In vivo pressure estimation using subharmonic contrast microbubble signals: proof of concept

A recent study . J. Acoust. Soc. Am. 130,[3137][3138][3139][3140][3141][3142][3143][3144][3145][3146][3147] showed that in contrast to the analytical result for free bubbles, the minimum threshold for subharmonic generation for contrast microbubbles does not necessarily occur at twice the resonance frequency. Here increased damping-either due to the small radius or the encapsulation-is shown to shift the minimum threshold away from twice the resonance frequency. Free bubbles as well as four models of the contrast agent encapsulation are investigated varying the surface dilatational viscosity. Encapsulation properties are determined using measured attenuation data for a commercial contrast agent. For sufficiently small damping, models predict two minima for the threshold curve-one at twice the resonance frequency being lower than the other at resonance frequency-in accord with the classical analytical result. However, increased damping damps the bubble response more at twice the resonance than at resonance, leading to a flattening of the threshold curve and a gradual shift of the absolute minimum from twice the resonance frequency toward the resonance frequency. The deviation from the classical result stems from the fact that the perturbation analysis employed to obtain it assumes small damping, not always applicable for contrast microbubbles.

Section: Introductionmentioning

confidence: 99%

Effects of encapsulation damping on the excitation threshold for subharmonic generation from contrast microbubbles

Katiyar

Sarkar

2012

“…They also generate super-and subharmonic signals under strong excitations, which can be used for harmonic (imaging at twice the excitation frequency) (Chang et al, 1996) and subharmonic (imaging at half the excitation frequency) imaging Shankar et al, 1998). Recently, the ambient pressure dependent variation of subharmonic response from contrast microbubbles has been proposed as a noninvasive estimator of the organ-level blood pressure (Adam et al, 2005;Andersen and Jensen, 2010;Forsberg et al, 2005;Frinking et al, 2010;Katiyar et al, 2011;Leodore et al, 2007;Shi et al, 1999). In vitro experiments with a number of contrast agents registered 5-15 dB decrease with 188 mm Hg increase in ambient pressure (Adam et al, 2005;Leodore et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

Excitation threshold for subharmonic generation from contrast microbubbles

Katiyar

Sarkar²

2011

Six models of contrast microbubbles are investigated to determine the excitation threshold for subharmonic generation. The models are applied to a commercial contrast agent; its characteristic parameters according to each model are determined using experimentally measured ultrasound attenuation. In contrast to the classical perturbative result, the minimum threshold for subharmonic generation is not always predicted at excitation with twice the resonance frequency; instead it occurs over a range of frequencies from resonance to twice the resonance frequency. The quantitative variation of the threshold with frequency depends on the model and the bubble radius. All models are transformed into a common interfacial rheological form, where the encapsulation is represented by two radius dependent surface properties-effective surface tension and surface dilatational viscosity. Variation of the effective surface tension with radius, specifically having an upper limit (resulting from strain softening or rupture of the encapsulation during expansion), plays a critical role. Without the upper limit, the predicted threshold is extremely large, especially near the resonance frequency. Having a lower limit on surface tension (e.g., zero surface tension in the buckled state) increases the threshold value at twice the resonance frequency, in some cases shifting the minimum threshold toward resonance.

“…11 Furthermore, in 2005 the same group presented in vivo results for proof of concept of the capabilities of the subharmonic response. 12 As the measurements were performed directly on the abdominal cavity and the aorta by incision of two dogs, this can hardly be characterized as noninvasive. However, the results still showed that the subharmonic component decreased as the ambient pressure increased and thereby indicated that the subharmonic response of UCAs can be used for ambient pressure measurements.…”

Section: Introductionmentioning

confidence: 99%

Ambient pressure sensitivity of microbubbles investigated through a parameter study

Andersen

Jensen

2009

Measurements on microbubbles clearly indicate a relation between the ambient pressure and the acoustic behavior of the bubble. The purpose of this study was to optimize the sensitivity of ambient pressure measurements, using the subharmonic component, through microbubble response simulations. The behavior of two microbubbles corresponding to two different contrast agents was investigated as a function of driving pulse and ambient overpressure pov. Simulations of Levovist using a rectangular driving pulse show an almost linear reduction in the subharmonic component as pov is increased. For a 20 cycle driving pulse, a reduction of 4.6 dB is observed when changing pov from 0 to 25 kPa. Increasing the pulse duration makes the reduction even clearer. For a pulse with 64 cycles, the reduction is 9.9 dB. This simulation is in good correspondence with measurement results presented in the literature. Further simulations of Levovist show that also the shape and the acoustic pressure of the driving pulse are very important factors. The best pressure sensitivity of Levovist was found to be 0.88 dB/kPa. For Sonazoid, a sensitivity of 1.14 dB/kPa has been found, although the reduction is not completely linear as a function of the ambient pressure.