1. A method of identifying a desired region of interest in a medical image of a subject where said medical image has a first maximum region having a first maximum value of a given variable inside of which said region of interest is to be identified, and a second maximum region spaced from said first maximum region and with a second maximum value of said given variable where the second maximum value is greater than the first maximum value, comprising the steps of: registering a user-selected initial voxel in the image which is in said first maximum region; also determining a first threshold which is a given fraction of said first maximum value and also defining a maximum tolerated distance between said user-selected voxel and said region of interest to be identified; using said first threshold, identifying values of said given variable of the image which are higher than said first threshold so as to define a first region of interest; checking whether said first region of interest includes both the first and the second maximum regions, and if so increasing the first threshold to a second threshold which is said given fraction of the second maximum value; using said second threshold, identifying values of the given variable of the image which are higher than said second threshold to define a second region of interest; if a distance of said second region of interest from said user-selected initial voxel is greater than said tolerated distance then determining a third threshold which is a higher fraction of said first maximum value than said given fraction; and using said third threshold, identifying values of the given variable of the image which are higher than said third threshold to define a third region of interest as said desired region which lies inside said first maximum region.