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WIREs Nanomed Nanobiotechnol
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Recent advances in photoacoustic contrast agents for in vivo imaging

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Abstract Photoacoustic imaging (PAI) is a noninvasive hybrid imaging modality offering rich optical contrast and high depth‐to‐resolution ratio deep‐tissue imaging. Endogenous chromophores present in the body such as hemoglobin, lipid, melanin, and so on provide strong photoacoustic contrast due to their strong light absorption in certain optical window. To enhance the performance of PAI further, researchers have developed several exogenous contrast agents such as metallic nanoparticles, carbon‐based nanomaterials, quantum dots, organic small molecules, semiconducting polymer nanoparticles, and so on. These exogenous contrast agents not only help improving the imaging contrast, but also make targeted molecular imaging possible. In this review article, we first discuss the state‐of‐the‐art PAI techniques with endogenous contrast mechanism. Later, we provide an overview of recent progress in the development of exogenous photoacoustic contrast agents for in vivo imaging applications. Finally, we present the pros/cons of the existing PA contrast agents along with future challenges of contrast agent‐based PAI for biomedical applications. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vivo Nanodiagnostics and Imaging
(a) Schematic illustration of the preparation of Polipo‐IR NPs. (b) Optical spectra of Polipo‐IR NPs in water and the free IR‐1061 dye in CCL2. The inset image shows the appearance of free IR‐1061 dye in CCL2 (left) and Polipo‐IR NPs in water (right). (c) Photograph of the chicken phantom and embedded NPs for deep‐tissue imaging. (d) Comparison of PA images at 808 and 1,064 nm. (e) SNR as a function of tissue depth at 1,064 and 808 nm. PA imaging of nude mouse bearing orthotopic HCC: photograph (f), ultrasound image (g), and PA images of preinjection (h), 0.5 hr (i), 2 hr (j), 3 hr (k), and 4 hr (l), respectively. (Reproduced by permission of The Royal Society of Chemistry (Q. Chen, Chen, et al., )). HCC, hepatocellular carcinoma; SNR, signal‐to‐noise ratio
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(a) Schematic diagram of the adopted procedure for ICG‐labeling and in vivo detection of MSCs. (b) Raw average PA intensity over time after spectral unmixing. (c) Representative PA monitoring of the cell engraftment over days (excitation wavelength: 810 nm) after spectral unmixing of signal in the right hindlimb. (Reproduced with permission from Filippi et al. ()). ICG, indocyanine green; MSC, medicinal signaling cell
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(a) TEM image, (b) HR‐TEM image of MoO3 − x Quantum Dots. Inset in (a) is the schematic of the quantum dot. (c) Optical absorption spectra of MoO3 − x QDs at different concentrations. (d) PA images of MoO3 − x QD at different concentrations. (e and f) in vivo photoacoustic images of mouse bearing HeLa tumor before and after injection of MoO3 − x QDs at different time points. (Reproduced by permission of The Royal Society of Chemistry Ding et al. ())
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Gold plasmonic blackbody (AuPB) for high‐contrast photoacoustic imaging: (a) one‐pot synthesis of AuPBs, (b) optical absorbance spectra of dopamine and AuPBs at different stages of synthesis; inset: photographs of dopamine solution (left) and AuPB dispersion (right), (c) simulation results of the heat power density inside the AuPBs illuminated by light at 808 and 1,064 nm with same power density 1.0 W/cm2, (d) in vivo PA imaging of mouse tumor (highlighted by yellow circles) at 1,064 nm excitation at different times after intravenous injection of 100 μl of AuPBs (μg/ml). (Reproduced with permission from J. Zhou, Jiang, et al. ())
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(a) Schematic and TEM images of AuNRs, Au/AgNRs, and etched Au/AgNRs. Photoacoustic spectra (b) and in vitro photoacoustic image (c) of AuNRs, Au/AgNRs, and etched Au/AgNRs. The addition of the Ag shell effectively quenches the PA signal from AuNRs. However, the PA contrast is regenerated upon the etching of the Ag shell. B‐mode ultrasound (gray scale) and photoacoustic (red) images of subcutaneously injected Au/AgNRs (d) and Au/AgNRs with successive addition of silver etchant (e). PA imaging was done at 800 nm (NIR‐I). (Reproduced with permission from T. Kim et al. ())
