Investigating ultrasound–light interaction in scattering media

Abstract

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation's "tagging efficiency." Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature.Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process.Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results:Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4 mm −1 for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion:Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques.

First principle simulation package for arbitrary acousto-optic interaction in scattering materials

“…2. We further found, in contrast to the findings of Huang et al [1], that A and thus the acoustic movement of scatterers can be neglected without any loss in accuracy. We attribute this difference to A being calculated as having a uniform envelope shape in Huang et al's source code, while we set it to have the same envelope shape as the pressure wave.…”

“…The design and development questions remaining for UOT are many and the need for an accurate simulation tool is evident if we are to avoid endless experimental trial and error. In this regard, we at Lund University and Spectracure have, based on the work by Huang et al [1], built a simulation package for the interaction between arbitrary optical and ultrasonic fields in scattering media [2,3]. This model has been further validated with measurements of tagging efficiencies in intralipid phantoms using fully characterized ultrasound pulses [2].…”

Snapshot time-reversed ultrasonically encoded optical focusing guided by time-reversed photoacoustic wave

“…To describe the locations of any particles, we model the ultrasound-induced displacement of particle at position and time by using the following equation: where is the displacement amplitude, is the unit vector of , and is particular phase offset between particle displacement and ultrasound pressure. Recently, Huang et al have investigated the ultrasound–light interaction in scattering media [27] , and we can know from their theory that = , where is the density of the background medium and is the velocity of acoustic wave. In their theory, phase offset can be approximately replaced by between particle displacement and ultrasound pressure when particles move.…”

“…Because in MC simulation, each photon is independent, we did not treat the photon as a wave phenomenon and ignore features such as phase and polarization. So we can calculate power spectrum by the following equation [27] : where represents expectation and we can calculate power spectrum by . With the above definition, we can estimate the tagging efficiency from the overall average of the power spectrum of all speckles.…”

Interactions of a short-pulsed plane acoustic wave with complex rigid objects: a numerical study

“…Light is widely used in biomedical imaging because it is considered relatively safe compared to other imaging modalities such as X-ray computed tomography [1]. However, the turbid nature of biological tissues causes light to severely scatter, preventing high-resolution imaging deep in the tissues.…”