The thermoelastic mechanism of optoacoustic wave generation is based on absorption of light energy in a medium followed by temperature rise and thermal expansion in the medium. The thermal expansion of the irradiated medium induces mechanical stress (pressure rise). A short optical pulse with the incident fluence, F_o, induces a pressure rise, P(z), in the medium upon condition of stress confinement:

where β [1/°C] is the thermal expansion coefficient; c_s [cm/s] is the speed of sound; C_p [J/g°C] is the heat capacity at constant pressure; F(z) [J/cm²] is the fluence of the optical pulse; and μ_a [cm⁻¹] is the absorption coefficient of the medium. The generated optoacoustic pressure can be expressed in J/cm³ or in bar (1 J/cm³ = 10 bar). The combination of the thermophysical parameters, βcs2/C_p in Equation (1) represents the Grüneisen parameter, Γ (dimensionless). The exponential light attenuation in the medium is represented by exp(−μ_az).

The Equation (1) is valid upon the stress-confinement condition when pressure relaxation is negligible during the heat deposition. The stress-confined condition is satisfied when light pulse duration, τ_p, is shorter than the stress relaxation time in the irradiated volume,τ_str:

Nanosecond pulses can be used to generate conditions of stress confinement for many biomedical optoacoustic applications including monitoring of physiologic variables (Esenaliev et al., 1993, 1996, 1998a; Larin et al., 2001, 2002, 2005; Larina et al., 2005; Petrova et al., 2005, 2009; Petrov et al., 2005, 2006; Brecht et al., 2007).

According to Equation (1), optoacoustic pressure amplitude is proportional to the Grüneisen parameter, fluence, and absorption coefficient of the medium, while the pressure spatial profile is dependent on the absorption coefficient. Since z and t are related by the simple equation:

the spatial distribution of optoacoustic pressure P(z) is detected by a wide-band acoustic transducer as a temporal profile P(t):

Therefore, by analyzing the amplitude and/or temporal profile of optoacoustic waves, one can measure the absolute value of the absorption coefficient of the irradiated medium.

Most tissues are strongly scattering media in the optical spectral range. In addition to the absorption coefficient, two other major optical parameters are responsible for distribution of light in tissues: scattering (μ_s) and effective attenuation (μ_eff) coefficients. Attenuation of diffusively scattered light depends on the effective attenuation coefficient which is related to μ_a, μ_s, and the anisotropy factor (g) as:

where μ_s(1–g) is the reduced scattering coefficient, μ^s′ (Welch and van Gemert, 2011). Light penetration depth in tissues is defined as 1/μ_eff. Distribution of laser fluence and, therefore, pressure in tissue (not very close to the surface) is dependent on optical absorption and effective attenuation coefficients:

where k is the parameter resulted from multiple scattering in tissue and depends on absorption and scattering coefficients (Welch and van Gemert, 2011).

Absorption and reduced scattering coefficients of tissues are low in the near-IR spectral range (from 700 to 1,300 nm), that results in deeper penetration of near-IR radiation compared with that of other parts of the spectrum. Application of near-IR radiation allows for sufficient penetration of light in tissues for optoacoustic measurements of physiologic variables.

Accurate temperature mapping with sub-mm spatial resolution may provide precise thermotherapy of abnormal tissues with minimal damage to surrounding normal tissues. Amplitude of optoacoustic pressure waves induced in many tissues increases with temperature, mostly due to temperature dependence of the thermal expansion coefficient (Esenaliev et al., 1998b; Larin et al., 2002, 2005; Larina et al., 2005). The temperature-dependent Grüneisen parameter can be expressed with an equation:

where A and B are constants and T is temperature. One can modify (Equation 1) for the case of absorbing media without scattering as:

and for the case of strongly scattering media in deeper (not in the subsurface) areas as:

where T(z) is the temperature distribution in tissue. One can rearrange (Equation 9) to obtain temperature distribution in tissue:

where P(z)_{T = To} is the optoacoustic pressure profile recorded at the initial temperature T_o and C and D are parameters that are dependent on tissue properties. Therefore, by recording and analyzing the temporal optoacoustic pressure profile, one can reconstruct distribution of temperature during hyperthermia.

We experimentally demonstrated linear increase with temperature of optoacoustic pressure amplitude in tissue phantoms and tissues such as liver and myocardium (Esenaliev et al., 1998b). In another set of experiments, using rapid heating, we produced temperature gradients in tissue and tissue-like sample (Larina et al., 2005). During the heating we monitored the temperature distribution with optoacoustic system, while a multi-sensor temperature probe inserted in the samples measured actual temperature distribution. These studies demonstrated that the accuracy of optoacoustic temperature was better than 1°C at the spatial resolution less than 1 mm.

Tissue optical and thermophysical properties change due to coagulation or freezing. This may provide fast and accurate optoacoustic feedback during thermotherapy with heating or cooling agents. Because the optoacoustic wave amplitude and temporal parameters are dependent on tissue properties (Equation 6), detection and analysis of the optoacoustic waves during thermotherapy may be used for real-time monitoring of the extent of tissue coagulation or freezing with high resolution and contrast (Esenaliev et al., 1998a; Larin et al., 2002, 2005; Larina et al., 2005).

