The nutcracker phenomenon (NCP) is an accentuated phenomenon of normal subtle compression of the left renal vein (LRV) between the abdominal aorta and superior mesenteric artery. The nutcracker syndrome (NCS) is a syndrome caused by NCP, resulting in hematuria, proteinuria, or left flank pain.  
The established diagnostic criterion of NCP is a pressure gradient more than 3 mmHG across the compression site, and NCS has been known to be rare, but probably is not rare and in fact quite common. The most important reason why NCS is known to be rare is that the diagnosis is difficult unless invasive venous catheterization and pressure measurement confirm the pressure gradient. Doppler ultrasound is useful in measuring the flow velocity of LRV. From the velocity measured by Doppler ultrasound, we may estimate the pressure gradient. Doppler ultrasound of LRV needs technical skills and experience. Computed tomography (CT) is a popular imaging technique for patients with hematuria, and we need to be familiar with the findings of the NCP at CT. There are variations of NCP associated with vascular anatomy and posture of the patients. This book will illustrate the concepts of NCP and NCS, findings at Doppler ultrasound and CT, and variations of NCP.