Functional Photoacoustic and Ultrasonic Assessment of Osteoporosis: A Clinical Feasibility Study

Simultaneous photoacoustic and ultrasound imaging: A review

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that combines the advantages of optical and ultrasound imaging, enabling the generation of images with both optical resolution and acoustic penetration depth. By leveraging similar signal acquisition and processing methods, the integration of photoacoustic and ultrasound imaging has introduced a novel hybrid imaging modality suitable for clinical applications. Photoacoustic-ultrasound imaging allows for non-invasive, high-resolution, and deep-penetrating imaging, providing a wealth of image information. In recent years, with the deepening research and the expanding biomedical application scenarios of photoacoustic-ultrasound bimodal systems, the immense potential of photoacoustic-ultrasound bimodal imaging in basic research and clinical applications has been demonstrated, with some research achievements already commercialized. In this review, we introduce the principles, technical advantages, and biomedical applications of photoacoustic-ultrasound bimodal imaging techniques, specifically focusing on tomographic, microscopic, and endoscopic imaging modalities. Furthermore, we discuss the future directions of photoacoustic-ultrasound bimodal imaging technology.

Development of a semi-anthropomorphic photoacoustic calcaneus phantom based on nano computed tomography and stereolithography 3D printing

Osteoporosis is a major public health threat with significant physical, psychosocial, and financial consequences. The calcaneus bone has been used as a measurement site for risk prediction of osteoporosis by noninvasive quantitative ultrasound (QUS). By adding optical contrast to QUS, our previous studies indicate that a combination of photoacoustic (PA) and QUS, that is, PAQUS, provides a novel opportunity to assess the health of human calcaneus. Calibration of the PAQUS system is crucial to realize quantitative and repeatable measurements of the calcaneus. Therefore, a phantom which simulates the optical, ultrasound, and architectural properties of the human calcaneus, for PAQUS system calibration, is required. Additionally, a controllable phantom offers researchers a versatile framework for developing versatile structures, allowing more controlled assessment of how varying bone structures cause defined alterations in PA and QUS signals. In this work, we present the first semi-anthropomorphic calcaneus phantom for PAQUS. The phantom was developed based on nano computed-tomography (nano-CT) and stereolithography 3D printing, aiming to maximize accuracy in the approximation of both trabecular and cortical bone microstructures. Compared with the original digital input calcaneus model from a human cadaveric donor, the printed model achieved accuracies of 71.15% in total structure and 87.21% in bone volume fraction. Inorganic materials including synthetic blood, mineral oil, intralipid, and agar gel were used to model the substitutes of bone marrow and soft tissue, filling and covering the calcaneus phantom. The ultrasound and optical properties of this phantom were measured, and the results were consistent with those measured by a commercialized device and from previous in vivo studies. In addition, a short-term stability test was conducted for this phantom, demonstrating that the optical and ultrasound properties of the phantom were stable without significant variation over 1 month. This semi-anthropomorphic calcaneus phantom shows structural, ultrasound, and optical properties similar to those from a human calcaneus in vivo and, thereby, can serve as an effective source for equipment calibration and the comprehensive study of human patients.

On the imaging depth limit of photoacoustic tomography in the visible and first near-infrared windows

It is well known that photoacoustic tomography (PAT) can circumvent the photon scattering problem in optical imaging and achieve high-contrast and high-resolution imaging at centimeter depths. However, after two decades of development, the long-standing question of the imaging depth limit of PAT in biological tissues remains unclear. Here we propose a numerical framework for evaluating the imaging depth limit of PAT in the visible and the first near-infrared windows. The established framework simulates the physical process of PAT and consists of seven modules, including tissue modelling, photon transportation, photon to ultrasound conversion, sound field propagation, signal reception, image reconstruction, and imaging depth evaluation. The framework can simulate the imaging depth limits in general tissues, such as the human breast, the human abdomen-liver tissues, and the rodent whole body and provide accurate evaluation results. The study elucidates the fundamental imaging depth limit of PAT in biological tissues and can provide useful guidance for practical experiments.

