Medical Imaging
Since its introduction in the late 1950s, ultrasound has become one of the most useful tools in obstetric medicine, a branch of medicine focusing on the care of pregnant women. Using high-frequency sound waves, undetectable to the human ear, ultrasound allows doctors to see inside the body of an expectant mother and take images of her baby. It uses the same type of technology as in SONAR, which ships use to look for underwater objects. While most common in obstetric medicine, ultrasound is also employed in other medical disciplines, especially when looking at soft organs, since it is safer and often more effective than x-rays.
An ultrasound machine uses a phased array to send sound pulses back and forth across a patient. These pulses are partially reflected by organs and other matter, and the reflected signals are turned into an image. The Ultrasound activity lets students explore how ultrasound works and take images of a hypothetical "patient." The activity contains three sections, plus an introduction, all of which help demonstrate the important aspects of ultrasound technology.
Arrays
Instead of using moving parts, ultrasound uses a phased array to send sound waves sweeping from side to side. In this activity, the students can adjust a phased array to control the ultrasound signal, or they can observe the Auto mode to see how an actual transducer sends signals. By gradually changing the phase shift of several sources, a region of constructive interference moves back and forth. This allows the transducer, which acts as both the transmitter and receiver, to send and receive signals from several directions, so it can scan a large area of the body.
Reflection
A sound wave consists of several small packets, called phonons, which are occasionally reflected by matter. Use this activity to explore the reflection of phonons as a sound pulse passes through matter, and study how the density and thickness of the matter affects the reflection. When the reflected phonons return to the transducer, it records them as a signal. As the transducer sweeps around, it combines all of these signals to create a final image.
Images
Ultrasound images are created by combining the reflected signals as the phased array scans across the patient. In the final activity, you can use ultrasound to scan a series of blocks. Try moving the blocks around inside the "Patient," and observe the final image.
Advanced Discussion Questions
- Arrays: Discuss why it is easy for the transducer to detect the regions of constructive interference.
- Reflection: Discuss why the pulse is partially reflected by matter. What are some real-life examples of sound reflection and of sound traveling through solid matter?
- Reflection: Discuss how the density and the thickness of the matter affects the amount of reflection.
- Images: Discuss why the image does not change instantly when you move a block.
Additional Resources
Obstetric Ultrasound
(in-depth information on ultrasound as used in obstetric medicine)
A search using Britannica
(a good list of links related to X-rays)