The editorial commemorates Leon Esterowitz, founding director of the NSF Biophotonics program, and his pivotal role in advancing biomedical optics research. It reflects on his scientific contributions, visionary support of emerging research, and enduring impact on the researchers and field he helped shape.
SignificanceThe stiffness and viscosity of biological tissue determine how mechanical forces are transmitted and dissipated within it, and both are increasingly used to study tumor growth and treatment response. Measuring these properties within small three-dimensional samples is challenging: it requires shear waves at kilohertz frequencies, which attenuate over short distances, while conventional excitation methods introduce additional signal loss that is difficult to separate from the intrinsic dissipation of the tissue.AimTo enable sub-millimeter viscoelastic characterization of three-dimensional biological constructs in the multikilohertz frequency range using quasi-planar wave excitation and optical detection.ApproachWe developed an optical microelastography platform that combines quasi-planar shear-wave excitation using a piezoelectric actuator, high-speed imaging (up to 210,000 fps), and k-space dispersion analysis to estimate frequency-dependent viscoelastic parameters through a Kelvin–Voigt model. The system was validated using polyacrylamide phantoms with tunable viscoelastic properties at five concentrations (15% to 26% v/v).ResultsValidation across polyacrylamide concentrations yielded effective shear moduli of 0.70 to 19.31 kPa, with a minimum resolvable shear wavelength of ∼110 μm in the polyacrylamide phantoms. Experimental phase-velocity dispersion showed good agreement with finite-element predictions (RMSE<0.16 m/s, mean relative error <3%, R2>0.96). Measurements on four melanoma spheroids (237 to 823 µm diameter) produced effective viscoelastic parameters of μ=0.45 to 0.71 kPa and η=0.033 to 0.097 Pa·s (R2≥0.96), obtained within specimen-specific analysis bands and consistent with values reported for three-dimensional tumor spheroid models.ConclusionsThis platform demonstrates kilohertz-range viscoelastic characterization of three-dimensional biological constructs at sub-millimeter scales, validated in polyacrylamide phantoms and demonstrated in proof-of-concept measurements on four melanoma tumor spheroids of different sizes. The quasi-planar excitation approach reduces geometric spreading artifacts associated with conventional excitation methods and provides a noninvasive optical tool for studying frequency-dependent mechanical behavior in mechanobiology and tissue engineering.
SignificanceNear-infrared (NIR) fluorescence lifetime imaging (FLI) provides molecular and functional contrast that encodes local biochemical and biophysical environments beyond what fluorescence intensity alone can reveal. Realizing this potential across biological imaging and emerging clinical applications requires rigorous fluorophore characterization and a critical assessment of the photophysics, measurement capabilities, and conditions governing reliable lifetime interpretation.AimWe comprehensively survey the contemporary NIR FLI landscape, spanning fluorophore photophysics, imaging instrumentation, lifetime analysis, and biomedical applications, to identify application-specific criteria for fluorophore selection, quantitative measurement, and translational validation.ApproachWithin the NIR range, we critically compare organic dyes, fluorescent proteins, inorganic fluorophores, and endogenous emitters, integrating photophysical evidence with acquisition and analysis methods for multiplexing, Förster resonance energy transfer (FRET), environmental sensing, and clinical imaging.ResultsReported NIR fluorophore lifetimes span tens of picoseconds to over 100 ns and depend on molecular structure, solvent, binding state, and aggregation. Multiplexing requires reproducible, distinguishable decay signatures; sensing requires selective, calibrated lifetime responses; and FRET requires reliable donor-only references. Uncorrected instrumental effects, tissue photon migration, and inappropriate analysis assumptions can distort lifetime estimates. Incomplete characterization and inconsistent reporting further limit reproducibility and comparison across studies.ConclusionsQuantitative NIR FLI requires precise probe characterization, standardized measurement validation, and application-specific performance assessment. We provide practical recommendations for probe selection, lifetime contrast interpretation, and clinical utility evaluation, establishing a framework for reproducible NIR FLI from mechanistic cell studies to image-guided interventions.
SignificanceContinuous, noninvasive assessment of myocardial ischemia in intact mouse models poses a significant technical challenge. Conventional diffuse optical techniques are mostly limited to ex vivo hearts or open-chest animal models due to severe signal attenuation and tissue interference caused by the complex thoracic anatomy. Empirical placement of photoelectric probes over the cardiothorax further undermines the sensitivity and specificity of cardiac signal detection.AimThis study aims to systematically characterize photon transport dynamics within the mouse thorax, determine the optimal light Source–Detector (S-D) spatial configuration, and quantitatively evaluate the theoretical feasibility of transthoracic diffuse optics for detecting varying degrees of myocardial ischemia.ApproachUsing a high-fidelity 3D voxelized model derived from realistic mouse anatomical data, we performed Monte Carlo photon transport simulations at 800 nm. A high-density virtual array of sources and detectors was deployed on the body surface to determine the optimal S-D positions for cardiac signal extraction. Myocardial ischemia was simulated by modulating the optical coefficients of myocardial tissue, enabling the investigation of the relationship between surface-captured optical signals and ischemia severity.ResultsThe simulations identified an optimized S-D configuration that maximizes the detection sensitivity for deep myocardial signals while minimizing interference from overlying tissues. Notably, under this configuration, the diffuse optical signals collected at the body surface showed a strong linear correlation with the degree of myocardial ischemia.ConclusionsAn optimized S-D configuration was demonstrated for establishing a linear correlation between optical signal variations and ischemic severity. This study provides a theoretical framework toward diffuse optics for in vivo assessment of cardiac hemodynamics and tissue oxygenation in murine models.
SignificanceDeep learning (DL)-based image reconstruction methods for photoacoustic computed tomography (PACT) have developed rapidly in recent years. However, most existing methods have not employed standardized datasets, and their evaluations rely on traditional image quality (IQ) metrics that may lack clinical relevance. The absence of a standardized framework for clinically meaningful IQ assessment hinders fair comparison and raises concerns about the reproducibility and reliability of reported advancements in PACT.AimA benchmarking framework is proposed that provides open-source, anatomically plausible synthetic datasets and evaluation strategies for DL-based acoustic inversion methods in PACT.ApproachThe datasets each include over 10,000 two-dimensional (2D) stochastic breast objects with clinically relevant lesions and paired measurements at varying modeling complexity. The evaluation strategies incorporate both traditional and task-based IQ measures to assess fidelity and clinical utility. A preliminary benchmarking study is conducted to demonstrate the framework’s utility by comparing DL-based and physics-based reconstruction methods.ResultsThe benchmarking study demonstrated that the proposed framework enabled comprehensive, quantitative comparisons of reconstruction performance and revealed important limitations in certain DL-based methods. Although they performed well according to traditional IQ measures, they often failed to accurately recover lesions. This highlights the inadequacy of traditional metrics and motivates the need for task-based assessments.ConclusionsThe proposed benchmarking framework enables systematic comparisons of DL-based acoustic inversion methods for 2D PACT. By integrating clinically relevant synthetic datasets with rigorous evaluation protocols, it enables reproducible, objective assessments and facilitates method development and system optimization in PACT.
