Medical imaging already has a long history. First it was the X-ray machine, which has roots dating back to the late 19th century. Another great leap came in the 1960s through ultrasound, which uses bouncing sound waves to get a picture of what's going on inside the body. Computer-assisted tomography, or CT, which uses X-rays to image cross-sections of tissue and computer power to reconstruct them into readable form, was patented in the 1970s. Magnetic resonance imaging, MRI, arrived in the early 1980s. The technology uses powerful magnets to reorient certain atoms in the body and then capture the data produced when they return to their normal positions. Many new generations of these technologies have given radiologists and other physicians stunning views of the internal workings of the human body. Three-dimensional images available from positron emission tomography, or PET, provide yet another diagnostic tool being offered at a growing number of facilities across the country.