Ultrasound reveals capillaries and cells in living organs

Ultrasound is one of the most widely used imaging techniques in medicine, but so far it has not been capable of observing body tissues at the scale of cells. Now a team of scientists from Delft University of Technology (TU Delft), the Netherlands Institute for Neuroscience and the California Institute of Technology (Caltech) has managed to image specifically labelled cells in 3D with ultrasound, with their results published in the journal Science.

Current light-based microscopes often require imaging of non-living samples, meaning the samples need to be removed from the body and you lose the ability to track activity of cells over time. The present leading technology to image how living cells behave in 3D — for example, during the development of embryos — is called light-sheet microscopy, and this is limited to translucent or thin specimens because light cannot penetrate deeper than 1 mm in opaque tissue.

“Ultrasound can image centimetres deep in opaque mammal tissue, allowing non-invasive imaging of whole organs,” said lead researcher David Maresca, from TU Delft. “This gives us information about how cells behave in their natural environment, something that light-based methods can’t do in larger, living tissues.”

The team has successfully imaged living cells inside whole organs across volumes the size of a sugar cube. Key to their innovation in ultrasound imaging — a method called nonlinear sound-sheet microscopy — was the discovery of a sound-reflecting probe made in the Shapiro Lab at Caltech.

“This probe is a nanoscale gas-filled vesicle that lights up in ultrasound images, making cells visible,” explained first author Baptiste Heiles, previously from TU Delft and now based at Caltech. “These vesicles have a protein shell and we can engineer them to tune their brightness in images. We used these gas vesicles to track cancer cells.”

In addition to revealing cells, the team used ultrasound and microbubbles as probes circulating in the blood stream to detect brain capillaries. Heiles noted, “To our knowledge, nonlinear sound sheet microscopy is the first technique capable of observing capillaries in living brains. This breakthrough has tremendous potential to diagnose small vessel diseases in patients.”

Since microbubble probes are already approved for human use, this technique could be deployed in hospitals in a few years. Going beyond clinical practice, sound-sheet microscopy could greatly benefit biological research and the development of new cancer treatments in particular, according to Maresca.

“Our imaging technique can distinguish healthy versus cancer tissue,” Maresca said. “Furthermore, it can visualise the necrotic core of a tumour; the centre of the tumour where cells start dying due to a lack of oxygen. Thus, it could assist in monitoring the progression of cancer and the response to treatment.”

Image caption: Artist’s impression of the new ultrasound technique, called nonlinear sound sheet microscopy. The blue lines represent two ultrasound waves intersecting in an X-shape pattern to scan cells and capillaries in body tissue. The cells and capillary vessels are labelled with ultrasound probes (in cyan and pink), making them light up in ultrasound images. Image credit: Maayan Harel, Maresca Lab.