1. A method for very fast, in vivo, differential tissue characterization, said method comprising the steps of: a) providing a magnetic resonance imaging apparatus capable of operation at diffusion encoding levels (b-factors) at least between about 100 sec/mm 2 and about 5000 sec/mm 2 ; b) acquiring a set of signal decays from a patient using said magnetic resonance imaging apparatus at each of a selected plurality of encoding levels distributed across the range of b-factors within its capability, each said set of signal decays being representative of an image of a preselected anatomical cross-section of the patient, and each said signal decay of each said set corresponding to a pixel of its associated image; c) processing said acquired signal decays on a pixel by pixel basis to obtain the best possible fit between them and the mono-exponential equation: S&equals;S 0 exp(− bD ) wherein S 0 is a constant that depends on (i) constant values associated with the magnetic imaging apparatus, (ii) the spin-spin relaxation time T2, (iii) the spin-lattice relaxation time T1, and (iv) the spin density &rgr;, wherein b is the diffusion encoding level, and wherein D is the diffusion coefficient representing the slope of the natural logarithm of S vs b; d) quantifying the differences between the best fit determined in step c and the measured signal decay for each pixel; and e) displaying the quantified differences determined in step d as the amplitudes of a resultant image such that a difference of zero is displayed as a black pixel, and the larger the difference is the brighter the display of the associated pixel becomes.