A technique for determining whether cancer has spread could save many patients from surgery.
The technique uses sophisticated scans
It involves injecting tiny magnetic particles designed to accumulate within certain cancerous tissues.
By tracking their movement on a scan, doctors can identify whether a tumour has spread, without having to resort to taking biopsies.
The Harvard Medical School study is published online by the Public Library of Science Medicine.
The researchers hope it will make life easier for patients and improve the accuracy of cancer diagnoses.
Dr Ralph Weissleder, who worked on developing the technique, said: "This method of cancer staging provides unprecedented accuracy and will spare unnecessary surgery."
Once somebody has been diagnosed with cancer, it is important that doctors establish how far advanced it is.
This is a process called staging and normally involves looking for signs that the cancer has spread to the lymph nodes.
These are fleshy, pea-sized structures that are found in groups throughout the body, such as the neck, armpits and groin.
When a tumour spreads, it normally targets one particular lymph node, known as the sentinel node.
By removing this one particular node, cancer specialists can check for malignant cells.
If there are none it is very unlikely that there will be any cancer cells deeper in the lymphatic system.
But lymph node biopsy can be alarming for patients and is not always accurate.
Researchers are looking for better ways to grade cancers more accurately.
The US team recruited 70 patients with a range of different cancers.
Each one was injected with magnetic nanoparticles that were designed to home in on the specific magnetic properties of cancerous cells in the lymph nodes.
Using MRI scans, they were able to establish that there were different patterns for malignant nodes and healthy nodes.
The researchers then developed a computer program that could detect which lymph nodes were cancerous and generate a three-dimensional reconstruction for doctors to look at.
A spokesman for Cancer Research UK said the breakthrough had the potential to dramatically improve cancer diagnosis.
Medical director Professor John Toy said: "The technique shows early promise that it may be able to pick up nodes that are involved with cancer spreading, without the need for a surgeon's scalpel.
"But it's not a very large study and the results showed that 8% of patients came up positive when they were not.
"So you could end up subjecting some patients to chemotherapy when they do not need it."