Frequency-domain photothermoacoustic imaging contrast enhancement with a CW laser and non-linear frequency modulation chirps

The application of photoacoustic (PA) phenomena to medical imaging has been investigated for more than a decade. To implement this modality, one may choose between two types of laser sources, pulsed or continuous wave (CW). This selection will affect all features of the imaging technique. Nowadays pulsed lasers are the state-of-the-art technique in the PTA research. In this work we report frequency-domain photothermoacoustic imaging using linear and non-linear frequency chirps with a CW laser. The images produced using turbid tissue phantoms with subsurface inclusions were compared according to their contrast and depth resolution of absorbing lesions. In the CW method, in addition to the image produced by the amplitude of the cross-correlation between input and output signals, another image which is generated by the phase of the correlation signal is also available. The application of nonlinear frequency modulation instead of the standard linear frequency chirps introduced in our laboratory is demonstrated. These features are additional degrees of freedom uniquely available to the CW (but not to the pulsed laser) method.