Acoustic Frequency-Based Approach for Identification of Photoacoustic Surgical Biomarkers

Abstract

Spectral unmixing techniques for photoacoustic images are often used to isolate signal origins (e.g., blood, contrast agents, lipids). However, these techniques often require many (e.g., 12–59) wavelength transmissions for optimal performance to exploit the optical properties of different biological chromophores. Analysis of the acoustic frequency response of photoacoustic signals has the potential to provide additional discrimination of photoacoustic signals from different materials, with the added benefit of potentially requiring only a few optical wavelength emissions. This study presents our initial results testing this hypothesis in a phantom experiment, given the task of differentiating photoacoustic signals from deoxygenated hemoglobin (Hb) and methylene blue (MB). Coherence-based beamforming, principal component analysis, and nearest neighbor classification were employed to determine ground-truth labels, perform feature extraction, and classify image contents, respectively. The mean ± one standard deviation of classification accuracy was increased from 0.65 ± 0.16 to 0.88 ± 0.17 when increasing the number of wavelength emissions from one to two, respectively. When using an optimal laser wavelength pair of 710–870 nm, the sensitivity and specificity of detecting MB over Hb were 1.00 and 1.00, respectively. Results are highly promising for the differentiation of photoacoustic-sensitive materials with comparable performance to that achieved with more conventional multispectral laser wavelength approaches.

Dual-wavelength photoacoustic atlas method to estimate fractional methylene blue and hemoglobin contents

“…Our initial dual-wavelength atlas method used LW-SLSC beamforming to extract meaningful photoacoustic data. 46 The method herein uses M-Weighted SLSC beamforming, rather than LW-SLSC beamforming to reduce the extensive processing times required to generate coherence masks for a total of 2200 frames (i.e., 11 concentrations wavelengths frames trials datasets) of photoacoustic data. However, considering that only a single ultrasound image is needed to show the structural detail surrounding the concentration maps in Figs.…”

“…We developed a novel photoacoustic-based, dual-wavelength atlas method that accurately estimates and generates concentration maps of a mixture of endogenous and exogenous chromophores (i.e., MB and Hb, respectively). The method builds on our previous dual-wavelength approach 46 and measures the photoacoustic spectra obtained from two laser wavelength emissions as a linear combination of the chromophore spectra stored in an atlas. Linearity and monotonicity were confirmed with analyses of acoustic spectra and estimated concentrations as the ground-truth concentration increased, as shown in Figs.…”

“…3(a) ] resulted in requiring only the first principal component for feature reduction, which agrees with previous optimization results of the dual-wavelength atlas method. 46 , 66 However, the number of principal components as well as the axial kernel size required for accurate estimation may increase with noise and diverse photoacoustic responses obtained from different factors, such as fiber tip geometries, 73 – 75 vessel size, 76 – 78 absorber size, 79 – 81 ex vivo , 82 and in vivo 83 , 84 data. Future studies will investigate the effect of these factors in the optimization process of our method.…”

“…1(b) , based on the dual-wavelength atlas method previously proposed for the identification of individual chromophores. 46 With this setup, frequency domain information is expected to be useful because the optical fiber is placed inside the PVCP chamber, thus generating fluence maps that diminish radially from the tip of the optical fiber. This fluence distribution generates unequal acoustic frequency responses, as volume regions of different sizes are being excited.…”

“…We previously proposed an acoustic frequency-based method to discriminate photoacoustic responses from different materials and overcome challenges with traditional spectral unmixing techniques. 46 By measuring the photoacoustic response from only two laser wavelengths, the initial version of our dual-wavelength atlas method achieved comparable sensitivity, specificity, and accuracy to traditional spectral unmixing methods 47,48 and related classifiers. 49,50 Based on the linear relationship between contrast agent concentration and photoacoustic amplitude, [51][52][53] we hypothesize that our method can be extended to locally characterize the volumetric ratio between two photoacoustic-sensitive materials.…”