Deep-Tissue Photoacoustic Tomography of a Genetically Encoded Near-Infrared Fluorescent Probe

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

SIGNIFICANCE

Methylene blue (MB) is an exogenous contrast agent that has the potential to assist with visualization and penetration challenges in photoacoustic imaging. However, monitoring the local concentration between MB and endogenous chromophores is critical for avoiding unnecessary MB accumulations that could lead to adverse effects such as hemolysis when exposed to increased dose and photodamage when exposed to high laser energies.

AIM

We developed a modified version of a previously proposed acoustic-based atlas method to estimate concentration levels from a mixture of two photoacoustic-sensitive materials after two laser wavelength emissions.

APPROACH

Photoacoustic data were acquired from mixtures of 100-μM MB and either human or porcine blood (Hb) injected in a plastisol phantom, using laser wavelengths of 710 and 870 nm. An algorithm to perform linear regression of the acoustic frequency response from an atlas composed of pure concentrations was designed to assess the concentration levels from photoacoustic samples obtained from 11 known MB/Hb volume mixtures. The mean absolute error (MAE), coefficient of determination (i.e., R2), and Spearman's correlation coefficient (i.e., ρ) between the estimated results and ground-truth labels were calculated to assess the algorithm performance, linearity, and monotonicity, respectively.

RESULTS

The overall MAE, R2, and ρ were 12.68%, 0.80, and 0.89, respectively, for the human Hb dataset and 9.92%, 0.86, and 0.93, respectively, for the porcine Hb dataset. In addition, a similarly linear relationship was observed between the acoustic frequency response at 2.3 MHz and 870-nm laser wavelength and the ground-truth concentrations, with R2 and | ρ | values of 0.76 and 0.88, respectively.

CONCLUSIONS

Contrast agent concentration monitoring is feasible with the proposed approach. The potential for minimal data acquisition times with only two wavelength emissions is advantageous toward real-time implementation in the operating room.

Multi-segmented feature coupling for jointly reconstructing initial pressure and speed of sound in photoacoustic computed tomography

SIGNIFICANCE

Photoacoustic computed tomography (PACT) is a fast-growing imaging modality. In PACT, the image quality is degraded due to the unknown distribution of the speed of sound (SoS). Emerging initial pressure (IP) and SoS joint-reconstruction methods promise reduced artifacts in PACT. However, previous joint-reconstruction methods have some deficiencies. A more effective method has promising prospects in preclinical applications.

AIM

We propose a multi-segmented feature coupling (MSFC) method for SoS-IP joint reconstruction in PACT.

APPROACH

In the proposed method, the ultrasound detectors were divided into multiple sub-arrays with each sub-array and its opposite counterpart considered to be a pair. The delay and sum algorithm was then used to reconstruct two images based on a subarray pair and estimated a direction-specific SoS, based on image correlation and the orientation of the subarrays. Once the data generated by all pairs of subarrays were processed, an image that was optimized in terms of minimal feature splitting in all directions was generated. Further, based on the direction-specific SoS, a model-based method was used to directly reconstruct the SoS distribution.

RESULTS

Both phantom and animal experiments demonstrated feasibility and showed promising results compared with conventional methods, with less splitting and blurring and fewer distortions.

CONCLUSIONS

The developed MSFC method shows promising results for both IP and SoS reconstruction. The MSFC method will help to optimize the image quality of PACT in clinical applications.

Review on photoacoustic imaging of the brain using nanoprobes

Photoacoustic (PA) tomography (PAT) is a hybrid imaging modality that integrates rich optical contrasts with a high-ultrasonic spatial resolution in deep tissue. Among various imaging applications, PA neuroimaging is becoming increasingly important as it nicely complements the limitations of conventional neuroimaging modalities, such as the low-temporal resolution in magnetic resonance imaging and the low depth-to-resolution ratio in optical microscopy/tomography. In addition, the intrinsic hemoglobin contrast PA neuroimaging has also been greatly improved by recent developments in nanoparticles (NPs). For instance, near-infrared absorbing NPs greatly enhanced the vascular contrast in deep-brain PAT; tumor-targeting NPs allowed highly sensitive and highly specific delineation of brain tumors; and multifunctional NPs enabled comprehensive examination of the brain through multimodal imaging. We aim to give an overview of NPs used in PA neuroimaging. Classifications of various NPs used in PAT will be introduced at the beginning, followed by an overview of PA neuroimaging systems, and finally we will discuss major applications of NPs in PA neuroimaging and highlight representative studies.

Optical-resolution photoacoustic microscopy for volumetric and spectral analysis of histological and immunochemical samples

Optical-resolution photoacoustic microscopy (OR-PAM) is an imaging modality with superb penetration depth and excellent absorption contrast. Here we demonstrate, for the first time, that this technique can advance quantitative analysis of conventional chromogenic histochemistry. Because OR-PAM can quantify the absorption contrast at different wavelengths, it is feasible to spectrally resolve the specific biomolecules involved in a staining color. Furthermore, the tomographic capability of OR-PAM allows for noninvasive volumetric imaging of a thick sample without microtoming it. By immunostaining the sample with different chromogenic agents, we further demonstrated the ability of OR-PAM to resolve different types of cells in a coculture sample with imaging depths up to 1 mm. Taken together, the integration of OR-PAM with (immuno)histochemistry offers a simple and versatile technique with broad applications in cell biology, pathology, tissue engineering, and related biomedical studies.

Ultrasound-guided photoacoustic imaging: current state and future development

Photoacoustic imaging, frequently coregistered with ultrasonic imaging, can provide functional and cellular/molecular information about tissue within the anatomical landmarks of an imaged region. This review details the fundamentals of photoacoustic imaging and its most promising imaging applications. Particular attention is paid to photoacoustic imaging's relationship with ultrasound, focusing on distinct differences and similarities between the two modalities and highlighting the mutual benefit of using both concurrently in certain preclinical and clinical applications. Much like its origins as an imaging modality were intertwined with ultrasonic imaging (namely, its acoustic transducers and hardware), the future of photoacoustic imaging-particularly in the clinical arena-similarly depends on ultrasound and its time-tested ability to provide real-time visualization of soft tissue.

In vitro characterization of genetically expressed absorbing proteins using photoacoustic spectroscopy

Genetically expressed fluorescent proteins have been shown to provide photoacoustic contrast. However, they can be limited by low photoacoustic generation efficiency and low optical absorption at red and near infrared wavelengths, thus limiting their usefulness in mammalian small animal models. In addition, many fluorescent proteins exhibit low photostability due to photobleaching and transient absorption effects. In this study, we explore these issues by synthesizing and characterizing a range of commonly used fluorescent proteins (dsRed, mCherry, mNeptune, mRaspberry, AQ143, E2 Crimson) and novel non-fluorescent chromoproteins (aeCP597 and cjBlue and a non-fluorescent mutant of E2 Crimson). The photoacoustic spectra, photoacoustic generation efficiency and photostability of each fluorescent protein and chromoprotein were measured. Compared to the fluorescent proteins, the chromoproteins were found to exhibit higher photoacoustic generation efficiency due to the absence of radiative relaxation and ground state depopulation, and significantly higher photostability. The feasibility of converting an existing fluorescent protein into a non-fluorescent chromoprotein via mutagenesis was also demonstrated. The chromoprotein mutant exhibited greater photoacoustic signal generation efficiency and better agreement between the photoacoustic and the specific extinction coefficient spectra than the original fluorescent protein. Lastly, the genetic expression of a chromoprotein in mammalian cells was demonstrated. This study suggests that chromoproteins may have potential for providing genetically encoded photoacoustic contrast.