What is the life-saving imaging modality in this case?
Popliteal US with Doppler assessment is mandatory for popliteal fossa cystic lesions to exclude aneurysmal sacs and to avoid the wrong decision of a biopsy that could be fatal.
What do you think about the cause of knee pain in this case?
Mostly the soft tissue congestion caused by increased blood flow and venous return.
Contrast filled aneurysmal sac in the popliteal fossa communicated anteriorly with the popliteal artery via a narrow curved short neck. No leakage of contrast out of the sac. No thrombosis and or arterial branches originating from it.
Incidentally the liver is cirrhotic. Splenectomy. Contrast filled oesophageal varices with mesenteric and retroperitoneal edema.