Corneal laser refractive surgery is a method for correcting vision using lasers to reshape the cornea and change its curvature and thickness. Femtosecond laser corneal cutting is widely used in ophthalmic refractive surgery as a precise, minimally invasive, and controllable surgical technique. In femtosecond laser refractive surgery, the numerical aperture of the optical system determines the focal spot size and required single-pulse energy, which are critical parameters that influence the corneal cutting quality. In this study, we built a femtosecond laser surgery system with an adjustable numerical aperture. We investigated the effect of numerical aperture on cutting quality in the corneal stroma by analyzing the differences in bubble morphology, smoothness of flap separation, and proportion of damaged stromal cells. This study aimed to assist clinicians in selecting the appropriate surgical parameters more effectively.

Freshly enucleated pig eyeballs and New Zealand white rabbits were selected as experimental subjects. By adjusting the diameter of the incident beam, corneal flaps were formed on the pig eyeballs using a femtosecond laser with numerical aperture values of 0.16, 0.30, and 0.80. The morphology of the bubbles after cutting was recorded, and the smoothness of the separation was observed when the corneal flaps were lifted. Cell damage experiments were conducted by cutting New Zealand white rabbit eyeballs with a femtosecond laser at numerical aperture values of 0.16, 0.30, and 0.80. After creating the flap with the femtosecond laser, the rabbit eyeballs were placed in corneal active medium (DX solution) and incubated at 4 ℃ for 6 h to induce apoptosis in the damaged corneal stromal cells. Subsequently, the rabbit eyeballs were removed and prefixed in a 4% paraformaldehyde (PFA) solution for 2 h. After dewaxing and rehydration, the corneal sections were double-stained with DAPI (4,6-diamidino-2-phenylindole, D9542, Sigma-Aldrich) and TUNEL (TdT-mediated dUTP nick end labeling, C1088, Beyotime). Finally, the apoptotic cell counts were determined by imaging the sections under a fluorescence microscope.

Under the three different numerical aperture values (0.16, 0.30, and 0.80), as the numerical aperture increases, the volume of the bubbles decreases gradually, and the density of the bubble layer increases (Figs.5 and 6). This is mainly attributed to the decreasing volume of the focal spot with an increasing numerical aperture, which decreases cavitation bubbles. Corneal flaps formed at a higher numerical aperture are easier to separate. This is primarily because smaller cavitation bubbles result in a denser bubble layers, which facilitates the separation of the interlamellar space with less adhesions between the tissue layers. In the cell damage experiment, as numerical aperture increases, the number of apoptotic cells decreases significantly, as shown in Fig. 7. This is attributed to the decreased single-pulse energy and decreased focal spot size associated with an increase in numerical aperture, which results in smaller photodisruption zones and reduced damage to the surrounding tissues. Therefore, increasing the numerical aperture is beneficial for reducing the extent of stromal cell damage.

The effects of femtosecond laser corneal cutting for different numerical aperture values were investigated experimentally. The morphological differences in cavitation bubbles induced by a femtosecond laser at different numerical aperture values were analyzed, and the ease of separation of the lamellar layers and the extent of cell damage were compared. The results of the experiment show that during femtosecond laser corneal cutting, a higher numerical aperture yields smaller bubbles, denser bubble layers, easier separation of corneal flaps, and lower levels of damage to stromal cells. Therefore, a higher numerical aperture is beneficial in femtosecond laser refractive surgery. Overall, this study provides valuable insights into the effects of numerical aperture on femtosecond laser corneal cutting and highlights the importance of optimizing the numerical aperture to achieve improved treatment outcomes in corneal procedures.

Currently, most commercial optical coherence angiography (OCTA) systems lack a real-time display of en face OCTA images, which makes it difficult for operators to obtain intuitive feedback on data quality and adjust the system quickly and accurately in a single acquisition of OCTA volume data. In the process of dynamic acquisition of OCTA volume data, determining the state changes of the subjects is difficult, resulting in invalid data acquisition. In an experiment on flicker light-induced functional retinal hyperemia, which provides a new perspective for the early screening of human diabetic retinopathy, the continuous collection of multiple groups of three-dimensional data may be invalid because of the poor quality of one group, thereby wasting data processing time. Therefore, a real-time display of the experimental results is required. Although GPU-based OCTA data real-time processing methods have been proposed, the speed of the existing real-time processing methods still needs to be improved to adapt to high-speed scanning OCTA systems.

