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Dr Lu says it helps to think of each microbubble as a car in traffic.

Other winning images included extreme close-ups of kitchen sink soap bubbles, a drug-coated microbubble, and a butterfly wing.

"But as soon as it hits a very small microbubble of gas, there's a big reflection. It's a very good scatterer of ultrasound."

Alzheimer's mice in the control group, which received microbubble injections but no stimulation, showed no improvement.