Invited Speakers

The biomedical imaging field seeks quantitative visualization of morphological and, more importantly, functional, cellular and molecular properties of tissue. This presentation will introduce an integrated approach utilizing ultrasound (US), photoacoustic (PA) and elasticity (E) imaging, augmented with theranostic agents. Through specific examples, we will elucidate how US/PA/E imaging has the potential to revolutionize both fundamental medical science and the clinical management of diseases. Various applications of the multi-scale non-ionizing US/PA/E imaging will be explored, spanning from cancer detection and diagnosis to cell tracking and image-guided molecular and adoptive cell transfer immunotherapy. The role of nanoconstructs in these applications will be highlighted, and the methods tailored for high contrast, background-free US/PA imaging will be discussed. The presentation will conclude by addressing current challenges and concerns associated with theranostic agents and US/PA/E imaging, along with proposed solutions.

In vivo muscle biomechanical properties are classically deduced from inverse dynamics or measures of joint torque performed using ergometers. However, this provide information about the combined behavior of several structures. Thus, it is complicated to isolate the behavior of an individual muscle. Ultrasound elastography techniques have been developed with the aim to non-invasively assess localized stiffness. In this presentation we focused on the supersonic shear imaging technique that allows to recover stiffness by looking at the propagation of shear waves. In the case of isotropic, homogeneous and quasi-incompressible biological tissues, the propagation velocity is directly linked to the shear elastic modulus. However, this link is not so obvious in muscles due to their anisotropy. Here, we present how stiffness as well as viscosity can be estimated in fusiform muscles but also in pennate muscles by using specific ultrasound sequences that allows to recover directly anisotropy in a 2D imaging plane. Finally, we show how it is possible to estimate nonlinear elastic properties of muscle by deriving the acoustoelasticity theory in a transverse isotropic media. Our hope is to combine all of these mechanical parameters to understand the behavior of a single muscle and deduce its force during movement.

Cardiovascular disease is a systemic, pathologic condition characterized by several structural changes of the arteries. Modifications of arterial structure lead to changes in arterial elasticity, which can predate clinical manifestations of occlusive atherosclerotic disease. To evaluate vascular mechanical properties, we utilize ultrasound-based methods to noninvasively perturb and measure the response of the arterial wall. We use acoustic radiation force (ARF) from focused ultrasound beams to generate propagating waves with high frequency bandwidth in the arterial wall. We then measure the propagation motion with ultrafast ultrasound imaging. These measurements are made with high temporal resolution (< 20 milliseconds) such that we can explore how the mechanical properties change through the course of the cardiac cycle. Additionally, we use computational approaches to understand how the wave motion in the arterial wall corresponds to the elastic and viscoelastic properties using time- and frequency-domain methods. These methods provide a comprehensive approach for exploring arterial biomechanics for the purposes of assessing cardiovascular health. 

Metrology is the science of measurement and its application, as defined in the International Vocabulary of Metrology (VIM). Metrology encompasses some fundamental aspects that are pretty often disregarded in general. The most important consequence is a lack of reliability in many experimental work. Some terms, such as true quantity value, repeatability, measurement error, accuracy, precision, and uncertainty, are regularly incorrectly used. The worst outcome of that misunderstanding is the possible flaw in the reproducibility of experimental research. The metrological concepts are not complex to understand. On the contrary, the statistics behind the uncertainty assessment, for instance, are as simple as standard deviations and t-Student distribution. This lecture aims to disclose the most important terms and concepts presented in VIM and an overview of uncertainty budget development as defined in the Guide for Uncertainty in Measurement (GUM). The talk will present and discuss the abovementioned terms and a few more based on VIM and GUM. The formal analysis of the quantities of influence for two experimental applications will illustrate the concepts. They are speed of sound in homogeneous material and ultrasonic power measurement using the radiation force balance. By the end of the presentation, the audience can develop an uncertainty budget for virtually any experimental method.

The acoustic radiation force generated by an array of transducers can be applied for suspending and manipulating small objects in mid-air. To simulate the radiation force that acts on the levitated object, most authors employ the Gor’kov equation, which assumes the acoustic radiation force is a conservative force. However, there are cases where the acoustic radiation force is not conservative, resulting in an incorrect evaluation of the radiation force. In this study, we implement a numerical model for simulating the acoustic radiation force on a small rigid sphere suspended by an array of 40 kHz transducers operating in mid-air. With the purpose of estimating the errors caused by the Gor’kov approach, we compare the results obtained by two versions of the model: A model based on the Gor’kov equation and another model based on the radiation force calculated by Silva (J. Acoust. Soc. Am. 136, 2405, 2014) and rewritten by Abdelazis and Grier (Phys. Rev. E 2, 013172, 2020). 

Comparison between the acoustic radiation force obtained by the Gor’kov approach and by the Silva/Abdelazis and Grier approach shows that the Gor’kov equation can be accurately used for calculating the radiation force generated by standing wave fields, but the Gor’kov approach can lead to an incorrect evaluation of the force when the sphere is subjected to a focused beam or a vortex beam. For these cases, the model based on Silva/Abdelazis and Grier approach seems more appropriate for simulating the acoustic radiation force that acts on small spheres.