Super-Resolution Photoacoustic Imaging

These images compare the imaging of blood flowing through channels with various approaches. At top are single photoacoustic images from the image stack the researchers analyzed. At bottom images show the fluctuation analysis, with five channels clearly resolved in the final fluctuation analysis. (Source: B. Arnal, Grenoble U.)

Researchers have reported an approach to photo­acoustic imaging that offers vastly improved reso­lution, setting the stage for detailed in vivo imaging of deep tissue. The technique is based on compu­tational improve­ments, so it can be performed with existing imaging hardware, and thus could provide a practical and low-cost option for improving bio­medical imaging for research and diag­nostics.

After further refine­ments, the approach could offer the oppor­tunity to observe the minute details of processes occurring in living tissue, such as the growth of tiny blood vessels, and therefore provide insights on normal develop­ment or disease processes such as cancer. “Our main goal is to develop a micro­scope that can see the micro­vasculature and capillary vessels,” said Ori Katz, a researcher with the Hebrew Uni­versity of Jerusalem, Israel. “It’s important to be able to watch these grow with nearby tumors, for example.”

The researchers report overcoming the acoustic dif­fraction limit, a barrier that previously limited the resolution obtainable with photo­acoustic imaging, by exploiting signal fluc­tuations stemming from the natural motion of red blood cells. Such fluc­tuations might other­wise be considered noise or viewed as detrimental to the measurem­ents. “With photo­acoustic imaging you can see much deeper in tissue than you can with an optical micro­scope, but the resolution is limited by the acoustic wave­length,” Katz said. “What we have dis­covered is a way to obtain photo­acoustic images with consi­derably better reso­lution, without any change in the hardware.”

Photo­acoustic imaging combines optical illumination and ultra­sound to image biolo­gical samples in ways that would not be possible with either modality alone. Optical methods can provide excellent resolution but often only near the surface as light is highly scattered in tissue. Ultra­sound can go much deeper but does not offer the same contrast as optical imaging. By inte­grating the two modalities, researchers have been able to overcome the drawbacks of each to advance a host of appli­cations. However, the imaging technique does have certain limi­tations. Photo­acoustic imaging relies on acoustic detection, so the image resolution is determined by the acoustic wavelength. While optical micro­scopy, for example, can see objects on the scale of less than a micron, photo­acoustic imaging is limited to tens of microns. This means that photo­acoustic imaging cannot resolve small objects like micro­vessels or capil­laries.

Katz devised the method for surpassing the acoustic dif­fraction limit in colla­boration with Emmanuel Bossy, now at Univer­sité Grenoble Alpes in Grenoble, France. At the heart of their work is an advanced statistical analysis framework that they apply to images of red blood cells flowing through the vessels; the blood cells faci­litate imaging by absorbing light at particular wave­lengths. By increasing the reso­lution computa­tionally, they avoided the need for any addi­tional hardware, so the advances described can be attained using existing photo­acoustic imaging systems.

The tools needed to achieve super-resolution with photo­acoustic imaging were described nearly a decade ago in a work in optical micro­scopy with the technique super-reso­lution optical fluc­tuation imaging (SOFI). Katz and colleagues came to this work after grappling with the problem of the acoustic dif­fraction limit and discovered that the same mathe­matics used with SOFI could be used for improving photo­acoustic imaging. “Someone just needed to make the connection,” Katz said. “It’s the same equation – the wave equation. Mathe­matically, you could say it’s the same problem.”

Last year, Katz and his colleagues demonstrated the ability to surpass the acoustic diffraction limit using a SOFI-inspired photo­acoustic imaging technique. That work had two main limi­tations. First, it required the use of a long-coherence laser, not a standard part of photo­acoustic imaging systems, in order to form dynamic structured inter­ference patterns called speckle to create the signal fluc­tuations. Second, due to their small dimensions, the use of speckles as dynamic illu­mination resulted in the fluc­tuations having a low amplitude with respect to the mean photo­acoustic signal, which in turn made it difficult to resolve the specimen in question.

Now, the researchers showed that they could overcome these limi­tations by applying the sta­tistical analysis framework to the inherent signal fluc­tuations caused by the flow of red blood cells. So the researchers didn’t need to rely on coherent structured illu­mination and furthermore demonstrated experi­mentally that they could perform super-resolution photo­acoustic imaging using a conven­tional imaging system. The demon­stration served as a proof of principle for the new technique. The researchers are now focused on developing it further, to fulfill its potential for in vivo appli­cations.

Katz described two main challenges in reaching this goal. The first is the problem of motion artifacts. In their demon­stration, the researchers imaged blood streaming through small tubes. In animal models and in humans, though, blood flow is only one of the motions they would have to consider. The technique would also need to account for the heartbeat, the changing volume of the vessels and even micro­scale movements of the tissue itself.

The other main challenge relates to signal levels. In recent experi­ments blood was the only absorber in play, but in real-world scenarios other absorbers would be present. The researchers are now working on ways to better see the signal origi­nating from flow while sup­pressing any back­ground signals. In addition to tackling these challenges, the team is working to apply sophis­ticated recon­struction algo­rithms that will further increase the resolution and back­ground reduction by taking into account prior infor­mation about blood flow, the imaging system response and other factors. (Source: OSA)

Reference: T. Chaigne et al.: Super-resolution photoacoustic imaging via flow-induced absorption fluctuations, Optica 4, 1397 (2017); DOI: 10.1364/OPTICA.4.001397

Link: NeuroCure Cluster of Excellence, Berlin, Germany • Dept. of Applied Physics, Hebrew University of Jerusalem, Jerusalem, Israel

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