But the identity of a patient’s orthopedist explains a lot more about where he or she got her M.R.I. than any other factor considered, including price and distance. Less than 1 percent of patients in the study sample availed themselves of a price comparison tool to shop for M.R.I.s before receiving one.

By this reasoning, the authors concluded that doctors sent people to more expensive locations than they had to. On the way to their M.R.I., patients drove by an average of six other places where the procedure could have been done more cheaply.

“Many patients are going to very expensive providers when lower-price options with equal quality are available,” said Zack Cooper, a health economist at Yale and a co-author of the study. Though patients seem to follow the advice of their doctors on where to go, their doctors don’t have all the information on hand to make the best decisions for the patient either.

There are over 15 M.R.I. locations within a half-hour drive for most patients. As with many health care services, there is a large variation of prices across these locations, which means a tremendous opportunity to save money by selecting lower-priced ones. In one large, urban market, prices for the procedure are as low as about $280 and as high as about $2,100.

If patients went to the lowest-cost M.R.I. that was no farther than they already drove, they’d save 36 percent. Savings rise if they’re willing to travel farther. Within an hour’s drive, for example, savings of 55 percent are available. Savings are split between patient and insurer, depending on cost sharing. On average, patients pay just over $300 toward the cost of the procedure.

There is no evidence that the quality of low- and high-priced M.R.I.s differs, at least enough to be clinically meaningful. The study found that virtually none of the M.R.I.s at any price level had to be repeated — strong evidence that the doctors relying on them are satisfied even with the lower-priced images.

At almost $1,500, the average price of a hospital M.R.I. is more than double that of one at an imaging center. The study found that doctors who work for hospitals (rather than independently) are more likely to send their patients for more expensive hospital-based imaging. Just getting all patients to use M.R.I.s that are no farther away and not in a hospital could save 16 percent.