Molecular specificity in photoacoustic microscopy by time-resolved transient absorption

Photoacoustic Imaging in Biomedicine and Life Sciences

Photo-acoustic imaging, also known as opto-acoustic imaging, has become a widely popular modality for biomedical applications. This hybrid technique possesses the advantages of high optical contrast and high ultrasonic resolution. Due to the distinct optical absorption properties of tissue compartments and main chromophores, photo-acoustics is able to non-invasively observe structural and functional variations within biological tissues including oxygenation and deoxygenation, blood vessels and spatial melanin distribution. The detection of acoustic waves produced by a pulsed laser source yields a high scaling range, from organ level photo-acoustic tomography to sub-cellular or even molecular imaging. This review discusses significant novel technical solutions utilising photo-acoustics and their applications in the fields of biomedicine and life sciences.

Molecular Photoacoustic Contrast Agents: Design Principles & Applications

Photoacoustic imaging (PAI) is a rapidly growing field which offers high spatial resolution and high contrast for deep-tissue imaging in vivo. PAI is nonionizing and noninvasive and combines the optical resolution of fluorescence imaging with the spatial resolution of ultrasound imaging. In particular, the development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation ranging from small molecules to macromolecular proteins, polymeric and inorganic nanoparticles. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented and a review of prior reports spanning from early-to-current literature is put forth.

Photophysical and Photoacoustic Properties of π-Extended Curcumin Dyes. Effects of the Terminal Dimethylamino Electron-donor and the Bridging Aryl Ring

The synthesis, photophysical and photoacoustic characterization for a series of nine π-extended quadrupolar curcumin dyes is presented. A systematic evaluation of the π-bridging unit including the p-phenyl, naphth-4-yl, thien-2-yl and hybrid 4-naphthathien-2-yl groups is presented. Furthermore, evaluation of the strongly donating donor-π-acceptor-π-donor quadrupolar dimethylamino terminated derivatives is also included. Select dyes exhibit excited state absorption at increased laser fluence which translates to the production of a nonlinear enhanced photoacoustic response. In particular, the bis-4-dimethylaminonaphtha-2-thien-5-yl curcuminBF₂ contrast agent DMA-5 exhibits an excellent molar photoacoustics (PA) emission at both low (9.4 × 10³  V M^-1 ) and high (1.47 × 10⁵  V M^-1 ) laser fluence which is confirmed by its strong contrast by photoacoustic tomography (PAT). In summary, the strong absorbance and enhanced photoacoustic properties of naphthyl and thienyl curcuminoids here presented provides great promise for future photoacoustic imaging applications as demonstrated by preliminary PAT studies.

Emerging concepts in functional and molecular photoacoustic imaging

Providing the specific imaging contrast of optical absorption and excellent spatial scalability across the optical and ultrasonic dimensions, photoacoustic imaging has been rapidly emerging and expanding in the past two decades. In this review, I focus on a few latest advances in this enabling technology that hold the potential to transform in vivo functional and molecular imaging at multiple length scales. Specifically, multi-parametric photoacoustic microscopy enables simultaneous high-resolution mapping of hemoglobin concentration, oxygen saturation and blood flow-opening up the possibility of quantifying the metabolic rate of oxygen at the microscopic level. The pump-probe approach harnesses a variety of photoinduced transient optical absorption as novel contrast mechanisms for high-specificity molecular imaging at depth and as nonlinear excitation strategies for high-resolution volumetric microscopy beyond the conventional limit. Novel magneto-optical and photochromic probes lead to contrast-enhanced molecular photoacoustic imaging through differential detection.

Shining light on the dark side of imaging: excited state absorption enhancement of a bis-styryl BODIPY photoacoustic contrast agent

A first approach toward understanding the targeted design of molecular photoacoustic contrast agents (MPACs) is presented. Optical and photoacoustic Z-scan spectroscopy was used to identify how nonlinear (excited-state) absorption contributes to enhancing the photoacoustic emission of the curcuminBF2 and bis-styryl (MeOPh)2BODIPY dyes relative to Cy3.

Simplified method for ultra high-resolution photoacoustic microscopy via transient absorption

Photoacoustic microscopy (PAM) is a hybrid imaging modality that combines optical illumination with ultrasonic detection to achieve absorption contrast imaging of endogenous and exogenous chromophores. Optical resolution PAM achieves high lateral-resolution by tightly focusing the excitation light; however the axial resolution is still dependent upon the bandwidth of the ultrasonic transducer. As a result, PAM images have highly asymmetric voxels with submicron lateral resolution and axial resolution typically limited to tens of microns. We have previously reported on a resonant multiphoton approach to PAM called transient absorption ultrasonic microscopy (TAUM), which enables high axial resolution by frequency encoding the photoacoustic signal at the overlap of a pump and a probe beam. This approach enables photoacoustic imaging with subcellular resolution on par with other multiphoton microscopy techniques. Here, we report on an innovation that enables TAUM imaging with a much less sophisticated optical system than previously reported. If we allow the time delay between the pump and probe to collapse to zero, the pump and probe optical paths can be combined. An amplitude modulator in the single beam path is sufficient to encode the TAUM signal at the second harmonic of the modulation frequency. The resulting system is essentially a standard optical resolution PAM system that incorporates an amplitude modulator and utilizes a Fourier post processing algorithm to improve the axial resolution by approximately an order of magnitude. A prototype system based on this approach has been assembled and tested on fixed bovine erythrocytes.

Molecular Imaging in Optical Coherence Tomography

Optical coherence tomography (OCT) is a medical imaging technique that provides tomographic images at micron scales in three dimensions and high speeds. The addition of molecular contrast to the available morphological image holds great promise for extending OCT's impact in clinical practice and beyond. Fundamental limitations prevent OCT from directly taking advantage of powerful molecular processes such as fluorescence emission and incoherent Raman scattering. A wide range of approaches is being researched to provide molecular contrast to OCT. Here we review those approaches with particular attention to those that derive their molecular contrast directly from modulation of the OCT signal. We also provide a brief overview of the multimodal approaches to gaining molecular contrast coincident with OCT.