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.

Keywords: acousto-optics; scattering; tomography.

Fig. 1

A cartoon illustration of tagging efficiency. Only light passing through the ultrasound field is considered. Tagged light refers to the light whose frequency is shifted by ultrasonic modulation.

Fig. 2

Flowchart of the simulation process. (1) Generate trajectories using Monte Carlo methods, (2) calculate OPL and the electric field considering refractive index changes and particle displacement comprehensively, (3) calculate power spectrum for each individual speckle, and (4) average to get the ensemble power spectrum and tagging efficiency.

Fig. 3

Diagram of the experimental setup. (a) The experimental setup consisted of a reference arm and a sample arm to detect light passing through the ultrasound field. ISO, isolator; HWP, half-wave plate; BB, beam block, M, mirror; PBS, polarized beam splitter; BE, beam expander; L, lens, AOM, acousto-optic modulator; ID, iris diaphragm; FPC, fiber port coupler; P, polarizer; FP, fiber port; PC, polarization controller; FC, fiber coupler; BPD, balanced photo-detector; DAQ, data acquisition card. (b) A zoomed-in view of the sample. (c). The ultrasound pressure field distribution of the three transducers used in the study.

Fig. 4

Tagging efficiency for 1, 2.25, and 3.5 MHz ultrasound versus ultrasound peak pressure. Dotted line shows simulation results and solid line shows experiment results. Each data point is an average tagging efficiency of 250 speckles. Error bar indicates the standard deviation of five measurements.

Fig. 5

Normalized power spectrum of light passing though 1-MHz ultrasound field in scattering medium. The blue numbers in the plot indicate the order of the peak (with a frequency of $n{f}_{\text{us}}\pm f_0$). (a) Ensemble power spectrum of 1250 speckles. (b) Power spectrum of one speckle arbitrarily chosen from the speckle field. (c) Power spectrum of another speckle arbitrarily chosen from the speckle field.