CHICAGO — A shipment of 18 frozen human heads discovered and seized by customs officials during routine X-ray screening of cargo arriving at O’Hare International Airport in Chicago were used for medical research in Italy and were being returned for cremation in Illinois, officials said Tuesday. The heads are used for training in fields such as dentistry, ophthalmology and neurology, where they are used for Alzheimer’s research. They are also used to train plastic surgeons and by students learning to perform facial reconstructions.