Extending Biological Imaging to the Fifth Dimension: Evolution of volumetric small animal multispectral optoacoustic tomography

Despite the ancient discovery of the basic physical phenomenon underlying optoacoustic imaging and tomography [1], the lack of suitable laser sources, ultrasound detection technology, data acquisition, and processing capacities has long hindered the realization of efficient imaging devices. In fact, the first high-quality images from living animals were obtained about a decade ago (Figure 1), which was followed by an exponential growth of technical developments in instrumentation, algorithms, and biomedical applications surrounding this fascinating field. The ability of optoacoustics to probe optical contrast along a wide domain of penetration scales while maintaining excellent spatiotemporal resolution representative of ultrasound imaging, as shown in Figure 2, is unparalleled among the other optical imaging modalities [2], [3].

Dimensionality in Optoacoustic Imaging

A large variety of approaches have been proposed for in vivo small animal optoacoustic imaging. Naturally, a single optoacoustic waveform represents one-dimensional information along the axis of the ultrasound detection element. Thus, two-or three-dimensional images can be rendered by raster scanning the detector in the two remaining spatial dimensions, as performed in acoustic or optical resolution optoacoustic microscopy [7]. Another technique consists in scanning an optical probe beam along a Febry-Pérot interferometric film to tomographically detect optoacoustically generated sound using an all-optical approach [8]. An alternative method for whole-body optoacoustic tomography was reported by Brecht et al., which was able to render the three-dimensional distribution of vasculature structures and blood-rich organs such as the liver, spleen, and kidney by rotating a matrix array transducer around the imaged mouse [9]. The real-time imaging capacity in whole-body observations was demonstrated by a cross-sectional MSOT system based on an array of cylindrically focused transducers [10]. This imaging geometry enabled capture of two-dimensional slices representing an entire cross section of living mouse at video rate (see Figure 3). Figure 3

(a) A schematic drawing of the cross-sectional MSOT system. A curved array of wideband and cylindrically focused ultrasound transducers enables parallel data acquisition. Optical fibers are used to homogeneously illuminate the object. (b)–(d) MSOT images of mouse anatomy taken at 750 nm. 1: kidneys; 2: spine; 3: spleen; 4: vena cava; 5: liver; and 6: brain. (Figure adapted in parts from [10].)