In one of the first moves by Congress to control expenditures in doctors' offices, the Senate voted 50 to 45 yesterday to require doctors to get local health planning agencies' approval before buying any equipment worth more than $100,000.

The measure was intended to stem a buying rush that has seen the nation's hospitals and doctors buying more than 1,000 "CAT" scanners - computerized X-ray machines - at a cost of around $525 million. Health planning officials believe many of them are unneeded.

The new proposal was backed by Sen. Edward M. Kennedy (D-Mass.) - and opposed by the American Medical Association, in full lobbying force - only two days after Kennedy and the AMA jointly opened a conference here on preventive medicine in what they called a new spirit of cooperation.

A Senate aide called yesterday's vote "the first time the Senate has voted against the AMA in a very long time.

The vote came during consideration of a bill to extend the 1974 health planning act. That law has led to a nationwide system of state and local health planning agencies that review all major expenditures by hospitals, and proposals to build new ones.

Over the years, doctors have fought any attempt to extend such spending controls to them too. At the same time, some doctors who traditionally use costly hospital equipment have begun moving their bases of operations to avoid these and other controls.

In Senate debate, Kennedy called physicians "the only major health group" trying to avoid the controls. Without controls, he said, health planning will fail and "waste, fraud and abuse" will increase costs.

The Senate finally passed the health planning extension by voice vote. A similar measure awaits a House vote.

The House commerce health subcommittee chairman, Rep. Paul Rogers (D-Fla.), sponsored a doctors office control proposal much like the Senates. AMA opposition led to a compromise, and the version passed by the full committee would control doctors' equipment costing over $150,000, and only if it is used for a hospitals bed-patients. Much present use of CAT scanners and other expensive equipment is for hospitals' out-patients or those who never occupy a bed.