Transplanted kidney. Coloured ultrasound scan of a transplanted kidney in a 24-year-old patient. The kidney is the grey area. Transplantation replaces a failed or failing kidney with a donor kidney. The kidneys filter waste from the blood and excrete it as urine. Ultrasound scanning 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.