New methods for viewing diseased structures inside the body improved diagnosis of disease beginning in the 1970s. A gamma camera detects radioactive medication that attaches to certain forms of cancer cells. Computed tomography (CT) scanners use X rays to produce lifelike three-dimensional images of body structures. Magnetic resonance imaging (MRI) scanners produce highly detailed images without X rays. Positron emission tomography (PET) detects very early warning signs of disease. Sonograms, or ultrasound, taken with high-frequency sound waves diagnose disease and monitor the progress of pregnancies. X rays and high-energy particles emitted by linear accelerators also are used to treat cancer. Lithotripsy uses high-frequency sound waves to destroy some kidney stones and gallstones, conditions that once required surgery.