It is developed on a spectral-domain OCT (SD-OCT) system. Limited by the frame grabber, the maximum acquisition line speed of the system was 120 kHz in the high-bit-depth mode and 250 kHz in the low-bit-depth mode. An optical coherence angiography algorithm based on the inverse signal-to-noise ratio (SNR) and complex-valued decorrelation (ID-OCTA) was used to extract blood signals by adaptive SNR and achieve high-quality angiography. The sum of absolute differences (SAD) algorithm was used to register OCT images, and the retinal OCT images were segmented by a vertical gradient distribution, which is convenient for fast parallel processing on a Graphics Processing Unit (GPU). This study proposes a real-time processing framework based on a GPU (Fig.1), which uses texture memory to realize fast interpolation and filtering calculations and the CUDA stream to mask the time delay of data transmission between the host and GPU. We developed a real-time processing program using C++ and CUDA and a multithread system control program using the C++ and MFC libraries. To compare the guiding effect of the real-time data processing method in this study and the method using only a CPU, two real-time display modes were used for data acquisition: en face OCTA images and cross-sectional OCT images. Moderately experienced operators collected multiple groups of data in these modes within 40 s. Three sets of data were collected continuously in 12 s to simulate the dynamic acquisition of OCTA volume data. The quality of the collected data was evaluated using the en face OCTA image quality index. In the flicker light-induced functional retinal hyperemia experiment in mice, the experimental success criteria and quantification parameters were set. Operators conducted multiple experiments to compare the experimental success rates of the two real-time display modes.

The en face OCTA image real-time display was realized in the system with a 250 kHz line scanning speed (Fig.2), and the line-processing rate was 365 kHz (Table 1). Compared with the real-time display of the cross-sectional OCT image, the real-time en face OCTA image can guide system refraction and eye position adjustment more accurately and quickly (Fig.3, Table 2). In dynamic OCTA acquisition, the real-time display of en face OCTA images can reflect the movement of the mouse eye and its jitter, which is not evident in cross-sectional OCT images (Fig.4). In an experiment on functional retinal hyperemia, the real-time display video generated immediately after the experiment (Fig.6) can be used as a preview of the experimental results. Compared with 66.7% in the cross-sectional OCT image real-time display mode, the experimental success rate of the en face OCTA image real-time display mode was 93.3%, which proves that this mode helps avoid the situation where system adjustment and subject status problems lead to experimental failure (Table 3, Fig.5). The system can help the experimenter screen unqualified data and quickly judge the experimental results. In the future, the system could replace the frame grabber that supports a higher acquisition speed to improve its scanning speed.

We realized the real-time display of en face OCTA images in a 250 kHz SD-OCT system. Compared to the real-time display of cross-sectional OCT images using a CPU, the real-time display method in this study helps the operator adjust the system more quickly and accurately during the single acquisition of OCTA volume data and provides feedback on the subject eye state during the dynamic acquisition of OCTA volume data. The proposed real-time display method was confirmed to have a data quality feedback function in the experiment of flicker light-induced functional retinal hyperemia, which improved the experimental success rate. The line processing rate reaches 365 kHz, which can be adapted to a high-speed scanning OCTA system.

Skin diseases are common human conditions, and their detection and diagnosis are necessary. Because of the influence of doctors' subjectivity and skin trauma, traditional detection methods are inadequate for accurate and effective diagnosis of skin diseases. Therefore, skin imaging techniques are gradually being used for diagnosis. The photoacoustic imaging technique is an emerging imaging method that combines the high contrast of optical imaging with the deep imaging advantages of ultrasound imaging. Photoacoustic images provide structural and functional information to assist doctors in diagnosing diseases and to improve the accuracy of assessment and treatment. Photoacoustic skin imaging technology can satisfy imaging requirements of different hardware configurations, and its variety of hardware forms ensures that the technology can achieve microscopic and macroscopic imaging, with the potential to respond to diverse clinical needs. By using this technology, melanin and hemoglobin can be detected when capturing images of melanin particles and microvessels at different depths in the palm of the human hand, which can be used for diagnosing pigmented and vascular skin diseases. Photoacoustic skin imaging provides high-resolution images of all skin layers, which is crucial for the early diagnosis and evaluation of skin diseases. Therefore, this paper reviews the systematic classification of existing photoacoustic skin imaging modalities and the performance enhancement methods of image reconstruction algorithms, describes several applications of photoacoustic imaging in clinical human research, and analyzes the advantages and clinical potential of photoacoustic skin imaging as an emerging imaging technology. Thus, readers can gain a detailed and comprehensive understanding of photoacoustic skin imaging technology.

