Multi-spectral optoacoustic tomography (MSOT), also known as functional photoacoustic tomography (fPAT), is an imaging technology that generates high-resolution optical images in scattering media, including biological tissues. MSOT illuminates tissue with light of transient energy, typically light pulses lasting 1-100 nanoseconds. The tissue absorbs the light pulses, and as a result undergoes thermo-elastic expansion, a phenomenon known as the optoacoustic or photoacoustic effect. This expansion gives rise to ultrasound waves (photoechoes) that are detected and formed into an image. Image formation can be done by means of hardware (e.g. acoustic focusing or optical focusing) or computed tomography (mathematical image formation). Unlike other types of optoacoustic imaging, MSOT involves illuminating the sample with multiple wavelengths, allowing it to detect ultrasound waves emitted by different photoabsorbing molecules in the tissue, whether endogenous (oxygenated and deoxygenated hemoglobin, melanin) or exogenous (imaging probes, nanoparticles). Computational techniques such as spectral unmixing deconvolute the ultrasound waves emitted by these different absorbers, allowing each emitter to be visualized separately in the target tissue. In this way, MSOT can allow visualization of hemoglobin concentration and tissue oxygenation or hypoxia. Unlike other optical imaging methods, MSOT is unaffected by photon scattering and thus can provide high-resolution optical images deep inside biological tissues.V. Ntziachristos, D. Razansky, "Molecular imaging by means of multispectral optoacoustic tomography (MSOT)", Chem. Rev. 110 (2010) 2783-2794.
Description MSOT has been described as a 6-dimensional (6-parametric) method, in which the three geometrical dimensions (x, y, z) are complemented by time, illumination wavelengths and band of ultrasound frequencies detected. MSOT can measure over time, allowing longitudinal studies of dynamic processes.A. Taruttis, V. Ntziachristos, "Advances in real-time multispectral optoacoustic imaging and its applications", Nat Photon 9 (2015) 219-227. Illumination wavelengths in MSOT can cover the entire spectrum from ultraviolet (UV) to infrared (IR). The wavelength defines the photoabsorbers that can be seen and the imaging depth. High-energy ion beamsW. Assmann, S. Kellnberger, S. Reinhardt, S. Lehrack, A. Edlich, P.G. Thirolf, M. Moser, G. Dollinger, M. Omar, V. Ntziachristos, K. Parodi, "Ionoacoustic characterization of the proton Bragg peak with submillimeter accuracy", Med. Phys. 42 (2015) 567-574. and energy in the radiofrequency rangeD. Razansky, S. Kellnberger, V. Ntziachristos, "Near-field radiofrequency thermoacoustic tomography with impulse excitation", Med. Phys. 37 (2010) 4602-4607.K. Stephan, H. Amir, R. Daniel, N. Vasilis, "Near-field thermoacoustic tomography of small animals", Phys. Med. Biol. 56 (2011) 3433. have also been used. The choice of ultrasound frequency band defines resolution and overall size range of the objects that can be resolved. This choice of frequency band dictates whether the imaging will be in the macroscopic regime, involving resolution of 100-500 microns and penetration depth >10 mm, or mesoscopic range, involving resolution of 1-50 microns and penetration depth <10 mm. Microscopic resolution is also possible using multi-spectral optoacoustics.L.V. Wang, S. Hu, "Photoacoustic Tomography: In Vivo Imaging from Organelles to Organs", Science 335 (2012) 1458-1462. Like optical microscopy, they use focused light to form images and offers fundamentally the same capabilities (submicrometer resolution, <1mm penetration depth).
MSOT has now been used in a broad range of biological applications, including cardiovascular disease research,A. Taruttis, M. Wildgruber, K. Kosanke, N. Beziere, K. Licha, R. Haag, M. Aichler, A. Walch, E. Rummeny, V. Ntziachristos, "Multispectral optoacoustic tomography of myocardial infarction", Photoacoustics 1 (2013) 3-8.B. Wang, E. Yantsen, T. Larson, A.B. Karpiouk, S. Sethuraman, J.L. Su, K. Sokolov, S.Y. Emelianov, "Plasmonic Intravascular Photoacoustic Imaging for Detection of Macrophages in Atherosclerotic Plaques", Nano Lett. 9 (2009) 2212-2217. neuroimagingN.C. Burton, M. Patel, S. Morscher, W.H.P. Driessen, J. Claussen, N. Beziere, T. Jetzfellner, A. Taruttis, D. Razansky, B. Bednar, V. Ntziachristos, "Multispectral Opto-acoustic Tomography (MSOT) of the Brain and Glioblastoma Characterization", Neuroimage 65 (2013) 522-528.J. Yao, J. Xia, K.I. Maslov, M. Nasiriavanaki, V. Tsytsarev, A.V. Demchenko, L.V. Wang, "Noninvasive photoacoustic computed tomography of mouse brain metabolism in vivo", Neuroimage 64 (2013) 257-266.L. Xiang, L. Ji, T. Zhang, B. Wang, J. Yang, Q. Zhang, M.S. Jiang, J. Zhou, P.R. Carney, H. Jiang, "Noninvasive real time tomographic imaging of epileptic foci and networks", Neuroimage 66 (2013) 240-248.S. Gottschalk, T. Felix Fehm, X. Luís Deán-Ben, D. Razansky, "Noninvasive real-time visualization of multiple cerebral hemodynamic parameters in whole mouse brains using five-dimensional optoacoustic tomography", J. Cereb. Blood Flow Metab. 