Superresolution ultrasound

Ultrasound waves are well suited to biomedical imaging because they can penetrate deep into tissue without losing their coherence or causing damage. But because of diffraction, conventional ultrasound imaging, like conventional optical microscopy, is limited in resolution by wavelength. In clinical ultrasound, which uses wavelengths between 200 µm and 1 mm, that limit precludes the imaging of many important structures, including small blood vessels. As highlighted by the 2014 Nobel Prize in Chemistry (see Physics Today, December 2014, page 18 ), several recently developed optical fluorescence techniques can beat the diffraction limit. Inspired by that work, Mickael Tanter and his colleagues at the Langevin Institute (affiliated with ESPCI, Inserm, and CNRS) in Paris have now developed a superresolution ultrasound technique, which they’ve used to image the blood vessels in a rat’s brain with 10-µm resolution. They injected the rat bloodstream with a solution of inert gas microbubbles (a safe and established means of enhancing acoustic contrast), imaged the brain at 500 frames per second, and looked at the differences between successive images. In each difference image, the small number of microbubbles that moved or disintegrated between frames appeared as wavelength-sized blobs. Because the blobs were sparse enough not to overlap, their centers, which mark the bubbles’ positions, could be located accurately. By superposing the bubble positions from thousands of difference images, Tanter and colleagues built up composite images such as the one shown in the figure. Applying the technique in humans could help to detect cancer and other diseases that alter blood-vessel patterns. (C. Errico et al., Nature 527, 499, 2015 .)