This paper reviews and summarizes the photoacoustic skin imaging technology. First, photoacoustic skin systems are classified based on the imaging modality. Existing photoacoustic skin imaging systems are divided into photoacoustic microscopic and other photoacoustic skin imaging systems. The latter includes photoacoustic tomography and ultrasound/photoacoustic multimodal-imaging systems. This article summarizes research with superior performance in terms of imaging principles, resolution, imaging depth, scanning modes, and other hardware specifications. The corresponding system components are outlined. Subsequently, the research progress in photoacoustic microscopic skin imaging systems and other photoacoustic skin imaging systems is summarized in terms of comparison of the overall system performance.

In studies on photoacoustic microscopy imaging systems, significant progress has been achieved in improving photoacoustic dermoscopy systems, in-vivo skin microimaging, and multiscale skin microimaging, resulting in advancements in system performance (Table 1). Moreover, for other types of photoacoustic skin imaging systems, studies have focused on various aspects, such as photoacoustic tomography of subcutaneous blood vessels in the extremities, three-dimensional photoacoustic tomography, diode laser-based skin tomography systems, and ultrasound/photoacoustic multimodal imaging systems. These investigations lead to noteworthy research outcomes and enhancements in the hardware performance of skin imaging systems (Table 2). In addition, the inclusion of commercial skin imaging systems in the listings validates the practical application value of photoacoustic skin imaging systems.

This paper summarizes existing methods and strategies for enhancing the performance of photoacoustic imaging systems, with a focus on advancements in reconstruction algorithms. The analysis categorizes and discusses these methods based on three main aspects: imaging resolution enhancement algorithms, imaging depth enhancement algorithms, and noise removal algorithms. Each category is analyzed chronologically, starting with an overview of conventional and pivotal performance enhancement algorithms. Subsequently, the discussion encompasses performance enhancement algorithms that integrate deep-learning techniques. Finally, existing specialized algorithms are discussed.

This paper summarizes research on the clinical application of photoacoustic skin imaging technology, classifies skin diseases into two categories (skin cancer and other skin diseases), and summarizes the photoacoustic skin imaging methods for detecting typical diseases. On the one hand, the research teams are currently focusing on detecting melanin and collagen content in the detection and investigation of skin cancer. On the other hand, the imaging of blood vessel shape and distribution pattern can be used as a criterion to determine whether the detected area is diseased and to identify the lesion boundary. In this paper, other skin diseases are classified as inflammatory, vascular, or pigmented according to their causative factors. The pathological features of the diseases and detection methods based on photoacoustic skin imaging technology are described using typical diseases as examples. A summary of the clinical applications of this technology for diverse skin diseases demonstrates its unique advantages and potential for clinical applications.

The concluding section of this article highlights the prevailing challenges of photoacoustic skin imaging and outlines the corresponding research directions aimed at addressing these issues. These challenges include resolving the issue of dynamic changes in clinical data, advancing multimodal imaging capabilities, developing user-friendly imaging devices, and establishing standardized imaging protocols and data analysis techniques.

Photoacoustic skin imaging is an emerging technique with several advantages, such as excellent imaging quality, cost-effectiveness, tissue safety, and promising clinical potential. Photoacoustic imaging is anticipated to become widely used in the future for clinical skin examinations. Further exploration and development of photoacoustic skin imaging technology are required to advance its clinical applications and facilitate its integration into medical practice.

