The invention provides a method and apparatus for monitoring a thermal effect of a conformal thermal treatment to diseased tissue in a target volume. The method includes acquiring a reference image of the volume containing the diseased tissue at a first position of the device, determining a baseline temperature distribution of the reference image, acquiring an image of the volume containing the diseased tissue while delivering a treatment to the volume, performing a phase subtraction between the image and the reference image to determine a change in phase between the image and the reference image and determining a current spatial temperature distribution in the volume containing the diseased tissue.
