Are there any regulations that state what needs to be evaluated (structures of the heart) when the definition of cardiac magnetic resonance imaging (MRI) codes 75557–75565 state “morphology”? There is no mention of “complete” or “limited” within the definition.
As you say, since "complete" and "limited" are not part of the code description and are not defined, then complete evaluation is not needed to assign these codes. As with other computed tomography (CT) and MRI codes, imaging enough to get the diagnosis is enough.
In CPT Changes 2008, when these codes were introduced, the example for 75557 stated "cardiac magnetic resonance imaging (MRI) is ordered to assess the right ventricular structure and function." While there was no description of what EXACTLY would be imaged, this tells us that these studies can be done for certain elements alone.