Because there was LAB work long before there were CAT scans.
CAT = Computed Axial Tomography
CT = Computed Tomography
Same thing. Just less wordy.
Does anyone remember when MRI scans were called NMR (Nuclear Magnetic Resonance)??? Since the word Nuclear is politically incorrect, the medical industry relabeled them MRI (Magnetic Resonance Imaging). Same thing. Just less scarry.