New York ? When Mike Stevens learned his lungs were riddled with cancer, it took only a week to start chemotherapy — but six weeks to find out if it was doing any good.

“You’re going through all this suffering and stuff and you want to know, am I going to survive? Is this stuff working?” said Stevens, 48, of La Jolla, Calif. “Your whole life is in sort of a limbo.”

Doctors typically must wait weeks or months to see if a treatment is shrinking tumors or at least halting their growth. But researchers are exploring a new use for medical imaging that could shorten the stay in purgatory, possibly revealing within a few days whether chemo is working.

That speed could save both lives and money. It would allow doctors to switch more quickly from an ineffective drug to a different one, and save health care dollars by waving doctors off expensive but futile treatments.

The same approach may also prove useful for monitoring radiation therapy.

This experimental imaging relies on a familiar hospital workhorse: PET scans, typically used for things like detecting cancer or revealing the effects of a heart attack. Unlike CT scans or MRIs, PET scans can show a tumor’s internal activity, not just its size.

When used to assess the effects of cancer treatment, it can reveal inside information about what the therapy is doing to a tumor even when there’s no outward sign.

To do a PET scan, doctors inject a patient with a radioactive substance that shows up on the scan in places where certain processes are happening — like hungry cancer cells gobbling up a lot of blood sugar. Think of it as looking around your neighborhood late at night for light in bedroom windows to see who is still awake.

Many cancer patients get PET scans now to assess their disease before treatment, or to spot recurrences later on. But except for lymphoma, PET scans aren’t routinely used to get a quicker answer on how cancers are responding to therapy.

The new research tests both standard PET scans and a newer approach that involves injecting a different tracer substance.

The standard scan, which looks for blood sugar usage, has gotten good results in tests with a variety of tumors including breast, prostate, colorectal and esophageal cancers, said Dr. Steven Larson of the Memorial Sloan-Kettering Cancer Center in New York.

Some experiments have revealed chemo’s effects within 10 days to two weeks.