WAUWATOSA, Wis. — When doctors in a remote African town warned a 20-something pregnant woman she was well past her due date, the Liberian patient agreed to have labor induced.

But Dr. Simon Kotlyar wanted to confirm the diagnosis first. So the visiting doctor performed an ultrasound test using a new system — a machine miniaturized to the size of a laptop computer — and discovered the Monrovia woman was only 32 weeks pregnant, not 40 weeks as anecdotal evidence had led doctors to believe.

"Having that system made a pretty big difference," said Kotlyar, chief resident in the department of emergency medicine at Yale-New Haven Hospital in Connecticut. "I told her to go home and come back in a few weeks."

As ultrasound machines become more compact and their image quality more precise, doctors have begun carrying the body-imaging technology to rural U.S. hospitals and developing countries. No longer is ultrasound available only to hospitals with reliable power supplies and room for bulky equipment.

The fledgling industry of portable ultrasound units has grown rapidly the past two years and it's expected to become an even bigger part of the ultrasound market. It is dominated by SonoSite Inc. of Bothell, Wash., and GE Healthcare, based in the Milwaukee suburb of Wauwatosa. Each commands about 40 percent of the worldwide market, according to industry expert Harvey Klein.

SonoSite pioneered the technology of hand-carried units in 1999. Industry leader GE Healthcare entered that market later — in 2002. Officials there said they had already planned their own line of portable units and SonoSite's entrance didn't alter their long-term strategy.

Cruder versions of portable ultrasound equipment have been around for almost 30 years but SonoSite was the first to produce images of usable quality, said Klein, president of Klein Biomedical Consultants in New York City.

The portable battery-operated machines aren't expected to completely replace standard console-sized units, just as laptop computers haven't rendered desktop models obsolete. But Klein said the compact machines are proving popular among doctors outside the traditional areas of radiology, cardiology and prenatal care.

Ultrasound produces real-time imaging of a beating heart or developing fetus by interpreting sound waves bounced off solid internal objects.

Other common techniques for exploring inside the body without surgery include MRI — magnetic resonance imaging — systems that yield vivid results but whose scans require the patient to lie still for minutes. X-rays are still popular for producing images of bones, but the technology exposes a patient to potentially harmful radiation.

Ultrasound has its own drawbacks. For example, its effectiveness depends on operators knowing precisely how to position patients and where to place the imaging probes to reveal the best views.

To doctors, image quality is key. The newer compact units can now produce images comparable to those of the higher-end console units about 90 percent of the time, said Dr. Craig Sable of the Children's National Medical Center in Washington, D.C.

GE Healthcare said the market for portable ultrasound is small but its business is growing rapidly, with sales of $174 million in 2006, a 74 percent increase over 2005.