Photoacoustic imaging is an imaging modality that enables imaging optical absorption in biological tissues with acoustic resolution. Many (but not all) PA implementations integrate active pulse/echo ultrasound in a hybrid imaging system to capitalize on ultrasound contrast for anatomical information. Because of this relationship, it is envisioned that Photoacoustic images will often be presented side-by-side with or fused with ultrasound images (for a real-world presentation example, see Figure AAAAA.4-1).