  While the multiexponential nature of T2 decays measured in vivo is well
known, characterizing T2 decays by a single time constant is still very useful
when differentiating among structures and pathologies in MRI images. A novel,
robust, fast and very simple method is presented for both estimating and
displaying the average time constant for the T2 decay of each pixel from a
multiecho MRI sequence. The average time constant is calculated from the
average of the values measured from the T2 decay over many echoes. For a
monoexponential decay, the normalized decay average varies monotonically with
the time constant. Therefore, it is simple to map any normalized decay average
to an average time constant. This method takes advantage of the robustness of
the normalized decay average to both artifacts and multiexponential decays.
Color intensity projections (CIPs) were used to display 32 echoes acquired at a
10ms spacing as a single color image. The brightness of each pixel in each
color image was determined by the intensity of the corresponding pixel in the
earliest image and the hue was determined by the normalized decay average.
Examples demonstrate the effectiveness of using CIPs to display the results of
a multiecho sequence for a healthy subject and a multiple sclerosis patient.
