Breakthroughs in Photonics 2013: Photoacoustic Tomography in Biomedicine

Photoacoustic tomography (PAT) is one of the fastest growing biomedical imaging modalities in the last decade. Building on its high scalability and complementary imaging contrast to other mainstream modalities, PAT has gained substantial momentum in both preclinical and clinical studies. In 2013, PAT has grown markedly in both its technological capabilities and biomedical applications. In particular, breakthroughs have been made in super-resolution imaging, deep blood flow measurement, small animal resting state brain mapping, video rate functional human imaging, and human breast imaging. These breakthroughs have either successfully solved long-standing technical issues in PAT or significantly enhanced its imaging capability. This review will summarize state-of-the-art developments in PAT and highlight a few representative achievements of the year 2013

Advances in super-resolution photoacoustic imaging

Photoacoustic (PA) imaging (PAI), or optoacoustic imaging, is a hybrid imaging modality that combines optical absorption contrast and ultrasound image formation. In PAI, the target is excited by a short laser pulse and subsequently absorbs the photon energy, leading to a transient local temperature rise. The temperature rise induces a local pressure rise that propagates as acoustic waves. As acoustic waves generally undergo less scattering and attenuation in tissue compared with light, PAI can provide high-resolution images in both the optical (quasi)ballistic and (quasi)diffusive regimes (1,2). Based on the image formation methods, PAI can be classified into two categories: photoacoustic microscopy (PAM) and photoacoustic computed tomography (PACT). PAM uses a focused excitation light beam and/or a focused single-element ultrasonic transducer for direct image formation through position scanning (1,2). PAM has a maximum imaging depth ranging from a few hundred micrometers to a few millimeters with spatial resolution ranging from sub-micrometer to sub-millimeter (2,3). PAM can be further classified into optical-resolution PAM (OR-PAM) and acoustic-resolution PAM (AR-PAM). For both OR-PAM and AR-PAM, the axial resolution is determined by the bandwidth of the ultrasonic transducer (4). OR-PAM works in the optical (quasi)ballistic regime, whereas the light is tightly focused that it can penetrate about one optical transport mean free path (~1 mm in soft tissue). Therefore, the lateral resolution of OR-PAM is mainly determined by the optical focal spot size (4-6). The optical focusing is diffraction-limited as λ/2NA, where λ is the light wavelength, and NA is the numerical aperture of objective lens. On the contrary, in AR-PAM, the laser is loosely focused to fulfill the entire acoustic focal spot, thereby penetrating a few optical transport mean free paths, i.e., in the quasi-diffusive regime. The lateral resolution of AR-PAM is thus determined by the size of acoustic focus (4,7,8), limited by acoustic diffraction. In PACT, the object is illuminated with a wide-field laser beam in the diffusive regime, and the generated acoustic waves are detected at multiple locations or by using a multi-element transducer array. The image formed by PACT is reconstructed by an inverse algorithm. The spatial resolution of PACT is fundamentally limited by acoustic diffraction, and additionally affected by the directionality and spacing of the detector elements (9). Recently, several studies have shown that sub-diffraction imaging of biological samples can be achieved through PAI by breaking optical-diffraction limit in the (quasi)ballistic regime or acoustic-diffraction limit in the (quasi)diffusive regime, which have opened new possibilities for fundamental biological studies. Yao et al. developed a photoimprint PAM using the intensity-dependent photobleaching effect and acquired a melanoma cell PA image with a lateral resolution of 90 nm (10). Danielli et al. reported a label-free PA nanoscopy based on the optical-absorption saturation effect and acquired a mitochondria PA image with a lateral resolution of 88 nm (11). Chaigne et al. exploited the sample-dynamics-induced inherent temporal fluctuation in the PA signals and achieved a resolution enhancement of about 1.4 over conventional PACT (12). Murray et al. broke the acoustic diffraction limit by implementing a blind speckle illumination and block-FISTA reconstruction algorithm and achieved a resolution close to the acoustic speckle size (13). Dean-Ben et al. also overcame the acoustic diffraction limit by incorporating rapid sequential acquisition of 3D PA images of flowing absorbing particles and further enhanced the visibility of structures under limited-view tomographic conditions (14). Conkey et al. optimized wavefront shaping with photoacoustic feedback and achieved up to ten times improvement in signal-to-noise ratio and five to six times sub-acoustic-diffraction resolution (15). In this concise review, we summarize and analyze the recent development in super-resolution (SR) PAI (SR-PAI) in both the optical (quasi)ballistic and (quasi)diffusive regime, as well as their representative applications. We also discuss the current challenges in SR-PAI and envision the potential breakthroughs

