Normal scanners can spot cancerous lumps and tumours sitting beside normal flesh - but the computerised scanner can spot tiny numbers of microscopic cancer cells in as many as 10m normal cells.
Doctors at Hammersmith Hospital in London are preparing for clinical trials of a system which uses a sophisticated bone marrow scanner.
They believe that it will help them to see whether surgery and drugs have actually worked, and give an early warning of any return of the disease.
It is also hoped this will help them see the efficacy of a new drug, exemestane, which may help patients for whom existing chemotherapies have failed to have an effect.
Normal scanners can spot cancerous lumps and tumours sitting beside normal flesh - but the computerised scanner can spot tiny numbers of microscopic cancer cells in as many as 10m normal cells.
It is possible that this sensitivity may provide an "early warning" system to let doctors know if breast cancer is beginning to spread.
This means that existing medicines, such as tamoxifen, are not working - and could allow oncologists to start giving exemestane and then monitor how well it is working.
Professor Charles Coombes, a breast cancer expert working on the project, said: "It's an automated scanning system which can see very small numbers of cancer cells.
"50% of breast cancer patients have these small numbers of cells in their marrow when they first present with the disease.
Tamoxifen success
"The new drug could work in patients where tamoxifen has stopped working, and essentially we have a system which can then monitor cancer cells."
The drug has already proved useful in women in whom breast tumours have come back after initial treatment, so is likely to work well to halt the spread of cancer in women at an earlier stage of the illness.
But full clinical trials have yet to be carried out to prove this.
In the UK, approximately 35,000 women a year are diagnosed as having breast cancer.
Although drugs like tamoxifen have contributed greatly to large falls in death rates, approximately 13,000 women in the UK die from it each year.
The Cancer Research Campaign, which is funding the trials, says it hoped the combination could be available to women within five years.
Dr Lesley Walker, from the CRC, said: "The expense is not huge - the equipment is a fraction of the cost of current x-ray equipment.
"And the drug treatment could save the NHS money."