Renal dystrophy. Colour doppler ultrasound scan inside a human kidney with impaired renal function (dystrophy). The ureter (black, round) is dilated where it exits from the kidney, near the junction of kidney blood vessels (red). The ureter normally passes urine to the bladder, but dilation causes backflow into the kidney. This ultrasound scan was made by Dr Philippe Saada. Saada obtains high- definition images by linking doppler ultrasound to a numeric colour video. Doppler ultrasound uses high-frequency sound waves to determine whether organs are diseased, and can register moving particles like blood. Photographed at the Ultra- sound Centre of La Marjolaine at Montrouge, Paris.