Ultrasound-modulated optical tomography（UOT）is an imaging technology that uses Acousto-Optic effect, also called Acousto-Optic imaging（AO）

1. Sound and Light Effect

When the ultrasonic wave passes through the medium, periodic elastic deformation occurs in it, so that the refractive index of the medium changes periodically, which is equivalent to a moving phase grating, which is called the acousto-optic effect. If there is light passing through the medium at the same time, the light will be diffracted by the phase grating, which is called acousto-optic diffraction. Using this principle, the intensity, direction and frequency of the laser beam of light can be modulated.

As shown in the figure, in short, sound waves can affect the light path through the medium by changing the properties of the medium.

2. Principle of Acousto-Optic Imaging

As shown in the figure below, there are two main ways of acousto-optic effect:

1. Use ultrasound to change the pressure field of the medium and change the refractive index of the medium to change the light path through the medium.

2. Use ultrasound to move the scattering particles to change the light path.

Ultrasound (US) scatters much less in tissue than light, so we can use it for localization in tissue. Small ultrasound pulses of 1 µs length are irradiated with 1 µs laser pulses. During this 1 µs irradiation time, the US encapsulation is almost fixed in the tissue, and modulate the photons passing through this US pressure field. Of all the light that passes through the entire sample, only a small fraction is modulated by the ultrasound.

The modulation is carried out in the following ways:

1: The refractive index of the medium changes due to the action of ultrasonic pressure (Raman-Natter effect);

2: The scattering particles vibrate slightly as the US frequency changes the optical path. Scattering on vibrating particles causes the Doppler shift to vary with US frequency. Finally, the ultrasonic wave changes the optical path and frequency modulates the light, and this small part of the modulated light has the information of the local fluence. Then, by measuring the marked local information, the characteristic information of the medium sample can be obtained.

The acousto-optic signal acquisition device is shown in the figure below:

3. Common UOT System Structure

The basic system structure is shown in the figure. The US probe transmits ultrasonic waves to the biological tissue and focuses it on the biological tissue, and then the Laser emits the laser. When the laser passes through the biological tissue, the frequency and phase changes are sent at the point where the ultrasonic focus is focused to obtain the mark, and then the laser is transmitted through the biological tissue. The CCD receives the laser light. By analyzing the marking information in the laser, the changes sent by the biological tissue due to ultrasound can be obtained, so as to obtain the information of the biological tissue, and the received optical signal itself also carries the optical imaging information, so the acousto-optical imaging combines the acoustic signal and the optical signal. Information. Optical signals have high contrast, and acoustic signals have high resolution.

Two typical hardware diagrams of acousto-optic systems:

4. Acousto-Optic Image Reconstruction Algorithm

The current acousto-optic image reconstruction algorithm is mainly based on iteration

In the paper of Reconstruction and stability in acousto-optic imaging for absorption maps with bounded variation, a fast iterative method is proposed, as shown in the following figure:

In the paper of Ultrafast acousto-optic imaging with ultrasonic plane waves, it proposes an ultrafast imaging method using ultrasound phase waves, but requires two ultrasound probes