A substance that obstructs the passage of x-rays is called?

A substance that obstructs the passage of x-rays is called a radiopaque substance. Radiopaque substances are used in medical imaging to improve the visibility of certain body parts or structures on x-rays. They are typically made of heavy metals, such as barium or iodine, which absorb x-rays and create a white appearance on the x-ray image. This makes it easier for radiologists to see the outlines of organs and structures, and to identify any abnormalities. Radiopaque substances are often used in diagnostic procedures such as barium swallows, upper gastrointestinal (GI) series, and computed tomography (CT) scans.