As photoacoustic imaging (PAI) begins to mature and undergo clinical translation, there is a need for well-validated, standardized performance test methods to support device development, quality control, and regulatory evaluation. Despite recent progress, current PAI phantoms may not adequately replicate tissue light and sound transport over the full range of optical wavelengths and acoustic frequencies employed by reported PAI devices. Here we introduce polyacrylamide (PAA) hydrogel as a candidate material for fabricating stable phantoms with well-characterized optical and acoustic properties that are biologically relevant over a broad range of system design parameters. We evaluated suitability of PAA phantoms for conducting image quality assessment of three PAI systems with substantially different operating parameters including two commercial systems and a custom system. Imaging results indicated that appropriately tuned PAA phantoms are useful tools for assessing and comparing PAI system image quality. These phantoms may also facilitate future standardization of performance test methodology.

. Significance : Photoacoustic imaging (PAI) is a powerful emerging technology with broad clinical applications, but consensus test methods are needed to standardize performance evaluation and accelerate translation. Aim : To review consensus image quality test methods for mature imaging modalities [ultrasound, magnetic resonance imaging (MRI), x-ray CT, and x-ray mammography], identify best practices in phantom design and testing procedures, and compare against current practices in PAI phantom testing. Approach : We reviewed scientific papers, international standards, clinical accreditation guidelines, and professional society recommendations describing medical image quality test methods. Observations are organized by image quality characteristics (IQCs), including spatial resolution, geometric accuracy, imaging depth, uniformity, sensitivity, low-contrast detectability, and artifacts. Results : Consensus documents typically prescribed phantom geometry and material property requirements, as well as specific data acquisition and analysis protocols to optimize test consistency and reproducibility. While these documents considered a wide array of IQCs, reported PAI phantom testing focused heavily on in-plane resolution, depth of visualization, and sensitivity. Understudied IQCs that merit further consideration include out-of-plane resolution, geometric accuracy, uniformity, low-contrast detectability, and co-registration accuracy. Conclusions : Available medical image quality standards provide a blueprint for establishing consensus best practices for photoacoustic image quality assessment and thus hastening PAI technology advancement, translation, and clinical adoption.

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Skin consists of a lamellar structure with diverse cell types (e.g., immune cells, melanocytes, and basal cells) that periodically detach from the basement membrane, move to the surface, and die for self-renewal. [3] Melanocytes are a critical cell type that generate melanin to absorb UV light (290-400 nm), which is a major risk for skin diseases (e.g., melanoma) due to DNA damage. [4][5][6][7] Here, melanin-containing organelles called melanosomes are transferred to the surrounding keratinocytes. This increase in melanosome concentration leads to darker skin phototypes, and darker phototypes can be a function of racial background or previous sun exposure, that is, tanning. [8] Indeed, skin pigmentation depends on variations in the size, number, clustering phase, and the proportions between melanin species (e.g., eumelanin and pheomelanin). [9] Skin pigmentation has been quantified using melanosome volume fraction (M f ) parameter: 1.3-6.3% for lightly pigmented adults, 11-16% for moderately pigmented adults, and 18-43% for darkly pigmented adults. [10] Variations in skin phototypes can complicate biomedical optics. Melanin absorption increases linearly from 800 to 600 nm and exponentially from 600 to 300 nm. [11,12] Darker skin phototypes can absorb and scatter more photons: as a result, incident light is attenuated before it reaches the target of interest, and signal transmission can be impeded back to the sensor. Therefore, variations in skin phototypes have negatively affected many forms of medical optic technology including pulse oximetry, [13,14] cerebral tissue oximeters, [15] optical coherence tomography, [16] wearable electronics, [17][18][19] photoacoustic (PA) imaging, [20] fluorescence imaging, [21] and photothermal therapy. [22] One recent study compared 48 097 pairs of oxygen saturation levels measured by pulse oximetry and arterial blood gas test obtained from 8675 white patients and 1326 black patients. [13] The results found that pulse oximetry had trouble in diagnosing hypoxemia in 11% Black patients and 3% white patients due to light absorption by melanin. [13,14] Furthermore, wearable electronics (e.g., smartwatches) have reported inaccuracies in heart rate readings occurring more often in users with dark skin than light skin. [17,18] Clearly, the impact of differences in skin phototypes underscore the ongoing need to understand and correct racial bias in optical technologies. While larger 3D-bioprinted skin-mimicking phantoms with skin colors ranging across the Fitzpatrick scale are reported. These tools can help understand the impact of skin phototypes on biomedical optics. Synthetic melanin nanoparticles of different sizes (70-500 nm) and clusters are fabricated to mimic the optical behavior of melanosome. The absorption coefficient and reduced scattering coefficient of the phantoms are comparable to real human skin. Further the melanin content and distribution in the phantoms versus real human skins are validated via photoacoustic (PA) imaging. The PA signal of the phantom can be improved ...

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