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Dipolar surface plasmon resonances of (a) gold nanospheres, (b) gold nanorods, and (c) PA spectra of gold nanospheres and nanorods. Au nanospheres show absorption peak only in the visible region. Au‐nanorods show weak absorption in the visible region corresponding to transverse mode (TM) and strong absorption in the NIR region corresponding to longitudinal mode (LM). (Reproduced with permission from El‐Brolossy et al. ())
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(a) Schematic of the dual‐element ultrasound transducer with central frequencies at 20 and 40 MHz. in vivo sO2 mapping of a mouse brain with the scalp removed and the skull intact: (b) optical absorption spectra of the oxyhemoglobin (HbO2), deoxyhemoglobin (HbR), and miRFP670. Measured sO2 maps of the mouse brain vasculature at 532 and 640 nm by (c) OR‐HF (optical resolution‐high frequency) mode, and (d) OR‐LF mode. Molecular imaging of bacteria expressing miRFP670 protein: (e) Top‐view projection image of the mouse ear injected with bacteria, obtained by AR‐LF with the dual‐wavelength illumination at 532 and 640 nm. (f) Cross‐sectional B‐scan images obtained by AR‐LF across the dashed line in (e). (Reproduced with permission from W. Liu et al. ())
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(a) 1,064 nm PACT system with latitudinal mode illumination for in vivo imaging of human hand. PACT of blood vasculature of the human hand (b) MAP image, (c) color‐encoded depth image. (Reproduced by permission from Wray et al. ()). MAP, maximum amplitude projection; PACT, photoacoustic computer tomography
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(a) Schematic of the small‐animal whole‐body photoacoustic computed tomography system with scanning spherical detection array. (b–d) Whole‐body images acquired from a living intact mouse. (Legend): 1, left atrium; 2, cardiac ventricles; 3, liver; 4, spleen; 5, thoracic vessels; 6, kidney; 7, spine; 8, brown adipose tissue. (Reproduced by permission from Fehm et al. ())
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Absorption coefficient of endogenous (intrinsic) tissue chromophores at their typical concentrations in the human body as a function of wavelength in the 400–1,400 nm region, which covers the UV, visible, near‐infrared I (NIR‐I), and near‐infrared II (NIR‐II) windows, respectively. Adapted from http://omlc.ogi.edu
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In vivo PA imaging of rat brain vasculature using PIGD NPs: (a) schematic of the synthesis of PIGD nanoparticles, brain vascular images at (b) 0 min (before injection), (c) 40 min, (d) 70 min postinjection of PIGD NPs (2 mg∕ml, 0.25 ml per rat). Energy density used was 5 mJ∕cm2. Reproduced with permission from P. K. Upputuri et al. ())
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(a) Schematic of SPN‐II nanoparticles, (b) optical absorption spectra of SPN‐I and SPN‐II (40 μg/ml), in vivo PA images of rat brain at 750 nm (c) and 1,064 nm (d) acquired 70 min after injecting SPN‐II (6 mg/ml, 0.3 ml per rat). Energy density used was 5.5 mJ/cm2. (Reproduced with permission from Jiang, Upputuri, et al. ()). SPN, semiconducting polymer nanoparticle
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(a) Schematic of BVNP synthesis: The use of water, MES, and NaCl results in water−BVNPs (iii), MES–BVNPs (i), and NaCl−BVNPs (ii), respectively. Dialysis of MES–BVNPs with water (iv) results in water−BVNPs. Nanoparticles degrade in the presence of biliverdin reductase (v). (b) MES–BVNPs have a red‐shifted fluorescence compared to the water−BVNPs and NaCl−BVNPs. All compositions exhibit high absorbance at 365 and 680 nm, resulting in fluorescent and photoacoustic properties. (c) Photoacoustic imaging of sentinel lymph nodes using BVNPs at 680 nm wavelength. PA images of mouse before nanoparticle injection, 10 and 30 min after injection. Scale bar‐5 mm (white), 1 mm (yellow). (Reproduced with permission from Fathi et al. ()). BVNP, biliverdin nanoparticle; LN, lymph node; LV, lymphatic vessel; MES, 2‐(N‐morpholino) ethanesulfonic acid
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In vivo image of the cortical layer of a mouse brain with conventional PACT using hemoglobin as the contrast (a), with super‐resolution PACT image with localizing single droplets. (c) PA amplitude profiles of conventional (blue) and super‐resolution (red) images along the lines that are shown in (a) and (b). (d) and (e) are magnified images of the selected regions in (a) and (b). (f) PA amplitude profiles along the dotted lines in (d) and (e). (Reproduced with permission from P. Zhang et al. ()). PACT, photoacoustic computer tomography
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