Characterization of continuous wave ultrasound for acousto-optic modulated diffuse correlation spectroscopy (AOM-DCS)

Abstract

Intra and post-operative blood flow monitoring of tissue has been shown to be effective in the improvement of patient outcomes. Diffuse correlation spectroscopy (DCS) has been shown to be effective in measuring blood flow at the bedside, and is a useful technique in measuring cerebral blood flow (CBF) in many clinical settings. However, DCS suffers from reduced sensitivity to blood flow changes at larger tissue depths, making measurements of CBF in adults difficult. This issue can be addressed with acousto-optic modulated diffuse correlation spectroscopy (AOM-DCS), which is a hybrid technique that combines the sensitivity of DCS to blood flow with ultrasound resolution to allow for improved spatial resolution of the optical signal based on knowledge of the area which is insonified by ultrasound. We present a quantitative model for perfusion estimation based on AOM-DCS in the presence of continuous wave ultrasound, supported by theoretical derivations, Monte Carlo simulations, and phantom and human subject experiments. Quantification of the influence of individual mechanisms that contribute to the temporal fluctuations of the optical intensity due to ultrasound is shown to agree with previously derived results. By using this model, the recovery of blood-flow induced scatterer dynamics based on ultrasound-modulated light is shown to deviate by less than one percent from the standard DCS measurement of scatterer dynamics over a range of optical scattering values and scatterer motion conditions. This work provides an important step towards future implementation of AOM-DCS setups with more complex spatio-temporal distributions of ultrasound pressure, which are needed to enhance the DCS spatial resolution.

Optimizing a two-layer method for hybrid diffuse correlation spectroscopy and frequency-domain diffuse optical spectroscopy cerebral measurements in adults

“…47 Although achieving an adequate SNR at source-detector distances beyond 2.5 cm is challenging, recent studies have demonstrated promising new strategies to boost brain sensitivity. [48][49][50][51][52][53][54][55][56][57][58][59][60][61] Previous studies that used a 2L approach for DCS analysis assumed homogeneous or fixed optical properties from the literature. 12,15,17,20,21,26 This approach has the advantage of simplicity.…”

Diffuse correlation spectroscopy: current status and future outlook

“…These include time-of-flight selection (time-resolved/time-domain DCS 54 and the related iNIRS technique 55 ), pathlength selection through coherence gating, 56 and acoustic (ultrasound) tagging. 57 , 58 A major advantage of these techniques is that large source-detector separations are no longer needed, enabling compact probe design and/or dense spatial sampling and increased resolution, for example, for functional brain imaging. Further, time-domain DCS and iNIRS intrinsically sample the optical properties of the sample as well, providing both spectroscopic and flow property measurement.…”

Fourier domain diffuse correlation spectroscopy with heterodyne holographic detection

“…These benefits include improved monitoring of cerebrovascular autoregulation dynamics, more robust identification of motion artefacts, and increased measurement throughput that could enable high spatial resolution with fewer detectors. Improvements in SNR are also motivated by the ability to perform flow measurements with improved spatial resolution and depth penetration through the use of acousto-optic tomography (AOT) [3,[8][9][10], which is an emerging hybrid imaging modality that makes use of the ultrasound modulation of light [11]. The modulation efficiency of photons by ultrasound is low at biologically safe power levels, and therefore significantly improved SNR is required in order to detect this modulated component [9].…”