Abstract
.Photoacoustic (PA) imaging is an emerging medical imaging modality that combines optical excitation and ultrasound detection. Because ultrasound scatters much less than light in biological tissues, PA generates high-resolution images at centimeters depth. In recent years, wavelengths in the second near-infrared (NIR-II) window (1000 to 1700 nm) have been increasingly explored due to its potential for preclinical and clinical applications. In contrast to the conventional PA imaging in the visible (400 to 700 nm) and the first NIR-I (700 to 1000 nm) window, PA imaging in the NIR-II window offers numerous advantages, including high spatial resolution, deeper penetration depth, reduced optical absorption, and tissue scattering. Moreover, the second window allows a fivefold higher light excitation energy density compared to the visible window for enhancing the imaging depth significantly. We highlight the importance of the second window for PA imaging and discuss the various NIR-II PA imaging systems and contrast agents with strong absorption in the NIR-II spectral region. Numerous applications of NIR-II PA imaging, including whole-body animal imaging and human imaging, are also discussed.
Highlights
In biomedical imaging, there is a huge demand for a noninvasive imaging modality that can provide high spatiotemporal resolution, high penetration depth, and anatomical and functional contrasts at an affordable price
We focus on the NIR-II photoacoustic imaging (PAI) systems, contrast agents, and their preclinical and clinical applications
The review is organized in the following order: Sec. 2 discusses the motivation for NIR-II PAI, Sec. 3 discusses about in vitro imaging, Sec. 4 discusses about in vivo small animal imaging, Sec. 5 is dedicated to in vivo human imaging, Sec. 6 discusses about high-resolution imaging in the second window, Sec. 7 presents the summary and perspectives, and in Sec. 8, we conclude the review
Summary
There is a huge demand for a noninvasive imaging modality that can provide high spatiotemporal resolution, high penetration depth, and anatomical and functional contrasts at an affordable price. Optical imaging modalities, including fluorescence, multiphoton fluorescence, bioluminescence, optical projection tomography, optical coherence tomography (OCT), and diffuse optical tomography (DOT), were demonstrated for in vivo animal imaging.[10,11,12] These methods used nonionizing radiation, unlike CT scan (x-ray), SPET, and PET (gamma rays), and can provide structural as well as functional information at high resolution.[13,14,15,16] due to the strong optical absorption and scattering of tissue, the imaging depth was limited to ∼0.1 mm (roughly the mean free path) in conventional widefield optical microscopy.[17,18] OCT, on the other hand, used ballistic and quasi-ballistic photons for imaging and can image up to the transport mean free path (∼1 mm in biological tissue) It is currently in use for skin and eye imaging applications with an imaging depth of up to 2 mm.[19,20] DOT offered several. The review is organized in the following order: Sec. 2 discusses the motivation for NIR-II PAI, Sec. 3 discusses about in vitro imaging, Sec. 4 discusses about in vivo small animal imaging, Sec. 5 is dedicated to in vivo human imaging, Sec. 6 discusses about high-resolution imaging in the second window, Sec. 7 presents the summary and perspectives, and in Sec. 8, we conclude the review
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