Figure 1. Photon and ultrasound propagation regimes in soft tissue.

(a) Photon propagation regimes in soft tissue, which are approximately related to the penetration depths of representative optical imaging modalities [5, 28]. The four regimes are divided at photon propagation depths of 0.1 mm, 1 mm, 10 cm and 1 m, with typical optical absorption coefficient of 0.1 cm⁻¹, optical scattering coefficient of 100 cm⁻¹ and anisotropy of 0.9. The classification holds in optical scattering dominant media. (b) Ultrasound propagation regimes at typical ultrasound frequencies in soft tissue, with corresponding biomedical applications [34]. The ultrasound attenuation coefficient is approximately proportional to the ultrasound frequency up to at least 300 MHz for skin [35] and 100 MHz for kidney [36]. Here, the −10 dB propagation depth at the frequency up to 100 MHz is estimated with an ultrasound attenuation coefficient of 0.5 dB/cm/MHz. The propagation depths at 300 MHz for kidney and 1 GHz for skin are extrapolated based on the literature data [35, 36], taking into account the ultrasound attenuation by water.