In vivo photoacoustic molecular imaging of the distribution of serum albumin in rat burned skin

Seeing through the Skin: Photoacoustic Tomography of Skin Vasculature and Beyond

Skin diseases are the most common human diseases and manifest in distinct structural and functional changes to skin tissue components such as basal cells, vasculature, and pigmentation. Although biopsy is the standard practice for skin disease diagnosis, it is not sufficient to provide in vivo status of the skin and highly depends on the timing of diagnosis. Noninvasive imaging technologies that can provide structural and functional tissue information in real time would be invaluable for skin disease diagnosis and treatment evaluation. Among the modern medical imaging technologies, photoacoustic (PA) tomography (PAT) shows great promise as an emerging optical imaging modality with high spatial resolution, high imaging speed, deep penetration depth, rich contrast, and inherent sensitivity to functional and molecular information. Over the last decade, PAT has undergone an explosion in technical development and biomedical applications. Particularly, PAT has attracted increasing attention in skin disease diagnosis, providing structural, functional, metabolic, molecular, and histological information. In this concise review, we introduce the principles and imaging capability of various PA skin imaging technologies. We highlight the representative applications in the past decade with a focus on imaging skin vasculature and melanoma. We also envision the critical technical developments necessary to further accelerate the translation of PAT technologies to fundamental skin research and clinical impacts.

Optoacoustic imaging of the skin

Optoacoustic (OA, photoacoustic) imaging capitalizes on the synergistic combination of light excitation and ultrasound detection to empower biological and clinical investigations with rich optical contrast while effectively bridging the gap between micro and macroscopic imaging realms. State-of-the-art OA embodiments consistently provide images at micron-scale resolution through superficial tissue layers by means of focused illumination that can be smoothly exchanged for acoustic-resolution images at diffuse light depths of several millimetres to centimetres via ultrasound beamforming or tomographic reconstruction. Taken together, this unique multi-scale imaging capacity opens unprecedented capabilities for high-resolution in vivo interrogations of the skin at scalable depths. Moreover, diverse anatomical and functional information is retrieved via dynamic mapping of endogenous chromophores such as haemoglobin, melanin, lipids, collagen, water and others. This, along with the use of non-ionizing radiation, facilitates a clinical translation of the OA modalities. We review recent progress in OA imaging of the skin in preclinical and clinical studies exploiting the rich contrast provided by endogenous substances in tissues. The imaging capabilities of existing approaches are discussed in the context of initial translational studies on skin cancer, inflammatory skin diseases, wounds and other conditions.

In vivo dual-scale photoacoustic surveillance and assessment of burn healing

Accurate diagnoses of superficial and deep dermal burns are difficult to make even by experienced investigators due to slight differences in dermis damage. Many imaging technologies have been developed to improve the burn depth assessment. But these imaging tools have limitations in deep imaging or resolving ability. Photoacoustic imaging is a hybrid modality combining optical and ultrasound imaging that remains high resolution in deep imaging depth. In this work, we used dual-scale photoacoustic imaging to noninvasively diagnose burn injury and monitor the burn healing. Real-time PACT provided cross-sectional and volumetric images of the burn region. High-resolution PAM allowed for imaging of angiogenesis on the hyperemic ring. A long-term surveillance was also performed to assess the difference between the two damage degrees of burn injuries. Our proposed method suggests an effective tool to diagnose and monitor burn injury.

Photoacoustic diagnosis of pharmacokinetics and vascular shutdown effects in photodynamic treatment with indocyanine green-lactosome for a subcutaneous tumor in mice

Indocyanine green lactosome (ICG-lactosome) is an attractive new-generation agent for photodynamic therapy (PDT) that is characterized by a near-infrared excitation wavelength and high stability in the bloodstream. Fluorescence imaging has been used to examine its pharmacokinetics in vivo, but no depth-resolved information can be obtained with this method. In this study, we applied photoacoustic (PA) imaging to visualize the depth distribution of ICG-lactosome in a mouse subcutaneous tumor model. With this method, the depth distribution of blood vessels can also be visualized, enabling detection of vascular shutdown effects due to PDT. We performed PA imaging of both the distributions of ICG-lactosome and blood vessels in a tumor before and after PDT, and we found that PA signals originating from ICG-lactosome were greatly increased at 18 h after drug injection but rapidly decreased after PDT. These results indicate efficient accumulation of ICG-lactosome and rapid photobleaching due to the PDT reaction in the tumor, respectively. After PDT, PA amplitudes of hemoglobin were significantly decreased, being attributable to vascular shutdown effects. These results show the usefulness of PA imaging for monitoring not only photosensitizer accumulation and bleaching but also vascular responses in PDT with ICG-lactosome. This method can be applied to the diagnosis of many types of PDT processes.

Preclinical detection of liver fibrosis using dual-modality photoacoustic/ultrasound system

Liver fibrosis is a major cause for increasing mortality worldwide. Preclinical research using animal models is required for the discovery of new anti-fibrotic therapies, but currently relies on endpoint liver histology. In this study, we investigated a cost-effective and portable photoacoustic/ultrasound (PA/US) imaging system as a potential non-invasive alternative. Fibrosis was induced in mice using CCl₄ followed by liver imaging and histological analysis. Imaging showed significantly increased PA features with higher frequency signals in fibrotic livers versus healthy livers. This corresponds to more heterogeneous liver structure resulting from collagen deposition and angiogenesis. Importantly, PA response and its frequency were highly correlated with histological parameters. These results demonstrate the preclinical feasibility of the PA imaging approach and applicability of dual PA/US system.

Imaging Techniques for Clinical Burn Assessment with a Focus on Multispectral Imaging

Significance: Burn assessments, including extent and severity, are some of the most critical diagnoses in burn care, and many recently developed imaging techniques may have the potential to improve the accuracy of these evaluations. Recent Advances: Optical devices, telemedicine, and high-frequency ultrasound are among the highlights in recent burn imaging advancements. We present another promising technology, multispectral imaging (MSI), which also has the potential to impact current medical practice in burn care, among a variety of other specialties. Critical Issues: At this time, it is still a matter of debate as to why there is no consensus on the use of technology to assist burn assessments in the United States. Fortunately, the availability of techniques does not appear to be a limitation. However, the selection of appropriate imaging technology to augment the provision of burn care can be difficult for clinicians to navigate. There are many technologies available, but a comprehensive review summarizing the tissue characteristics measured by each technology in light of aiding clinicians in selecting the proper device is missing. This would be especially valuable for the nonburn specialists who encounter burn injuries. Future Directions: The questions of when burn assessment devices are useful to the burn team, how the various imaging devices work, and where the various burn imaging technologies fit into the spectrum of burn care will continue to be addressed. Technologies that can image a large surface area quickly, such as thermography or laser speckle imaging, may be suitable for initial burn assessment and triage. In the setting of presurgical planning, ultrasound or optical microscopy techniques, including optical coherence tomography, may prove useful. MSI, which actually has origins in burn care, may ultimately meet a high number of requirements for burn assessment in routine clinical use.