Woman being treated for kidney stones using lithotripsy, a non-invasive technique that uses ultrasound waves to break the stones down into smaller fragments that may be passed out in urine. The patient is immersed in a water bath, together with with the two cylindrical devices that focus sound waves into the kidneys in a series of repeated blasts". Urinary stone formation is a metabolic disorder which results in precipitation of mineral ions (notably calcium) in urine. Normal urine contains polymer substances which inhibit crystal growth and prevents stones from growing to a size where they are too large to be expelled in the flow of urine."