The invention is based on the realisation that inflamed plaques in blood vessel walls, which are potentially vulnerable to rupture, can be detected at elevated temperatures which are lower than previously realised. In particular, the low temperatures at which inflamed plaque can be detected are often associated with systems in which blood flow is generally close to normal in particular temperature differences above zero but not more than 0.39.degree. C. have been found to be indicative of the presence of inflamed plaque.