Do all arachnoid cysts cause slight compression? how could a doctor tell if a cyst is causing compression by just looking at a scan? left parietal lobe arachnoid cyst 30mm by 18mm?

If the cyst is big enough it may compress the brain or spine but it most often does not cause any symptoms. Yes, we can tell the compression when we look at the CT or MRI brain images. Base of the clinical symptoms and the images, we will decide if the cyst needs to be taken care of.

Arachnoid cysts are typically coincidental and non-pathological findings, and only rarely expand sufficiently to cause significant brain or spinal cord compression. Imaging typically allows objectification of the presence of a mass effect.