Ultrasound imaging uses high frequency sound waves to look inside the body without using ionizing radiation. A handheld probe sends pulses of sound into tissue and listens for echoes that return from boundaries between different materials. The timing and strength of those echoes are converted into a grayscale image on a monitor.
This technology matters because it helps doctors view organs, blood flow, muscles, and developing babies in real time.
Key Facts
- Ultrasound frequency is typically 2 MHz to 15 MHz, far above the human hearing range of about 20 Hz to 20,000 Hz.
- Wave speed in soft tissue is approximated as v = 1540 m/s.
- Echo depth is found from d = vt/2 because the sound travels to the boundary and back.
- Wave relationship: v = fλ, where v is wave speed, f is frequency, and λ is wavelength.
- Higher frequency gives better resolution but less penetration because more sound is absorbed and scattered.
- Brighter pixels usually represent stronger returning echoes from larger changes in acoustic impedance.
Vocabulary
- Ultrasound
- Ultrasound is sound with a frequency above the range of human hearing, used in medicine to create images from echoes.
- Transducer
- A transducer is the probe component that converts electrical signals into sound waves and returning sound waves into electrical signals.
- Echo
- An echo is a reflected sound wave that returns to the probe after hitting a boundary between tissues.
- Acoustic impedance
- Acoustic impedance is a measure of how strongly a material resists sound wave motion and affects how much sound is reflected at a boundary.
- Resolution
- Resolution is the ability of an imaging system to distinguish two nearby structures as separate objects.
Common Mistakes to Avoid
- Using d = vt instead of d = vt/2 for echo depth is wrong because the measured time includes both the outgoing and returning trip.
- Thinking ultrasound images are photographs is wrong because the image is calculated from echo timing and strength, not visible light.
- Assuming the highest frequency is always best is wrong because high frequency improves detail but reduces penetration into deeper tissue.
- Ignoring tissue boundaries is wrong because most useful echoes come from changes in acoustic impedance between materials, not from uniform tissue.
Practice Questions
- 1 An ultrasound echo returns to the probe 52 microseconds after a pulse is sent. Using v = 1540 m/s, how deep is the reflecting boundary?
- 2 A probe emits ultrasound at 5.0 MHz in soft tissue where v = 1540 m/s. What is the wavelength of the sound wave in millimeters?
- 3 A doctor switches from a 3 MHz probe to a 12 MHz probe for a shallow scan. Explain how this change affects image detail and penetration depth.