Multiscale Functional and Molecular Photoacoustic Tomography

Photoacoustic tomography (PAT) combines rich optical absorption contrast with the high spatial resolution of ultrasound at depths in tissue. The high scalability of PAT has enabled anatomical imaging of biological structures ranging from organelles to organs. The inherent functional and molecular imaging capabilities of PAT have further allowed it to measure important physiological parameters and track critical cellular activities. Integration of PAT with other imaging technologies provides complementary capabilities and can potentially accelerate the clinical translation of PAT

Optical clearing-aided photoacoustic microscopy with enhanced resolution and imaging depth

Due to strong light scattering in tissue, both the spatial resolution and maximum penetration depth of optical-resolution photoacoustic microscopy (OR-PAM) deteriorate sharply with depth. To reduce tissue scattering, we propose to use glycerol as an optical clearing agent in OR-PAM. Our results show that the imaging performance of OR-PAM can be greatly enhanced by optical clearing both in vitro and in vivo

Photoacoustic tomography: fundamentals, advances and prospects

Optical microscopy has been contributing to the development of life science for more than three centuries. However, due to strong optical scattering in tissue, its in vivo imaging ability has been restricted to studies at superficial depths. Advances in photoacoustic tomography (PAT) now allow multiscale imaging at depths from sub-millimeter to several centimeters, with spatial resolutions from sub-micrometer to sub-millimeter. Because of this high scalability and its unique optical absorption contrast, PAT is capable of performing anatomical, functional, molecular and fluid-dynamic imaging at various system levels, and is playing an increasingly important role in fundamental biological research and clinical practice. This Review discusses recent technical progress in PAT and presents corresponding applications. It ends with a discussion of several prospects and their technical challenges

Photoacoustic microscopy

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (∼1 mm in soft tissue). With its excellent scalability, PAM can provide high-resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering-based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence-based methods, such as wide-field, confocal, and multi-photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state-of-the-art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies

Photoacoustic brain imaging: from microscopic to macroscopic scales

Human brain mapping has become one of the most exciting contemporary research areas, with major breakthroughs expected in the coming decades. Modern brain imaging techniques have allowed neuroscientists to gather a wealth of anatomic and functional information about the brain. Among these techniques, by virtue of its rich optical absorption contrast, high spatial and temporal resolutions, and deep penetration, photoacoustic tomography (PAT) has attracted more and more attention, and is playing an increasingly important role in brain studies. In particular, PAT complements other brain imaging modalities by providing high-resolution functional and metabolic imaging. More importantly, PAT’s unique scalability enables scrutinizing the brain at both microscopic and macroscopic scales, using the same imaging contrast. In this review, we present the state-of-the-art PAT techniques for brain imaging, summarize representative neuroscience applications, outline the technical challenges in translating PAT to human brain imaging, and envision potential technological deliverables

In vivo Photoacoustic Tomography of Total Blood Flow and Potential Imaging of Cancer Angiogenesis and Hypermetabolism

Blood flow is a key parameter in studying cancer angiogenesis and hypermetabolism. Current photoacoustic blood flow estimation methods focus on either the axial or transverse component of the flow vector. However, the Doppler angle (beam-to-flow angle) is needed to calculate the total flow speed, and it cannot always be estimated accurately in practice, especially when the system's axial and lateral resolutions are different. To overcome this problem, we propose a method to compute the total flow speed and Doppler angle by combining the axial and transverse flow measurements. The method has been verified by flowing bovine blood in a plastic tube at various speeds and Doppler angles. The error was experimentally determined to be less than 0.3 mm/s for total flow speed, and less than 15° for the Doppler angle. In addition, the method was tested in vivo on a mouse ear. We believe that the proposed method has the potential to be used for cancer angiogenesis and hypermetabolism imaging