93306 is a complete transthoracic echo with Doppler and Colorflow
93308 is a followup or limited transthoracic echo ONLY( no doppler or colorflow)
And you know a limited echo is anything less then a complete which would mean all of the structures that need to be mentioned for a full echo are not. Below is what would need to be documented for a complete echo.
LV
RV
LA
RA
mitral,aortic,tricuspid valves
pericardium
adjacent portion of the aorta
did I miss anything?