The words heterogeneous and heterogenous, and their antonyms homogeneous and homogenous, respectively, are commonly used in radiology reports and medical literature. There is a seemingly widely held misconception that they represent alternative spellings of the same word, possibly with "heterogeneous" representing the British spelling whereas "heterogenous" being the US spelling. This is incorrect. They are, in fact, different words with distinct meanings.
Heterogeneous refers to a structure with dissimilar components or elements, appearing irregular or variegated. For example, a dermoid cyst has heterogeneous attenuation on CT. It is the antonym for homogeneous, meaning a structure with similar components.
Heterogenous refers to a structure having a foreign origin. For example, heterogenous bone formation is bone where bone should not exist. To make matters worse, heterogenous bone formation is often also heterogeneous! Heterogenous is, therefore, the antonym of homogenous, meaning the tissue or structure is located or originates from an expected location. Supposedly, one could say that hepatocellular carcinoma is homogenous, as it arises within the liver. There is, however, little use for this word.
How to use...
If you are confused or bored by such semantic pedantry or just do not care about it, you are not alone and congratulations on reading this far(!), but there is a simple rule: always use the version with the extra "e" (homogeneous and heterogeneous). If you do that, you will be right 99% of the time.