[1] It is used in imaging of biological soft tissues and has potential applications for early cancer detection.
[2] As a hybrid modality which uses both light and sound, UOT provides some of the best features of both: the use of light provides strong contrast and sensitivity (both molecular and functional); these two features are derived from the optical component of UOT.
However, the difficulty of tackling the two fundamental problems with UOT (low SNR in deep tissue and short speckle decorrelation time) have caused UOT to evolve relatively slowly; most work in the field is limited to theoretical simulations or phantom / sample studies.
Eventually, despite the strength of optical scattering in tissue, some of these photons will pass through the ROI.
[4] Sufficiently coherent light traveling through a medium creates a speckle pattern (Fig 1 of next citation).
The changes of these speckle patterns are used to derive various properties of the tissue during reconstruction and analysis.
[5] Optical imaging modalities typically rely on ballistic photons to collect and convey information.
[7] DOT has fantastic penetration, on the scale of centimeters, but suffers from inferior resolution (~1cm).
Both imaging modalities use diffuse photons, which typically cannot be used to transmit information from deep within the tissue.
UOT and PAI have different ways to effectively transfer information from the diffuse photons within the tissue back to the system, which allows for centimeter imaging depths (deeper than DOT) while retaining high spatial resolutions (millimeters to 100s of micrometers).
Thus, UOT can achieve imaging depths (exploiting diffuse photons) deeper than 9 cm into the body while retaining high spatial resolutions (from the dimensions of the ultrasound focus), ~mm scale, as of 2017.
The target within the testing medium will be irradiated by a laser beam and a focused ultrasonic wave.
These properties will be used to reconstruct images showing the inside view of the medium.
1 Incoherent Modulation of Light Due to Ultrasound-Induced Variations in Optical Properties of Medium.
This variation in mass density will then influence the local optical properties.
With different optical properties, the reemitted light features (like intensity) will be modified.
2 Variations in Optical Phase in Response to Ultrasound-Induced Displacements of Scatterers.
3 Variations in Optical Phase in Response to Ultrasonic Modulation of Index of Refraction of Background Medium.
This effect can further influence the free-path phases when light passes through the ultrasonic region and form a speckle pattern.
[5] These three mechanisms are the fundamental building blocks required to design a UOT system.
Es demsontrates the unit-amplitude electric field of the scattered light of a path of length s, and p(s) denotes the probability density function of s.[13] In analytic UOT model, we treat the light source as an optical plane wave.
represents how ultrasound averagely change the light per free path via index of refraction and displacementare respectively.
[14] For simplicity, the Fourier transformed term exp(-iω0t) is dropped, and the ω here represents relative angular frequency of unmodulated light.
In conclusion, in UOT analytic model, with the help of weak-scattering approximation, weak-modulation approximation, diffusion theory, and Wiener-Khinchin theorem, the relationship between acoustic amplitude and modulated light can be successfully observed.
To improve the axial resolution, Ultrasonic frequency-swept UOT model is designed.
Basically, a function generator will produce a frequency signal relating to time.
The electrical signal will then pass through an amplifier, an Oscilloscope and be stored in the data base.
In the end, all the 1D image will be pieced together to generate a full view inside the medium.
In summary, a frequency-swept (chirped) ultrasonic wave can encode laser light traversing the acoustic axis with various frequencies.
[17] Recent advances in UOT (2020 onwards) include 1) the development of Coded Ultrasound Transmissions for SNR gain in AOI,[18] 2) the development of Homodyne Time of Flight AOI,[9] 3) the use of super-resolution techniques to improve UOT beyond the acoustic diffraction limit,[11] and 4) the use of coaxial interferometry to better enable modern high-performance cameras for parallel detection of UOT signals.