Recent trends in bioacoustophotonics and biothermophotonics of tissues are presented. The presentation is centered on the development of well-known frequency-domain photothermal and photoacoustic techniques to address issues associated with diffuse photon density waves during optical excitation of turbid media, both in hard tissues (teeth) and soft tissues. These methods have concrete advantages over the conventional pulsed-laser counterparts. In Part I we present biothermophotonic principles and applications to the detection of the carious state in human teeth as embodied
by laser photothermal radiometry supported by modulated luminescence. The emphasis is on the abilities of these techniques to approach important problems such as the diagnosis of occlusal pits and fissures and interproximal lesions between teeth which normally go undetected by x-ray radiographs. In Part II we present theoretical and experimental results in frequency-domain bioacoustophotonics of turbid media, such as soft tissues, and we describe the development of sensitive sub-surface imaging methodologies which hold the promise for sensitive diagnostics of cancerous lesions in e.g. a human breast. Results using tissue phantoms and ex-vivo specimens are discussed and the current level of subsurface lesion sensitivity compared to state-of-the-art pulsed photoacoustic techniques is examined. In summary, advances in coupled frequency-domain diffuse-photon-density-wave and thermal or thermoelastic responses of turbid media constitute new trends in bioacoustophotonics and biothermophotonics promising for their signal quality and high dynamic range.
Frequency-domain correlation and spectral analysis photothermoacoustic (FD-PTA) imaging is a promising new technique, which is being developed to detect tumor masses in turbid biological tissue. Unlike conventional biomedical photoacoustics which uses time-of-flight acoustic information induced by a pulsed laser to indicate the tumor size and location, in this research, a new FD-PTA instrument featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength is constructed and tested for its depth profilometric capabilities in turbid media imaging. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser-fluence modulation frequencies with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. A report on two-dimensional spatial scans, performed on tissue mimicking control phantoms with various optical, acoustical and geometrical properties will be presented. Combining with the depth information carried by the back-propagated chirp signal at each scanning position, one could rapidly generate sub-surface three-dimensional images of the scanning area, a combination of tasks that is difficult or impossible by use of pulsed photoacoustic detection. It is concluded that frequency domain photothermoacoustics using a linear frequency sweep method and heterodyne lock-in detection has the potential to be a reliable tool for biomedical depth-profilometric imaging.