KEYWORDS: Chromophores, 3D modeling, Spherical lenses, Absorption, Photoacoustic tomography, Scattering, Monte Carlo methods, Blood, Data modeling, Photons
Quantitative photoacoustic tomography aims to recover maps of the local concentrations of tissue chromophores from multispectral images. While model-based inversion schemes are promising approaches, major challenges to their practical implementation include the unknown fluence distribution and the scale of the inverse problem. We describe an inversion scheme based on a radiance Monte Carlo model and an adjoint-assisted gradient optimization that incorporates fluence-dependent step sizes and adaptive moment estimation. The inversion is shown to recover absolute chromophore concentrations, blood oxygen saturation, and the Grüneisen parameter from in silico three-dimensional phantom images for different radiance approximations. The scattering coefficient is assumed to be homogeneous and known a priori.
KEYWORDS: 3D modeling, 3D image processing, Photoacoustic spectroscopy, Chromophores, Monte Carlo methods, Scattering, Light scattering, Calibration, Photoacoustic tomography, Tomography
Quantitative photoacoustic (PA) tomography aims to recover absolute chromophore concentrations from multiwavelength PA images. Challenges include the accurate prediction of the fluence, the accuracy of the initial pressure distribution reconstructed from measured data, and the large scale of the inverse problem involving high resolution 3D images. In this study, a radiance Monte-Carlo (RMC) light model was used to predict the fluence inside tissue phantoms. Gradients of the scattering coefficient and the chromophore concentrations were calculated using the adjoint formalism. The gradient descent efficiency was significantly improved by using adaptive moment estimation. 3D maps of chromophore concentrations and the scattering coefficient were recovered from measured PA images. The inversion scheme was validated on measured images of a tissue phantom consisting of a scattering liquid and chromophore-filled polymer tubes immersed at different depths. The images were acquired at visible and near-infrared wavelengths using a Fabry-Perot scanner with a planar detection geometry. Amplitude mismatches in the reconstructed initial pressure images due to limited view detection were corrected using an ad hoc correction method. The inversion was stabilized by introducing a calibrated absorber in the imaged volume, or an absolute calibration of the setup. 3D maps of absolute chromophore concentrations, their ratios, and the global scattering coefficient were accurately recovered. The recovery of chromophore concentrations in the image background where SNR is low was identified as a significant new challenge for quantitative PA imaging.
The goal of quantitative photoacoustic tomography (qPAT) is to recover maps of the chromophore distributions from multiwavelength images of the initial pressure. Model-based inversions that incorporate the physical processes underlying the photoacoustic (PA) signal generation represent a promising approach. Monte-Carlo models of the light transport are computationally expensive, but provide accurate fluence distributions predictions, especially in the ballistic and quasi-ballistic regimes. Here, we focus on the inverse problem of 3D qPAT of blood oxygenation and investigate the application of the Monte-Carlo method in a model-based inversion scheme. A forward model of the light transport based on the MCX simulator and acoustic propagation modeled by the k-Wave toolbox was used to generate a PA image data set acquired in a tissue phantom over a planar detection geometry. The combination of the optical and acoustic models is shown to account for limited-view artifacts. In addition, the errors in the fluence due to, for example, partial volume artifacts and absorbers immediately adjacent to the region of interest are investigated. To accomplish large-scale inversions in 3D, the number of degrees of freedom is reduced by applying image segmentation to the initial pressure distribution to extract a limited number of regions with homogeneous optical parameters. The absorber concentration in the tissue phantom was estimated using a coordinate descent parameter search based on the comparison between measured and modeled PA spectra. The estimated relative concentrations using this approach lie within 5 % compared to the known concentrations. Finally, we discuss the feasibility of this approach to recover the blood oxygenation from experimental data.
KEYWORDS: 3D modeling, Sensors, Image segmentation, Tissue optics, Absorption, Acoustics, Scattering, Nickel, Photoacoustic tomography, Chromophores, Blood oxygen saturation, Fabry–Perot interferometers, Data modeling, Photoacoustic imaging, Monte Carlo methods, Inverse problem on medical image
Quantitative photoacoustic tomography (qPAT) aims to extract physiological parameters, such as blood oxygen saturation (sO2), from measured multi-wavelength image data sets. The challenge of this approach lies in the inherently nonlinear fluence distribution in the tissue, which has to be accounted for by using an appropriate model, and the large scale of the inverse problem. In addition, the accuracy of experimental and scanner-specific parameters, such as the wavelength dependence of the incident fluence, the acoustic detector response, the beam profile and divergence, needs to be considered. This study aims at quantitative imaging of blood sO2, as it has been shown to be a more robust parameter compared to absolute concentrations. We propose a Monte-Carlo–based inversion scheme in conjunction with a reduction in the number of variables achieved using image segmentation. The inversion scheme is experimentally validated in tissue-mimicking phantoms consisting of polymer tubes suspended in a scattering liquid. The tubes were filled with chromophore solutions at different concentration ratios. 3-D multi-spectral image data sets were acquired using a Fabry-Perot based PA scanner. A quantitative comparison of the measured data with the output of the forward model is presented. Parameter estimates of chromophore concentration ratios were found to be within 5 % of the true values.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.