In many ways, ultrasound waves are ideally suited to noninvasive biomedical imaging. They’re easy and inexpensive to produce and detect, and 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 to about half a wavelength. In clinical ultrasound applications, which use wavelengths between 200 µm and 1 mm, that limit precludes the imaging of many important structures, including small blood vessels. Shorter wavelengths yield better resolution, but they also penetrate less deeply into tissue.

For optical applications, innovative fluorescence techniques have been devised to overcome thelimit, as recognized by the 2014 Nobel Prize in Chemistry (see December 2014, page 18 ). Inspired by that work, Mickael Tanter and his colleagues at the Langevin Institute (affiliated with ESPCI, Inserm, and CNRS) in Paris have now developed atechnique,which they’ve used tothe blood vessels in a rat’swith 10-µm resolution, as shown in figure. Applying the technique in humans could help to detectand other diseases that alter blood-flow patterns.

Fluorophores and microbubbles Section: Choose Top of page ABSTRACT Fluorophores and microbub... << Twisty vessels REFERENCES CITING ARTICLES

In conventional fluorescence microscopy, one decorates a specimen of interest with fluorescent molecules, or fluorophores, that emit photons at a characteristic wavelength when they’re optically excited. Each fluorophore produces a diffraction-limited blur of light, several hundred nanometers in diameter; the blurs from all the fluorophores overlap and combine to yield a low-resolution image.

imaging techniques based on that principle. 2 et al. , Science 313, 1642 (2006); 3, 793 (2006); 91, 4258 (2006). 2. E. Betzig, Science, 1642 (2006); https://doi.org/10.1126/science.1127344 M. J. Rust, M. Bates, X. Zhuang, Nat. Meth., 793 (2006); https://doi.org/10.1038/nmeth929 S. T. Hess, T. P. Girirajan, M. D. Mason, Biophys. J., 4258 (2006). https://doi.org/10.1529/biophysj.106.091116 researchers repeatedly imaged the specimen, with a different set of fluorophores activated each time, to build up a high-resolution picture. But if the blur from a single fluorophore can be somehow isolated (for example, by rendering all the surrounding fluorophores temporarily unable to fluoresce), then its center—the fluorophore’s position—can be pinpointed precisely. Within a few months of one another in 2006, three groups—led, respectively, by Eric Betzig of Janelia Farm, Xiaowei Zhuang of Harvard University, and Samuel Hess of the University of Maine—publishedtechniques based on that principle.In each case, therepeatedlythe specimen, with a different set of fluorophores activated each time, to build up a high-resolution picture.

Tanter learned about the new superresolution optical techniques from Hess in 2009, when the two researchers were invited lecturers at a summer course at Cold Spring Harbor Laboratory in New York. Tanter got the idea for a similar technique based on ultrasound and worked with his Langevin Institute colleagues, including Olivier Couture and PhD student Claudia Errico, to bring it to fruition.

In the role of the fluorophores the ultrasound technique uses micron-sized bubbles of inert gas. Because they’re strong scatterers of ultrasound and safe when injected into the bloodstream, microbubbles are an established means of enhancing acoustic contrast in medical imaging. But unlike fluorophores, their scattering can’t readily be turned off and on. Another approach would have to be found to make the bubbles behave as nonoverlapping point sources—the key requirement for superresolution imaging.

ultrasound signals don’t overlap. That approach has recently been tried by researchers at Imperial College London and Kings College London. 3 et al. , Phys. Med. Biol. 58, 6447 (2013); et al. , IEEE Trans. Med. Imag. 34, 433 (2015). 3. O. M. Viessmann, Phys. Med. Biol., 6447 (2013); https://doi.org/10.1088/0031-9155/58/18/6447 K. Christensen-Jeffries, IEEE Trans. Med. Imag., 433 (2015). https://doi.org/10.1109/TMI.2014.2359650 imaged the blood vessels in a mouse’s ear with subwavelength resolution, but it took a long time—on the order of an hour—to build up the composite images. One possible solution is simply to use fewer of them: If the bubbles themselves are hundreds of microns apart, theirsignals don’t overlap. That approach has recently been tried byat Imperial College London and Kings College London.Theythe blood vessels in a mouse’s ear with subwavelength resolution, but it took a long time—on the order of an hour—to build up the composite