We performed high-resolution, real-time optoacoustic monitoring of tissue coagulation during conductive heating (Larina et al., 2005) or interstitial heating by CW laser light (Larin et al., 2005). Analysis of optoacoustic signal slopes was used for monitoring tissue heating and dimensions of coagulation lesions. Coagulation was induced in liver, myocardium, and prostate by interstitial CW Nd:YAG laser irradiation of the samples or by conductive heating. The optical properties did not change up to the coagulation temperature (about 53°C), but sharply increased during heating up to 70°C. The interstitial coagulation front was monitored in freshly excised canine tissues in real time with spatial resolution of about 0.6 mm. These results suggested that this technique may be used for real-time precise thermotherapy of malignant and benign lesions at depths of the order of centimeter.

Real-time monitoring of cooling and freezing of tissues, cells, and other biological objects with a high spatial and temporal resolution is necessary for selective cryoablation of cancer and benign tumors and for organs, tissues, and other biological objects in medicine and cryobiology. Using liquid nitrogen, we demonstrated that tissue hypothermia and freezing can be monitored with the optoacoustic technique because both amplitude and profile of the optoacoustic waves change with temperature during cooling and freezing (Larin et al., 2002). Sharp increase of the optoacoustic signal slope was detected between −2 and −4°C resulted from the formation of the frozen zone in liver tissue. High spatial resolution (better than 0.5 mm) was obtained in these studies. Such resolution is sufficient for cryoablation monitoring with high precision.

Optical absorption contrast can be used for monitoring in tissues of exogenous dyes and nanoparticles with high temporal and spatial resolution. Indocyanine green (ICG), an FDA-approved dye for intravenous injections has a high absorption in the near IR spectral range with a maximum at 800–805 nm. Optoacoustic measurements of ICG-produced signal can be useful for monitoring of cardiac output (CO), cardiac index (CI), blood volume (BV), and hepatic function. We measured amplitude and peak-to-peak amplitude of optoacoustic signals induced in whole arterial blood in vitro at clinically relevant ICG concentrations. Both amplitude and peak-to-peak amplitude of the signals linearly increased with ICG concentration with high correlation coefficients (R² = 0.990 and 0.991, respectively) (Prough et al., 2008). The blood effective attenuation coefficient derived from the optoacoustic signal slopes also increased linearly with ICG concentration with high correlation coefficient of R² = 0.986.

Nano- and microparticles that strongly absorb light can be used for photothermal therapy of abnormal tissues including tumors (Esenaliev, 1999a, 2000, 2016a). High sensitivity of the optoacoustic technique to absorption changes provides basis for monitoring of nanoparticle delivery into tumors and for tumor coagulation. We used the optoacoustic technique for real-time monitoring of nanoparticle accumulation in human tumors of nude mice and the nanoparticle-induced laser thermotherapy of these tumors (Esenaliev et al., 2007).

Hemoglobin has a high absorption coefficient in the visible and near-IR spectral range (Welch and van Gemert, 2011). We experimentally demonstrated that both the amplitude and spatial distribution of the generated optoacoustic pressure induced in blood are dependent on total hemoglobin concentration [THb] (Esenaliev et al., 2004a,b; Petrova et al., 2005).

High z-axial resolution of the optoacoustic technique permits direct [THb] measurements in blood vessels because the optoacoustic waves induced in blood arrive at the acoustic transducer at the time defined by Equation (3). Since the hemoglobin absorption coefficient is dependent on hemoglobin saturation (i.e., oxygenation), we used the wavelength of 805 nm (isobestic point) where oxy- and deoxygenated hemoglobin have same absorption (Esenaliev et al., 2004a,b; Petrova et al., 2005). This allows for [THb] measurement at any oxygenation.

One of the most important optoacoustic applications is oxygenation imaging, monitoring, and sensing (Esenaliev et al., 2002a,b). They can be used for diagnostics and management of large populations of patients including those with traumatic brain injury (TBI), circulatory shock, stroke, patients undergoing surgery, anemic patients, neonatal patients, and fetuses during late-stage labor (Petrova et al., 2005, 2009; Petrov et al., 2005, 2006, 2012a,b, 2014, 2016, 2017a,b; Brecht et al., 2007; Esenaliev et al., 2016b; Esenaliev, 2017; Herrmann et al., 2017).

We proposed and developed a noninvasive, optoacoustic diagnostic platform for measurements of oxygenation, hemoglobin concentration, and other important physiological parameters in tissues and specific blood vessels (Petrova et al., 2005, 2009; Petrov et al., 2005, 2006, 2012a,b, 2014, 2016, 2017a,b; Brecht et al., 2007; Prough et al., 2008; Herrmann et al., 2017). Because hemoglobin is a major chromophore in the near IR spectral range and its absorption depends on oxygenation, optoacoustics is suitable for monitoring of these physiologic variables.

RE is a co-owner of Noninvasix, Inc., a UTMB-based startup that has licensed the rights to optoacoustic technology.