What does effacement of the ventricles mean and the left ventricle being smaller than the right is this normal or worrysome? can an arachnoid cyst cause this?

It is not feasible to interpret imaging studies in absence of the films themselves, the indications for the studies, and clinical findings prompting such. Suggest this be discussed with the doctors involved in the assessment. Alternatively, you could schedule a virtual consult at HT, provide your films, and obtain an opinion (www.connect@healthtap.com)