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.
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. 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 beams and energy in the radiofrequency range 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.[6] Microscopic resolution is also possible using multi-spectral optoacoustics. 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, neuroimaging and cancer research. The development of real-time hand-held imaging systems has enabled clinical use of MSOT for imaging the breast, vasculature, lymph nodes and skin.
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).
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