg/C to the calibration at 22/spl deg/C show that within the limits of precision of this experiment, there is no difference in hydrophone sensitivity at these four temperatures.

Interest in transcranial ultrasound has motivated numerous ultrasonic studies of the skull and brain but reports of the ultrasonic properties of the scalp are limited. Scalp is a layered structure of dermis/epidermis followed by a fatty subcutaneous layer, a fibrous layer (galea aponeurotica) and lastly loose connective tissue. The goal of this study is to ultrasonically characterize the transmural tissue structure of the scalp. Sixty-four formalin-fixed specimens were prepared from four human donors from four scalp regions in two different orientations and scanned with a 25-MHz transducer. Apparent integrated backscatter (AIB) and integrated backscatter coefficient (IBC) were measured over a 10 × 12 mm region in the transmural plane. Parametric images and autocorrelated profile plots reveal how these parameters change throughout the layered structure. Regions of interest in the parametric images in each scalp layer were used to compute average values. The measured parameters were significantly lower in the connective tissue layer compared to both the epidermis/dermis and subcutaneous layers. Values varied from −45.7 ± 3.05 dB and 0.117 ± 0.102 cm⁻¹str⁻¹ in the dermis/epidermis layer to −51.2 ± 3.94 dB and 0.042 ± 0.073 cm⁻¹str⁻¹ in the connective tissue layer for AIB and IBC, respectively.

In this work, the frequency dependence of shear wave attenuation in micellar fluid was investigated. The micellar fluid used was a concentrated (>100 mM) aqueous solution of a surfactant (cetyltrimethylammonium bromide) and a salt (sodium salicylate) combined in a 5:3 ratio. The fluid was seeded with polyethylene microspheres ranging from 212 to 250 μm in diameter. The fluid is viscoelastic thus supporting the propagation of shear waves. Shear waves were generated in the frequency range 50–120 Hz in 10 Hz increments by a plate touching the fluid surface driven by a function generator. The motion of the microspheres was captured using a high-speed camera. The recorded video was divided into frames and particle tracking was used to determine the horizontal and vertical coordinates of the microspheres. The amplitude attenuation coefficient, α, was determined from the slope of the plot of ln(A(y) versus A₀ ) versus y where A(y) is the displacement amplitude, A₀ is the reference amplitude and y is the propagation distance of the wave. A(y) was determined by selecting bins of thickness Δy and determining the maximum horizontal displacement of particles in the bins. No dependence of attenuation on frequency was observed in the studied frequency range.

Interest in transcranial ultrasound has motivated numerous ultrasonic studies of the skull and brain; however, the ultrasonic properties of the scalp are relatively unknown. The goal of this study was to ultrasonically characterize the transmural structure of scalp tissue. Sixty-four formalin fixed specimens were prepared from four human donors and scanned in a water tank with a 25 MHz transducer to create parametric images of the speed of sound (SOS), frequency slope of attenuation (FSA), and integrated attenuation coefficient (IAC). Images revealed three distinct layers: a dermis/ epidermis layer, a subcutaneous layer, and a connective tissue layer. A statistically significant difference was observed between all layers for all parameters in most cases. Exceptions were between the dermis/epidermis layer and the subcutaneous layer for FSA and between the subcutaneous layer and the connective tissue layer for SOS.

The University of Mississippi provides a unique opportunity to study physical acoustics in two departments: the Department of Physics and Astronomy and the Department of Mechanical Engineering. It is one of the few universities in the United States where students can pursue PhD and MS degrees in Physics with a focus on physical acoustics research. The National Center of Physical Acoustics (NCPA) is an 85 000 square foot standalone facility that houses many laboratories on the campus of the University of Mississippi solely dedicated to the physics and engineering applications of acoustics over the entire frequency range, from infrasound to ultrasound. Students can study a wide range of topics including the atmosphere, ocean, weather, jet noise, porous media, acoustic metamaterials, ultrasonic properties of materials, nondestructive testing, and fluid dynamics. Our alumni work in academia, national labs, industry, and the medical field among other occupations. The University of Mississippi is a PhD granting institution with an R1 Carnegie designation placing it among schools with the highest level of research activity. It is located in Oxford, MS, a cultural mecca of the southern United States, home to William Faulkner (one of the most celebrated American authors), many artistic festivals and sporting events.

In this talk, I will present two perspectives of Rich Raspet; one from the point of view as a graduate student and the other from the point of view as a faculty colleague. As a PhD student in the University of Mississippi Physics program, he was my instructor in two courses and gave me advice whenever I requested it. Later, as a faculty member in the same department, I became Rich’s colleague where I continued to learn much from him. I will reflect on what it was like to know Rich in both modes and what I learned from him as a student and as a faculty colleague.