Many men diagnosed with early prostate cancer undergo "active surveillance" where doctors monitor their condition with biopsies and blood tests and only start treatment if the tumour grows.
But biopsies are painful, invasive and carry side effects, while the standard blood test can be inaccurate. The new technique, called diffusion-weighted magnetic resonance imaging is thought more patient-friendly and reliable.
Lesley Walker of Cancer Research UK said: "Imaging like this has great potential to provide non-invasive, accurate ways to monitor patients to help doctors limit the number of men who undergo unnecessary treatment.
"It now needs to be confirmed in much larger studies before this test should be used routinely in a clinical setting."