Spiral volumetric optoacoustic tomography visualizes multi-scale dynamics in mice

Abstract

Imaging dynamics at different temporal and spatial scales is essential for understanding the biological complexity of living organisms, disease state and progression. Optoacoustic imaging has been shown to offer exclusive applicability across multiple scales with excellent optical contrast and high resolution in deep-tissue observations. Yet, efficient visualization of multi-scale dynamics remained difficult with state-of-the-art systems due to inefficient trade-offs between image acquisition time and effective field of view. Herein, we introduce the spiral volumetric optoacoustic tomography technique that provides spectrally enriched high-resolution contrast across multiple spatiotemporal scales. In vivo experiments in mice demonstrate a wide range of dynamic imaging capabilities, from three-dimensional high-frame-rate visualization of moving organs and contrast agent kinetics in selected areas to whole-body longitudinal studies with unprecedented image quality. The newly introduced paradigm shift in imaging of multi-scale dynamics adds to the multifarious advantages provided by the optoacoustic technology for structural, functional and molecular imaging.

Keywords: multi-scale dynamics; multi-spectral imaging; optoacoustic tomography; real-time imaging; whole-body imaging.

Figure 1

The spiral volumetric optoacoustic tomography (SVOT) approach. (a) Whole-body tomographic data acquisition is performed along a spiral (helical) scanning trajectory by means of a spherical matrix ultrasound detection array, further capable of real-time three-dimensional (3D) imaging. (b) Representative single 3D image acquired from the mouse’s abdomen. The system can render single (~1 cm³) volumes at a frame rate of 100 Hz, only limited by the pulse repetition rate of the excitation laser. (c) Multi-spectral (or spectrally unmixed) 3D images can be generated for each position of the matrix array after acquiring volumetric image data at multiple excitation wavelengths of the laser. By fast sweeping of the laser wavelength, it only takes 50 ms to generate a volumetric multi-spectral dataset at five wavelengths. (d) Volumetric optoacoustic image rendered from a larger area by performing a partial scan for 5 s. (e) It takes ~5 min to acquire whole-body image data by combining all images acquired along the entire spiral trajectory.

Figure 2

High-frame-rate volumetric imaging at the whole-organ level. (a) Maximum intensity projection (MIP) of the three-dimensional (3D) optoacoustic images acquired from a living murine heart after tail-vein injection (at t₁=0 s) of 100 nmol of indocyanine green (ICG). (b) Optoacoustic signal traces (dimmed lines) along with their respective low-pass-filtered equivalents (bold lines) for the respective regions indicated in a. The pulmonary transit time Δt_p and the delay Δt_ca between the ICG appearance at a coronary artery versus the thoracic artery are indicated. (c) Zoom into the signal traces marked with equivalent color in b along with their respective Fourier transforms—the two distinctive peaks correspond to the breathing rate (BR) and heart rate (HR). (d) 3D color mapping of the time-to-peak values corresponding to the ICG bolus appearance.

Figure 3

Contrast agent kinetics in larger areas. (a) Spectrally unmixed images (green) for three representative instants following tail-vein injection of 100 nmol of the agent at 0 s. The kinetic images were superimposed onto the whole-body optoacoustic anatomical reference (acquired at 800 nm wavelength). (b) Optoacoustic signal traces from the regions indicated by circles in a. Note the indicated delay Δt_v between the indocyanine green (ICG) appearance in a lateral vein versus the tumor area. (c) Spectrally unmixed images (green) superimposed onto an anatomical reference image of the entire kidney region. Tail-vein injection of 10 nmol was done at t=0 s. (d) Box plots indicate distribution of the optoacoustic signal values in the corresponding volumes of interest labeled in c.