There's a reason the dental assistant leaves the room while your teeth are being X-rayed. Prolonged exposure to the radiation can be harmful. But if you assume that the dentist has done everything possible to minimize your exposure too, guess again.

Effective and low-cost technology to protect dental patients from low-level radiation has been around for as long as 25 years, but most dentists don't use it.

The next time you're in the chair, drop the term "rectangular collimation" on your dentist. He or she probably heard it in dental school and then promptly forgot it. In common language, rectangular collimation is the process for targeting X-rays only to the film used for the exposure. Without it, the face and neck absorb much of the radiation for no medical or diagnostic purpose.

The process is simple. Dentists can use a small metal film holder or an attachment to their X-ray machine with collimation plates. Several varieties are available from various manufacturers, usually at a one-time cost to the dentist of as little as $20.

Dental experts told us that less than 1 percent of American dentists use rectangular collimation. The process is usually mentioned in American dental schools, but only a minority of those schools stress it.

It isn't the only safeguard dentists ignore. X-ray radiation could be limited if dentists used type-E film, which has been available for nearly a decade. It requires less than half the exposure time than the more popular type-D film.

According to Eastman-Kodak sales figures, type-D film is still the X-ray film of choice for American dentists. They find it easier to use, and it produces marginally better exposures than type-E. But for finding hidden cavities, type-E film works just fine.

Dentists sacrifice safety for convenience in other ways too. Many dentists and their assistants opt to develop X-ray exposures manually, dipping the film into vats of chemicals in the office. The process results in a dim image, so the assistants turn up the power on the X-ray machines to get a darker image. Automatic film developers are available, but some dentists would rather not pay the price.

Doctors and professors of dentistry told our associate Tim Warner that the cumulative effect of the safety devices available could reduce the amount of radiation exposure by 10 times.

The failure of dentists to catch on has Theodore P. Croll and other experts grinding their teeth. Croll, a specialist in pediatric dentistry from Doylestown, Pa., has made the use of safer techniques his personal crusade, but with little success. Croll has appealed to many consumer groups, which have shown some interest but weren't willing to pressure the dental industry to change its ways.

"The best situation is to have no radiation," Croll told us. "The second best is to have as little radiation as possible. It's just a matter of convincing dentists that the time to change is long past due." Stuart C. White, a professor at the UCLA School of Dentistry, thinks dentists will change when patients demand it. "Patients are going to become informed and they're going to take the information to the office and dentists are going to have to be responsive."