Optoacoustic and Ultrasound Imaging

Medical imaging plays a central role in modern healthcare, providing invaluable structural and functional information of the inside of the body. Ultrasonography (US) is broadly available due to its portability, safe use, real-time display, and low price. On the downside, the classical contrast mechanism—echo intensity—restricts its diagnostic value to limited scenarios. At the Optoacoustic and Ultrasound Imaging Team, we are advancing innovative US-based imaging modes aimed at combining the availability and flexibility of US imaging with complementary diagnostic power.

• In opto (or photo-) acoustic imaging (OA), short laser pulses—as opposed to sound—are sent into the tissue where they generate US signals upon optical absorption. Detection of these signals allows to create maps of the tissue’s optical contrast, with promise in, e.g., the diagnosis and monitoring of vascular diseases, arthritis, and cancer. Our team focuses on the combination of OA with classical US systems, where we are leading in the development of techniques that enable a clinically useful imaging depth.

• Conventional US maps the intensity of echoes, which represents the tissue’s microstructure and allows a diagnosis of disease that affects this structure. There is—however—much more information contained in US signals than just echo intensity. The tissue’s speed of sound (SoS) is a promising marker for disease that influences tissue composition. Our team has invented and develops computed Ultrasound Tomography in Echo mode (CUTE) which maps the SoS using conventional US equipment. Together with our collaborators and the Inselspital (Prof. Dr. med. Annalisa Berzigotti) we have already shown the promise of this technique for liver diagnosis where it allows a quantification of liver fat concentration.

Our final goal is the combination of conventional ultrasound, OA, CUTE, and other novel modalities in a single device which will unify the virtues of real-time operability and patient safety with the diagnostic accuracy of multimodal imaging.

Quantitative Spectral Optoacoustic Imaging

Spectral OA imaging exploits the wavelength-dependent optical absorption properties of specific chromophores in tissue (e.g. hemoglobin), with as goal to provide quantitative estimates of their spatially varying concentrations. A physiologically important example is the determination of local blood oxygen saturation, based on the distinct absorption spectra of oxy- and deoxyhemoglobin in the near-infrared range. This is of particular relevance for the study of oxygenation heterogeneity in tumors, the early detection and monitoring of cerebral ischemia in brain, and of other abnormalities characterized by a change in tissue oxygenation or perfusion status.

A central issue, which makes quantitative oxygenation measurements challenging, is the unknown extent of wavelength-dependent optical attenuation inside biological tissue. It engenders a spectral distortion of the recorded OA signals relative to the absorption spectrum of the chromophore's of interest. Only when this distortion is taken into account (which is referred to as "spectral correction"), a recovery of accurate quantitative information is possible.

In our research on quantitative deep OA imaging, we therefore investigate techniques to correct for the spectral distortion of the recorded OA signals.

Optoacoustic Clutter Reduction

For clinical combined optoacoustic (OA) and ultrasound (US) imaging, maximum flexibility is provided when the irradiation optics is integrated with the ultrasound detector into a handheld probe. The drawback of this epi-illumination geometry is that the high laser intensity near the detection aperture generates strong background signals that clutter the OA image, thus limiting imaging depth (see Figure, phantom study). Our niche is the development of techniques for reducing this clutter with the goal to enable deep OA imaging. One of these techniques is localized vibration tagging (LOVIT): a long pulsed ultrasound beam generates acoustic radiation force (ARF) that induces localized tissue displacement at its focus. Subtraction of OA images acquired before and after the ARF push preserves true OA signal in the displacement focus while eliminating the clutter background (see Figure).

Speed of Sound Imaging

Left: conventional echo ultrasound provides gray-scale images of echo intensity representing tissue microstructure. Right: a map of the spatial distribution of speed of sound (SoS) reveals differences in tissue composition, here skin (s), subcutaneous fat (sf), muscle (m) muscle, extraperitoneal fat (pf), and liver (l).

The speed of sound (SoS) inside tissue depends on tissue composition and is therefore a diagnostic marker for disease that affects tissue composition. Conventional pulse-echo ultrasound reconstructs the location of acoustic reflectors inside the tissue based on the round-trip propagation time of the echoes. Computed Ultrasound Tomography in Echo mode (CUTE) goes beyond that: it senses the phase shift of local echoes when detected under varying insonification and detection angles. This echo phase shift is related to the changing round-trip time representing line integrals of SoS, and thus the spatial distribution of SoS can therefore be reconstructed by solving the corresponding inverse problem. The Figure shows a typical example of combined conventional ultrasound (left, shows the echo intensity) and CUTE (right side) in the abdomen: The different SoS of different tissue layers are nicely resolved.