Photoacoustic (PA) Imaging is an emerging imaging modality that enables imaging via optical absorption in biological tissues with acoustic resolution.  Many implementations integrate active pulse/echo ultrasound in a hybrid imaging system to capitalize on well-established contrast for anatomical information.  PA Imaging provides a combination of functional and morphologic information. The functional component of PA can show the neoangiogenesis and relatively greater degree of deoxygenation that occurs within and around invasive malignant masses, and thus, can help to better distinguish between benign and malignant masses than can gray scale ultrasound using only morphologic information. 

The currently FDA PMA-approved  indication for PA is for adjunctive diagnosis, i.e., better distinction between benign and malignant breast masses than can be achieved with gray scale ultrasound alone. The feasibility of other prospective applications within the breast imaging arena, such as using PA as a prognostic biomarker, for monitoring response to neoadjuvant therapy, and for assessment of axillary lymph nodes are being investigated or are planned to be investigated. Prospective, applications within organ systems outside of breast imaging include adjunctive diagnosis in thyroid nodules and prostate nodules/masses. 

Example PA image from a handheld acquisition: This is a 6 mm grade II invasive duct carcinoma that is ER positive, PR positive, HER2 negative, and has a ki67 of 30 (luminal B molecular subtype. As is most common in luminal B subtypes, the PA exam is positive in all 3 zones. The white ROI defines the border between the internal zone (i) and boundary zone (bz). In invasive malignancies, the bz represents the thick, echogenic rim. The aqua colored ROI shows the border between the bz and the peripheral zone (pz). There is intense deoxygenated hemoglobin (red) within the internal and boundary zones and there are multiple perpendicularly oriented “radiating” green vessels (arrows in bottom center frame) within the pz. 

  https://pubs.rsna.org/doi/10.1148/radiol.2017172228 

Example PA image in microscopy: