how turn an image to a percentile image

Hello I have a medical image and I want to turn all the values in the image into percentile values. I am not sure how to do this. The way it has been explained is that the image is binned into percentiles. What I know I should do is that for each voxel value in the image I have to calculate the percent that falls bellow that certain value. However I dont know how to perform this and also keep all voxels in the original coordinate space when doing it. It is basically suppossed to normalize the image intensity. Im not very advanced in Matlab, hope I can get some help. Thank you :)