Quantitative Photoacoustic Tomography Research Articles

Multi-source quantitative photoacoustic tomography in a diffusive regime

Photoacoustic tomography (PAT) is a novel hybrid medical imaging technique that aims to combine the large contrast of optical coefficients with the high-resolution capabilities of ultrasound. We assume that the first step of PAT, namely the reconstruction of a map of absorbed radiation from ultrasound boundary measurement, has been done. We focus on quantitative photoacoustic tomography, which aims at quantitatively reconstructing the optical coefficients from knowledge of the absorbed radiation map. We present a non-iterative procedure to reconstruct such optical coefficients, namely the diffusion and absorption coefficients, and the Grüneisen coefficient when the propagation of radiation is modeled by a second-order elliptic equation. We show that PAT measurements allow us to uniquely reconstruct only two out of the above three coefficients, even when data are collected using an arbitrary number of radiation illuminations. We present uniqueness and stability results for the reconstructions of two such parameters and demonstrate the accuracy of the reconstruction algorithm with numerical reconstructions from two-dimensional synthetic data.

Multispectral quantitative photoacoustic imaging of osteoarthritis in finger joints

We present in vivo experimental evidence that multispectral quantitative photoacoustic tomography (qPAT) has the potential to detect osteoarthritis (OA) in the finger joints. In this pilot study, two OA patients and three healthy volunteers were enrolled, and their distal interphalangeal (DIP) joints were examined photoacoustically by a multispectral PAT scanner. Images of tissue physiological/functional parameters including oxyhemoglobin, deoxyhemoglobin, oxygen saturation, and water content, along with the tissue acoustic velocity of all the examined joints, were simultaneously recovered using a finite element reconstruction algorithm for multispectral photoacoustic measurements. The recovered multispectral photoacoustic images show that the OA joints have significantly elevated water content, decreased oxygen saturation, and increased acoustic velocity compared to the normal joints.

Quantitative two-dimensional photoacoustic tomography of osteoarthritis in the finger joints

We present for the first time in vivo experimental evidence that quantitative photoacoustic tomography (qPAT) has the potential to detect osteoarthritis (OA) in the finger joints. In this pilot study, 2 OA patients and 4 healthy volunteers were enrolled, and their distal interphalangeal (DIP) joints were examined photoacoustically by a PAT scanner. Tissue absorption coefficient images of all the examined joints, which were unavailable from the conventional PAT, were recovered using our finite element based qPAT approach. The recovered quantitative photoacoustic images revealed significant differences in the tissue absorption coefficient of the joint cavity (cartilage and synovial fluid) between the OA and normal joints.

Quantitative photoacoustic tomography with multiple optical sources

Quantitative imaging of optical properties of biological tissues with high resolution has been a long-sought-after goal of many research groups. Photoacoustic imaging is a hybrid bio-optical imaging technique offering optical absorption contrast with ultrasonic spatial resolution. While photoacoustic methods offer significant promise for high-resolution optical imaging, quantification has thus far proved challenging. In this paper, a noniterative reconstruction technique for producing quantitative photoacoustic images of absorption perturbations is introduced for the case when the optical properties of the turbid background are known and when multiple optical illumination locations are used. Through theoretical developments and computational examples it is demonstrated that multiple-optical-source photoacoustic imaging can produce quantitative optical absorption reconstructions. The combination of optical and photoacoustic measurements is shown to yield improved reconstruction stability.

Transport-based quantitative photoacoustic tomography: simulations and experiments

In this paper we describe a method for quantitative photoacoustic tomography (qPAT) based on the photon radiative transfer equation (RTE) coupled with the Helmholtz photoacoustic wave equation. Considerable simulations and tissue-like phantom experiments are conducted to evaluate transport-based qPAT in comparison with diffusion-based qPAT. In these comparative simulations and experiments, we systematically examine the effects of the ratio of μa/μ′s (absorption/reduced scattering coefficient), the anisotropy factor (g) and the imaging domain size on the transport- and diffusion-based photoacoustic image reconstruction. The results obtained show that transport-based qPAT allows for clearly more accurate recovery of the absolute absorption coefficient images of heterogeneous media over diffusion-based qPAT for all the cases examined and provides considerably improved image quality for cases where the photon diffusion approximation (DA) is invalid.

Image reconstruction in photoacoustic tomography with variable speed of sound using a higher-order geometrical acoustics approximation

Previous research correcting for variable speed of sound in photoacoustic tomography (PAT) based on a generalized radon transform (GRT) model assumes first-order geometrical acoustics (GA) approximation. In the GRT model, the pressure is related to the optical absorption, in an acoustically inhomogeneous medium, through integration over nonspherical isochronous surfaces. Previous research based on the GRT model assumes that the path taken by acoustic rays is linear and neglects amplitude perturbations to the measured pressure. We have derived a higher-order GA expression that takes into account the first-order effect in the amplitude of the measured signal and higher-order perturbation to the travel times. The higher-order perturbation to travel time incorporates the effect of ray bending. Incorrect travel times can lead to image distortion and blurring. These corrections are expected to impact image quality and quantitative PAT. We have previously shown that travel-time corrections in 2-D suggest that perceivable differences in the isochronous surfaces can be seen when the second-order travel-time perturbations are taken into account with a 10% speed-of-sound variation. In this work, we develop iterative image reconstruction algorithms that incorporate this higher-order GA approximation assuming that the speed of sound map is known. We evaluate the effect of higher-order GA approximation on image quality and accuracy.

