A method of detecting vulnerable arterial plaque formed on the vessel wall in vivo is disclosed. The method comprises obtaining an image of the arterial plaque, measuring the temperature of the arterial plaque, and determining whether the plaque exhibits an elevated temperature compared to the temperature of at least one adjacent vessel wall site. The method may include determining whether the plaque exhibits an elevated temperature of between 0.4 to 4 C. above at least one adjacent vessel wall site.