Optoacoustic tomography: Systems and theory
Optoacoustic tomography (OAT), also known as photoacoustic tomography, is a hybrid imaging modality that enables the visualization of optical contrast with high resolution at tissue depths in which light propagation is diffusive. In OAT, high-energy laser pulses are sent into the tissue, leading to local energy deposition and (almost) instantaneous thermal expansion of optically absorbing tissue constituents. This rapid expansion propagates outwards in the form of acoustic waves. To form an image of the optical absorption in the tissue, one needs to measure the acoustic waves emanating from the imaged object in a tomographic fashion and apply acoustic inversion. In multi-spectral optoacoustic tomography (MSOT), the optical excitation is performed in several wavelengths, enabling the detection of various chromophores based on their spectral signature and quantifying oxygen saturation in tissue. The additional dimension offered by MSOT makes it a highly promising technique for pre-clinical research and clinical applications.
The design of MSOT systems is a multi-facet problem that requires an understanding of the physics of light and ultrasound propagation in tissue, characteristics of ultrasound detectors, the mathematical properties of the inverse problems involved, and the technical constraints of the imaging scenario. At LBIS, our goal is to develop new system designs and algorithmic tools that enable imaging applications.
Experimental reconstruction of an optoacoustic point source when (a) the deterctor’s geometry is not modelled and (b) when it is. (taken from A. Rosenthal et al., Med. Phys., 2011)
Detection of ultrasound via optical interferometry
The detection of ultrasound is conventionally performed by using piezoelectric transducers. Despite the ubiquity of this approach, it suffers from several drawbacks that limit its application. In particular, in the field of optoacoustic imaging, the opacity of piezoelectric materials in use puts constraints on possible illumination patterns. In addition, piezoelectric transducers generally lose sensitivity upon miniaturization, hindering the development of minimally invasive optoacoustic endoscopes. At LBIS, we develop ultrasound detectors based on interferometric principles to enable new imaging devices. Our approach relies on miniature optical resonators in silica and silicon platforms and on a unique interrogation approach called pulse interferometry.
A scheme for a pulse-interferometry setup (taken from A. Rosenthal et al., Las. Photonics Rev., 2014)