BOSTON, DEC. 5 -- Doctors take four times as many X-rays when they own X-ray machines and make money on them, according to a study that provides new evidence of how profits may influence the way physicians practice medicine.

Doctors make a profit on every picture they take with their equipment. But they earn nothing when they send patients to other specialists for X-rays.

In recent years, many doctors have purchased X-ray and ultrasound machines so they can make diagnostic images in their offices. While they argue that this is convenient for them and their patients, critics suspect that the chance to make more money is also a big incentive to buy and operate these machines.

The new study provides circumstantial evidence that doctors who own imaging machines may overuse the equipment, simply to collect fees. Compared with those who send their patients to radiologists, those who own X-ray and ultrasound machines take four times as many diagnostic images for such common complaints as colds and backaches, according to the study published in Thursday's New England Journal of Medicine.

Bruce J. Hillman of the University of Arizona, who directed the study, cautioned that it could not determine whether the extra X-rays were needed. It also could not delve into doctors' reasons for taking them.

However, he said, "Almost certainly one has to wonder whether it's an issue of having these machines and realizing that incomes can be increased by their utilization. That doesn't mean I believe that physicians are sinisterly abusing imaging. But when they come to the crossroads of making the decision and they have the machine in the office, it's awfully easy to decide that the patient needs it."

Ethical guidelines agree that doctors' decisions should be based solely on what is best for patients' health, not physician's finances.

An editorial last year in the Journal of the American Medical Association said that while doctors are free to own equipment, "the patient's interest must always be uppermost. Exploitation for any reason is reprehensible."

Primary care physicians who own X-ray machines were critical of the study and its implications. James Weber, a family practitioner in Jacksonville, Ark., said doctors who do more X-rays may be practicing better medicine.

"The assumption is inferred that these tests are ordered to benefit the physician rather than the patient," he said. "The vast majority of physicians have their patients' interests at heart. This article makes the assumption that they don't."

J. Harold Morris, an internist in Kansas City, said doctors who buy the equipment may be inclined to take more X-rays because of their training or medical outlook.

"It's my belief that it's more physician behavior in terms of accepting uncertainty and needing test results rather than greed," he said. "I suspect most doctors using it this way were trained this way or are afraid of malpractice."