Recent advances in photoacoustic endoscopy

Table 1 Overview of currently available photoacoustic imaging technologies

Technology	Full name	Brief physics	Current applications	Future applications	Additional value to
					standard endoscopy
PAT	Photoacoustic tomography	Optical excitation of light absorbers in tissues by a pulsed laser and ultrasonic detection using mechanical scanning or detector arrays	Three major implementations include PAM, PACT, PAE	Functional information with the aid of an exogenous contrast	-
MSOT	Multispectral optoacoustic tomography	Utilization of multiple illumination wavelengths, spectral separation of optical reporter of interest from background absorption	Functional imaging of blood vessels, melanoma imaging of primary tumors and metastasis, characterization of atherosclerotic plaques, etc.	Tissue anatomy, function, molecular biomarkers, and gene expression	-
PAM	Photoacoustic microscopy	Based on a scanning focused ultrasonic transducer	Anatomical images of cutaneous microvasculature	Noninvasive imaging of individual cell nuclei	-
PACT	Photoacoustic computed tomography	Based on an array of unfocused ultrasonic transducers, use of an inverse algorhithm to reconstruct a tomographic image	Tumor boundaries and connections with surrounding blood and lymphatic vessels	Same as PAT	-
PAE	Photoacoustic endoscopy	Probe that combines light delivery, acoustic sensing, and mechanical scanning in one small unit placed at the distal end of the endoscope	Gastrointestinal tract imaging	Improve the accuracy of cancer staging	Optical absorption-based contrast with high spatial resolution at depths
PAE-EUS	Photoacoustic endoscopy and Endoscopic ultrasound	Integrated system for ultrasonic images produced with conventional pulse-echo imaging and photoacoustic images formed through detection of acoustic waves	Gastrointestinal tract and lymphovascular imaging	Early-stage tumor detection or in situ characterization of diseased tissues	Angiographic and spectral imaging function would enhance EUS’s role

PAT: Photoacoustic tomography; MSOT: Multispectral optoacoustic tomography; PAM: Photoacoustic microscopy; PACT: Photoacoustic computed tomography; PAE: Photoacoustic endoscopy; EUS: Endoscopic ultrasound.