Theoretical and experimental study of attenuation in cancellous bone

Significance

Photoacoustic (PA) technology shows great potential for bone assessment. However, the PA signals in cancellous bone are complex due to its complex composition and porous structure, making such signals challenging to apply directly in bone analysis.

Aim

We introduce a photoacoustic differential attenuation spectrum (PA-DAS) method to separate the contribution of the acoustic propagation path to the PA signal from that of the source, and theoretically and experimentally investigate the propagation attenuation characteristics of cancellous bone.

Approach

We modified Biot's theory by accounting for the high frequency and viscosity. In parallel with the rabbit osteoporosis model, we build an experimental PA-DAS system featuring an eccentric excitation differential detection mechanism. Moreover, we extract a PA-DAS quantization parameter-slope-to quantify the attenuation of high- and low-frequency components.

Results

The results show that the porosity of cancellous bone can be evaluated by fast longitude wave attenuation at different frequencies and the PA-DAS slope of the osteoporotic group is significantly lower compared with the normal group (**p < 0.01 ).

Conclusions

Findings demonstrate that PA-DAS effectively differentiates osteoporotic bone from healthy bone, facilitating quantitative assessment of bone mineral density, and osteoporosis diagnosis.

Spatially-variant image deconvolution for photoacoustic tomography

Photoacoustic tomography (PAT) system can reconstruct images of biological tissues with high resolution and contrast. However, in practice, the PAT images are usually degraded by spatially variant blur and streak artifacts due to the non-ideal imaging conditions and chosen reconstruction algorithms. Therefore, in this paper, we propose a two-phase restoration method to progressively improve the image quality. In the first phase, we design a precise device and measuring method to obtain spatially variant point spread function samples at preset positions of the PAT system in image domain, then we adopt principal component analysis and radial basis function interpolation to model the entire spatially variant point spread function. Afterwards, we propose a sparse logarithmic gradient regularized Richardson-Lucy (SLG-RL) algorithm to deblur the reconstructed PAT images. In the second phase, we present a novel method called deringing which is also based on SLG-RL to remove the streak artifacts. Finally, we evaluate our method with simulation, phantom and in vivo experiments, respectively. All the results show that our method can significantly improve the quality of PAT images.

Photoacoustic Imaging and Characterization of Bone in Medicine: Overview, Applications, and Outlook

Photoacoustic techniques have shown promise in identifying molecular changes in bone tissue and visualizing tissue microstructure. This capability represents significant advantages over gold standards (i.e., dual-energy X-ray absorptiometry) for bone evaluation without requiring ionizing radiation. Instead, photoacoustic imaging uses light to penetrate through bone, followed by acoustic pressure generation, resulting in highly sensitive optical absorption contrast in deep biological tissues. This review covers multiple bone-related photoacoustic imaging contributions to clinical applications, spanning bone cancer, joint pathologies, spinal disorders, osteoporosis, bone-related surgical guidance, consolidation monitoring, and transsphenoidal and transcranial imaging. We also present a summary of photoacoustic-based techniques for characterizing biomechanical properties of bone, including temperature, guided waves, spectral parameters, and spectroscopy. We conclude with a future outlook based on the current state of technological developments, recent achievements, and possible new directions.

Biomechanical structure-function relations for human trabecular bone - comparison of calcaneus, femoral neck, greater trochanter, proximal tibia, and vertebra

MicroCT-based finite element models were used to compute power law relations for uniaxial compressive yield stress versus bone volume fraction for 78 cores of human trabecular bone from five anatomic sites. The leading coefficient of the power law for calcaneus differed from those for most of the other sites (p < 0.05). However, after normalizing by site-specific mean values, neither the leading coefficient (p > 0.5) nor exponent (p > 0.5) differed among sites, suggesting that a given percentage deviation from mean bone volume fraction has the same mechanical consequence for all sites investigated. These findings help explain the success of calcaneal x-ray and ultrasound measurements for predicting hip fracture risk.

Nakagami statistics-based photoacoustic spectroscopy used for label-free assessment of bone tissue

Quick identification of abnormal molecular metabolism of bone tissues is challenging. Photoacoustic (PA) spectroscopy techniques have great potential in molecular imaging. However, most of them are amplitude-dependent and easily affected by the light deposition, especially for bone tissues with high optical scattering. In this Letter, we propose a Nakagami statistics-based PA spectroscopy (NSPS) method for characterizing molecules in bone tissues. We indicate that the NSPS curve can intelligently identify changes in the content of molecules in bone tissues, with a high disturbance-resisting ability. The NSPS has remarkable potential for use in the early and rapid detection of bone diseases.

Extracellular vesicles as a novel photosensitive drug delivery system for enhanced photodynamic therapy

Photodynamic therapy (PDT) is a promising non-invasive therapeutic approach that utilizes photosensitizers (PSs) to generate highly reactive oxygen species (ROS), including singlet oxygen, for removal of targeted cells. PDT has been proven efficacious for the treatment of several diseases, including cancer, cardiovascular disease, inflammatory bowel disease, and diabetic ocular disease. However, the therapeutic efficacy of PDT is limited and often accompanied by side effects, largely due to non-specific delivery of PSs beyond the desired lesion site. Over the past decade, despite various nanoparticular drug delivery systems developed have markedly improved the treatment efficacy while reducing the off-target effects of PSs, concerns over the safety and toxicity of synthetic nanomaterials following intravenous administration are raised. Extracellular vesicles (EVs), a type of nanoparticle released from cells, are emerging as a natural drug delivery system for PSs in light of EV's potentially low immunogenicity and biocompatibility compared with other nanoparticles. This review aims to provide an overview of the research progress in PS delivery systems and propose EVs as an alternative PS delivery system for PDT. Moreover, the challenges and future perspectives of EVs for PS delivery are discussed.

Photoacoustic characterization of bone physico-chemical information

Osteoporosis usually alters the chemical composition and physical microstructure of bone. Currently, most clinical techniques for bone assessment are focused on the either bone microstructure or bone mineral density (BMD). In this study, a novel multi-wavelength photoacoustic time-frequency spectral analysis (MWPA-TFSA) method was introduced based on the optical absorption spectra and photoacoustic effects of biological macromolecules, which evaluates changes in bone chemical composition and microstructure. The results demonstrated that osteoporotic bones had decreased BMD, more lipids, and wider trabecular separation filled with larger marrow clusters, which were consistent with multiple gold-standard results, suggesting that the MWPA-TFSA method has the potential to provide a thorough bone physico-chemical information evaluation noninvasively and nonradiatively.

Characterization of multi-biomarkers for bone health assessment based on photoacoustic physicochemical analysis method

Photoacoustic (PA) techniques are potential alternatives to histopathology. The physicochemical spectrogram (PCS) generated by the PA measurement at multiple wavelengths can presents the morphology and chemical composition target at multi-biomarkers simultaneously. In this work, via multi-wavelength PA measurements performed on rabbit bone models, we investigated the feasibility of using PCSs for bone health assessment. A comprehensive analysis of the PCSs, termed PA physicochemical analysis (PAPCA), was conducted. The "slope" and "relative content" were used as the PAPCA-quantified parameters to characterize the changes in the physical and chemical properties of bone tissue, respectively. The findings are consistent well with the gold-standard imaging results. It demonstrated that the PAPCA can be used to characterize both the microstructure and content of multi-biomarkers which highly related with bone health. Considering the PA technique is noninvasive and radiation-free, it has great potential in the implementation and monitoring of bone diseases progression.

