Photoacoustic tomography (PAT) can create multiscale multicontrast images of living biological structures ranging from organelles to organs. This emerging technology overcomes the high degree of scattering of optical photons in biological tissue by making use of the photoacoustic effect. Light absorption by molecules creates a thermally induced pressure jump that launches ultrasonic waves, which are received by acoustic detectors to form images. Different implementations of PAT allow the spatial resolution to be scaled with the desired imaging depth in tissue while a high depth-to-resolution ratio is maintained. As a rule of thumb, the achievable spatial resolution is on the order of 1/200 of the desired imaging depth, which can reach up to 7 centimeters. PAT provides anatomical, functional, metabolic, molecular, and genetic contrasts of vasculature, hemodynamics, oxygen metabolism, biomarkers, and gene expression. We review the state of the art of PAT for both biological and clinical studies and discuss future prospects.

Photoacoustic Tomography: In Vivo Imaging from Organelles to Organs

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Ambient gas density and fuel vaporization effects on the penetration and dispersion of diesel sprays were examined over a gas density range spanning nearly two order of magnitude. This range included gas densities more than a factor of two higher than top-dead-center conditions in current technology heavy-duty diesel engines. The results show that ambient gas density has a significantly larger effect on spray penetration and a smaller effect on spray dispersion than has been previously reported. The increased dependence of penetration on gas density is shown to be the result of gas density effects on dispersion. In addition, the results show that vaporization decreases penetration and dispersion by as much as 20% relative to non-vaporizing sprays; however, the effects of vaporization decrease with increasing gas density. Characteristic penetration time and length scales are presented that include a dispersion term that accounts for the increased dependence of penetration on ambient density. These penetration time and length scales collapse the penetration data obtained over the entire range of conditions examined in the experiment into two distinct non-dimensional penetration curves: one for the non-vaporizing conditions and one for the vaporizing conditions. Comparison of the two nondimensional penetration curves to a theoretical penetration correlation for non-vaporizing sprays helped isolate and explain the effects of droplets and vaporization on penetration. The theoretical penetration correlation was derived using the penetration time and length scales and simple model for a non-vaporizing spray that has been previously presented in the literature. The correlation is in good agreement with the non-vaporizing data from this experiment and other commonly quoted penetration data sets. It also provides a potential explanation for much of scatter in the penetration predicted by various correlations in the literature.

/pdf/effects-of-gas-density-and-vaporization-on-penetration-and-53r9byyd3t.pdf

Effects of Gas Density and Vaporization on Penetration and Dispersion of Diesel Sprays

The life sciences can benefit greatly from imaging technologies that connect microscopic discoveries with macroscopic observations. One technology uniquely positioned to provide such benefits is photoacoustic tomography (PAT), a sensitive modality for imaging optical absorption contrast over a range of spatial scales at high speed. In PAT, endogenous contrast reveals a tissue's anatomical, functional, metabolic, and histologic properties, and exogenous contrast provides molecular and cellular specificity. The spatial scale of PAT covers organelles, cells, tissues, organs, and small animals. Consequently, PAT is complementary to other imaging modalities in contrast mechanism, penetration, spatial resolution, and temporal resolution. We review the fundamentals of PAT and provide practical guidelines for matching PAT systems with research needs. We also summarize the most promising biomedical applications of PAT, discuss related challenges, and envision PAT's potential to lead to further breakthroughs.

/pdf/a-practical-guide-to-photoacoustic-tomography-in-the-life-3ot96ziog0.pdf

A practical guide to photoacoustic tomography in the life sciences

The Foundations of Statistics By Prof. Leonard J. Savage. (Wiley Publications in Statistics.) Pp. xv + 294. (New York; John Wiley and Sons, Inc.; London: Chapman and Hall, Ltd., 1954.) 48s. net.

Probability and Statistical Inference

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time.

https://www.pnas.org/content/pnas/99/17/10994.full.pdf

FAST CARS: Engineering a laser spectroscopic technique for rapid identification of bacterial spores

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time consuming microscopic, chemical and biological assays. Current research into real time laser spectroscopic detectors of such contaminants is based on e.g. resonant Raman spectroscopy. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. The connection with previous studies based on "Coherent Anti-Stokes Raman Spectroscopy" (CARS) is to be noted. However generating and utilizing maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is much more challenging. This extension of the CARS technique is called FAST CARS (Femtosecond Adaptive Spectroscopic Techniques for Coherent Anti-Stokes Raman Spectroscopy), and the present paper proposes and analyses ways in which it could be used to rapidly identify pre-selected molecules in real time.

https://arxiv.org/pdf/physics/0203007.pdf

FAST CARS: Engineering a Laser Spectroscopic Technique for Rapid Identification of Bacterial Spores

Nitrogen and hydrogen CARS temperature measurements in a hydrogen/air flame using a near-adiabatic flat-flame burner

The mixing characteristics of an underexpanded sonic jet injected into a Mach 1.6 crosse ow are studied experimentally. Shadowgraph photography and planar laser-induced e uorescence from acetone are used to obtain temporally resolved e owe eld visualizations of a side viewof the barrel shock region, a side view of the downstream plume, and an end view cross section of the plume. Instantaneous images, mean and standard deviation images, and image-intensity probability density functions (PDFs) are used to study the mixing produced by large-scale turbulent structures. PDFs show that time-averaged mixture fraction measurements greatly overestimate the actual level of instantaneous mixing in the e owe eld. Comparisons of the instantaneous images and PDFs show the role of large-scale rolling structures and jetlike plumes in transporting coherent packets of e uid across the threedimensional shear layer formed between the jet and crosse ow. Signie cant instantaneous mixing in this e owe eld occursin the wakeregiondownstream ofthebarrel shock region and below thejetcenterline.Thecounter-rotating streamwisevortex pairin thejetplumeplaysan importantrole in the scalarmixing processes becauseit transports jet e uid down toward the wake and entrains crosse ow e uid from below up into the jet.

Robert P. Lucht

Papers

FAST CARS: Engineering a laser spectroscopic technique for rapid identification of bacterial spores

FAST CARS: Engineering a Laser Spectroscopic Technique for Rapid Identification of Bacterial Spores

Nitrogen and hydrogen CARS temperature measurements in a hydrogen/air flame using a near-adiabatic flat-flame burner

Mixing of a Sonic Transverse Jet Injected into a Supersonic Flow

Effect of Damkohler number on superequilibrium OH concentration in turbulent nonpremixed jet flames