Deep in vivo photoacoustic imaging of mammalian tissues using a tyrosinase-based genetic reporter

Abstract

Photoacoustic imaging allows absorption-based high-resolution spectroscopic in vivo imaging at a depth beyond that of optical microscopy. Until recently, photoacoustic imaging has largely been restricted to visualizing the vasculature through endogenous haemoglobin contrast, with most non-vascularized tissues remaining invisible unless exogenous contrast agents are administered. Genetically encodable photoacoustic contrast is attractive as it allows selective labelling of cells, permitting studies of, for example, specific genetic expression, cell growth or more complex biological behaviours in vivo. In this study we report a novel photoacoustic imaging scanner and a tyrosinase-based reporter system that causes human cell lines to synthesize the absorbing pigment eumelanin, thus providing strong photoacoustic contrast. Detailed threedimensional images of xenografts formed of tyrosinase-expressing cells implanted in mice are obtained in vivo to depths approaching 10 mm with a spatial resolution below 100 μm. This scheme is a powerful tool for studying cellular and genetic processes in deep mammalian tissues.O ptical techniques such as fluorescence or bioluminescence imaging are widely used to visualize biological tissues in vivo 1-4 . However, strong optical scattering fundamentally limits the penetration depth or spatial resolution. Microscopy and other techniques that utilize ballistic photons 4 can provide cellular resolution, but only to sub-millimetre penetration depths, while diffuse optical methods such as fluorescence optical tomography 1 can provide greater penetration depths (on the scale of centimetres) but with only limited spatial resolution (on the scale of millimetres). Photoacoustic imaging (PAI) offers the prospect of overcoming these limitations 5-8 . Here, ultrasound waves generated by the absorption of laser light by tissue chromophores are used to produce images of biological tissues based on optical absorption. Because acoustic waves are scattered much less than photons in soft tissues, PAI avoids the depth and spatial resolution limitations of purely optical imaging techniques: depths of a few centimetres with scalable spatial resolution ranging from tens to hundreds of micrometres (depending on depth) are readily achievable.Although strong absorption by haemoglobin enables the acquisition of exquisite three-dimensional photoacoustic (PA) images of the vasculature 9-14 , most cells and tissues are relatively weakly absorbing at visible and near-infrared wavelengths and are thus indistinguishable in the absence of exogenous contrast. The latter can be provided by nanoparticle-or dye-based targeted contrast agents 15-17 , but these can present challenges in achieving effective specific targeting and clearance. The use of reporter genes to provide genetically encoded exogenous PA contrast would avoid these limitations and has the further advantage of providing opportunities to study more complex biological behaviours such as cell growth dynamics and intracellular processes such as gene expressi...

Deep tissue photoacoustic computed tomography with a fast and compact laser system

“…Photoacoustic (PA) computed tomography (PACT) has emerged as a novel imaging modality that provides high resolution imaging of optical absorption in deep tissue [1][2][3][4][5][6]. Over the past few years, various preclinical and clinical applications of the technique have been demonstrated, including functional brain imaging [7], small-animal whole-body imaging [8], breast cancer screening [9], and guidance of lymph node biopsy [10].…”

Simultaneous in vivo imaging of diffuse optical reflectance, optoacoustic pressure, and ultrasonic scattering

“…The pecularities of vascular development within various tumor models are often used to demonstrate in vivo performance of the novel OA imaging systems [18,19]. Figure 2 represents the results of simultaneous DR/OA/US imaging of SKBR3 tumor model.…”

Three-dimensional thermographic imaging using a virtual wave concept

“…In photoacoustic imaging, ultrasonic waves are generated in internal structures by means of laser irradiation, whereas in laser ultrasonics, the ultrasonic waves generated at a sample surface are scattered by the internal structures. 14,44 For both cases, F-SAFT reconstruction can be used. In laser ultrasonics, the propagation distances have to be doubled compared to photoacoustics, or alternatively, half of the sound velocity is taken.…”