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Kidney. Coloured 3-D ultrasound scan of a healthy human kidney. The kidneys filter nitrogenous waste from the blood, excreting it as urine. The kidneys are located in the rear of the abdominal cavity just above the waist. There is one kidney on each side of the body. Ultrasound is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure. 3-D scanning uses computer technology to return more detailed images than conventional 2-D scans. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR NAJEEB LAYYOUS)