Developing a technique for combining light and ultrasound for deep tissue imaging

Abstract

Biological tissues are strongly light-scattering and absorbing media that limit the depth of optical-based imaging. Even though, optical imaging provides good optical contrast information of biological content that is beneficial for clinical diagnostics application. Ultrasound was used to assist the optical-based imaging technique in overcoming the poor spatial resolution and shallow imaging depth difficulties. Ultrasound can penetrate deep into tissue. When the tissue is illuminated, the ultrasound modulates the light inside the ultrasound focus. The modulated light that shifted by one ultrasound frequency is called the tagged photons. Measure of the tagged photons intensity distribution would provide local light irradiance information in the tissue, and this is the principle of ultrasound optical tomography (UOT). The UOT system can have a spatial resolution as good as the ultrasound focus. The goal of this thesis work is to check the feasibility of UOT at 606 nm. The thesis work was first started with characterizing tissue models with controlled optical properties, homogeneity and macroscopic geometry. Then the performance of an ultrasound scanner was investigated. The UOT experiment was carried out on multiple tissue models with different thicknesses but the same optical properties. When the laser energy was 30 nJ, the current UOT system can detect UOT signal for a 40 mm thick tissue model. And it was also capable of performing a 1D line measurement on a 10 mm thick highly scattering medium with an absorbing inclusion in the center, the dimension of the inclusion can be estimated from the UOT measurements.

Popular Abstract

The discovery of X-rays was the key to developing medical imaging methods. Since then, medical imaging techniques have advanced to the point where it can provide real-time, 2D or 3D, visual representations of the internal structures of a human body. The three most commonly used techniques are MRI, CT, and PET scans, all with their advantages and disadvantages. The MRI uses a strong magnetic field to reconstruct an image from resonating atoms, but this means that patients with metal implants, like pacemakers, cannot perform an MRI scan. Patients can also be allergic to the radiotracers used in the PET scan, and exposure to the X-rays during a CT-scan carries a risk of cancer. Thus, the world is still looking for
other imaging techniques that can overcome those limitations.

Medical imaging shares the same basis as photography. If using a regular camera, to be able to take a picture of an object, you first need to be able to see it. In a pitch-black room, you would not be able to image the object without using the flash on the camera, there needs to be enough light to register an image on the camera sensors. If you enable the flash mode on your camera and block the flash with your hand, you should notice that your hand glows red when the flash is on. The reason your hand glows red color is due to the absorption of the tissue is wavelength dependent. For human tissue, more blue light is absorbed compare to red light. Even though light travels through tissue in such a way that it appears that light fills the inside, but it does not allow an observer from outside to see any structure within it. This is because light scatters in all directions multiple times while inside the tissue, there is no way we can directly trace back where the light we observed came from. If all the light particles, called photons, that passed through a certain region inside the tissue were labeled, even if they were scattered away afterward, we can still determine the origin of these labeled photons. That is the fundamental principle of developing the imaging technique called ultrasound optical tomography (UOT). Focused ultrasound and light are simultaneously applied to the tissue. Photons inside the ultrasound focus are tagged; the intensity of tagged photons is dependent on the encountered biological contents optical properties. An UOT tissue image can be acquired by moving the ultrasound focus while recording the tagged photons intensity changes.

• author: Li, Meng
• supervisor: Stefan Kröll
• organization: Atomic Physics; Department of Physics
• course: FYSM60 20181
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: UOT, Ultrasound Optical Tomography, Monte Carlo, TCSPC, PTOF, Photon time of flight, Ultrasound, US, AOT, Acousto-optical tomography, Spectral hole burning, SHB, ultrasound modulation
• report number: LRAP-546
• language: English
• additional info: LRAP-546
• id: 8953171
• date added to LUP: 2018-07-12 20:35:58
• date last changed: 2018-07-12 20:35:58