Dear MS. Writeacher - Thank You
1.(4). Since the previous scan, the patient has undergone surgery leaving a distorted and small right hemithorax.
I put the comma here because you could start a new sentence with The Patient .....
2.(5). The remainder of the visualized aspects of the upper abdomen were unremarkable.
Changed it to were unremarkable
1 = correct
The problem with #2 is that the subject (remainder) is singular, but the verb (whether it's "are" or "were") is plural. It would be better to rephrase the sentence like this:
The remaining visualized aspects of the upper abdomen are unremarkable. (You could also use "were" in this sentence.)
Now the subject is "aspects" (plural) and the verb agrees/matches it. Here is a link explaining the specific error in the original sentence:
http://grammar.ccc.commnet.edu/grammar/sv_agr.htm
See #8.