Since 2020, breast cancer has emerged as the most prevalent cancer globally and a leading cause of cancer-related deaths among women. Affected by various genetic or environmental carcinogenic factors, breast cells undergo irreversible gene mutations, initiating the uncontrolled proliferation of malignant cells that crowd into clusters to form breast tumors. The in-situ tumors induce local tissue hypoxia in their internal and surrounding areas, leading to vascular hyperplasia, which propels the growth of cancer cells and their invasion into normal tissues.

Medical imaging is the primary tool for breast cancer screening, diagnosis, and treatment assessment. Early screening plays important roles in reducing mortality; accurate diagnosis is essential for effective treatment; and treatment assessment is critical to provide timely feedback and prognosis of cancer responses. Conventional imaging methods for breast cancer, such as mammography, ultrasonography, and magnetic resonance imaging, though widely used in clinics, exhibit limitations including low diagnostic specificity, slow imaging speed, ionizing radiation, or the need of contrast agent injection. For instance, more than 75% of patients receive benign biopsy results after ultrasound diagnosis. Furthermore, current imaging modalities lack the capacity to provide real-time monitoring, evaluation, and prognosis of the cancer responses during neoadjuvant therapy. New imaging modalities with complementary advantages are crucial to address the evolving clinical demands.

Photoacoustic imaging (PAI) is an emerging technology in the biomedical imaging field and has garnered significant attention owing to its exceptional performance. In addition to its high imaging speed, high spatiotemporal resolution, ionizing-free radiation, and abundant penetration, PAI can provide rich functional optical contrast to reveal physiological characteristics of the tumor microenvironment underneath the skin.

Multiple research groups in the PAI field have achieved notable technical breakthroughs for breast cancer screening, diagnosis, and treatment assessment. Regarding early screening, advanced PAI devices have been developed based on customized ultrasonic arrays. These devices aim to detect physiological characteristics such as vascular proliferation, increased hemoglobin concentration, and abnormal blood oxygen saturation in breast tumor areas through entire breast scanning. Some teams have explored the integration of PAI and ultrasonography, utilizing the complementary anatomical information. As concerns breast tumor diagnosis, numerous clinical studies have demonstrated that physiological characteristics in the microenvironment of a tumor can improve the distinction between benign and malignant breast tumors, facilitating accurate BI-RADS classification and reducing the chance of benign biopsy. The high imaging contrast of PAI also enables the guidance of breast sentinel lymph node biopsy with better clearance. While the combination of PAI with exogenous contrast agents and molecular probes is still in the preclinical stage, it holds the potential for more specific diagnosis in future. Regarding treatment assessment, PAI proves efficient and safe in recording physiological dynamics of the cancer microenvironment in response to therapy, offering crucial prognostic information and seamless feedback to the treatment. In addition, the label-free nature of ultraviolet PAI also provides H&E-like images without the need for staining, exhibiting early promise for accurate and rapid detection of tumor margins intraoperatively.

Regardless of the numerous advantages and multiple niche applications, PAI still faces several challenges to achieve wide clinical usage. First, the spread of PAI technologies depends on the established standards of system design, operation, and data processing to reduce the significant performance disparities among devices developed by different teams. Second, several feasibility studies have been conducted in the PAI field but large-scale clinical studies are still lacking. The PAI indicators revealed from breast cancer images have not been systematically documented or incorporated into clinical practice. Third, a gap still exists between the technical teams and clinical needs. For instance, while three-dimensional PAI exhibits better image clarity for lesion measurement, clinical practices and diagnostic analyses still heavily rely on real-time two-dimensional sectional imaging. Accordingly, to further establish its clinical value, PAI researchers need to evolve from scattered and small-scale feasibility studies to large-scale clinical trials addressing fundamental medical questions. This involves improving existing diagnostic and treatment methods and ultimately integrating them into the existing clinical framework.

Nickel-based single-crystal superalloys are widely used in the manufacture of single-crystal turbine blades in the combustion chambers of aircraft engines owing to their excellent high-temperature mechanical properties. However, these components often suffer from severe damage, such as edge erosion, cracking, and pitting, owing to the harsh operating conditions, including high temperatures and pressures, requiring repairs to extend their service life. Laser additive manufacturing technology has garnered significant attention for repairing nickel-based single-crystal superalloys owing to its unique advantages, including controllable heat input, the ability to fabricate complex structures, and reparability. However, the appearance of grain defects during the repair process may lead to serious failure phenomena, such as crack propagation and component fracture, during component operation, thereby posing potential risks to the safety and reliability of aircraft engines. Therefore, this study selected a DD6 second-generation nickel-based single-crystal superalloy as the substrate material and used lasers with powers of 1200 and 1500 W to re-melt the substrate, revealing the mechanisms of stray grain formation at the fusion line, top of the molten pool, and at the intersection of the dendrites, thereby providing a theoretical basis and technical support for the laser additive repair of nickel-based single-crystal superalloys.

