Ultrasound, by definition, is sound that lies beyond the range of human hearing. So how can a fetus hear an ultrasound scan?

As explained by the researchers Mostafa Fatemi and colleagues at the Mayo Foundation, Minnesota, traditional imaging systems produce ultrasound as sequences of short-duration, high-energy bursts called "pulse trains."

When the pulses enter the body, they tap internal organs at a regular rate. When the ultrasound points at the head of the fetus, its sensitive hearing structure gets vibrated at a rate equal to the number of pulses per second. (Typically, several thousand pulses are transmitted per second in a pulse train, a rate equal to several thousand Hertz.)

The fetus senses these vibrations as tones, equivalent to the high notes of a piano. The sound can get loud -- about the equivalent of 100-120 decibels of airborne sound, or the level of sound of an approaching subway train.

Rather than being akin to a sound from the outside world, though, the sensation is more like what you hear when your finger taps a spot close to an ear -- which is why it's inaudible to others, including the mother.

What's more, the sound is focused on a tiny, square-millimeter spot, and the sound diminishes rapidly from that spot, so that the fetus could quickly adjust its position to avoid the loudness.

Fatemi stresses that their findings do not suggest that this sound is harmful to a fetus.

These studies can help explain physicians' observations that a fetus moves vigorously when ultrasound is directed at its head. They eliminate the notion that ultrasound is a passive observation technique, but they may also inspire new ultrasound exams for testing normal fetal function. (Paper 1pBB6 at meeting; abstract at this URL.)

(Editor's Note: This story, with only minor editing, is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 568, December 7, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon.)


[Contact: Mostafa Fatemi]

10-Dec-2001