Joan Brennan was 33 weeks pregnant and needed X-rays to show what was blocking her intestine. Because she had received several conventional X-rays elsewhere, doctors at the University of Connecticut Health Center worried that more radiation might hurt her baby. So they used a new X-ray system, called Micro-Dose, that cuts exposure to as little as one thousandth the usual level, depending on the type of examination.

Using Micro-Dose, doctors shot a beam as small as a pencil point back and forth over Brennan and fed the picture into a computer one tiny dot at a time. Doctors watching on a computer screen could stop the beam when it got too close to the fetus.

Micro-Dose is the latest commercially available medical X-ray system from American Science & Engineering Inc. (AS&E), a Cambridge, Mass., company with $19 million in sales. Although it uses digital electronics, as do computed tomographic (CT) scanners and nuclear magnetic resonance systems, Micro-Dose is aimed at the conventional X-ray market, which accounts for about 85% of a typical hospital's radiology work. So far, four hospitals have bought the $450,000 system, which costs almost twice as much as a conventional one.

Micro-Dose pictures don't show as much detail as conventional X-rays. " It's the difference between a very dull light bulb and a bright one," says Thomas Spackman, professor and head of radiology at the Connecticut center. MicroDose, he says, must prove it can take good chest X-rays, the most common type. But because the computer can calculate angles and lengths, Spackman says, the system is good for diagnosing curvature of the spine or other conditions for which doctors need to take measurements. Sometimes, too, Micro-Dose helps doctors see things they might miss in conventional images. Adjusting the contrast, for example, might reveal patches of lung cancer hidden by the heart and diaphragm.

Tighter government cost-control measures have made some potential customers reluctant to buy the system. But AS&E says that Micro-Dose saves money because the X-rays don't have to be printed out on film. The radiologist can adjust the contrast on the computer screen and produce a paper printout, avoiding the expense of over- and underexposed pictures that would otherwise have to be retaken.

Still, old habits die hard. Spackman says Connecticut's orthopedic group prefers to print out Micro-Dose pictures on film.