Magnified kidney stone chips
W.D. Auer / Alamy
Beams of ultrasound could be used to remove kidney stones by steering them through the body. In experiments on pigs, a team using the approach was able to move glass beads along a predefined course.
Kidney stones arise when minerals dissolved in urine form crystals. They can get stuck inside the kidneys or in the ureter, the narrow tube that leads from the kidneys to the bladder, becoming painful.
One treatment involves breaking the stones up into smaller pieces, so they can more easily exit the body in urine. This may be done by pushing a long thin tube up into the ureter from the bladder, or by firing ultrasound shock waves from outside the body, but both methods can leave fragments behind that encourage more stones to grow. More severe cases require surgery.
A team led by Michael Bailey at the University of Washington in Seattle has been exploring a different approach, using gentler ultrasound waves to release stones from where they have got stuck.
The first idea the team tried was giving a stone a small nudge, to boost its chances of moving along naturally. The first test of this, in 15 people, suggests that smaller stones can usually be made to move a little using this approach, helping them to be cleared from the body, although it wasn’t a placebo-controlled trial.
Now Bailey’s group has found a way to better control how the stone moves, using carefully targeted sound waves to create a ring of high pressure around the stone, trapping it in place. If the ring is moved, it drags the stone along with it. “Even moving it just a small distance will help,” says Bailey.
The team tested the approach on three anaesthetised pigs, using glass beads that had been placed inside their bladders to stand in for kidney stones. The beads were successfully steered along pre-chosen routes with over 90 per cent accuracy. “It’s really quite controllable,” says Bailey, who consults for US firm SonoMotion, which has licensed the technology.
Moving spherical beads within a large space such as the bladder is likely to be easier than steering irregular-shaped crystals through a narrow tube, says Ben Turney at the University of Oxford. But he does think the technique could help push stones out of places in the kidney where they tend to lodge. “Then you would hope that the system will flush them through.”
Journal reference: PNAS, DOI: 10.1073/pnas.2001779117
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