This study employed a DD6 nickel-based single-crystal high-temperature alloy prepared via directional solidification as an experimental substrate material. Subsequently, a laser cladding additive manufacturing device equipped with a 2 kW German Rofin fiber-coupled semiconductor laser was utilized. The process parameters included laser powers of 1200 and 1500 W, a scanning rate of 3 mm/s, and a spot diameter of 4 mm for the single-track remelting experiments on the substrate (001) crystal surface. To prevent sample oxidation during remelting, argon gas was used as a protective gas at a flow rate of 10 L/min. The dendritic morphologies of the substrate and remelted region were observed using an OLYMPUS DSX510 optical microscope. Simultaneously, scanning electron microscopy and X-ray energy-dispersive spectroscopy were employed for detailed analysis of the molten pool morphology and elemental distribution. Electron backscatter diffraction (EBSD) analysis was performed to further investigate the crystallographic properties of the specimens. The sample tilt angle was set to 70°, with a scan step size of 3 μm, using nickel as the calibration phase. HKL Channel 5 post-processing software was employed for texture and orientation deviation analysis of the samples.

The results show that, after laser remelting, the molten pool could be divided into four regions ([001], [100], [010], and [01¯0]) based on the growth direction of the grains (Fig.3), and the primary dendrite spacing increased with increasing laser power. From the crystallographic texture characteristics of the molten pool (Fig.5), it is evident that carbides mainly occurred at the fusion line, top of the molten pool, and intersection of the turning dendrites. Carbides at the fusion line occurred owing to the local collapse of the rough growth interface, causing a deviation in the crystal growth direction. This also increased the diffusion rate of the solute atoms and enhanced the degree of undercooling at the solidification interface, providing heterogeneous nucleation sites for grain nucleation, thereby inducing carbide formation. The carbides at the top of the molten pool were caused by the columnar-to-equiaxed transition, where the numerical simulation showed that the temperature gradient gradually decreased and the solidification rate increased from the bottom to the top of the molten pool (Fig.11), promoting the columnar-to-equiaxed transition. Carbides at the intersection of turning dendrites occur because of collisions resulting from the lower temperature gradient at this location compared with adjacent areas and changes in the direction of the temperature gradient. The thermal stress numerical simulation results (Fig.12) indicate that a low laser power input effectively increases the temperature gradient and reduces the residual stress level, which is beneficial for inhibiting carbide formation in single-crystal repairs.

This article utilized two distinct laser powers, 1200 and 1500 W, to conduct single-track laser remelting on DD6 nickel-based single-crystal superalloys, in conjunction with numerical simulations, to investigate the mechanisms behind stray grain formation in various regions within the molten pool. The principal findings are as follows:

1) Stray grains predominantly emerged in three areas: the fusion line of the molten pool, top of the molten pool, and convergence point of diverging dendrites.

2) At a laser power of 1500 W, there was an increased quantity and larger size of the stray grains. No stray grains were observed at the smooth interface of the fusion line, whereas carbides formed at rough interfaces could lead to the collapse of the solid-liquid interface, causing a deviation in the dendritic growth direction and inducing stray grain formation.

3) The development of stray grains at the apex of the molten pool was closely related to the temperature gradient of the molten pool and movement rate of the solid-liquid solidification interface. The minimal temperature gradient at the summit of the molten pool enhanced the likelihood of a columnar-to-equiaxed transition, fostering the generation of stray grains. Furthermore, it was discovered that a reduced laser power could increase the temperature gradient and suppress the formation of stray grains.

4) The genesis of stray grains at the confluence of diverging dendrites was attributed to collisions caused by the temperature gradient at the dendrite junction, which was lower than that in the surrounding areas, coupled with a shift in the direction of the temperature gradient.