Nice case of renal vein thrombosis (RVT). Whenever you look at the kidneys, and notice that one side is asymmetrically enlarged, make sure to look at the parenchyma to evaluate its enhancement. Remember - you have a control side (the other kidney), which will often be normal. In this case, the left kidney has delayed enhancement with perinephric stranding. Unfortunately, one of our guys missed the renal vein thrombosis and called this pyelonephritis.
Honestly, I think vascular structures outside of the Aorta are blind spots on CT for most radiologists, especially if it's not a CTA or CTV. Start including the other vascular structures (portal vein, mesenteric and renal vessels, and iliac veins) in your search pattern now so that you can buzz through them on every case looking for hidden abnormalities.
RVT is, much like portal vein thrombosis, related to either bland or tumor thrombus. In this case, there was no renal tumor present (other than some simple cysts), so this was a bland thrombus. According to radiopedia, the majority of bland thrombi are related to hypercoaguability from nephrotic syndrome (something I didn't know!), but I have most commonly seen thrombus in renal cell carcinoma and post trauma. The thrombus can break off and result in pulmonary embolism, so keep that complication in your back pocket. If you aren't sure if the vein is thrombosed, a renal doppler ultrasound would be a quick and helpful exam.
