Optoacoustic (OA) imaging [1] (also may be referred to as photoacoustic imaging) is a method of acquisition and reconstruction of visual representation of biological tissue based on time-resolved detection of acoustic pressure profiles induced in tissue through absorption of optical pulses under irradiation conditions of temporal pressure confinement during optical energy deposition.
[2]. The term “irradiation conditions of temporal pressure confinement” means that optical energy (or other heat-generating energy) must be delivered to tissue faster than resulting acoustic wave can propagate the distance in tissue equal to the desirable spatial resolution. For example, having desirable resolution of optoacoustic images of 15 µm, and the speed of sound propagation in tissue of 1.5 mm per µs, one needs optical pulses shorter than 10 ns. Thus, utilization of short (nanosecond) optical pulses represents necessary (but not sufficient) condition to achieve desirable spatial resolution on OA images.
The sufficient condition to obtain desirable spatial resolution is to employ detectors of acoustic waves with impulse response profile of not longer than duration of the optoacoustic pulse emitted by a single voxel that needs to be resolved. Satisfaction of irradiation conditions of temporal pressure confinement is also required for the optoacoustic signals to accurately resemble profiles of absorbed optical energy in tissue. Distribution of absorbed optical energy can be used to visualize and characterize quantitatively various tissue structures and their physiological functions based on variations in tissue optical properties.
In order to relate tissue structure, functional state or quantitative measure of chromophore concentrations to optoacoustic images, the acoustic detectors must be capable of resolving not only rapid changes in optoacoustic signals associated with sharp edges and boundaries in tissues, but also reproduce slow changes associated with smooth variation in optical properties within one type of tissue. In other words, acoustic detectors have to detect both high and low ultrasonic frequencies of acoustic pressure at once. These types of acoustic detectors are called ultrawide-band acoustic transducers
[3]. These transducers have relatively equal detection sensitivity over the entire ultrasonic range from about 100 kHz to 10 MHz (and in some cases even higher up to 100 MHz). The ultrasonic detection bandwidth of acoustic transducers defines the limits of depth resolution. The lateral resolution, on the other hand, is defined by dimensions of each acoustic transducer, dimensions and geometry of the acoustic transducers in array. Only an array of transducers provides lateral resolution of optoacoustic images. The array of transducers can be simulated by scanning a single transducer along tissue surface.
[1] AA. Oraevsky, S.L. Jacques, R.O. Esenaliev: “Laser Optoacoustic Imaging System for Medical Diagnostics”, USPTO Serial # 05,840,023
[2] A.A. Oraevsky, A.A. Karabutov: “Optoacoustic Tomography”, in Biomedical Photonics Handbook, ed. by T. Vo-Dinh, CRC Press, Boca Raton, Florida, 2003, Vol. PM125, Chapter 34, pp. 34/1-34/34.
[3] A.A. Oraevsky: “Optoacoustic Tomography: From Fundamentals to Diagnostic Imaging of Breast Cancer”, in Biomedical Photonics Handbook, Second Edition: Fundamentals, Devices, and Techniques, ed. by T. Vo-Dinh, CRC Press, Boca Raton, Florida, 2014, Vol. PM222, Chapter 21, pp. 715-757.