Acute Deep Vein Thrombosis: On ultrasound, the vein in distended by hypoechoic thrombus and will not compress or may partially compress, without collaterals. In the below image showing acute deep vein thrombosis the common femoral vein is enlarged (image A yellow arrow) and non-compressible (image B yellow arrow). Image C shows a lack of flow within the vein.

Chronic Deep Vein Thrombosis: An ultrasound demonstrating chronic deep vein thrombosis will exhibit a vein that is incompressible, narrow and irregular and shows echogenic thrombus attached to the venous walls with development of collaterals. The below image shows the right popliteal vein with an echogenic vein wall and a compressible lumen with echogenic material within (image A). Image C shows flow around the echogenic material within the lumen, but flow is partially blocked and diverted around the material.

Ultrasound can be used to detect deep vein thrombosis as well as differentiate acute from chronic. 

Superior Vena Cava Syndrome: CT is the imaging modality of preference for evaluation of SVC Syndrome. The superior vena cava is responsible for approximately one-third of venous return to the heart, making it extremely important and any compression or blockage a serious condition. SVC Syndrome is a process where venous return through the SVC is impaired. In this particular case this is occurring by compression of the SVC (blue arrow) due to the large mediastinal lymph nodes shown by the red arrow. The SVC is compressed because of its think walls compared to arterial walls.

Renal Vein Thrombosis: This CT image displays a normal kidney and renal vein on the right which provides a great comparison for the left side. The left kidney is enlarged and the left renal vein is dilated with no evidence of contrast filling at the venous phase. This is indicative of renal vein thrombosis. This particular patient had anti-phospholipid antibody syndrome which results in hypercoagulability of the blood and development of renal vein thrombosis.

Venous Aneurysm: Ultrasound with the use of color Doppler is often the initial and best choice of imaging to evaluate/diagnose a venous aneurysm. The below black and white image shows distention of the vein and the color image suggests that there is abnormal flow.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5220209/#:~:text=In%20acute%20thrombosi %2C%20vein%20is,of%20collaterals%20(Figure%202).

https://www.swjpcc.com/imaging/2020/12/2/medical-image-of-the-month-superior-vena-cava-syndrome.html

https://radiopaedia.org/cases/renal-vein-thrombosis-1

https://radiopaedia.org/articles/popliteal-venous-aneurysm?lang=us