If you need a qualitative heat map only, I can suggest you an approximation. The idea is that the mean/std of the normalized monoexponential decay is directly related to the tau (decay parameter, as I understand). So, first normalize the stack to the first image. Then take a z-average over the stack. This will give you a heat map, where lower mean values mean faster decay . Hope it helps.

I think Shavkat's proposal is the best option you have if you want to stick to Imagej. If you have access to a more complex software like Origin, you can impotr the tif file as a 3d dataset, and perform the fit there.

Anyway i would suggest as the easiest solution, a scripting language, like Python or Matlab. You can find attached a very simple Python code, which would just need as external libraries Python image library to open the tif file, and Scipy and Numpy to perform the fits.

I hope it helps!

Thank you both for your answers and efforts. Some googleing revealed that there is actually already a great plugin for ImageJ called PixBleach. It allows fitting mono-, bi- or stretched exponential functions, and allows some adjustments of analysis parameters. In addition to providing a (TIF) 2D pseudo image of the tau values, it provides a corresponding image of the amplitude of the exponential, which I find very useful. It is very easy to install and use. I only used it in the monoexponential mode, but I assume the others work equally well. 

Hello dear Jan, I am wondering if you have a MATLAB code for this kinds of analysis. Thanks.

Hi Mohammad

No, I am afraid I dont have a MatLab code. i was looking for an imageJ solution, and when I found it, I stopped exploring the subject further.

Best

Jan