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Photoacoustic computed tomography (PACT) is a non-invasive hybrid imaging technique that excites biological tissues with light and detects the subsequently generated ultrasound to form images. PACT combines the advantages of both optical imaging—high optical contrast, and ultrasonic imaging—high resolution and deep penetration in biological tissues. PACT has been widely used for vascular network mapping, functional brain imaging, and tumor detection in deep tissues.
However, by detecting ultrasonic waves, PACT cannot escape the doom that is faced by all wave-based imaging techniques: the diffraction of waves presents a fundamental limit on its spatial resolution. Due to ultrasonic diffraction, an absorbing point source is spread out as a disk (point spread function) in its image, which has a size comparable with the ultrasound wavelength. Therefore, structures in tissues are smeared out by this disk and blurred, and any features that are separated by a distance smaller than the ultrasound wavelength cannot be resolved. Although finer resolution can be achieved by detecting ultrasound with shorter wavelengths, the attenuation of ultrasound in tissues becomes stronger accordingly, limiting penetration.
Researchers - Caltech - Optical - Imaging - Laboratory
Recently, researchers at the Caltech Optical Imaging Laboratory, directed by Lihong Wang, developed a technique for in vivo super-resolution PACT. It breaks the acoustic diffraction limit by localizing the centers of single dyed droplets flowing in blood vessels. This technique resolves brain blood vessels at a six-fold finer resolution. The research has been published in Light: Science and Applications.
The researchers fabricated 'photoacoustically bright' oil-in-water droplets using a solution of a hydrophobic dye, namely, IR-780 iodide in oil. The sizes of the droplets range from 4 to 30 microns, which are much smaller than the wavelengths of the detected ultrasound, making them excellent photoacoustic point sources. Taking advantage of their small sizes, liquid compliance, and high photoacoustic 'brightness', once injected into the bloodstream, the droplets flow smoothly in blood...
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