Ultrasound medical imaging | Mechanical waves and sound | Physics | Khan Academy

Key Concepts

• Ultrasound: Sound waves with frequencies above the human hearing range (above 20,000 Hz).
• Transducer: A device that converts electrical energy into sound energy (and vice versa).
• Reflection: The bouncing back of a wave when it encounters a boundary between two different media.
• Interface: The boundary between two different media.
• Speed of Sound: The speed at which sound waves travel through a medium, determined by the properties of the medium.
• Wavelength: The distance between two successive crests or troughs of a wave.
• Frequency: The number of complete cycles of a wave that pass a point in a given time (measured in Hertz).
• Diffraction: The spreading out of waves as they pass through an opening or around an obstacle.

Ultrasound for Medical Imaging

The Basic Principle

Ultrasound imaging utilizes high-frequency sound waves (ultrasound) to visualize internal structures of the human body. A transducer is placed on the skin and emits pulses of ultrasound. These pulses travel through the body and reflect at interfaces between different tissues or media (e.g., between organs and blood). The transducer detects the reflected pulses and measures the time it takes for them to return. Knowing the speed of sound in the body, the transducer calculates the distance to the reflecting interface. By analyzing the timing and intensity of the reflected pulses, an image of the internal structures can be created.

Step-by-Step Process

1. Transducer Placement: A transducer is placed on the skin over the area of interest.
2. Pulse Emission: The transducer emits a short pulse of ultrasound.
3. Wave Propagation: The ultrasound pulse travels through the body.
4. Reflection at Interfaces: When the pulse encounters an interface between two different media (e.g., tissue and blood), a portion of the wave is reflected back towards the transducer.
5. Time Measurement: The transducer measures the time it takes for the reflected pulse to return.
6. Distance Calculation: Using the speed of sound in the body, the transducer calculates the distance to the reflecting interface.
7. Image Construction: By emitting multiple pulses and analyzing the reflected signals, the transducer creates an image of the internal structures.

Why Ultrasound Frequencies?

The video highlights two primary reasons for using ultrasound frequencies in medical imaging:

1. Patient Comfort: Audible frequencies would be noisy and potentially disturbing to the patient.
2. Reduced Diffraction: Higher frequencies (and thus shorter wavelengths) result in less diffraction.

Diffraction and Image Clarity

Diffraction is the spreading out of waves as they pass through an opening or around an obstacle. In the context of ultrasound imaging, diffraction can cause sound waves to bend around corners and reflect off of unintended surfaces, leading to a blurry image. High-frequency ultrasound waves have shorter wavelengths, which means they diffract less and produce clearer images.

The video uses the analogy of waves passing through a small hole in a barrier to illustrate diffraction. Low-frequency waves spread out significantly after passing through the hole, while high-frequency waves spread out much less. This difference in diffraction explains why high-frequency ultrasound is preferred for medical imaging.

Equation

The video mentions the relationship between the speed of a wave (v), its wavelength (λ), and its frequency (f):

• v = λ * f

This equation shows that for a given speed of sound (v), a higher frequency (f) corresponds to a shorter wavelength (λ), and vice versa. The speed of sound is determined by the medium through which it is traveling.

Conclusion

Ultrasound imaging is a valuable medical tool that uses high-frequency sound waves to visualize internal structures. By emitting pulses of ultrasound and analyzing the reflected signals, a transducer can create an image of the body's interior. The use of ultrasound frequencies minimizes diffraction, resulting in clearer images and avoiding patient discomfort.