What principle does an ultrasound rely on?

Ultrasound relies on the principle that sound waves can be transmitted and reflected back when they encounter different media. When an ultrasound machine emits high-frequency sound waves into the body, these waves travel through various tissues and fluids. When they hit a boundary between different types of tissues, some of the waves are reflected back to the ultrasound transducer while others continue to propagate through the tissue. The time it takes for the echoes to return to the transducer is then used to create an image of the internal structures based on these reflections.

The other options don't fully capture how ultrasound operates. While it is true that sound waves travel in straight lines, that statement alone doesn’t encompass the essential function of ultrasound in medical imaging. High-frequency sound waves are indeed used to create detailed images, but the key action is the transmission and reflection of these waves. The notion that sound waves cannot penetrate liquids misrepresents how ultrasound works; in fact, ultrasounds can and do pass through various liquids effectively, which is crucial for imaging organs that contain fluid.