Quantitative multispectral photoacoustic tomography and wavelength optimization

We describe a reconstruction method that allows direct recovery of tissue chromophore concentrations and acoustic velocity using tomographic multi-spectral photoacoustic measurements. The method is tested in detail using extensive simulated data. Criteria for choosing optimum wavelength sets are analyzed for simultaneous reconstruction of tissue functional and acoustic properties.

Quantitative multispectral photoacoustic tomography and wavelength optimization

Quantitative multispectral photoacoustic tomography and wavelength optimization

Quantitative three-dimensional photoacoustic tomography of the finger joints: phantom studies in a spherical scanning geometry

In this paper, we present a series of phantom experiments that aim to systematically evaluate the possibility of quantitative three-dimensional (3D) photoacoustic imaging of the finger joints in a spherical scanning geometry. The 3D absorption coefficient images of the joint-mimicking phantoms are obtained using our finite-element-based photoacoustic image reconstruction algorithm coupled with the photon diffusion equation. The results show that a 1 mm thick ‘cartilage’ can be accurately differentiated from the ‘bones’ with a 1 MHz transducer. In addition, the absorption coefficient of the ‘cartilage’ can be effectively recovered when this optical property is varied from 0.015 to 0.04 mm−1. The phantom images shown in this paper suggest that 3D photoacoustic tomography in a spherical scanning geometry has the potential to image a realistic human joint.

Quantitative photoacoustic tomography based on the radiative transfer equation

We describe a method for quantitative photoacoustic tomography (PAT) based on the radiative transfer equation (RTE) coupled with the Helmholtz photoacoustic wave equation. This RTE-based quantitative PAT allows for accurate recovery of absolute absorption coefficient images of heterogeneous media and provides significantly improved image reconstruction for the cases where the photon diffusion approximation may fail. The method and associated finite element reconstruction algorithm are validated using a series of tissuelike phantom experiments.

Quantitative three-dimensional photoacoustic tomography of the finger joints: an in vivo study.

We present for the first time in vivo full three-dimensional (3-D) photoacoustic tomography (PAT) of the distal interphalangeal joint in a human subject. Both absorbed energy density and absorption coefficient images of the joint are quantitatively obtained using our finite-element-based photoacoustic image reconstruction algorithm coupled with the photon diffusion equation. The results show that major anatomical features in the joint along with the side arteries can be imaged with a 1-MHz transducer in a spherical scanning geometry. In addition, the cartilages associated with the joint can be quantitatively differentiated from the phalanx. This in vivo study suggests that the 3-D PAT method described has the potential to be used for early diagnosis of joint diseases such as osteoarthritis and rheumatoid arthritis.

Quantitative photoacoustic tomography from boundary pressure measurements: noniterative recovery of optical absorption coefficient from the reconstructed absorbed energy map

We describe a noniterative method for recovering optical absorption coefficient distribution from the absorbed energy map reconstructed using simulated and noisy boundary pressure measurements. The source reconstruction problem is first solved for the absorbed energy map corresponding to single- and multiple-source illuminations from the side of the imaging plane. It is shown that the absorbed energy map and the absorption coefficient distribution, recovered from the single-source illumination with a large variation in photon flux distribution, have signal-to-noise ratios comparable to those of the reconstructed parameters from a more uniform photon density distribution corresponding to multiple-source illuminations. The absorbed energy map is input as absorption coefficient times photon flux in the time-independent diffusion equation (DE) governing photon transport to recover the photon flux in a single step. The recovered photon flux is used to compute the optical absorption coefficient distribution from the absorbed energy map. In the absence of experimental data, we obtain the boundary measurements through Monte Carlo simulations, and we attempt to address the possible limitations of the DE model in the overall reconstruction procedure.

Quantitative photoacoustic tomography: Recovery of optical absorption coefficient maps of heterogeneous media

We report on experimental demonstration of photoacoustic tomography for reconstructing the optical absorption coefficient images of heterogeneous media. Photoacoustic images are obtained from a series of tissuelike phantom experiments using a finite element-based reconstruction algorithm coupled with a scanning photoacoustic imaging system. The experimental results show that optical absorption images can be quantitatively reconstructed when the photon diffusion model is coupled with the Helmholtz photoacoustic wave equation.

Simultaneous reconstruction of acoustic and optical properties of heterogeneous media by quantitative photoacoustic tomography

We report on experimental demonstration of photoacoustic tomography for simultaneous recovery of the optical and acoustic properties of heterogeneous media. Photoacoustic images are reconstructed from a series of tissue-like phantom experiments using a finite element-based reconstruction algorithm coupled with a scanning photoacoustic imaging system. The results show that the images obtained are quantitative in terms of the shape, size, location and optical and acoustic properties of the heterogeneities examined.

Quantitative photoacoustic imaging in the radiative transport regime

The objective of quantitative photoacoustic tomography (QPAT) is to reconstruct optical and thermodynamic properties of heterogeneous media from data of absorbed energy distribution inside the media. There have been extensive theoretical and computational studies on the inverse problem in QPAT, however, mostly in the diffusive regime. We present in this work some numerical reconstruction algorithms for multi-source QPAT in the radiative transport regime with energy data collected at either single or multiple wavelengths. We show that when the medium to be probed is non-scattering, explicit reconstruction schemes can be derived to reconstruct the absorption and the Gruneisen coefficients. When data at multiple wavelengths are utilized, we can reconstruct simultaneously the absorption, scattering and Gruneisen coefficients. We show by numerical simulations that the reconstructions are stable.