Two-dimensional cross-sectional PA images at various depths in biological tissues. Photographs of (A) the cross section of chicken breast tissues where three pairs of tubes (whole blood, ICG in water, and ICG in blood) are embedded and (B) the entire sample. Two-dimensional PA images using a 2.25-MHz ultrasonic transducer at depths of (C) 1.3, (D) 2.6, (E) 4.2, and (F) 5.2 cm. Two-dimensional PA images at a 5.2-cm depth acquired by (G) 3.5- and (H) 5.0-MHz ultrasonic transducers. (Reprinted with permission from ref 56. Copyright 2005 The Optical Society of America.)