ULTRASOUND scans can stop cells from dividing and make them commit suicide. A
research team in Ireland say this is the first evidence that routine scans,
which have let doctors peek at fetuses and internal organs for the past 40
years, affect the normal cell cycle.
A team led by Patrick Brennan of University College Dublin gave 12 mice an
8-megahertz scan lasting for 15 minutes. Hospital scans, which reflect inaudible
sound waves off soft tissue to produce images on a monitor, use frequencies of
between 3 and 10 megahertz and can last for up to an hour.
The researchers detected two significant changes in the cells of the small
intestine in scanned mice compared to the mice that hadn鈥檛 been scanned. Four
and a half hours after exposure, there was a 22 per cent reduction in the rate
of cell division, while the rate of programmed cell death or 鈥渁poptosis鈥� had
approximately doubled.
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Brennan believes there will be similar effects in humans. 鈥淚t has been
assumed for a long time that ultrasound has no effect on cells,鈥� he says. 鈥淲e
now have grounds to question that assumption.鈥�
Brennan stresses, however, that the implications for human health are
uncertain. 鈥淭here are changes happening, but we couldn鈥檛 say whether they are
harmful or harmless,鈥� he explains. The intestine is a very adaptable organ that
can compensate for alterations in the cell cycle, says Brennan.
It is possible that the sound waves damage the DNA in cells, delaying cell
division and repair. Brennan suggests that ultrasound might be switching on the
p53 gene which controls cell deaths. This gene, dubbed 鈥渢he guardian of
the genome鈥�, produces a protein that helps cells recognise DNA damage and then
either self-destruct or stop dividing.
Studies in the early 1990s by researchers at the University of Rochester in
New York and the Batelle Pacific Northwest Laboratories in Richland, Washington,
showed that tissue heating due to ultrasound can cause bleeding in mouse
intestines. Ultrasonographers now tune the power of scans to reduce such
heating.
But Brennan鈥檚 work is the first evidence that scans create changes in cells.
鈥淥ur results are preliminary and need further investigation,鈥� he says. The team
presented their results at the Radiology 1999 conference in Birmingham last
month and are now preparing them for submission to a peer-reviewed journal.
Alex Elliott, a researcher in clinical physics at the University of Glasgow,
thinks that Brennan鈥檚 results are important and should be followed with further
studies. 鈥淚f the conditions of his experiments really compare to the clinical
use of ultrasound,鈥� he says, 鈥渨e may have to review the current safety limits.鈥