The png file you posted is 8 bits per channel, uint8, 3 channel, with a transparency (alpha) channel that is set to all 255 (uint8) which is completely opaque.
The .mat file you posted relates to a different image, and is a matrix of uint16 values.
Are you asking how the uint16 is represented inside the file? If so then the answer is that they are stored in http://stackoverflow.com/questions/2384111/png-file-format-endianness "Network Byte Order", more commonly known as "Big Endian". That is a byte order in which the most significant of the bytes occurs first and then the less significant. That is the opposite of the order used by the x86 architecture, but it is the same as the order reflected by dec2hex and sprintf() with '%x' format.
What do the values correspond to, what units are they? I would put it to you that the images have been extracted from DICOM files without the relevant headers. See http://www.idlcoyote.com/fileio_tips/hounsfield.html
