Frequency-domain photoacoustic microscopy with resonant transducer and interferometric modulation for high-reliability sO2 imaging

Frequency-domain photoacoustic microscopy (FD-PAM) could leverage a high-Q resonant ultrasound transducer to suppress out-of-band noise and lower the noise-equivalent pressure (NEP) for truly simultaneous multi-wavelength functional imaging. In practice, however, conventional continuous-wave direct modulation makes it difficult to stay within the narrow resonance, leading most implementations to rely on low-Q broadband detectors instead. To obtain accurate and stable hemoglobin oxygen saturation (sO₂) measurements, we implement an acousto-optic modulator (AOM)-based interferometric modulation scheme in FD-PAM, enabling the practical use of a high-Q resonant ultrasound transducer while preserving truly simultaneous dual-wavelength excitation. FD-PAM with interferometric modulation operates robustly within the narrow bandwidth of the high-Q transducer and supports clean spectral separation (SSIM = 0.97 between frequency-separated channels), as well as quantitative sO₂ estimation with < 2% within-group variation across oxygenated, mixed, and deoxygenated conditions. We anticipate that this approach will be useful for monitoring drug-induced changes in hemoglobin oxygen saturation and quantitatively evaluating therapeutic response.