Both are the same. Both are the short form/abbreviation for Computerized Axial Tomography. The short form ``CAT`` might have been further shortened as ``CT`` for the sake of brevity and easiness. That is all.
CAT scan or CT scan is the computerized axial tomography which is a combination of many x-rays with the help of a computer which helps to view the cross sectional images of the internal organs .It is used to detect the abnormalities in the body.
As far as I know they are the same thing. CAT stands for Computerized Axial Tomography. Which later became simply know as Computerized Tomography. CT is basically just a shorted version of a CAT scan, since people are lazy and want to say things in as shortened terms as possible.