Fourier-domain methodology for depth-selective photothermoacoustic imaging of tissue chromophores

Development of a novel photothermoacoustic (PTA) imaging technique utilizing a frequency-modulated (chirped) optical excitation and Fourier-domain methodology for depth-selective imaging of tissue chromophores is presented. Use of frequency-domain signal detection rather than short-pulse time-resolved measurements of pressure transients give an advantage of higher SNR typical to coherent detection techniques. Additionally, we introduce chirped optical excitation to generate linear frequency modulated PTA response which enables unambiguous and precise depth measurements of tissue chromophores. In order to obtain depth profilometric information from the frequency-domain PTA (FD-PTA) measurements, we describe implementation of two signal processing algorithms: matched filter compression and heterodyne mixing with coherent detection. We show that direct relationship between chromophore depth and spectrum of photothermoacoustic signals can be established to enable depth-selective tissue imaging. Application of amplitude and phase FD-PTA imaging is demonstrated in experiments with light-scattering phantoms and chicken breast tissues containing subsurface inclusions. The potential of the FD-PTA method for noninvasive tissue tomography and molecular imaging is discussed.