35 (2015) 531-535. and cancer research.E. Herzog, A. Taruttis, N. Beziere, A.A. Lutich, D. Razansky, V. Ntziachristos, "Optical Imaging of Cancer Heterogeneity with Multispectral Optoacoustic Tomography", Radiology 263 (2012) 461-468.J. Laufer, P. Johnson, E. Zhang, B. Treeby, B. Cox, B. Pedley, P. Beard, "In vivo preclinical photoacoustic imaging of tumor vasculature development and therapy", J. Biomed. Opt. 17 (2012).Q. Ruan, L. Xi, S.L. Boye, S. Han, Z.J. Chen, W.W. Hauswirth, A.S. Lewin, M.E. Boulton, B.K. Law, W.G. Jiang, H. Jiang, J. Cai, "Development of an anti-angiogenic therapeutic model combining scAAV2-delivered siRNAs and noninvasive photoacoustic imaging of tumor vasculature development", Cancer Lett. 332 (2013) 120-129. The development of real-time hand-held imaging systemsA. Buehler, M. Kacprowicz, A. Taruttis, V. Ntziachristos, "Real-time handheld multispectral optoacoustic imaging", Opt. Lett. 38 (2013) 1404-1406. has enabled clinical use of MSOT for imaging the breast,M. Heijblom, D. Piras, M. Brinkhuis, J.C.G. van Hespen, F.M. van den Engh, M. van der Schaaf, J.M. Klaase, T.G. van Leeuwen, W. Steenbergen, S. Manohar, "Photoacoustic image patterns of breast carcinoma and comparisons with Magnetic Resonance Imaging and vascular stained histopathology", Scientific Reports 5 (2015) 11778.S. Manohar, A. Kharine, J.C.G. van Hespen, W. Steenbergen, T.G. van Leeuwen, "The Twente Photoacoustic Mammoscope: system overview and performance", Phys. Med. Biol. 50 (2005) 2543-2557.S. Manohar, S.E. Vaartjes, J.C.G.v. Hespen, J.M. Klaase, F.M.v.d. Engh, W. Steenbergen, T.G.v. Leeuwen, "Initial results of in vivo non-invasive cancer imaging in the human breast using near-infrared photoacoustics", Opt. Express 15 (2007) 12277-12285. vasculature,A. Taruttis, A.C. Timmermans, P.C. Wouters, M. Kacprowicz, G.M. van Dam, V. Ntziachristos, "Optoacoustic Imaging of Human Vasculature: Feasibility by Using a Handheld Probe", Radiology (2016) 152160.H.F. Zhang, K. Maslov, M.L. Li, G. Stoica, L.H.V. Wang, "In vivo volumetric imaging of subcutaneous microvasculature by photoacoustic microscopy", Opt. Express 14 (2006) 9317-9323. lymph nodesI. Stoffels, S. Morscher, I. Helfrich, U. Hillen, J. Lehy, N.C. Burton, T.C.P. Sardella, J. Claussen, T.D. Poeppel, H.S. Bachmann, A. Roesch, K. Griewank, D. Schadendorf, M. Gunzer, J. Klode, "Metastatic status of sentinel lymph nodes in melanoma determined noninvasively with multispectral optoacoustic imaging", Science Translational Medicine 7 (2015). and skin.M. Schwarz, A. Buehler, J. Aguirre, V. Ntziachristos, "Three-dimensional multispectral optoacoustic mesoscopy reveals melanin and blood oxygenation in human skin in vivo", Journal of biophotonics 9 (2016) 55-60.B. Zabihian, J. Weingast, M. Liu, E. Zhang, P. Beard, H. Pehamberger, W. Drexler, B. Hermann, "In vivo dual-modality photoacoustic and optical coherence tomography imaging of human dermatological pathologies", Biomed Opt Express 6 (2015) 3163-3178.
Etymology Multi-spectral. MSOT collects images at multiple wavelengths and resolves the spectral signatures in each voxel imaged, making it a multi-spectral method. Typically, MSOT is used to generate three images: one anatomical image at a single wavelength, one functional image resolving oxy- and deoxy-hemoglobin concentrations, and a third image resolving additional target photoabsorber(s). These additional photoabsorbers include melanin, fat, water and other endogenous or exogenous agents.
Optoacoustic. This term denotes the combination of optical (Greek, oπτικός) and acoustic (Greek, ακουστικός) energy (or components) in a single modality, which distinguishes optoacoustic imaging from optical imaging. Photoecho denotes the combination of light (Greek, Φως <phos>) and sound ( Ήχος <echos>) or reflection of sound Hχώ <echo>). The term photoacoustic is also widely used, and it denotes the generation of acoustic energy by light. Photoecho and photoacoustic are to optoacoustics what photon is to optics: optical methods rely on photons, whereas optoacoustic methods rely on photoechoes or photoacoustic responses.
Tomography. This term denotes images formed by combining raw measurements from multiple points around the specimen in a mathematical inversion scheme. This process is analogous to x-ray computed tomography, except that tomographic mathematical models describe light and sound propagation in tissues.
Fig. 1: Operational capabilities of MSOT. Hybrid image showing an optical micrograph of part of a mouse kidney cross-section (gray), overlaid with the distribution of an exogenous fluorescent agent imaged using MSOT (right). Distribution of oxy-hemoglobin (red) and deoxy-hemoglobin (blue) in the tumor, imaged using MSOT (left).
Operating principles