Detection of collagen by multi-wavelength photoacoustic analysis as a biomarker for bone health assessment

Collagen is an important biomarker of osteoporosis progression. Noninvasive, multispectral, photoacoustic (PA) techniques use pulsed laser light to induce PA signals to facilitate the visualization of chemical components that are strongly related to tissue health. In this study, the feasibility of multi-wavelength PA (MWPA) measurement of the collagen in bone, using the wavelength range of 1300-1800 nm, was investigated. First, the feasibility of this approach for detecting the collagen content of bone was demonstrated by means of numerical simulation. Then, ex vivo experiments were conducted on both animal and human bone specimens with different bone densities using the MWPA method. The relative collagen content was extracted and compared with the results of micro-computed tomography (micro-CT) and histology. The results showed that the "relative collagen content" parameter obtained using the MWPA approach correlated well with the bone volume ratio obtained from micro-CT images and histological analysis results. This study highlights the potential of the proposed PA technique for determining the collagen content of bones as a biomarker for bone health assessment.

The feasibility study of the transmission mode photoacoustic measurement of human calcaneus bone in vivo

The photoacoustic (PA) technique is uniquely positioned for biomedical applications primarily due to its ability to visualize optical absorption contrast in deep tissue at ultrasound resolution. In this work, via both three-dimensional (3D) numerical simulations and in vivo experiments on human subjects, we investigated the possibility of PA measurement of human calcaneus bones in vivo in a non-invasive manner, as well as its feasibility to differentiate osteoporosis patients from normal subjects. The results from the simulations and the experiments both demonstrated that, when one side of the heel is illuminated by laser with light fluence under the ANSI safety limit, the PA signal generated in the human calcaneus bone can be detected by an ultrasonic transducer at the other side of the heel (i.e. transmission mode). Quantitative power spectral analyses of the calcaneus bone PA signals were also conducted, demonstrating that the microarchitectural changes in calcaneus bone due to osteoporosis can be detected, as reflected by enhanced high frequency components in detected PA bone signal. Further statistical analysis of the experimental results from 10 osteoporosis patients and 10 healthy volunteers showed that the weighted frequency as a quantified PA spectral parameter can differentiate the two subject groups with statistical significance.

Wavelet transform-based photoacoustic time-frequency spectral analysis for bone assessment

In this study, we investigated the feasibility of using photoacoustic time-frequency spectral analysis (PA-TFSA) for evaluating the bone mineral density (BMD) and bone structure. Simulations and ex vivo experiments on bone samples with different BMDs and mean trabecular thickness (MTT) were conducted. All photoacoustic signals were processed using the wavelet transform-based PA-TFSA. The power-weighted mean frequency (PWMF) was evaluated to obtain the main frequency component at different times. The y-intercept, midband-fit, and slope of the linearly fitted curve of the PWMF over time were also quantified. The results show that the osteoporotic bone samples with lower BMD and thinner MTT have higher frequency components and lower acoustic frequency attenuation over time, thus higher y-intercept, midband-fit, and slope. The midband-fit and slope were found to be sensitive to the BMD; therefore, both parameters could be used to distinguish between osteoporotic and normal bones (p < 0.05).

Spatial resolution in photoacoustic computed tomography

Photoacoustic computed tomography (PACT) is a novel biomedical imaging modality and has experienced fast developments in the past two decades. Spatial resolution is an important criterion to measure the imaging performance of a PACT system. Here we survey state-of-the-art literature on the spatial resolution of PACT and analyze resolution degradation models from signal generation, propagation, reception, to image reconstruction. Particularly, the impacts of laser pulse duration, acoustic attenuation, acoustic heterogeneity, detector bandwidth, detector aperture, detector view angle, signal sampling, and image reconstruction algorithms are reviewed and discussed. Analytical expressions of point spread functions related to these impacting factors are summarized based on rigorous mathematical formulas. State-of-the-art approaches devoted to enhancing spatial resolution are also reviewed. This work is expected to elucidate the concept of spatial resolution in PACT and inspire novel image quality enhancement techniques.