It can be very difficult to determine if a hyperdense cyst in polycystic disease contains blood or proteinaceous fluid based on the CT findings.   MRI is better able to differentiate the two. Blood in this case has a morphological pattern of a peripheral bright T1 (low signal T2) rim indicative of intracellular methemoglobin and large core of predominately T1 and T2 hypointensity indicative of deoxyhemoglobin.  This is not typical of a uniform fluid-filled structure ie cyst.  Note there are adjacent cysts containing clear fluid ie low T1 and high T2 signal and proteinaceous fluid ie high T1 and low T2.